2002/106/EC: Commission DecisionShow full title

CHAPTER II U.K. Description of classical swine fever with emphasis on differential diagnosis

A. Introduction U.K.

1. Classical swine fever is caused by an enveloped RNA virus which belongs to the genus pestivirus of the flaviviridae family. This virus is related to the ruminant pestiviruses causing bovine viral diarrhoea (BVDV) and border disease (BDV). This relationship has serious diagnostic consequences as cross reactions occur and may lead to false positive results of the laboratory tests. U.K.

2. Classical swine fever virus is relatively stable in moist excretions of infected pigs, pig carcasses and fresh pig meat and some pig meat products. It is readily inactivated by detergents, lipid solvents, proteases and common disinfectants. U.K.

3. The main natural route of infection is oro-nasal by direct or indirect contact with infected pigs or by feeding of virus contaminated feed. In areas with a high density of pigs, spread of virus easily occurs between neighbouring pig holdings. Disease transmission via semen of infected boars may also occur. U.K.

4. The incubation period in individual animals is about one week to ten days, but under field conditions clinical symptoms may only become evident in a holding two to four weeks after virus introduction or even more if only adult breeding pigs or mild strains of virus are concerned. U.K.

5. The clinical signs of classical swine fever are extremely variable and it may be confused with many other diseases. Severity of symptoms depends mainly on the age of the animal and virus virulence. Usually young animals are affected more severely than older animals. In older breeding pigs the course of the infection is often mild or even sub-clinical. U.K.

6. Acute, chronic and prenatal forms of classical swine fever can be distinguished. U.K.

B. Acute form U.K.

1. Weaners and fattening pigs most often display the acute form of classical swine fever. The initial signs are anorexia, lethargy, fever, conjunctivitis, swollen lymph nodes, respiratory signs and constipation followed by diarrhoea. U.K.

The typical haemorrhages of the skin, are usually observed on the ear, tail, abdomen and the inner side of the limbs during the second and third week after infection until death. Neurological signs are frequently seen, such as a staggering hind limb gait, in coordination of movement, and convulsions.

A constant finding is fever. This is usually higher than 40 °C, but in adult pigs fever may not exceed 39,5 °C.

2. Classical swine fever virus causes severe leukopenia and immunosuppression, which often leads to enteric or respiratory secondary infections. The signs of these secondary infections can mask or overlap the most typical signs of classical swine fever and may mislead the farmer or the veterinarian. U.K.

Death occurs usually within one month. Recovery with production of antibodies does occur, most often in adult breeding animals which do not display severe clinical signs. Antibodies against classical swine fever virus are detectable from 2 to 3 weeks post infection onwards.

3. Pathological changes visible on post-mortem examination are most frequently observed in lymph nodes and kidneys. The lymph nodes become swollen, oedematous and haemorrhagic. Haemorrhages of the kidney may vary in size from hardly visible petechiae to ecchymotic haemorrhages. Similar haemorrhages can also be observed in the urinary bladder, larynx, epiglottis and heart and sometimes widespread over the serosa of the abdomen and chest. A non-purulent encephalitis is often present. Lesions due to secondary infections may also be seen which may mislead the veterinarian. Infarcts in the spleen are considered pathognomonic but are infrequently seen. U.K.

4. In general the acute form of African swine fever leads to a clinical and pathological picture very similar to that of classical swine fever. When present, haemorrhages on the skin and ears are quite easy to detect and lead to suspicion of acute African or classical swine fever. Few other diseases cause similar lesions. U.K.

Acute classical swine fever must also be considered in case of suspected erysipelas, porcine reproductive and respiratory syndrome, coumarin poisoning, purpura haemorragica, post-weaning multisystemic wasting syndrome, porcine dermatitis and nephropathy syndrome, salmonella or Pasteurella infections or any enteric or respiratory syndromes with fever which do not respond to antibiotic treatment.

5. Classical swine fever virus is shed in saliva, urine and faeces from the onset of clinical signs until death. Classical swine fever virus can also be shed via semen. U.K.

C. Chronic form U.K.

1. The chronic course of infection occurs when pigs are not able to develop an effective immune response against classical swine fever virus. Initial signs of a chronic infection are similar to the acute infection. Later, predominately non-specific signs are present, i.e. intermittent fever, chronic enteritis and wasting. The typical haemorrhages of the skin are missing. U.K.

These pigs may show clinical signs of disease for 2 to 3 months before death. Classical swine fever virus is constantly shed from the onset of clinical signs until death. Antibodies may be temporarily detected in serum samples.

2. Pathological changes are less typical, especially haemorrhages in organs and serosa may not be observed. In animals showing chronic diarrhoea, necrotic lesions are common on the ileum, the ileocaecal valve and the rectum. U.K.

3. As clinical signs of chronic classical swine fever are rather non-specific, many other diseases must be considered for differential diagnosis. The increased body temperature is not necessarily present in every animal, but in an infected holding fever can be detected at least in some pigs. U.K.

D. Prenatal form and late onset of disease U.K.

1. Classical swine fever virus is able to pass across the placenta of pregnant animals, and infect foetuses, but in the sows the disease is often sub-clinical. U.K.

The outcome of trans-placental infection of foetuses depends largely on the stage of gestation and viral virulence. Infection during early pregnancy may result in abortions and stillbirths, mummification and malformations. All this leads to a reduction of the fertility index in the holding.

Infection of sows at up to 90 days of pregnancy can lead to the birth of persistently viraemic piglets, which may be clinically normal at birth and survive for several months. After birth, they may show poor growth, wasting or occasionally congenital tremor. This course of infection is referred to as "late onset classical swine fever". These piglets may play a crucial role in spreading the disease and in the maintenance of virus persistence within a population, as they constantly shed virus until death.

2. Detection of classical swine fever may be particularly difficult in breeding pig holdings, as the course of the infection may be very mild and may be confused with many other pathological conditions. Reduced fertility and abortions can be caused by classical swine fever virus as well as parvovirus infection, PRRS, leptospirosis, and Aujeszkys disease. Material aborted due to classical swine fever infection cannot be distinguished pathologically from abortions due to other disease agents. U.K.

In case of suspicion of an infectious disease of the reproductive tract, investigation for classical swine fever must be immediately carried out whenever the holding in question can be considered at risk (e.g. due to location of the holding in an area where classical swine fever occurs in feral pigs), and in any case as soon as more common infectious diseases of the reproductive tract have been excluded.