Diagnosis and differential diagnosis

Clinical signs and pathology

The incubation period for ASF is 5 – 15 days. The first indication of ASF is unusually high mortality among domestic pigs of all ages. Clinical signs are high fever, which is characterised by pigs huddling together as if cold, loss of appetite, lethargy and, in white-skinned pigs, marked skin flushing, particularly of the extremities and ventral body. As the disease progresses, haemorrhages become visible on the skin and mucosa.

In the acute stage of ASF white-skinned pigs become flushed to cyanotic, particularly the ears, lower legs, and ventral abdomen

In the acute stage of ASF white-skinned pigs become flushed to cyanotic,
particularly the ears, lower legs, and ventral abdomen

Recumbency, accompanied by a high fever, indicated by congestion, haemorrhages and cyanosis of the ventral area and extremities

Recumbency, accompanied by a high fever, indicated by congestion,
haemorrhages and cyanosis of the ventral area and extremities

Other, more variable signs include ocular and nasal discharges, vomiting, constipation followed by diarrhoea with blood-stained faeces, abdominal pain, hind limb weakness resulting in incoordination and a swaying gait, respiratory distress, and, in disease of longer duration, nervous signs. Sows may abort at any stage of pregnancy due to the high fever. At necropsy haemorrhages are observed in organs and on serosal surfaces. The body cavities generally contain blood-tinged fluid. The spleen is usually enlarged and congested, and occasionally infarcts may be present. The lymph nodes, in particular submandibular, hepatogastric and mesenteric nodes, are markedly enlarged and haemorrhagic, often resembling blood clots. Sometimes there is severe lung oedema. Severe congestion of the gastric fundus, sometimes with ulceration, and haemorrhagic enteritis may be present. Histologically the most characteristic changes are widespread karyorrhexis of lymphoid tissues, effacement of the SS sheaths in the spleen, and fibrinoid vasculitis. The virus targets macrophages, especially in lymphoid tissues but also in other organs and in bone marrow, and most of the changes observed can be ascribed to the release of cytokines by macrophages in which virus is replicating. These cause inflammation with increased vascular permeability, resulting in oedema and haemorrhage. Although the virus does not replicate in lymphocytes, widespread apoptosis of both B and T cells occurs. Death is usually due to shock and disseminated intravascular coagulation, but some pigs die as a result of massive lung oedema.

Fibrinoid necrosis of arterioles

Fibrinoid necrosis of arterioles
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Pyknosis and karyorrhexis in lymphoid tissues



Fibrous pericarditis seen in cases of subacute and chronic ASF
The lungs do not collapse and are enlarged due to the accumulation of fluid, resulting in prominent interlobular septa and straw-coloured to blood-tinged fluid in body cavities

The lungs do not collapse and are enlarged due to the
accumulation of fluid, resulting in prominent interlobular
septa and straw-coloured to blood-tinged fluid in body cavities

Pinpoint haemorrhages in the renal cortex

Pinpoint haemorrhages in the renal cortex
The mucosa of the stomach is often deeply congested to haemorrhagic and sometimes necrotic

The mucosa of the stomach is often deeply congested
to haemorrhagic and sometimes necrotic


Clinical signs of African swine fever - staggering gait

As indicated earlier, the subacute and chronic forms of ASF appear to be rare. Subacute ASF is characterised by a fluctuating fever, and death, which occurs within weeks, is often due to pneumonia, generally caused by secondary bacterial invaders. Pigs that suffer chronic ASF are generally in poor condition, with long dull coats, ulcerous skin lesions including over bony points, and often have arthritis, pneumonia, and cardiac damage that can lead to congestive heart failure, with lesions such as fibrous epi- and pericaridtis evident at necropsy. Ulcers at the ileo-caecal junction, as described for classical swine fever, have been described in what apparently were chronic cases of ASF in Angola.

Chronically infected pigs are usually severely emaciated and stunted, with a long dull hair coat. Signs of pneumonia may be present as well as lameness and ulcers over bony points

Chronically infected pigs are usually severely emaciated and stunted,
with a long dull hair coat. Signs of pneumonia may be present
as well as lameness and ulcers over bony points

A field diagnosis is based on high mortality in pigs of all ages and the typical clinical signs and lesions. Although described as typical, none are pathognomonic, and laboratory confirmation is essential.

Laboratory confirmation

The samples of choice are spleen and lymph nodes on ice but not frozen. If maintenance of the cold chain is a problem the samples may be preserved in 50% glycerol-saline. Additionally, a set of samples from various organs (spleen, lymph nodes, lung, liver, kidney, brain) may be taken in 10% buffered formalin for histopathological examination and immunohistochemistry. If only live sick pigs are available, whole blood in anticoagulant (EDTA) may be submitted for PCR and blood in heparin-containing tubes for viral isolation. It should be noted that serum is not a useful sample for diagnosis of acute or peracute ASF, as most pigs die before antibodies can be detected. Commonly used laboratory tests to detect the presence of virus are polymerase chain reaction (PCR) and the direct fluorescent antibody test (FAT). For more detailed information on the diagnostic techniques, please refer to the OIE Manual. Although the OIE Manual points out that the FAT is not as sensitive as PCR, in countries where FAT is used it will detect ASF virus in an outbreak, when large amounts are present in blood or tissue samples, and is a more robust test than PCR when laboratory conditions are not ideal. The gold standard for diagnosis remains viral isolation and the observation of haemadsorption or cytopathic effects.

Differential diagnosis

The most important differential diagnosis for ASF is classical swine fever (CSF). The two diseases are indistinguishable on clinical signs and pathological lesions, and laboratory confirmation is essential. Bacterial septicaemia is probably the second most important differential diagnosis, in particular erysipelas, the acute form of which is characterised by high fever and a severely congestive picture. Usually younger age groups are affected, although acute erysipelas may occur in pigs of all ages, mortality is lower and there is a response to antimicrobial treatment. Other diseases that may be confused with ASF are possibly PRRS, particularly the more virulent forms that result in high mortality like the ‘high fever disease’ recently reported in China and Viet Nam, which is associated with a highly pathogenic strain of PRRS; porcine dermatitis and nephropathy syndrome, part of the porcine circovirus 2 associated disease (PCVAD) complex; and porcine trypanosomosis caused by Trypanosoma simiae. High mortality over a range of ages may occur with acute toxicoses, but these generally occur en masse rather than over a period of time as more pigs become infected with ASF. Poisoning with coumarins results in a haemorrhagic picture but usually only affects individual pigs. Stachybotryotoxicosis from mouldy feed results in widespread karyorrhexis that can be confused with the microscopic lesions of ASF.