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Classical swine fever

Classical swine fever (CSF), also known as hog cholera, is a highly contagious viral disease primarily affecting domestic pigs (Sus scrofa domesticus) and wild boar (Sus scrofa), caused by the classical swine fever virus (CSFV), an enveloped, positive-sense single-stranded RNA virus in the genus Pestivirus of the family Flaviviridae.^[1]^[2] The virus targets the mononuclear phagocytic system, lymphoid tissues, and endothelial cells, leading to immunosuppression, vascular damage, and hemorrhages that define its pathology.^[3] Clinically, CSF manifests in acute, subacute, chronic, or subclinical forms, with acute cases featuring high fever (up to 42°C), anorexia, lethargy, conjunctivitis, and progressive hemorrhages in skin, mucous membranes, and organs like the kidneys and spleen, often culminating in mortality rates exceeding 90% in naive piglets.^[4]^[5] Subclinical infections, more common with low-virulence strains, contribute to persistent shedding and silent spread, complicating detection.^[4] Transmission occurs via multiple routes, including direct contact with infected secretions, indirect exposure to contaminated fomites, feed, or semen, and vertical transmission from carrier sows, with wild boar serving as reservoirs in endemic areas.^[2]^[6] No specific antiviral treatment exists; control emphasizes strict biosecurity, movement restrictions, culling of infected herds (stamping-out), and vaccination with live-attenuated strains like C-strain, which has enabled eradication in regions such as North America (U.S. free since 1978) and parts of Europe.^[7]^[8] Epidemiologically, CSF remains endemic in Asia, Eastern Europe, and parts of Latin America, driving substantial economic losses—estimated in billions annually—from mortality, production halts, and international trade embargoes, as it is a notifiable disease under World Organisation for Animal Health (WOAH) standards.^[1]^[9] Genotypic diversity among CSFV isolates, classified into 1–7 subgroups based on E2 glycoprotein sequences, influences virulence and vaccine efficacy, underscoring ongoing challenges in global surveillance and marker vaccine development.^[8]^[4]

Etiology and Virus Properties

Taxonomy and Structure

Classical swine fever virus (Pestivirus suis, previously known as classical swine fever virus or hog cholera virus) belongs to the genus Pestivirus within the family Flaviviridae.^[10]^[11] This classification reflects its phylogenetic relationship to other pestiviruses, such as those causing bovine viral diarrhea, based on genomic and antigenic similarities established through sequence analysis and serological studies.^[1] The International Committee on Taxonomy of Viruses (ICTV) updated the species nomenclature to Pestivirus suis in 2023 to align with standardized binomial naming for pestiviruses, emphasizing its host specificity to swine.^[11]^[12] The virion of CSFV is an enveloped, spherical particle measuring 40–60 nm in diameter with icosahedral symmetry in its nucleocapsid.^[13] It consists of a single-stranded, positive-sense RNA genome approximately 12.3 kilobases in length, encapsidated by the core protein (C) and surrounded by a lipid envelope derived from host cell membranes.^[14]^[8] The envelope incorporates three glycoproteins—Erns (ribonuclease-associated), E1, and E2—that mediate receptor binding, membrane fusion, and host cell attachment, with E2 serving as the primary immunogen for neutralizing antibodies.^[4]^[15] These structural components enable the virus's entry via clathrin-mediated endocytosis and determine its tissue tropism in porcine hosts.^[13]

Genomic Features and Replication

The genome of classical swine fever virus (CSFV) consists of a single-stranded, positive-sense RNA molecule approximately 12.3 kilobases in length, flanked by a 5' untranslated region (UTR) of about 373 nucleotides containing an internal ribosome entry site (IRES) and a variable 3' UTR often featuring poly-uridine tracts.^[4]^[16] This genome encodes a single open reading frame (ORF) that translates into a polyprotein of roughly 3,898 amino acids.^[17] The polyprotein undergoes co- and post-translational proteolytic cleavage by viral (N^pro, NS2, NS3) and host cell proteases to yield 12 mature proteins: four structural (capsid protein C, envelope glycoproteins Erns, E1, and E2) and eight non-structural (N^pro, p7, NS2, NS3, NS4A, NS4B, NS5A, NS5B).^[4]^[16] The structural proteins facilitate virion assembly and host cell attachment, with C forming the nucleocapsid enclosing the RNA genome, Erns exhibiting ribonuclease activity and immunomodulatory effects, and E1/E2 heterodimers mediating entry; non-structural proteins support replication, with NS3 providing serine protease and helicase activities, NS5B acting as the RNA-dependent RNA polymerase, and others like NS4A/B/5A inducing membrane rearrangements for replication complexes.^[4] CSFV replication occurs entirely in the host cell cytoplasm, initiating with attachment via envelope glycoproteins Erns and E2 binding to cellular receptors such as CD46 and glycosaminoglycans.^[4]^[13] Entry follows through receptor-mediated endocytosis, where the E1-E2 heterodimer promotes membrane fusion and release of the viral core into the cytosol.^[4] The released genomic RNA is then translated in a cap-independent manner via the IRES in the 5' UTR, producing the polyprotein that is rapidly processed into functional units.^[4]^[16] Replication complexes form on rearranged endoplasmic reticulum membranes induced by non-structural proteins, where NS5B synthesizes a complementary negative-sense RNA strand serving as a template for multiple positive-sense genomic and subgenomic RNAs.^[4] New positive-sense RNAs associate with C protein to form nucleocapsids, which bud into the endoplasmic reticulum lumen acquiring the envelope glycoproteins, followed by trafficking through the Golgi apparatus for maturation and final release via exocytosis without overt cytolysis in many strains.^[4] This process exploits host lipid metabolism and trafficking pathways, with viroporin p7 potentially aiding ion homeostasis and membrane permeability during assembly.^[4]^[18] The efficiency of replication contributes to high viral yields in permissive cells like porcine macrophages and endothelial cells, underpinning the virus's pathogenesis.^[4]

