Epizootiology of swine fever

In countries with an intensive pig production system, swine fever (SF), where present, may be regarded as one of the economically most important pig diseases. Although reservoirs of virus and pathways of transmission are well understood, field experience has shown that the disease is extremely difficult to eradicate, especially in countries with intensive pig breeding. The virulence of the strains circulating in the field and the measures applied to control the disease determine to a large extent the course of an epizootic. Outbreaks of highly virulent strains are readily recognised. Epizootics caused by low virulent strains are characterised by indistinct signs of disease, slow spread of virus through the herd and the comparatively important role of the 'carrier sow syndrome'. The latter phenomenon may result in the birth of healthy looking but persistently infected and immune-tolerant piglets. This and the occurrence of chronic infections are largely responsible for the perpetuation of the virus in the pig population. The 1982-1985 epizootic of SF in the Netherlands showed an almost equal prevalence of outbreaks in breeding and fattening herds. In proportion to the number of herds in the country, there was in both categories an approximately linear relationship between the risk of infection and the size of the herds. The proportion of affected breeding herds increased as mass vaccination of areas at risk was started earlier in the epizootic. In close-knit communities with large breeding units, man should be regarded as the single most important factor in the herd-to-herd transmission of the virus, whereas in fattening units and in areas with small pig farms, transport and introduction of infected pigs into herds accounts for the majority of outbreaks and for the spread of the disease. Farms visited by persons handling pigs subsequent to contact with an infected herd should, therefore, be included in the epizootiological investigations. Virological and serological methods are indispensable in order to confirm or to rule out the possibility of infection of suspected contacts, especially in outbreaks caused by strains of low virulence. Continuous education of farmers regarding the importance of maintaining precautionary measures against the introduction of contagious diseases and, in the case of an advancing epizootic, special instructions to all people entering piggeries, would contribute greatly to reducing the untraceable pathways of SF spread.