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Although the incubation period for classical swine fever is generally less than ten days, in the field it may take up to four weeks for clinical signs to become apparent in a population. Disease severity varies with virulence, immune status and the age of the animal: this means that although acute, chronic and congenital forms of the disease can be appreciated, there is no "classic" disease presentation.

Although the incubation period for classical swine fever is generally less than ten days, in the field it may take up to four weeks for clinical signs to become apparent in a population. Disease severity varies with virulence, immune status and the age of the animal: this means that although acute, chronic and congenital forms of the disease can be appreciated, there is no "classic" disease presentation.

In the '''acute form''', animals are almost always pyrexic. In piglets the fever may exceed 40⁰C, but in adults temperatures may be no higher than 39.5⁰C. Lethargy, conjunctivitis and lymphomegaly may be seen, as well as respiratory signs and diarrhoea. Neurological signs such as gait abnormalities, incoordination and convulsions are also common. The most telling sign of classical swine fever is haemorrhage of the skin. Haemorrhages arise in the second or third week post-infection on the ear, tail, abdomen and medial aspect of the limbs and persist until death. CSF virus also causes severe leukopenia and immunosuppression, leading to secondary enteric or respiratory infections which may cause confusion by masking or overlapping the more typical signs of CSF. The clinical signs of acute CSF become less specific and diagnosis more difficult as the age of onset increases. Also, acute classical swine fever is clinically indistinguishable from African swine fever and so care must be taken when formulating a diagnosis. Other differential diagnoses for acute CSF are [[Erysipelas - Pig|erysipelas]], [[Porcine Reproductive and Respiratory Syndrome|PRRS]], [[Haemorrhage - Pathology#Purpura|purpura haemorragica]], [[Porcine Circoviruses|PMWS, PDNS]], [[Salmonellosis]] and [[Pasteurellosis - Pigs|Pasteurellosis]]. Classical swine fever should also be considered in any pyrexic enteric or respiratory disease case that is not responsive to antibiotics.  

In the '''acute form''', animals are almost always pyrexic. In piglets the fever may exceed 40⁰C, but in adults temperatures may be no higher than 39.5⁰C. Lethargy, conjunctivitis and lymphomegaly may be seen, as well as respiratory signs and diarrhoea. Neurological signs such as gait abnormalities, incoordination and convulsions are also common. The most telling sign of classical swine fever is haemorrhage of the skin. Haemorrhages arise in the second or third week post-infection on the ear, tail, abdomen and medial aspect of the limbs and persist until death. CSF virus also causes severe leukopenia and immunosuppression, leading to secondary enteric or respiratory infections which may cause confusion by masking or overlapping the more typical signs of CSF. The clinical signs of acute CSF become less specific and diagnosis more difficult as the age of onset increases. Also, acute classical swine fever is clinically indistinguishable from African swine fever and so care must be taken when formulating a diagnosis. Other differential diagnoses for acute CSF are [[Erysipelas - Pig|erysipelas]], [[Porcine Reproductive and Respiratory Syndrome|PRRS]], [[Haemorrhage#Purpura|purpura haemorragica]], [[Porcine Circoviruses|PMWS, PDNS]], [[Salmonellosis]] and [[Pasteurellosis - Pigs|Pasteurellosis]]. Classical swine fever should also be considered in any pyrexic enteric or respiratory disease case that is not responsive to antibiotics.  

The '''chronic form''' of classical swine fever develops when pigs fail to mount an effective immune response to viral infection. Initially, the signs are similar to the acute form of the disease, but symptoms become less specific as the course progresses. For example, pigs may display chronic enteritis, loss of condition, lameness or intermittent pyrexia. In a herd, mortality may be increased or there may be large numbers of runty pigs. Although animals may survive some months after contracting chronic CSF, the disease is always eventually fatal and animals continue to shed virus until death.

The '''chronic form''' of classical swine fever develops when pigs fail to mount an effective immune response to viral infection. Initially, the signs are similar to the acute form of the disease, but symptoms become less specific as the course progresses. For example, pigs may display chronic enteritis, loss of condition, lameness or intermittent pyrexia. In a herd, mortality may be increased or there may be large numbers of runty pigs. Although animals may survive some months after contracting chronic CSF, the disease is always eventually fatal and animals continue to shed virus until death.