Pathogenesis

Viral Entry and Tissue Tropism

Classical swine fever virus (CSFV) initiates infection through receptor-mediated endocytosis following attachment to host cell surfaces. The process involves the viral glycoproteins E^rms, E1, and E2, where E^rms binds to attachment factors such as heparan sulfates and the laminin receptor, while E2 facilitates interaction with entry receptors like CD46, leading to internalization via clathrin- or caveolin-dependent pathways that are pH-, dynamin-, and cholesterol-dependent.^[19]^[16] Additional host factors, including ADAM17, annexin II, and MERTK (which binds E2), promote attachment and entry, with MERTK enhancing viral uptake in porcine kidney cells.^[19] The virus uncoats in endosomes, releasing its RNA genome into the cytoplasm for replication.^[16] Tissue tropism of CSFV primarily targets the mononuclear phagocytic system and vascular endothelium in pigs, with initial replication occurring in the tonsils after oropharyngeal exposure.^[16] The virus exhibits high replication in lymphoid tissues such as the spleen, submandibular, inguinal, and mesenteric lymph nodes, as well as in the pancreas, ileum, and blood, where viral loads peak around day 8 post-infection in experimental models.^[20] Moderate replication occurs in the tonsils, liver, lungs, jejunum, and kidneys, while loads remain low in the heart, duodenum, and brain.^[20] Key target cells include monocytes, macrophages, dendritic cells, and endothelial cells, facilitating systemic spread via viremia and contributing to vascular damage and immunosuppression.^[16] Virus is detectable in all examined organs by day 1 post-infection, with progressive increases in permissive tissues.^[20]

Host Immune Response and Disease Mechanisms

Classical swine fever virus (CSFV) infection triggers an initial innate immune response in pigs, characterized by recognition via pattern recognition receptors such as RIG-I and TLR3, leading to type I interferon (IFN) production and cytokine release including IL-1β and IL-6.^[21] However, CSFV rapidly evades this through viral proteins like Npro, which degrades IRF3 to block IFN regulatory factor signaling, and Erns, whose RNase activity targets viral dsRNA to inhibit RIG-I and NF-κB pathways.^[21] NS5A further suppresses STAT1 phosphorylation and NF-κB activation, while NS4B disrupts TLR3 signaling, resulting in diminished antiviral states and unchecked viral replication in monocytes and macrophages, which are infected at rates up to 90% within 24 hours post-infection.^[21]^[22] In the adaptive immune response, CSFV targets lymphocytes indirectly, inducing activation-induced cell death (AICD) via upregulated Fas expression on T cells (>30% by days 3–5 post-infection) and TNF-α mediation, causing apoptosis detectable as early as 1 day post-infection with sub-G1 cell increases up to 6.1% by day 7.^[23] This leads to profound lymphopenia (lymphocyte counts dropping below 10,000/μl by day 2 and to <1/10 initial levels by day 5), impairing T-cell proliferation (4–5-fold reduction by day 3) and B-cell function, with no effective neutralizing antibody production.^[23] NS3 degrades MHC-I to hinder antigen presentation, depleting CD8+ T cells and reducing IFN-γ secretion, while elevated IL-10 and TGF-β1 further suppress adaptive immunity.^[21] Disease mechanisms stem from this immunosuppression and direct cytopathic effects, with CSFV inducing apoptosis in uninfected bystander lymphocytes (infection rate <3% in leukocytes) to evade clearance, while inhibiting it in permissive cells via Npro to sustain replication.^[23]^[21] Monocytic cells, primary targets, produce elevated prostaglandin E2 (5–20 times higher serum levels by day 5) and IL-1 (up to 5-fold in monocytes), driving vascular permeability, fever, and hemorrhagic diathesis including petechiae and spleen infarction by days 4–5.^[22] Autophagy, enhanced by E2 and NS5A via LC3 binding and mTORC1/AMPK modulation, supports virion assembly and mitophagy to reduce IFN-triggered apoptosis, while NLRP3 inflammasome activation promotes pyroptosis in lymphoid tissues, amplifying IL-1β release, inflammation, and lymphocyte depletion for persistent infection.^[24]^[21] These processes collectively cause leukopenia, secondary infections, and multisystemic pathology, with viremia peaking at 10^3–10^5 TCID50/ml by days 5–7.^[23]

Strain Variability

Genotypes and Subgenotypes

Classical swine fever virus (CSFV) strains are phylogenetically classified into three major genotypes—1, 2, and 3—based on nucleotide sequence divergence, primarily in the E2 glycoprotein gene and 5' untranslated region (5' UTR), with strains within a genotype sharing less than 15-20% divergence and subgenotypes reflecting finer clustering at 8-12% divergence thresholds.^[8]^[25] This system, established through maximum likelihood and Bayesian phylogenetic analyses of partial and full genomes, enables tracking of viral evolution and epidemiology without altering the single serotype status of CSFV.^[26]^[27] Genotype 1, the most genetically diverse, includes seven subgenotypes (1.1 to 1.7), encompassing early 20th-century isolates and vaccine strains like the C-strain, with subgenotype 1.1 historically dominant in Europe before 1990.^[8]^[28] Subgenotypes 1.2 through 1.7 emerged later, reflecting regional adaptations, such as 1.3 in Cuba forming a distinct clade.^[29] Genotype 2 consists of three subgenotypes (2.1, 2.2, and 2.3), with 2.1 prevalent in recent Asian outbreaks, including strains isolated as early as 1989 in China, and showing ongoing intra-subgenotype evolution via recombination and mutation.^[8]^[30] Genotype 3 features four subgenotypes (3.1 to 3.4), associated with strains from the Americas and Africa, including highly variable isolates from outbreaks in the 1970s-1980s, though less common in current global circulation compared to genotypes 1 and 2.^[8]^[31] Overall, this genotyping framework, refined through whole-genome sequencing of over 200 strains, reveals temporal shifts, with genotype 2 expanding since the 2000s.^[26]

