Swine vesicular disease (SVD)
Occurrence: Recent reports, Italy and Eastern Asia.
Age affected: All ages.
Causes: SVD virus.
Effects: Fever, inappetence, lameness, vesicles or ulcers on feet, mouth, udder, snout.
Swine vesicular disease (SVD) is caused by a picornavirus which grows readily in pig cell cultures. SVD virus is stable at pH 5 and more resistant to acid and alkaline conditions than foot and mouth disease virus. A pH below 2.5 or above 12.5 is necessary to destroy the virus. It is destroyed by moist heat at 60°C within 30 minutes, but in cool, humid conditions it can persist for more than 4 months and it survives indefinitely in infected refrigerated pig tissues. At least 7 antigenic types have been identified.
Infection enters through the tonsil in 90% of cases, but may also enter through the skin of the head, tongue, the foot, and the gut. It then multiplies in the draining lymph node and subsequently gives rise to a viraemia (virus carried in the blood) within a day followed by clinical signs within two days. The gastrointestinal tract may be exposed to 107.5 particles without infection, but the dose required to infect the coronary band of the foot is only 103.6 viable virus particles. Infections from swill may, therefore, take place through the skin. Neutralising antibody can be detected 7 days after infection and peaks at 28 days post-infection.
Mode of transmission
SVD is normally transmitted directly from pig to pig and affects only pigs, being shed in vesicle fluid and in the faeces. The virus enters by the oronasal route, usually through the tonsil, and also through the skin of the head, tongue, the foot especially of the coronary band, and via the gut. Since the virus is stable indefinitely when frozen and is resistant to heat, infection can result from the consumption of improperly cooked pig swill.
The virus can be carried mechanically on clothing and remain on pen furniture and in transport for months and is easily spread through a unit, from pen to pen. Farms become infected by the movement of pigs which may be short term carriers or mildly affected. The use of markets and farm transport allows pigs to contact infection and can give rise to outbreaks. Mechanical spread by birds can occur both within a farm and between farms.
The disease is indistinguishable from foot and mouth disease (FMD) in pigs. The incubation period is usually 2-4 days for local, primary lesions, 5-6 days after viraemia and 7-11 days after ingestion. The disease begins with fever (40-41°C, 104-106°F), 1 or 2 days’ inappetence, lameness and reluctance to move and affected pigs may arch their backs or group their feet beneath themselves. Severely affected animals scream or walk on their knees. Blanched areas of skin or epithelium develop into vesicles (blisters within 2-4 hours and then rupture to leave clean ulcers. Healing by granulation may be complete within 4 days. Lesions occur on the coronary band of the feet, the supernumerary digits and extend into the inter-digital space. Under-running of the horn and sloughing of the claw may occur. Vesicles also occur on the limbs, the mammary glands and in about 5% of animals, on the snout and in the mouth, especially on the dorsum of the tongue. The lesions may be mild and may pass unnoticed until later cases have been traced. From 25-70% of pigs may be affected, mainly those over 3 months of age, but there is little mortality. Some animals show no clinical signs but develop neutralising antibody.
The clinical signs of lameness and the presence of vesicles on the feet and elsewhere only allow a diagnosis of a vesicular condition to be made unless the case is linked to a confirmed outbreak of SVD. The pigs being examined must be restrained and the hooves cleaned for proper examination, as recovered animals are difficult to identify.
Recovered animals may have dark lines across each claw or evidence of ‘thimbling’ as hoof horn is shed. Samples of vesicular fluid are tested at reference laboratories to confirm the identity of the disease, and include completely fixation tests, virus isolation and, in long-stand cases, ELISA tests are useful for widespread screening and distinction between FMD and SVD. False positives in some ELISAs may result from antibody to other enteroviruses and neutralisation tests may be necessary to confirm their identity.
Nucleic acid from the virus can be detected using types of Polymerase Chain Reaction (RT-PCR and nested RT-PCR) and can distinguish SVD virus from foot and mouth virus in nasal swabs, tissues and faeces from day 7 to day 176 post infection. Contamination may be important with this method. Diagnosis is most difficult where seropositive herds occur with no contact with clinical signs of SVD (singleton reactor herds).
The post-mortem lesions are those described above. Old lesions may be seen as dark lines across the hoof horn, and gradually move down the hoof. Such lesions are not uncommon on a single foot, but their presence at the same height on all four feet suggests that a vesicular disease may have occurred.
Treatment and prevention
There is no treatment and no vaccine. SVD is important in international trade and is controlled by a slaughter policy in many countries. Suspicion of SVD leads immediately to notification of the state veterinarian and to isolation of the farm. When the disease is confirmed, the pigs are all slaughtered, buried or burned. Disinfection with 1% sodium hydroxide and flame guns is carried out, although certain other disinfectants, e.g. iodophors may be used. Animal movement in the infected area is halted and markets are stopped. The origin of the infection is investigated and the source of pigs brought to the farm is traced. A number of precautions are in place to stop SVD.
Heat treatment of waste foods containing meat is mandatory for swill feeders and swill feeding is currently totally forbidden in the EU. Pig movement can only be made under licence except for pigs going direct to slaughter from premises where no swill is fed. Pigs going to market or other farms must be licensed and come from farms which have received no pigs within the preceding 21 days. Markets for pigs of 21 day status and those for slaughter pigs cannot be held on the same premises. Strict rules for disinfection of lorries and premises are in force. The disease is managed as would be FMD.