Background and history

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. If you farm in any of these areas you should be aware of it. The infection is more widespread involving wild life including sea mammals.

It does not occur outside the Americas. The only confirmed reports outside the Americas were France during World War I and South Africa during the Boer war - both associated with horses.

Importance of VS

The VS virus produces a disease in pigs that is clinically indistinguishable FMD, SVD and VES. Most often however infection of pigs is subclinical.

In itself it is not very important in pigs. In severe outbreaks the foot lesions may be painful and make pigs lame and there may be a reduction in growth rate. On the other hand it can be so mild that the pigs do not appear lame or ill.

If you are concerned about VS read also the sections above on FMD and SVD (and if you work in California VES).

Species affected

In domesticated animals, VS is primarily a disease of horses and cattle only occasionally causing clinical disease in pigs. A range of wild animals including wild pigs, deer, raccoons and sea mammals. People can also be infected developing flu-like symptoms.

Clinical signs

The initial signs in affected swine are drooling saliva and a rise in body temperature to 40-41°C (106-107°F). Thereafter the 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. These burst leaving erosions and ulcers. Those on the nose and mouth then heal rapidly but those on the feet may become secondarily infected and permanent damage may result. 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. Most may not although many may seroconvert as a result of sub-clinical infection. 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.

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.

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.

Causes

The Americas can be divided into enzootic and epizootic regions. In the enzootic region the virus is present and cycling among animals and insects all the time. The enzootic region covers Central America, Mexico, the coastal plains of South Eastern USA and the North West of South America. The rest of the USA, Canada and South America constitute epizootic regions.

The virus is not present in these all the time but outbreaks occur seasonally and intermittently, several years often passing between epizootics. Epizootics usually start in the USA in late spring or early summer often after heavy rain, presumably reflecting the rise in the insect population. They usually end with the first frost although the 1982-83 outbreak in the USA persisted into winter.

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 erosions and ulcers are initially teeming with virus and although the virus is assumed to be carried around the body in the blood stream it is at very low undetectable levels.

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 is thought to be spread in these circumstances by getting into cuts and abrasions.

The virus can also be carried from herd to herd through the movement of pigs but pigs do not appear to become long term sub-clinical carriers.

Prevention

• Vaccination - It is possible to produce an effective live attenuated vaccine or an inactivated vaccine but in practice the low incidence of the disease in swine, even in the face of big outbreaks in cattle and horses makes vaccination uneconomical. Furthermore, in the USA vaccination of pigs against VS is not allowed.
• There are numerous serotypes of VS virus but only two (Indiana 1 and New Jersey 1) are known to affect pigs.

National precautions

• If your pig farm is in an epizootic area, which most states in the USA are for example, and VS is confirmed, the herd will not be slaughtered out to get rid of the virus because the disease is self limiting and disappears spontaneously. It is likely, however, that the authorities will quarantine your farm until they deem that the virus has gone.
• In the USA, animals in any state in which outbreaks of VS are occurring cannot be legally moved to any other state or to the EU without testing and quarantine.
• Countries which are completely free from VS (e.g. countries in Europe) apply national preventative measures against the introduction of VS. The main feature is control over the importation of cattle and pigs from countries in which VS occurs.
• In practice it is most unlikely that VS would get into such countries, spread and become established. Pig farmers in these countries should not worry about this disease
• If the disease did enter such a country, which could be through the movement of horses, a standstill on animal movement would probably be imposed and the affected animals would be isolated and might be slaughtered. Infact, it has only been identified once in Europe and that was in American military horses in France in the first world war. It did not spread or persist.

On-farm prevention

• As with other infectious diseases, if you are a pig farmer in an epizootic region be careful about the source of newly introduced pigs into your herd. Isolate them for a month to six weeks to ensure that they, or the source from which they have come, are not incubating this disease or any other.
• VS is not as contagious as, say, FMD or TGE, but if the disease breaks out in neighbouring pig herds you should tighten the biosecurity of your own farm.
• In the case of VS, the gap in the protective measures is that the virus is spread by insects and, of course, it is difficult to stop their movement.

Treatment

• If possible, make the affected pigs comfortable by providing clean bedding. Also, give them soft food. Put them in clean pens to reduce the likelihood of secondary infections. If secondary infections occur, treat with antibiotics.
• Use insect sprays and repellents to reduce the spread in your herd and to your neighbours' herds. If you supply other herds with stock, stop the movement of pigs from your herd for 30 days. (The authorities will probably insist on this anyway).
• If the disease breaks out in your herd the most urgent thing for you to do is to get an accurate diagnosis. Call your veterinarian immediately.