Virulence Differences and Evolution

Classical swine fever virus (CSFV) strains exhibit significant variation in virulence, ranging from highly pathogenic forms that induce acute hemorrhagic disease with mortality rates approaching 100% in domestic pigs to low-virulence or attenuated variants causing subclinical infections or persistent carriage, particularly in young animals.^[32] Highly virulent strains, such as the subgenotype 1.4 isolate Margarita from 1958, provoke severe clinical signs including prolonged fever, leukopenia, and high viremia in weaned pigs, leading to rapid systemic dissemination and endothelial damage.^[32] In contrast, low-virulence strains like the related PdR isolate from 2010 (also subgenotype 1.4) are avirulent in pigs over 3 months old, eliciting only mild or no symptoms, though they can establish chronic infections in neonates with altered 3' untranslated region (UTR) features.^[32] These virulence disparities are primarily driven by specific genetic alterations rather than broad phylogenetic clades, with key determinants including amino acid substitutions in the E2 glycoprotein (e.g., seven changes in PdR versus Margarita), nonstructural proteins NS5A and NS5B, and insertions in the 3' UTR poly-U tract (e.g., 36U in attenuated PdR versus 5U in virulent progenitors).^[32] Mutations in E2, such as those affecting glycosylation sites (e.g., position 986), reduce viral attachment to host cells and enhance attenuation, while positive selection at E2 sites like 34, 36, 49, and 72 facilitates immune evasion and modulates pathogenicity across strains.^[28]^[32] Virulence is not strictly tied to CSFV's three genotypes (1–3) or 11 subgenotypes, though certain lineages show associations; for instance, some genotype 2.1 strains in Asia have demonstrated heightened lethality post-2000.^[8] CSFV evolution, as an enveloped positive-sense RNA virus in the Flaviviridae family, proceeds via high mutation rates (approximately 7.09 × 10⁻⁴ substitutions/site/year genome-wide, varying by genotype: 2.66 × 10⁻⁴ for genotype 1, up to 1.31 × 10⁻² for genotype 3) and quasispecies dynamics, enabling adaptation to host immunity and selective pressures like vaccination.^[28] The virus likely originated around 1750 CE through a host jump from a ruminant pestivirus (e.g., akin to Tunisian sheep virus) to swine, with diversification into modern genotypes occurring circa 1800 CE under spatial and epidemiological constraints.^[28] In endemic settings, such as Cuba, suboptimal vaccination has fostered serial attenuation, as seen in the phylogenetic descent of low-virulent PdR from high-virulent Margarita over ~50 years, involving synergistic mutations that cumulatively dampen replication and tropism without abolishing infectivity.^[32] Recombination events are infrequent but contribute to subgenotype emergence, while positive selection on envelope proteins like E2 drives virulence shifts, often toward reduced pathogenicity in vaccinated populations to evade clearance.^[28]^[8]

Epidemiology

Transmission Pathways

Classical swine fever virus (CSFV) spreads primarily through horizontal transmission via direct contact between infected and susceptible pigs, with the oronasal route being the most efficient mechanism, involving exposure to virus-laden nasal, oral, or ocular secretions.^[4] Direct transmission also occurs through conjunctival exposure, skin abrasions, artificial insemination with contaminated semen, or venereal contact during mating.^[2] Infected pigs shed the virus in saliva, urine, feces, blood, and semen starting before clinical signs appear and continuing until death or development of antibodies, facilitating rapid within-herd dissemination.^[4] Chronically or persistently infected animals, often resulting from in utero exposure, excrete virus continuously or intermittently for weeks to months, amplifying transmission risk.^[2] Indirect horizontal transmission occurs via fomites such as contaminated equipment, vehicles, clothing, or human handlers, as the virus adheres to surfaces and survives in porcine tissues for up to nearly three months under refrigeration or over four years when frozen.^[2] Ingestion of uncooked swill or garbage containing infected pig tissues or meat products represents a major indirect route, historically linked to outbreaks, with virus persisting in such materials for extended periods under cool conditions.^[4] Aerosol transmission is possible over short distances (up to 1-2 meters) in droplets from respiratory secretions, particularly with virulent strains, though it is less efficient and mainly contributes to intra-herd spread under experimental or high-density conditions.^[4] Mechanical vectors like flies may transfer virus between animals, but arthropod-borne spread is not a primary pathway.^[33] Vertical transmission happens transplacentally from infected pregnant sows to fetuses, especially if infection occurs between days 50 and 70 of gestation, resulting in persistently infected piglets that shed virus lifelong without mounting an effective immune response.^[4] Such congenital infections sustain virus circulation in populations, including wild boar, where postnatal contact further perpetuates endemic cycles.^[2] Environmental persistence varies: CSFV remains viable in feces or slurry for up to 70 days at 17°C or 1-3 months at 4-5°C but inactivates within days to two weeks at 20°C or in sunlight-exposed conditions.^[2]

