This disease occurs mainly in South and Central America, occasionally in the USA and rarely as epidemics extending as far North as Canada and as far South as Argentina.
The VS virus produces a disease in pigs that is clinically indistinguishable FMD, SVD and VES. Most often however infection of pigs is subclinical.
Symptoms
All Pigs
- Drooling saliva.
- Foot lesions and lameness.
- May be a reduction in growth rate.
- Rise in body temperature to 40-41°C (106-107°F).
- Clinical signs are so closely similar to those of FMD, that they need not be repeated in detail here.
- Like FMD, the most striking feature is the appearance of vesicles (blisters) up to 30mm diameter on the nose, lips, and teats and around the coronets of the feet which may make the pigs lame.
- Mortality is usually low and most pigs recover in one to two weeks.
- One difference from FMD is the relatively small proportion of pigs in a herd outbreak that show vesicles. Another difference from FMD is that usually when an outbreak occurs in a pig herd it rarely if ever spreads to cattle and horses on the same farm and vice versa.
Causes / Contributing factors
- The virus is spread mechanically by a variety of insects and has been isolated from face flies, black flies, eye gnats, sand flies, leaf hoppers and mosquitoes.
- The virus may multiply in some of these insects and can pass vertically through the ovaries to the offspring.
- Insects are therefore thought to act as reservoirs, perpetuating the virus in the enzootic regions.
- It is thought that in the spread between pigs in epizootic regions insects get the virus on their mouth parts from feeding on the lesions left after the vesicles have burst and carry it mechanically to other pigs in the same herd or in neighbouring herds. It is unlikely that they get infected from sucking the pigs’ blood.
- The virus can also spread between pigs by direct contact particularly when pigs are tightly packed together, for example, during transport or when pigs fight after mixing
- The virus can also be carried from herd to herd through the movement of pigs.
Diagnosis
VS is notifiable in most epizootic areas, i.e. if you suspect it in the herd you or your veterinarian have to report it to the authorities.
The clinical signs of VS are similar to those of FMD and SVD both of which are subject to government slaughter and eradication policy in Canada, the USA, Mexico, Chile, South Brazil, and Argentina. It is therefore crucial to reach a fast accurate diagnosis. This can only be done by delivering samples to a laboratory equipped and capable of doing the appropriate tests. The aim is to eliminate the possibility of the disease being FMD or SVD (or in California VES).
The best samples to submit are vesicular fluid, if available, which has high concentrations of virus and/or vesicular tissue (e.g. the thin superficial skin layer over the vesicle) which also contains virus. If these samples are from pigs or cattle the authorities will probably only allow you to send them to a designated FMD laboratory in case the disease is FMD. If they are from a horse then they cannot be FMD (horses do not get FMD, SVD, or VES) and you may be allowed send them to other laboratories (e.g. in the USA, the USDA-NVSL at Ames or other State laboratories).
The possibility of the disease being FMD or SVD (or in California VES) should be eliminated and an accurate identification of the VS virus made. The first of these, namely, elimination of FMD, SVD and VES can probably only be done in the designated FMD laboratories. Other diagnostic laboratories may be able to do the second, namely, identification of the VS virus in samples from horses. This is done by demonstrating the presence of the VS virus in the vesicular fluid or tissue first by ELISA which is rapid giving an answer in a few hours time.
Paired blood samples (i.e. one sample taken during the early stage of the disease and one 10-14 days later) may also be taken. The authorities will probably allow these to be tested in non FMD-designated laboratories (e.g. in the USA, State laboratories). The tests used are generally ELISAs with back-up neutralisation and complement fixation tests. In horses, rising antibody levels to the VS virus have to be demonstrated in the blood samples to be sure that an active VS infection has taken place. This is because in epizootic regions some old horses may have positive antibodies from the last outbreak. Pigs generally do not live so long so single positive samples would be strongly indicative of active infection.
Unfortunately blood sampling and serology may mean a delay of at least two weeks which is too long.