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Epidemiology

CSF is strictly a disease of pigs. It does not affect humans or domestic livestock other than pigs. Its natural hosts are domestic pigs and European wild boars, which belong to the same species and are ancestral to domestic pigs. Limited experimental studies carried out after the CSF outbreaks in South Africa indicated that the virus can be transmitted to bush pigs and warthogs and can cause lethal disease in bush pigs.

It is highly contagious and is easily transmitted between pigs. The most important sources of virus are live infected pigs and meat and tissues derived from infected pigs. European wild boars are equally susceptible to infection with CSF virus and as in domestic pigs infections result in subclinical to acute fatal disease depending on the virulence of the virus and the immunity of the pigs. Like ASF, CSF affects pigs of all ages, but in endemic areas even infection with virulent virus may produce overt disease only in the younger pigs because the older pigs have developed a degree of immunity.

Molecular studies have revealed two major types of CSF virus, one in Europe and the other in Asia, which makes it possible to trace the probable origin of outbreaks.

Domestic and wild pigs

Maintenance of the virus in domestic pigs and wild boars is similar to that of ASF virus. There is no long-term carrier state in recovered pigs, but large continuous populations of either free-ranging domestic pigs or wild boars can maintain indefinite circulation of the virus. In North America and Western Europe CSF has been eradicated in domestic pigs, but it has not been possible to eradicate the virus from all the wild boar populations in Western Europe, and occasional outbreaks are registered when careless husbandry allows domestic pigs to come into contact with wild boars, or the entrails of wild boars infected with CSF are fed to domestic pigs. Such outbreaks are generally limited in extent and easily controlled. More serious outbreaks have occurred in recent decades in Belgium, The Netherlands and the United Kingdom, where molecular studies revealed the cause to be viruses of Asian origin that most probably came in with illegally imported pork that was subsequently fed as swill.



Wild boar

Maintenance of the virus outside the host

CSF virus is resistant to a wide range of temperature and pH changes. It is therefore able to persist for long periods in meat and tissues derived from viraemic pigs. More detailed studies have been made on the survival of CSF virus in meat than on ASF virus, and have demonstrated that the virus can survive for up to 85 days in chilled pork and more than four years in frozen pork. It can also survive for 140 – 312 days in cured hams depending on the curing process. It is destroyed by exposure to 70˚C for 30 minutes. It is sensitive to desiccation and exposure to sunlight and does not survive long in the environment in the absence of sufficient moisture and organic material. There are no biological vectors.

Transmission

Pigs are usually infected by the oronasal route either through contact with the excretions and secretions of infected pigs or by consuming the tissues of infected pigs. Fomites may transmit infection but this is considered to be less important than direct contact, scavenging or swill feeding. Vertical transmission occurs readily. In the case of infection with highly pathogenic viruses the foetuses usually die and will be resorbed or aborted. CSF is therefore considered to be one of the diseases that contribute to the SMEDI syndrome. Infection with less virulent viruses is not fatal to the piglets. If it occurs during the first trimester, before the foetuses have developed immune competence, it can result in the birth of persistently infected piglets. These piglets are born apparently normal and without antibodies to CSF. They shed virus intermittently until suddenly they develop clinical signs of acute CSF and die. Usually they do not survive for many months but survival up to 11 months has been recorded. These piglets are therefore potentially dangerous as unapparent transmitters of the virus. Sexual transmission has been demonstrated to occur and the virus can persist in chilled or frozen semen, but its importance in the epidemiology of the disease has not been established.

Other means of viral transmission have been investigated. Airborne transmission over a distance of less than 2 km may be possible in pig-dense areas under conditions of low temperature and high humidity. Mechanical transmission by biting flies may be possible. A study indicated that birds, rodents and domestic animals such as dogs and cats are not able to transmit the virus in infective quantities.