Global Distribution and Risk Factors

Classical swine fever (CSF) is endemic in parts of Asia (including China, Vietnam, Laos, Myanmar, and the Philippines), Central and South America (such as Mexico, Colombia, Ecuador, Peru, Bolivia, Paraguay, and non-free zones in Brazil), sub-Saharan Africa, and certain Caribbean islands.^[2]^[4] The virus persists in these regions due to ongoing circulation in domestic pig populations and wild boar reservoirs, with sporadic outbreaks reported as recently as 2023 in Brazil outside designated free zones and 2024 in parts of Asia.^[34] In contrast, CSF has been eradicated from North America (United States and Canada since 1978 and 1964, respectively), most of Western and Northern Europe, Australia, and New Zealand through sustained vaccination, stamping-out policies, and trade restrictions.^[1]^[2] Global surveillance by the World Organisation for Animal Health (WOAH) tracks self-declared free status, though reintroductions remain a threat in previously cleared areas, as evidenced by outbreaks in Japan starting in 2018.^[35] Key risk factors for CSF emergence or spread include direct contact between infected and susceptible pigs, indirect transmission via contaminated fomites (such as feed, vehicles, equipment, or clothing), and movement of live animals or uncooked pork products across borders.^[36] Wild boar populations serve as persistent reservoirs in endemic zones, facilitating spillover to domestic herds through shared environments or fence-line contact, particularly in Europe and Asia where sylvatic cycles sustain low-virulence strains.^[37] High-density backyard or smallholder farming systems with poor biosecurity amplify risks, as do gaps in vaccination coverage and surveillance in resource-limited settings; for instance, spatiotemporal analyses in the Andean region identify pig density, market connectivity, and proximity to borders as predictors of outbreaks.^[38] International trade in semen, embryos, or subclinically infected pigs poses additional introduction risks to free zones, underscoring the need for rigorous import controls and traceability.^[39]

Recent Outbreaks and Surveillance

In Japan, classical swine fever re-emerged in September 2018 after 26 years of absence, with outbreaks continuing through 2023 and into 2024, leading to the culling of approximately 368,000 pigs across 20 prefectures by the end of 2023.^[40] As of March 10, 2024, a total of 90 outbreaks had been confirmed in domestic pig farms, primarily involving genotype 2.1 strains of moderate virulence.^[41] A notable outbreak in May 2024 affected Hirono Town in Iwate Prefecture, prompting intensified tracing and culling measures, while a subsequent case in late 2024 marked the first reported since October of that year.^[40] ^[42] Outbreaks have also been reported in other regions since 2019, including South Korea, Colombia, Russia, and Brazil, often linked to genotype 1 or 2 strains introduced via infected animals or fomites, though specific incidence data for 2023–2025 remain limited outside Japan.^[43] In Europe, where eradication efforts since the 1990s have reduced prevalence, sporadic cases in wild boar populations persist, necessitating ongoing monitoring to prevent spillover to domestic herds, with no large-scale domestic outbreaks reported in 2023–2025.^[44] Surveillance for classical swine fever relies on a combination of active and passive systems coordinated by organizations like the World Organisation for Animal Health (WOAH) through its World Animal Health Information System (WAHIS), which mandates immediate reporting of outbreaks and enables global tracking.^[1] In the United States, the USDA Animal and Plant Health Inspection Service (APHIS) has conducted targeted surveillance since 2006, involving serological testing of cull sows, boars, and feral swine, alongside passive reporting from veterinarians and producers to detect introductions early.^[45] European programs emphasize routine pathological examinations, serological surveys in high-risk areas, and wild boar hunting samples, with integration into broader swine fever monitoring to differentiate from African swine fever.^[46] Emerging methods include oral fluid collection via ropes in pig pens for non-invasive virological detection and molecular assays like real-time RT-PCR for rapid genotyping, enhancing sensitivity in low-prevalence settings.^[2] In wild boar habitats, strategies incorporate hunter-submitted samples and bait-vaccination monitoring to assess prevalence and immunity waning, as demonstrated in Japanese programs evaluating oral vaccine efficacy against circulating strains.^[47] These efforts prioritize early detection to enable stamping-out policies, with WOAH standards guiding risk-based sampling to minimize false negatives in endemic or re-emerging areas.^[1]

Clinical Manifestations

Acute and Peracute Forms

The peracute form of classical swine fever (CSF) represents the most severe manifestation, characterized by sudden death occurring 3–4 days post-infection without prominent clinical signs. This form is associated with highly virulent strains and results in rapid viremia, leading to endothelial cell damage and disseminated intravascular coagulation. Mortality approaches 100% in affected herds, primarily in young pigs, with minimal opportunity for intervention.^[4] In contrast, the acute form follows an incubation period of 2–4 days, manifesting with high fever exceeding 41°C, anorexia, profound weakness, and alternating constipation or diarrhea. Additional signs include conjunctivitis, rhinitis, respiratory distress, and neurological symptoms such as ataxia, staggering gait, and posterior paralysis. Skin hyperemia or cyanosis, particularly on the ears, snout, and abdomen, is common, alongside leukopenia and thrombocytopenia. Death typically ensues 7–20 days after onset, with mortality rates of 80–100% in unvaccinated domestic pigs.^[4]^[48] Pathologically, both forms feature widespread vascular damage causing petechial and ecchymotic hemorrhages in multiple organs, including the skin, kidneys (evident as cortical petechiae resembling a "turkey egg"), lymph nodes, spleen, and gastrointestinal tract. Splenic enlargement with infarction, tonsillar necrosis, and lymphoid depletion are hallmarks, reflecting the virus's tropism for endothelial and mononuclear phagocytic cells. These lesions underscore the hemorrhagic diathesis central to disease progression.^[4]^[49] Strain virulence significantly influences presentation; historical highly virulent isolates like those from the 1990s European outbreaks induced predominantly acute/peracute disease, whereas modern moderate strains may shift toward subacute forms. Host factors, including age and concurrent infections, exacerbate severity, with piglets under 2 months most susceptible to peracute outcomes.^[4]^[8]

