African Swine Fever (ASF) is a highly contagious and often fatal viral disease of domestic and wild pigs, posing a severe threat to the swine industry. Its clinical signs and gross lesions can be non-specific, making an accurate and timely differential diagnosis crucial for effective disease control.
A robust differential diagnosis process is essential because ASF symptoms frequently mimic those of several other swine diseases. The similarity in gross lesions, particularly hemorrhages, cyanosis, and splenic enlargement, means that visual diagnosis alone is unreliable, necessitating laboratory confirmation.
Key Differential Diagnoses for African Swine Fever (ASF)
Based on clinical signs and gross lesions, the primary swine diseases that must be differentiated from ASF include:
- Classical Swine Fever (CSF) (also known as Hog Cholera)
- High-Pathogenic Porcine Reproductive and Respiratory Syndrome (HP-PRRS)
- Swine Erysipelas
- Septicaemic Salmonellosis
- Porcine Dermatitis Nephropathy Syndrome (PDNS)
Let's delve into the characteristics of each to understand their similarities and key distinctions from ASF.
1. Classical Swine Fever (CSF)
Classical Swine Fever, caused by a Pestivirus, is arguably the most challenging differential diagnosis for ASF due to their strikingly similar clinical manifestations and post-mortem lesions, especially in acute cases. Both are highly fatal hemorrhagic diseases.
- Similarities with ASF:
- Systemic Signs: High fever, anorexia, lethargy, and depression.
- Skin Lesions: Petechiae (pinpoint hemorrhages) and ecchymoses (larger hemorrhagic spots) on the skin, particularly on the ears, abdomen, and inner thighs, along with cyanosis.
- Lymph Nodes: Swollen and hemorrhagic lymph nodes, often described as having a "mottled" or "marbleized" appearance.
- Kidneys: Petechial hemorrhages, often leading to the "turkey egg" kidney appearance.
- Gastrointestinal Tract: Hemorrhages and inflammation.
- Key Distinguishing Features (often subtle):
- While both cause widespread hemorrhages, ASF typically results in a more profoundly enlarged, dark, and friable spleen.
- Chronic forms of CSF can present with characteristic "button ulcers" in the large intestine, which are generally absent in ASF.
- Neurological signs might be more variable and pronounced in some CSF forms.
- Further Reading: For more information, consult resources like the World Organisation for Animal Health (WOAH) on Classical Swine Fever.
2. High-Pathogenic Porcine Reproductive and Respiratory Syndrome (HP-PRRS)
High-Pathogenic PRRS represents a severe variant of Porcine Reproductive and Respiratory Syndrome, marked by high mortality rates (especially in young pigs) and a broad spectrum of respiratory, reproductive, and systemic signs. Its lesions can overlap with those of ASF.
- Similarities with ASF:
- Systemic Signs: High fever, anorexia, lethargy.
- Skin Lesions: Cyanosis of the ears, snout, and abdomen.
- Pulmonary Lesions: Severe interstitial pneumonia.
- Lymph Nodes: Enlargement and hemorrhage in lymph nodes.
- Key Distinguishing Features:
- HP-PRRS predominantly targets the respiratory and reproductive systems, leading to severe pneumonia, dyspnea, and abortions.
- While hemorrhages can occur, they are generally less extensive and severe than the widespread hemorrhages seen in ASF.
- ASF typically features more dramatic splenic changes and profound gastrointestinal hemorrhages.
- Further Reading: The USDA Animal and Plant Health Inspection Service (APHIS) provides information on PRRS.
3. Swine Erysipelas
Swine erysipelas, caused by the bacterium Erysipelothrix rhusiopathiae, can manifest in acute, subacute, or chronic forms. The acute septicaemic form, characterized by sudden onset and systemic signs, can be mistaken for ASF.
- Similarities with ASF:
- Systemic Signs: High fever, anorexia, depression.
- Skin Lesions: Reddening of the skin; in acute septicaemic cases, this can be diffuse and resemble the discoloration seen in ASF.
- Spleen: Splenomegaly (enlarged spleen).
- Key Distinguishing Features:
- The hallmark of erysipelas, though not always present in peracute cases, is the presence of distinctive diamond-shaped skin lesions (urticarial plaques).
- ASF is typically associated with more extensive and severe internal hemorrhages, including a very dark, friable spleen.
- Erysipelas is a bacterial infection and, if diagnosed early, may respond to antibiotic treatment.
- Further Reading: For more details, refer to the WOAH Swine Erysipelas overview.
4. Septicaemic Salmonellosis
Septicaemic salmonellosis, caused by Salmonella bacteria (e.g., Salmonella Choleraesuis or Salmonella Typhimurium), results in a systemic infection with high fever and severe enteric or respiratory involvement. The septicaemic form can closely mimic ASF.
- Similarities with ASF:
- Systemic Signs: High fever, depression, anorexia.
- Skin Lesions: Purple discoloration of the ears, snout, and ventral abdomen due to circulatory disturbances and cyanosis.
- Spleen: Splenomegaly.
- Lymph Nodes: Enlargement and congestion of lymph nodes.
- Gastrointestinal Tract: Can cause hemorrhagic gastroenteritis.
- Key Distinguishing Features:
- Salmonellosis often involves significant enteritis and necrotic lesions in the intestines, including button ulcers in the cecum and colon, which are distinctive (and shared with chronic CSF, but not typical of ASF).
- While hemorrhages occur, they are generally less severe and widespread than the extensive hemorrhages characteristic of ASF.
- ASF tends to cause more pronounced and hemorrhagic lymph node and splenic lesions, often with a friable spleen.
- Further Reading: The Merck Veterinary Manual provides detailed information on Salmonellosis in Swine.
