First, the bad news: Since the first U.S. cases of West Nile virus (WNV) were reported in the Northeast in 1999, this disease has spread to almost every part of the country. About a third of horses that become ill with WNV die or need to be put down, and a significant portion of those that survive are left with neurological damage.

Now the good news: You and your veterinarian have the tools to protect your horses from this disease. In fact, in many parts of the country, WNV is already on the run. The number of cases peaked nationally in 2002, when the U.S. Department of Agriculture (USDA) reported more than 14,000 equine cases in 40 states. In 2004, the virus was reported in every state except Washington, Alaska, and Hawaii, but the total number of cases fell. Parts of California and other Western states where the disease appeared for the first time saw the highest numbers of horse cases, but in other parts of the country, numbers were sharply down.

The dramatic drop is largely due to widespread vaccination against WNV, says Sandra Bushmich, DVM, a veterinary pathobiologist at the University of Connecticut, but other factors may be involved as well. Here’s Bushmich’s take on the status of this disease and efforts to fight it.

West Nile Risk

Mosquitoes transmit WNV. West Nile is primarily a disease of birds; mosquitoes that feed on infected birds pick up the virus and can pass it on when they bite other animals. Horses are affected much more often than any other animals. Although mosquitoes can also transmit the disease to people, infection and death rates are much lower for humans than for horses. The virus isn’t transmitted between horses or from horses to people.

WNV attacks the horse’s central nervous system, producing neurological signs that vary widely. “Some horses are a bit uncoordinated for a few days; some go into a coma and die,” says Bushmich. “Typically, the horse looks drunk, with a swaying, uncoordinated gait and a hanging head and other signs of depression.” Fever, muscle weakness, stumbling, partial paralysis, inability to stand, and convulsions are other signs. Similar signs are seen in other diseases that attack the central nervous system, including equine encephalitis and rabies, Bushmich notes.

There’s no cure for the disease, and even horses that survive may not recover fully. A University of Minnesota study of horses with acute WNV found that, six months after diagnosis, 40 percent of survivors had residual problems that their owners attributed to the disease. The most common problems were changes in gait and behavior that affected the horse’s use.

Still, not all horses that are exposed to WNV get sick. Horses that are exposed but don’t show symptoms may actually fight off the virus and develop some natural resistance to it, Bushmich says. That may be one reason why the number of WNV cases is declining in places where the disease has been around for a while. Rather than rolling the dice and betting that your horses won’t get this devastating disease, though, take these steps to protect them.

Step One: Vaccinate

A crash program developed a WNV vaccine and brought it to market in 2001. Currently two vaccines are available: the original West Nile Innovator (Fort Dodge Animal Health), and Recombitek (Merial), released in 2004. (Bushmich is currently testing a third equine vaccine, developed in conjunction with scientists at Yale University, that may lead to the development of a vaccine for humans.) The vaccines work in different ways. Innovator is a killed virus vaccine, while Recombitek is a recombinant vaccine—it contains bits of WNV proteins that have been inserted into harmless live viruses.

Both types prompt the horse’s body to mount an immune response that will protect against the real, live WNV should it show up. Studies have shown both to be safe. Both require an initial series of two shots, followed by periodic boosters (see the guidelines in the box on page 18). If you start with one type, it’s OK to switch and boost with the other, Bushmich says.

The vaccines work. Last year the University of California Center for Equine Health reported more than 550 confirmed cases of WNV, of which only 8 occurred in horses known to have been properly vaccinated. Bushmich notes that shots should be timed to provide strong immunity when mosquito populations peak. In much of the country, that’s during August and September. A booster given in April will provide good protection then, but the effects of one given in February may have already begun to fade. If you live in an area where mosquitoes are active year-round, or if you ship horses south in winter, you may need to boost more than once a year.

Bushmich discounts rumors, circulating on the Internet, that the vaccines are harmful for pregnant mares. “There is no data to suggest a problem with either vaccine,” she says. If you choose not to vaccinate pregnant mares, she adds, it’s extra important to vaccinate foals at the recommended intervals.

WNV vaccination has already become part of the regular program at many barns, especially in the East. It should stay that way, Bushmich says, even if the number of cases reported continues to fall, because WNV will likely remain a threat as long as there are mosquitoes to transmit it.

Step Two: Control Mosquitoes

This year, a wet winter and spring promised a bumper crop of mosquitoes in California and several other parts of the country, increasing concern about WNV. Mosquito control is a key part of the fight against the disease, Bushmich says.

Start by reducing mosquito breeding grounds. The insects lay their eggs in standing water, so empty and refill water troughs at least weekly. Get rid of old tires and anything else that catches rainwater, and turn over wheelbarrows and other containers when not in use. Keep wash-stall drains, storm drains, and roof gutters clean, so that water doesn’t back up in them, and cover cisterns and rain barrels. Drain or fill in puddles and ditches. In areas that can’t be drained, use mosquito dunks—blocks that dissolve in water, releasing microbes that kill mosquito larvae. Stock ponds with fish that eat the larvae (such as the mosquito fish, Gambusia affinis).

Once the larvae mature into flying insects, they’re harder to control. Ultraviolet and electric “bug zappers” don’t attract these insects. Mosquito traps that use carbon dioxide and other attractants work better, but they don’t catch enough mosquitoes to be the only means of control.

Protect your horses with fly repellents labeled for use against mosquitoes. Keep them indoors at dusk and dawn, prime times for mosquito activity. Mosquitoes are not strong fliers, so keeping fans running in the barn may discourage them from coming in. You can also put screens on the barn, or use a premises spray to reduce the number of mosquitoes inside. Choose a product that’s labeled for use around horses, and follow the directions carefully.

Mosquitoes will always be with us, but simple steps like these can help protect your horses from WNV—and other diseases, too.