Subacute and Chronic Forms

The subacute form of classical swine fever manifests with clinical signs resembling the acute form but of reduced severity and prolonged duration, typically lasting 20–30 days until death. Affected pigs exhibit fever ranging from 105–106°F (41°C) for 2–3 weeks, accompanied by weakness, anorexia, conjunctivitis, alternating diarrhea and constipation, and occasional neurological disturbances such as staggering gait, particularly in younger animals.^[49]^[2]^[50] Mortality is lower than in acute cases, with some recovery possible in older pigs, though viral shedding persists until death. Pathological findings include milder hemorrhagic lesions, such as petechiae in lymph nodes, tonsillitis, and reduced generalized vasculitis compared to acute disease.^[49]^[2] The chronic form, often linked to low-virulence strains or partial immunity in vaccinated herds, features intermittent and waxing-waning symptoms over weeks to months, frequently culminating in death within 1–3 months or up to a year. Initial phases mimic subacute illness with fever, depression, anorexia, leukopenia, and gastrointestinal disturbances, followed by temporary improvement and relapse marked by stunting, wasting, alopecia, skin lesions, and arched backs with disproportionate head size in juveniles.^[2]^[50]^[51] Secondary bacterial infections are common due to immunosuppression, and pregnant sows may transmit the virus transplacentally, resulting in stillbirths, mummification, or piglets with congenital tremors and malformations. Virus persists in tissues like lymph nodes, spleen, and lungs, enabling prolonged shedding via secretions, feces, urine, and semen, thus facilitating transmission.^[2]^[50] Pathologically, chronic cases show button ulcers—multifocal necrotic lesions with caseous centers—in the colonic mucosa, cecum, epiglottis, and larynx, alongside glomerulonephritis from immune complex deposition, chondrodysplasia in survivors beyond 30 days, thymus atrophy, and lymphoid depletion. Kidney hemorrhages and infarcts contribute to renal dysfunction. Recovery is rare and mostly in mature animals, but carriers pose ongoing epidemiological risks.^[49]^[2]^[50]^[51]

Variations in Domestic Pigs versus Wild Boar

Classical swine fever (CSF) presents with broadly similar clinical signs in domestic pigs and wild boar, including fever, anorexia, lethargy, diarrhea, and hemorrhages, but variations arise in disease progression, host immune responses, and persistence due to differences in population dynamics and potential genetic adaptations.^[52]^[2] In experimental infections with moderately virulent genotype 2.3 strains, both groups exhibit comparable viremia levels detectable from 7 days post-infection (dpi), but wild boar display sporadic fever peaks exceeding 40°C, contrasting with more sustained fever in domestic breeds like Landrace pigs.^[53] Host antibody responses differ notably, with wild boar developing detectable antibodies earlier (by 10 dpi) compared to domestic pigs, where Landrace pigs achieve high titers by 28 dpi and hybrids show delayed, lower responses from 14 dpi; this may contribute to occasional chronic cases in wild boar (observed in 17% of infected individuals in one study), facilitating viral persistence absent in most domestic infections.^[53] Pathological outcomes are acute-lethal in the majority (83% wild boar, 66-100% domestic depending on breed), featuring tonsillar hyperplasia, splenomegaly, and petechiae, though secondary bacterial infections (e.g., Actinobacillus) exacerbate severity more in hybrid domestic pigs.^[53]^[2] Epidemiologically, wild boar populations sustain endemic CSF cycles in high-density areas (>2,000 animals), where juveniles act as short-term reservoirs via transplacental transmission, yielding viremic offspring that die within weeks but enable ongoing spread; this contrasts with domestic pigs, lacking natural reservoirs and experiencing explosive acute outbreaks with near-100% mortality from virulent strains, often requiring culling for control.^[52] In Europe, wild boar serve as sources for 60% of domestic outbreaks (e.g., Germany 1993-1997), with persistence linked to reproductive rates rather than inherently milder disease, though circulating strains in wild hosts may evolve toward moderate virulence over time to avoid population collapse.^[52]^[2] Domestic pigs, in confined settings, amplify transmission rapidly via close contact, semen, and fomites, amplifying acute forms without the self-limiting dynamics seen in sparse wild populations (<2,000 animals).^[52]

Diagnosis

Virological and Molecular Tests

Virus isolation remains the gold standard for confirmatory diagnosis of classical swine fever virus (CSFV), involving inoculation of clinical samples such as tonsil, spleen, or blood into susceptible porcine cell lines like PK-15 or SPEV, followed by detection via immunofluorescence or immunoperoxidase staining.^[54] This method confirms viable virus but is labor-intensive, requiring 3-7 days for cytopathic effects to manifest, and is hindered by sample cytotoxicity or autolysis.^[55] According to World Organisation for Animal Health (WOAH) guidelines, isolation must be performed in biosafety level 3 facilities due to the virus's contagious nature.^[56] Molecular diagnostics, particularly reverse transcription polymerase chain reaction (RT-PCR) and real-time RT-PCR (qRT-PCR), provide rapid, sensitive detection of CSFV RNA from tissues, blood, oral fluids, or environmental samples, often within hours.^[57] WOAH-recommended protocols extract RNA from samples, amplify the 5' untranslated region (5' UTR) or E2 gene via RT-PCR, and visualize products by gel electrophoresis or quantify via TaqMan probes in qRT-PCR for enhanced specificity and avoidance of contamination.^[58] These assays detect as few as 10-100 viral genome copies, outperforming isolation in early infection stages (e.g., 2 days post-inoculation in serum) and degraded specimens.^[59] Validation studies confirm qRT-PCR sensitivity and specificity exceeding 99%, with no cross-reactivity to ruminant pestiviruses or African swine fever virus.^[57] ^[54] Multiplex real-time RT-PCR variants enable simultaneous detection and genotyping of CSFV alongside differentiation from vaccine strains (e.g., C-strain) or co-infections like African swine fever virus, targeting unique genomic markers such as the E2 glycoprotein.^[60] Insulated isothermal RT-PCR offers field-deployable alternatives without thermocyclers, achieving detection limits comparable to lab-based qRT-PCR.^[59] However, false negatives can occur in low-viruria chronic cases, necessitating serological confirmation, while PCR-positive results in vaccinated herds require sequencing for wild-type attribution.^[54] EU reference laboratory manuals emphasize combining molecular tests with virus isolation for definitive outbreaks.^[55]

