Understanding Equine Reportable Diseases

Learn the facts behind three equine diseases in recent news

by Dave Sauter, DVM

Today’s article will provide updates on three infectious diseases that have been in the news in recent months. Although these diseases are quite different, they have three important similarities: all three are diseases of horses, all three are “reportable diseases,” and all three are in the news.

Reportable Diseases

Many diseases can have devastating affects on public health, animal health, agriculture, industry and the economy. A recent example is the H1N1 human and swine influenza virus. Other examples that have gained much public attention in recent years include toxigenic E. coli, Mad Cow Disease and Foot and Mouth Disease.

On a global scale, the agency that tracks human reportable diseases is the World Health Organization (WHO). For animal disease, it is the World Organization for Animal Health (OIE, or Office International des Epizooties). In the U.S., the tradition traces back to at least 1878, when the CDC began tracking cholera, small pox, plague and yellow fever. Currently, much of the tracking responsibility falls onto state health agencies.

The lists of human and animal diseases these agencies track are long. Veterinarians, physicians, laboratories and other health care providers are required by law to report to appropriate government agencies whenever a reportable disease is encountered. Having a central reporting system in place enables rapid disease control measures to be implemented to minimize the spread of these diseases and their impact on human and animal health and the economy.

West Nile Virus (WNV)

Washington State topped the nation last year in the number of horses that tested positive for the WNV. According to Dr. Leonard Eldridge of the Washington Department of Agriculture, nearly all of the 41 positive horses were unvaccinated. Evidently, this year the WNV is making an early appearance. The state has been monitoring the mosquito population for the presence of the virus because it is the first signal that the virus is present. The Department of Health reported on June 3rd that it had detected the virus in mosquitoes collected in Yakima County. This represents one of the earliest detections of the virus since monitoring started in 2001.

The WNV was first identified in 1937 in Uganda and is established in Africa, West Asia, Europe and the Middle East. It made its first appearance in the U.S. in 1999 when it struck in the New York City area, resulting in 11 human deaths. Various species of birds serve as the natural host of the virus, which is transmitted by mosquitoes. Humans and horses are considered “dead end” hosts of the virus – we can get infected, but the disease does not spread from us, not even to uninfected mosquitoes that bite us. Horses are particularly susceptible to the infection. Although many horses that are infected do not show any symptoms, those that do get very sick, with a 30% or more mortality rate. Of the cases that become unable to stand due to the severe neurologic impairment, 60% die.

One of the reasons WNV is on the list of reportable diseases is the impact it can have on the equine industry. At its peak in 2002, over 15,000 horses that we know of in the U.S. died from WNV. Another reason it is important is because the incidence of the disease in the horse population gives human health care agencies a better indication of the incidence of the virus in the community. This assists them in making decisions concerning disease control measures, such as mosquito control.

At least three vaccines are available for WNV in the horse. They are all safe and reasonably effective at preventing WNV.

Contagious Equine Metritis (CEM)

CEM is a venereally transmissible disease of horses. There is no evidence to suggest that CEM affects people. The reason for its importance is because it is very contagious and could have devastating affects on the equine breeding industry.

The first case of CEM was diagnosed in 1977 in England. The following year it showed up in central Kentucky and in 1979 it was found in Missouri. The disease was eradicated from both states and no new cases were detected until 2006, when two post-quarantine imported Wisconsin stallions tested positive. The disease was rapidly contained again. Then, in December 2008, a central Kentucky stallion tested positive. Further investigation revealed that this outbreak spread far and wide prior to detection. To date, records indicate that at least 971 horses in 48 states have been exposed to the disease (Rhode Island and Hawaii are the only two states without at least one exposed horse). As of June 2009, the USDA reported that of the 971 exposed horses, 21 stallions and 5 mares have been diagnosed positive for CEM.

CEM is caused by a bacteria that is transmitted directly by intercourse but also indirectly through artificial insemination and through contact with contaminated surfaces, e.g., instruments, semen collection equipment, etc. Although acutely infected mares will develop a vulvar discharge, stallions and chronically infected mares do not have outward symptoms and can be the source of continued transmission of the disease. Since stallions do not have any symptoms and they can potentially breed tremendous numbers of mares, rapid dissemination can occur.

Vesicular Stomatitis (VS)

Vesicular Stomatitis occurs sporadically in the United States, principally in the western states. The disease causes blister-like lesions on the muzzle, tongue, teats and above the hooves of susceptible livestock, including horses. In fact, horses are often the first species of livestock to be confirmed positive in an area where the virus is active. Although the mortality rate is low, the morbidity rate is high. The virus spreads fast and furiously, commonly affecting upwards of 90% of the herd. Cattle and pigs also can get VS. The problem with VS in these species is that the lesions look very similar to the much more serious Foot and Mouth Disease (FMD). FMD is one of the most dreaded diseases of livestock owners because it is highly contagious, spreads rapidly over long distances, and causes severe economic losses. For example, a FMD outbreak in 2001 in the United Kingdom resulted in a reported 3.9 million animals, most of them healthy, being destroyed in order to contain and eradicate the disease. Compensation to farmers of the slaughtered animals cost the government more than one billion dollars. The last case of FMD in the U.S. was in 1929.

VS can cause serious losses also, but not to the level of devastation FMD would cause. Whenever VS does become active, it causes huge headaches because FMD must be considered a possibility until proven otherwise. Testing must be completed quickly because of the speed at which FMD spreads. Horses are not susceptible to FMD, so they can be helpful in the investigation.

According to Texas State Veterinarian, there is a current VS outbreak in that state involving just one horse. Prior to that, the most recent outbreak was in 2006 and only involved the state of Wyoming, where 17 horses and a dozen cattle on 13 premises were affected. In 2005, there was a VS outbreak involving livestock on at least 445 premises in nine states. To date, Washington State has never had a case of VS. Hopefully, prompt notification and containment of outbreaks in other states will keep it that way.