Recognizing African Swine Fever in the Field: Clinical Signs and Post-Mortem Findings Every Veterinarian Should Know

African swine fever (ASF) remains one of the most devastating infectious diseases affecting domestic pigs and wild boars because of its high mortality and significant impact on pig production. Early recognition is critical for limiting disease spread and initiating timely control measures. However, the clinical presentation is not always uniform. Disease severity varies according to the virulence of the virus, the infectious dose, route of exposure, and host factors, making field diagnosis challenging in some situations^1,2. 

For practicing veterinarians, understanding how ASF presents in different clinical forms and recognizing its characteristic post-mortem findings can greatly improve the speed and accuracy of field investigations. 

Understanding the Different Clinical Forms 

ASF can present as peracute, acute, subacute, or chronic disease, with each form showing distinct clinical characteristics^1,2. 

Acute ASF is commonly associated with highly virulent virus strains. Affected pigs typically develop high fever ranging from 40–42°C, accompanied by lethargy, anorexia, inactivity, and respiratory distress³. Cutaneous hyperemia, skin hemorrhages, nasal discharge, vomiting, and diarrhea may also occur. In some animals, diarrhea results in black staining around the perianal region^1,2. Pregnant sows may experience abortion⁴. 

In contrast, subacute ASF is generally caused by moderately virulent isolates. Clinical signs resemble those seen in acute disease but are usually less pronounced. Animals develop moderate to high fever, and mortality typically ranges from 30% to 70%, with deaths occurring between 7 and 20 days after infection. Vascular damage, particularly hemorrhage and edema, may be even more evident than in acute cases². 

Chronic ASF develops in animals infected with less virulent strains and is characterized by a prolonged disease course. Typical findings include multifocal skin necrosis, arthritis, growth retardation, progressive weight loss, respiratory distress, and abortion³. 

Field veterinarians should also be aware that initial outbreaks may not always present with dramatic mortality. Early cases may show only fever and hemorrhagic lymph nodes, making careful clinical examination essential^1,2. 

Post-Mortem Findings That Strengthen Clinical Suspicion 

Necropsy remains an important component of ASF investigation, especially when laboratory confirmation is pending. 

One of the most characteristic lesions in acute ASF is severe hemorrhagic splenomegaly. The spleen becomes markedly enlarged, dark red to black in color, with rounded edges, often occupying a substantial portion of the abdominal cavity². Severe hepatic congestion is frequently present alongside splenic enlargement. 

Another consistent finding is hemorrhagic lymphadenopathy. Gastrohepatic, renal, ileocecal, mesenteric, and other lymph nodes often appear enlarged with extensive hemorrhage, producing a distinctive marbled appearance². 

Additional lesions observed during post-mortem examination include: 

• Petechial hemorrhages in the kidneys, particularly within the cortex 

• Hemorrhages involving the epicardium and endocardium 

• Severe pulmonary edema 

• Gall bladder wall edema 

• Hydropericardium and ascites, particularly in subacute disease 

• Multifocal edema in the perirenal fat and gall bladder wall^1,2 

Subacute cases may exhibit partial splenomegaly rather than uniform enlargement, while multifocal hemorrhagic lymphadenitis involving multiple lymph nodes is commonly observed². 

Hematological Changes During Disease Progression 

Changes in circulating blood cells may provide supportive information during disease progression. 

As infection advances, lymphocyte numbers decline while neutrophil counts increase. Lymphocyte reduction typically becomes evident between 2 and 4 days post-inoculation, coinciding with a decrease in total white blood cell count. Neutrophil numbers increase progressively from 2 to 3 days post-inoculation, with many immature forms appearing in circulation². 

Blood smears may reveal lymphoblasts, monoblasts, metamyelocytes, reactive lymphocytes, and atypical lymphocytes early during infection⁵. As disease progresses, dead cells become increasingly prominent, reflecting extensive cellular damage⁵. Activated monocytes and macrophages release inflammatory mediators such as IL-1, IL-6, and TNF-α, contributing to tissue injury and lymphocyte apoptosis^2,6. 

Conclusion 

Successful field recognition of African swine fever depends on evaluating the complete clinical picture rather than relying on a single lesion or clinical sign. Appreciating the differences between acute, subacute, and chronic disease, together with careful post-mortem examination and interpretation of supportive hematological findings, enables veterinarians to identify suspect cases promptly and initiate appropriate disease control measures while awaiting laboratory confirmation.

References 

1. Sánchez-Vizcaíno JM, Mur L, Gomez-Villamandos JC, Carrasco L. An update on the epidemiology and pathology of African swine fever. Journal of comparative pathology. 2015 Jan 1;152(1):9-21. https://www.researchgate.net/profile/Librado-Carrasco/publication/269180289_An_Update_on_the_Epidemiology_and_Pathology_of_African_Swine_Fever/links/622b08309f7b324634211eba/An-Update-on-the-Epidemiology-and-Pathology-of-African-Swine-Fever.pdf 

2. Li Z, Chen W, Qiu Z, Li Y, Fan J, Wu K, Li X, Zhao M, Ding H, Fan S, Chen J. African swine fever virus: a review. Life. 2022 Aug 17;12(8):1255. https://www.mdpi.com/2075-1729/12/8/1255 

3. Salguero FJ. Comparative pathology and pathogenesis of African swine fever infection in swine. Frontiers in veterinary science. 2020 May 19;7:282. https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2020.00282/full 

4. Wu K, Zhang Y, Zeng S, Liu X, Li Y, Li X, Chen W, Li Z, Qin Y, Chen J, Fan S. Development and application of RAA nucleic acid test strip assay and double RAA gel electrophoresis detection methods for ASFV and CSFV. Frontiers in Molecular Biosciences. 2022 Jan 31;8:811824. https://www.frontiersin.org/journals/molecular-biosciences/articles/10.3389/fmolb.2021.811824/pdf 

5. Karalyan Z, Zakaryan H, Arzumanyan H, Sargsyan K, Voskanyan H, Hakobyan L, Abroyan L, Avetisyan A, Karalova E. Pathology of porcine peripheral white blood cells during infection with African swine fever virus. BMC veterinary research. 2012 Feb 28;8(1):18. https://link.springer.com/content/pdf/10.1186/1746-6148-8-18.pdf 

6. Pikalo J, Schoder ME, Sehl J, Breithaupt A, Tignon M, Cay AB, Gager AM, Fischer M, Beer M, Blome S. The African swine fever virus isolate Belgium 2018/1 shows high virulence in European wild boar. Transboundary and emerging diseases. 2020 Jul;67(4):1654-9. https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/tbed.13503