Serological and Pathological Methods

Serological methods detect antibodies against classical swine fever virus (CSFV) in serum or plasma, primarily used for surveillance, confirming exposure in convalescent animals, and differentiating infected from vaccinated pigs in eradication programs.^[61] Enzyme-linked immunosorbent assay (ELISA) serves as the initial screening test, targeting antibodies to glycoproteins like E2, with high sensitivity for herd-level detection but potential cross-reactivity with ruminant pestiviruses such as bovine viral diarrhea virus (BVDV).^[54] Confirmatory testing employs virus neutralization (VN) assays, considered the gold standard due to their specificity in measuring functional neutralizing antibodies; these tests, including plaque reduction or fluorescent antibody VN, require cell culture and typically take several days but achieve near-100% specificity when using reference strains.^[54] Samples are collected from live pigs in non-heparinized tubes, with VN often performed at a 1:5 dilution for trade certification or 1:10 for surveillance.^[61] Pathological methods involve postmortem examination of tissues to identify characteristic lesions and viral antigens, essential for confirming acute cases where virological detection may be challenging. Gross pathology in acute CSF includes petechial and ecchymotic hemorrhages on serosal surfaces (e.g., kidneys, bladder), splenomegaly with infarcts, and enlarged, marbled lymph nodes; chronic forms feature necrotic "button" ulcers in the intestinal mucosa and lymphoid atrophy.^[4] Histopathological findings reveal lymphoid depletion, vasculitis, and nonsuppurative encephalitis, though these are not pathognomonic and require differentiation from similar diseases like African swine fever.^[61] Antigen detection via direct fluorescent antibody test (FAT) or immunohistochemistry on cryostat sections of tonsil, spleen, kidney, or ileum uses fluorescein-labeled monoclonal antibodies specific to CSFV, providing rapid confirmation; fresh, chilled tissues are preferred to preserve antigenicity, with monoclonal antibodies aiding pestivirus differentiation.^[61] These methods are most effective in peracute or acute fatalities, where lesions are prominent, but serological correlation is advised for subacute cases.^[54]

Prevention and Control Measures

Biosecurity and Farm Management

Biosecurity protocols form the cornerstone of classical swine fever (CSF) prevention on pig farms, aiming to block viral introduction via direct contact with infected animals, indirect transmission through fomites, contaminated feed or water, and vectors such as personnel or equipment.^[62] Strict adherence to these measures has been shown to significantly reduce incursion risks, with studies identifying quarantine of incoming pigs and visitor restrictions as particularly effective in averting outbreaks.^[63] External biosecurity focuses on controlling inputs: farms should procure replacement stock solely from CSF-free certified sources, enforce a minimum 30-day quarantine with serological testing for new animals, and prohibit the use of swill feeding or unprocessed food waste, which can harbor the virus for extended periods.^[7] Vehicle and equipment disinfection using approved agents like sodium hypochlorite (1-3% solution) upon entry and exit, coupled with footbaths and dedicated farm clothing, minimizes fomite spread.^[64] Internal farm management emphasizes compartmentalization and hygiene to contain potential infections. All-in-all-out systems, where pens are fully depopulated, cleaned, and disinfected between batches, disrupt viral persistence in the environment, as CSF virus can survive in organic matter for weeks under cool, moist conditions.^[65] Routine pest control targets rodents and insects, which act as mechanical vectors, with integrated programs including bait stations and sealed facilities reducing transmission risks by up to 50% in modeled scenarios.^[63] Waste management requires secure burial or incineration of carcasses and manure to prevent scavenging by wild boar, while zoning farms into clean (office, feed storage) and dirty (animal housing) areas with unidirectional flow prevents cross-contamination.^[66] To mitigate wildlife interfaces, farms must implement physical barriers such as double fencing (at least 1.5 meters high with buried mesh) around perimeters, especially in regions with endemic CSF in wild boar populations, as contact between domestic pigs and infected feral swine has driven multiple outbreaks.^[65] Ongoing staff training on recognition of clinical signs—such as fever, lethargy, and hemorrhagic lesions—and rapid reporting protocols enhances early detection, while surveillance via periodic serological testing (e.g., ELISA for antibodies) in high-risk farms supports proactive management.^[7] Compliance audits and record-keeping of movements and treatments, as mandated in CSF-free zones, ensure traceability and regulatory adherence, with non-compliance linked to prolonged epidemics in historical cases.^[67]

Vaccination Approaches and Efficacy

Classical swine fever (CSF) vaccination primarily relies on live attenuated and subunit vaccines, with the choice depending on disease endemics, trade requirements, and the need for differentiating infected from vaccinated animals (DIVA). Live attenuated vaccines, such as the C-strain (a lapinized variant developed in the 1950s), are extensively used in endemic regions for their rapid and broad protection against diverse CSF virus (CSFV) genotypes.^[68] Subunit vaccines targeting the E2 glycoprotein serve as marker vaccines, enabling DIVA strategies via companion diagnostics that detect antibodies absent in vaccinated animals, which is crucial for surveillance and trade in CSF-controlled areas.^[4] The C-strain vaccine provides early onset immunity, with interferon-gamma responses detectable by day 6 post-vaccination (dpv) and full clinical and virological protection by 10 dpv, even via oral administration suitable for wild boar baiting.^[68] It confers sterilizing immunity, significantly reducing viremia and preventing transplacental transmission, with studies showing 100% survival against lethal challenges from various genotypes and long-lasting humoral and cellular responses lasting 6-11 months or longer.^[4] However, its lack of DIVA capability imposes trade barriers, and rare instances of reversion to virulence have been reported, such as with the LOM strain in South Korea in 2014.^[68] E2 subunit vaccines, often produced via baculovirus expression in insect cells, induce strong neutralizing antibodies without viral replication risk, offering 80-100% protection against clinical disease following one or two doses, depending on the formulation and presence of maternal antibodies.^[68] Efficacy trials demonstrate reduced virus transmission (up to 90% reduction) and full protection against genotype 2 strains, though onset is delayed compared to live vaccines, typically requiring 14-21 dpv for optimal effect, and vertical protection is limited.^[4] These vaccines, exemplified by products like Porcilis Pesti, support emergency vaccination in eradication programs but often necessitate adjuvants for robust responses in piglets.^[4] Chimeric and viral vector-based marker vaccines, such as CP7_E2alf (licensed in Europe in 2014), combine elements of pestiviruses with E2 for DIVA compatibility and have shown 100% survival in challenges with highly virulent strains like Koslov, with efficacy via intramuscular or oral routes and minimal shedding.^[4] Overall, while live vaccines dominate control in high-prevalence areas due to cost-effectiveness and speed, marker vaccines facilitate proof of freedom in low-risk settings, though no single vaccine achieves absolute sterilizing immunity across all strains without boosters.^[68]

