The outbreak of vesicular stomatitis in the United States which began in April last year appears close to being over, with figures indicating a great majority of the cases were in horses.
Currently, just one property remains under quarantine in the United States – in Yuma County, Colorado.
The US Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS), in an update dated February 10, said a total of 823 properties had recorded cases of the troublesome viral infection since the first case of the 2015 outbreak was confirmed in Grant County, New Mexico, on April 29.
An APHIS official confirmed to Horsetalk that about 90 percent of these properties had infected equines. Of the 823 premises, 86 were recorded as having only cattle affected, and five were reported as having both cattle and equines affected. The other 732 premises were reported as being “equine only”.
How many horses were infected across the 737 properties with known equine cases is not known.
In all, eight states recorded cases. The worst-affected was Colorado, with 441 premises affected across 36 counties. The other affected states were Arizona (36 premises in 3 counties), Nebraska (38 premises in 10 counties), New Mexico (52 premises in 13 counties), South Dakota (50 premises in 7 counties), Texas (4 premises in 4 counties), Utah (56 premises in 8 counties), and Wyoming (146 premises in 10 counties).
While just one property remains under quarantine, the virus may surge again in spring should it manage to winter over. The high-risk season in the US runs from spring to fall, with a peak in mid-summer.
Vesicular stomatitis can affect horses, mules, cattle, bison, sheep, goats, pigs, and camelids. It also has the ability to infect people, causing flu-like symptoms, but such cases are rare.
Clinical signs include vesicles, erosions and sloughing of the skin on the muzzle, tongue, teats and above the hooves of susceptible livestock. Vesicles are usually only seen early in the course of the disease.
Its transmission is not completely understood, but biting insects are known to be a key vector, and mechanical transmission and livestock movements are also believed to play a part.
The symptoms are similar to those of foot and mouth disease, which is why US authorities must legally be notified of cases. In cattle, animal health agencies must establish that the symptoms are not a result of foot and mouth disease, which has not been seen in the US since 1929.
Vesicular stomatitis is endemic in Mexico. Official figures collected and published online by agriculture officials in Mexico tended to show one or two infected cattle somewhere across the tropical coastal southern sweep of the country at any given time. The climate seemed to allow the virus to propagate in Mexico all year round.
Seasonal climate changes and ecological conditions allow the biting insects that carry the virus to spread north in some years when conditions are favorable to the insects.
When suitable conditions arise, the virus-carrying insects cross the US border, where they run the risk of infecting North American livestock.
Cases have been recorded across a wide area of the US since the first outbreak was confirmed in 1937. Some were recorded in feral pigs as far east as Ossabaw Island in Georgia.
US outbreaks have been sporadic, but in the last 30 years the American southwest has emerged as a hotspot for the virus.
Outbreaks in the region tend to start in the warmer south and spread north as the weather warms.
Many questions remain unanswered about the disease.
Some of the insect vectors that spread it among livestock are known, but there are probably many more.
Black flies, sand flies and Culicoides midges are known vectors.
It was possible that there may be reservoirs of the virus which have yet to be identified. These could be in the likes of mammals or insects. However, to date, no evidence of such reservoirs in the US has been found.
However, grasshoppers have been identified as a potential mechanical vector, arising from when drooling animals drop infected material on them or when they encounter infected saliva on grasses.
It is believed that major outbreaks occur when climatic and ecological factors combine in a way that favor the insect vectors, although the formula is not necessarily simple. One potential factor involved significant rainfall in the months before the major outbreaks.
Precisely how wet conditions may play a part in the rise of the disease is less clear. Is it, for example, related to subsequent higher humidity that may assist the vector insects, or does it create more breeding grounds or better survivability of insect eggs that are laid?
No vaccine is available.
The key measures that livestock owners could take during the high-risk season were insect control and the use of repellents on animals, especially targeting the more sensitive skin areas with little or no hair where the lesions are most likely to develop, including the muzzle, mouth, ears, around the teats, and above the hooves. Repellent is also recommended near any areas with broken skin, such as where fly strike or wounds are present.
Fly masks with ear protection can be effective, as can keeping horses indoors at dawn and dusk, when the insects are most active.
Research has shown that when animals are bitten by infected insects where their coats are thicker, they will develop an immune response but will not develop lesions at the site of the bites. In contrast, when bitten on the more sensitive areas, lesions can develop and can discharge infected fluid, which is capable of infecting other animals.
The vesicular stomatitis virus has not been detected in the bloodstream of infected horses and cattle. The only place scientists have been able to isolate the virus is from ruptured lesions, suggesting there is a very local transfer of virus.
Blood samples that have been taken from infected horse and cattle herds show, on average, that around 70 percent had been exposed to the virus. However, only 10 percent or fewer in the herd may develop lesions.
There was evidence, too, that infected vector insects could transfer the virus to more vector insects simply by co-feeding on an animal nearby one another – another compelling reason for livestock owners to institute insect control measures.