5. Porcine Dermatitis Nephropathy Syndrome (PDNS)
Porcine Dermatitis Nephropathy Syndrome (PDNS) is an immune-mediated condition often observed in pigs concurrently infected with Porcine Circovirus Type 2 (PCV2). It is primarily characterized by skin lesions and severe kidney damage.
- Similarities with ASF:
- Skin Lesions: Irregular red-brown to purple skin lesions, especially on the hindquarters and flanks, which may ulcerate. These can superficially resemble the skin discoloration or hemorrhages seen in ASF.
- Systemic Signs: Depression, anorexia, and fever (though usually less severe than in ASF).
- Key Distinguishing Features:
- PDNS primarily affects the kidneys (nephritis) and skin. Histopathological examination reveals vasculitis and glomerulonephritis.
- The skin lesions in PDNS are distinct: they are often raised, necrotic, and firm to the touch, differing from the diffuse cyanosis and petechial hemorrhages of ASF.
- Internal hemorrhages are not a prominent feature of PDNS, unlike ASF.
- Lymph node enlargement and splenic lesions are not as pronounced or hemorrhagic as observed in ASF.
- Further Reading: Information related to PDNS can often be found under resources for Porcine Circovirus Infections from WOAH, as PCV2 is a common underlying factor.
Comparative Table of ASF and its Key Differential Diagnoses
This table provides a concise summary of common clinical signs and gross lesions for ASF and its main differential diagnoses. However, due to significant overlap, laboratory testing remains essential for a definitive diagnosis.
| Feature | African Swine Fever (ASF) | Classical Swine Fever (CSF) | HP-PRRS | Swine Erysipelas | Septicaemic Salmonellosis | PDNS |
|---|---|---|---|---|---|---|
| Etiology | ASF Virus (Asfarviridae) | CSF Virus (Pestivirus) | PRRS Virus (Arteriviridae) | Erysipelothrix rhusiopathiae (bacterium) | Salmonella spp. (bacterium) | Immune-mediated (often PCV2-associated) |
| Clinical Signs | High fever, anorexia, lethargy, skin cyanosis/hemorrhages, staggering, vomiting, diarrhea. High mortality. | High fever, anorexia, lethargy, skin hemorrhages, neurological signs (variable), vomiting, diarrhea. High mortality. | High fever, severe respiratory distress, reproductive failure, cyanosis of ears/snout. High mortality in young pigs. | High fever, anorexia, diamond skin lesions (classic), lameness, arthritis. | High fever, depression, skin discoloration, diarrhea (often hemorrhagic), respiratory signs. | Irregular red-brown/purple skin lesions, depression, anorexia, mild fever. |
| Lymph Nodes | Markedly enlarged, severe hemorrhage, edematous, "marbleized" | Enlarged, often "marbleized" due to hemorrhage | Enlarged, congested, may have mild hemorrhage | Enlarged, congested | Enlarged, congested, may show necrosis | Enlarged, congested, no prominent hemorrhage |
| Spleen | Markedly enlarged, dark, friable, often hemorrhagic | Enlarged, congested, often with hemorrhagic infarcts | Mild to moderately enlarged, congested | Enlarged, congested (splenomegaly) | Enlarged, congested, may show necrosis | Normal to slightly enlarged |
| Kidneys | Petechiae, ecchymoses, sometimes infarcts | Petechiae ("turkey egg"), sometimes infarcts | Congested, occasional petechiae | Congested, occasional petechiae | Congested, occasional petechiae, sometimes nephritis | Markedly enlarged, pale, hemorrhagic, characteristic glomerulonephritis and vasculitis |
| Skin Lesions | Reddening, cyanosis (ears, snout, abdomen), petechiae, ecchymoses | Reddening, cyanosis, petechiae, ecchymoses | Cyanosis (ears, snout, abdomen) | Diamond-shaped raised lesions (urticaria), diffuse reddening in acute cases | Purple discoloration (ears, snout, abdomen), diffuse reddening | Irregular red-brown to purple macules/papules/plaques, often necrotic and firm to touch |
| Gastrointestinal | Hemorrhagic gastritis, enteritis; generally no characteristic ulcers | Hemorrhagic enteritis; button ulcers (cecum/colon) in chronic forms | Minimal GI lesions, sometimes mild enteritis | Mild gastroenteritis | Severe enteritis, necrosis, button ulcers (cecum/colon) | No specific GI lesions |
| Mortality | High (up to 100% in acute forms) | High | High (especially in young pigs) | Variable; moderate to high in acute septicaemic forms | Moderate to high | Low to moderate |
Importance of Laboratory Confirmation
Given the substantial overlap in clinical signs and gross lesions among ASF and its differential diagnoses, especially with CSF, definitive diagnosis cannot rely solely on visual inspection or field observations. Laboratory testing is indispensable for accurately identifying ASF.
- Recommended Diagnostic Tests:
- Real-time Polymerase Chain Reaction (RT-PCR): This is the gold standard for detecting ASF viral DNA in tissue samples (such as spleen, lymph nodes, tonsil), blood, or oral fluids. It offers high sensitivity and specificity.
- Antigen ELISA: Used to detect viral proteins directly.
- Antibody ELISA: Detects the presence of antibodies against the ASF virus, which can indicate previous exposure or chronic infection (though less common given ASF's high mortality).
- Virus Isolation: A highly specialized test performed in high-containment laboratories, involving the culture of the virus.
- Histopathology: Microscopic examination of tissues can reveal characteristic lesions suggestive of ASF but is not definitive without molecular tests.
Early and precise laboratory diagnosis is paramount for implementing swift and effective control measures, including stamping out policies, strict movement restrictions, and enhanced biosecurity protocols, to prevent the further spread of this devastating disease.