Stamping-Out and Eradication Strategies

Stamping-out policies for classical swine fever (CSF) entail the immediate culling of all pigs in affected herds, as well as those in epidemiologically linked contact herds, to halt virus transmission. This approach is mandated in CSF-free countries and regions, where it forms the cornerstone of outbreak response, supplemented by zoning into protection, surveillance, and free areas with strict movement restrictions. Proper carcass disposal, thorough premises disinfection using approved agents, and epidemiological tracing of potential sources and contacts are integral to preventing reintroduction.^[1]^[4] Eradication efforts build on stamping-out by incorporating enhanced active surveillance, such as serological testing in high-risk populations and passive reporting systems, to confirm absence of the virus over defined periods. In the European Union, this strategy eradicated CSF from domestic pig populations by the early 2000s, though wild boar reservoirs have necessitated ongoing measures like intensified hunting and oral bait vaccination in endemic pockets to achieve compartmentalization. The United States successfully eradicated hog cholera (the former name for CSF) in 1978 through a combination of compulsory vaccination and stamping-out, transitioning to a vaccination-free maintenance phase thereafter.^[52]^[69] Modified stamping-out variants, such as emergency vaccination followed by targeted culling of vaccinated animals, are employed in scenarios where mass culling poses excessive economic or logistical burdens, particularly in densely populated pig areas; this allows virus containment while preserving trade eligibility after seropositive animal removal and re-testing. However, routine prophylactic vaccination is prohibited in CSF-free zones to enable differentiation from infection via diagnostics, as vaccine-induced antibodies can confound serological surveillance. Success of eradication hinges on cross-border coordination, as demonstrated by regional programs under the World Organisation for Animal Health (WOAH), which recognize disease-free status only after verified stamping-out efficacy and sustained surveillance without outbreaks.^[70]^[71]^[1] Challenges to stamping-out include ethical concerns over large-scale culling and incomplete efficacy against subclinical or wild reservoirs, prompting hybrid strategies in some contexts; for instance, in feral swine, depopulation via hunting or trapping supplements domestic measures, though vaccination remains a contingency rather than primary tool. Long-term eradication requires biosecurity reinforcement to block re-entry via imports or fomites, with economic analyses indicating that stamping-out averts broader losses despite initial costs exceeding millions per outbreak in major producers.^[72]^[68]

Historical Context

Discovery and Initial Outbreaks

Classical swine fever, historically termed hog cholera in North America, was first documented in 1833 on a swine farm in southern Ohio, United States, marking the earliest reliable recognition of the disease as a distinct epizootic affecting pigs.^[73] Prior anecdotal reports suggest possible earlier occurrences, including an outbreak in Tennessee in 1810 and additional cases in Ohio during the early 1830s, though these lacked definitive diagnostic confirmation.^[48] The disease manifested with high fever, hemorrhagic lesions, and high mortality in domestic pigs, prompting initial observations of its contagious nature through direct contact and contaminated feed or premises. Following the Ohio index case, outbreaks proliferated across the United States, with reports emerging in at least 10 states during the first half of the 19th century; incidence escalated rapidly after 1860, becoming endemic and causing widespread economic disruption in the burgeoning swine industry.^[8] In Europe, the first substantiated outbreaks were recorded in 1862, often under the name Schweinepest, with spread facilitated by international trade in live pigs and pork products.^[43] Early containment efforts were rudimentary, relying on isolation and slaughter without understanding the viral etiology, which was not isolated until 1904 when the causative agent was identified in Europe.^[36] These initial epidemics underscored the pathogen's high transmissibility among domestic swine, though its origins remain uncertain, potentially tracing to North America.^[11]

Key Epidemics and Control Milestones

In the United States, classical swine fever (CSF), known domestically as hog cholera, was endemic throughout much of the 19th and early 20th centuries, causing significant economic losses estimated at $6 million in 1917 alone.^[74] The disease's persistence prompted the initiation of a national eradication program in 1961, involving widespread vaccination, surveillance, and stamping-out of infected herds, culminating in the last reported case in 1976 and official declaration of freedom on January 31, 1978.^[75]^[75] This achievement marked a major milestone in CSF control, achieved through coordinated federal-state efforts and the cessation of routine vaccination post-eradication to maintain disease-free status.^[76] In Europe, CSF re-emerged as a significant threat in the 1990s following the prohibition of prophylactic vaccination in 1990 to enable serological differentiation of infected from vaccinated animals (DIVA) under World Organisation for Animal Health (WOAH) guidelines.^[1] A notable outbreak began in Germany in 1993, linked to wild boar reservoirs, spreading to neighboring countries and persisting until 1998 with hundreds of affected herds culled.^[1] The most devastating European epidemic occurred in the Netherlands in 1997, affecting over 700 farms, necessitating the culling of approximately 12 million pigs—many for welfare reasons amid feed shortages—and resulting in economic losses exceeding $2.3 billion USD.^[77]^[2] These outbreaks underscored the risks of relying solely on stamping-out without emergency vaccination options, prompting revised EU contingency plans that incorporated targeted vaccination in high-risk scenarios.^[4] Key control milestones include the development of attenuated vaccines in the mid-20th century, such as the lapinized C-strain vaccine originating from a Chinese isolate in the late 1950s, which provided effective prophylaxis and facilitated eradication in regions like parts of Asia and Europe through mass immunization campaigns.^[78] In the 1990s, the advent of subunit marker vaccines, such as those based on the E2 glycoprotein, enabled DIVA strategies, allowing vaccinated populations to be distinguished from naturally infected ones via specific antibody tests, which supported CSF-free status certification in countries like those in the European Union by the early 2000s.^[79] Ongoing global efforts, coordinated by WOAH, have led to eradication in North America, Australia, and New Zealand, though the disease remains endemic in parts of Asia and persists in wild boar populations in Europe, complicating complete elimination.^[1]^[8]

Impacts and Consequences

Economic Losses and Trade Implications

Classical swine fever (CSF) outbreaks impose severe economic burdens on the swine industry through direct costs such as animal mortality, mass culling of infected and at-risk herds, and implementation of control measures including quarantine, disinfection, and surveillance. Indirect costs encompass reduced productivity, supply chain disruptions, and compensation payments to farmers, often amplifying losses in intensive pig-rearing regions. For instance, during the 1997–1998 epidemic in the Netherlands, authorities culled over 11 million pigs, resulting in total economic damages estimated at €2 billion (approximately USD 2.3 billion), encompassing culling expenses, lost income, and broader sectoral impacts.^[80]^[81] Similar modeling approaches applied to hypothetical outbreaks highlight that financial consequences can vary by farm size, outbreak scale, and policy responses, with smaller operations facing disproportionate relative losses due to limited reserves.^[82] Trade implications of CSF are profound, as the disease's World Organisation for Animal Health (WOAH) notifiable status prompts importing countries to impose stringent restrictions or outright bans on live swine, fresh pork, and certain pork products from affected nations or zones to prevent virus transmission via meat or fomites.^[1]^[58] These measures, enforced under international standards, can halt exports abruptly, leading to surplus domestic supply, depressed prices, and revenue shortfalls; for example, CSF-free certification enables access to high-value markets, while infection status reversal, as in historical European outbreaks, has prolonged recovery periods exceeding years and eroded competitive positioning.^[36] In the United States, where pork exports reached $6.67 billion in 2014, a single outbreak could trigger global market exclusion, compounding losses through retaliatory barriers from trading partners.^[83] Such restrictions underscore CSF's role in shaping pork trade flows, with affected exporters often redirecting to lower-margin domestic or regional outlets, further straining economies reliant on swine production.^[84]

Zoonotic Risk Assessment

Classical swine fever virus (CSFV) does not pose a zoonotic risk to humans, as there are no documented cases of natural or laboratory-acquired infections in people despite extensive global outbreaks in swine populations since its identification in the 19th century.^[2]^[85] Multiple veterinary authorities, including the World Organisation for Animal Health (WOAH) and the Center for Food Security and Public Health (CFSPH), affirm that CSFV lacks the capacity to infect or cause disease in humans, with the virus exhibiting host specificity restricted to suids such as domestic pigs and wild boar.^[56]^[2] Experimental studies and serological surveys have failed to detect CSFV replication in human cell lines or antibody responses in exposed individuals, underscoring the absence of transmissibility across species barriers.^[2] While CSFV can persist in porcine tissues and fluids, posing risks for indirect transmission among swine via fomites or contaminated feed, human handlers face no health threat from direct contact, ingestion, or aerosol exposure, as confirmed by risk assessments from bodies like the European Food Safety Authority (EFSA).^[86]^[87] Standard biosafety level 2 (BSL-2) precautions suffice for laboratory work with CSFV, reflecting its negligible human pathogen potential rather than enhanced containment for zoonotic agents.^[56] Public health surveillance data from regions with endemic CSF, such as parts of Asia and Europe, report zero human cases attributable to the virus, even in high-exposure scenarios like slaughterhouse operations or wild boar hunting.^[2] This contrasts with related pestiviruses like bovine viral diarrhea virus, which also show no zoonotic spillover, suggesting inherent virological barriers such as receptor incompatibility or inefficient cellular entry in non-suid hosts.^[85] Overall, the zoonotic risk assessment for CSF rates as negligible, with control efforts prioritizing porcine biosecurity over human protective measures.^[86]^[87]

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[81]
A model to estimate the financial consequences of classical swine ...
A model is presented aimed at a financial analysis of a Classical Swine Fever outbreak. Financial consequences are calculated for affected parties.
[82]
A model to estimate the financial consequences of classical swine ...
A model to estimate the financial consequences of classical swine fever outbreaks: principles and outcomes ... Due to its severe economic impact, CSF is ...
[83]
Modeling Classical Swine Fever Outbreak-Related Outcomes
The study was carried out to estimate classical swine fever (CSF) outbreak-related outcomes, such as epidemic duration and number of infected, vaccinated, and ...Materials And Methods · Selecting The Most Likely... · ResultsMissing: cost | Show results with:cost
[84]
Transboundary spread of pig diseases: the role of international trade ...
Feb 22, 2019 · Pig diseases spread through formal and informal trade (products, live animals), and fomites. Informal trade, including illegal smuggling, is ...
[85]
[PDF] Case Definition - Classical Swine Fever (CSF) - usda aphis
CSVF is present in wild boar in parts of Europe. ... 1 The pig (Sus scrofa, both domestic and wild) is the only natural host for classical swine fever virus.
[86]
Classical swine fever - Disease profiles - European Union
For official data on reported outbreaks, please visit the Geographical distribution section of this animal disease profile (menu on the top of the page). Data ...
[87]
Classical Swine Fever (CSF) - Agriculture.ks.gov
CSF has no treatment, as positive farms are required to depopulate. Classical Swine Fever does not affect human health and cannot be transmitted to humans.