West Nile Virus

Animals and West Nile

Courtesy of Hawaii Department of Health.

In the United States, West Nile virus strikes crows so dramatically that their deaths are used by health officials as an early detection method for the virus's advance.

But West Nile's impact on animals has moved well beyond the 1999 and 2000 news reports that focused on dying crows.

Although crows and the closely related blue jays are most susceptible to illness and death from West Nile infection, the virus affects many other birds to varying degrees. Illness and death has been reported in more than 170 species of birds in the United States. Among them are robins, cardinals, grackles, mockingbirds, and starlings.

Courtesy of June Chamberlain Auger,
Boston College.

There have been reports of declines in many populations of song birds around the country since the outbreak began. To assess movement of birds and the virus, the U.S. Geological Survey and the Centers for Disease Control and Prevention are carrying out a surveillance program in which migratory birds are captured and sampled for the virus.

It's Not Just Crows
Because crows become sick and die quickly, they are not good sentinel hosts for West Nile.

Many common birds, such as sparrows, do not become ill from the virus. These birds may harbor high levels of West Nile virus in their blood, and can and do serve as reservoirs, or the source of infection. Hungry mosquitoes feed on infected birds, then can pass the virus on to other birds, animals, and humans.

Map of southern migration routes for North American birds created by Chicago Department of Environment.

Not all species that become sick from West Nile build up enough virus in their blood to pass it on to mosquitoes. For example, humans and adult chickens are considered "dead-end" hosts for the virus, meaning that the transmission cycle ends with them. Although scientists think that infected people do not carry enough virus to pass it on to feeding mosquitoes, they can and have passed West Nile on to other humans via blood transfusions and organ transplants.

While birds appear to be the primary hosts for West Nile, the virus also has infected at least 27 other species of vertebrates, including horses, cats, dogs, bats, chipmunks, squirrels, rabbits, seals, and even alligators. Horses are affected by West Nile virus much more than any other domesticated animal.

In 2002, 40 states reported a total of 14,717 cases of equine illness caused by West Nile virus. Approximately 30% of horses infected with West Nile virus die from the illness.

Clinical signs of West Nile virus infection in horses include stumbling, lack of coordination and weakness of limbs. Not all infected horses exhibit symptoms.

West Nile Vaccine Available for Horses

The U.S. Department of Agriculture advises horse owners to vaccinate their animals against West Nile. A conditional licensed equine vaccine for West Nile virus is available through veterinarians.

Other precautions include eliminating mosquito breeding sites in and around stables and barns, and installing screens. Fans also may reduce the ability of mosquitoes to feed on horses.

West Nile infection can occur in cats and dogs, but illness appears to be extremely rare. Since West Nile arrived in the United States, the American Veterinary Association has reported only one case of an elderly dog that died from West Nile encephalitis, and three cats that became severely ill and died.

Pets with neurological disease should be examined for other causes of illness, including rabies. Notify a veterinarian when pets show signs of infection, such as fever, depression, lack of coordination, muscle weakness or spasms, or seizures or paralysis. A veterinarian also can provide guidance about products that protect animals from more common mosquito-borne diseases, such as heartworm in dogs.

Animal owners should not use human repellents that contain DEET on their pets or livestock. Because some animals are more sensitive than humans to DEET, check with your veterinarian about what products you can use to protect your pet.

Protecting Animals from Infection

Some zoos and animal parks have been hard hit by the virus. In fact, the first cases of West Nile Virus infection in the United States were identified in flamingoes and pheasants at the Bronx Zoo in 1999. Zoos from across the nation have lost flamingoes, penquins, and other birds, and seals.

Photos of healthy alligator and flamingoes courtesy of Jessie Cohen, Smithsonian's National Zoo.

Some 300 alligators at a Florida alligator farm died from West Nile disease in fall 2002.

A national zoo surveillance program for West Nile virus was established in late 2001. Zoo officials now share data on West Nile with one another and the U.S. Centers for Disease Control and Prevention for incorporation into a national public health database.

Many zoos and animal parks used the equine West Nile vaccine last year in an effort to protect their birds and other animals. Late in 2002, an experimental recombinant DNA vaccine was used in an effort to protect endangered condors in California zoos.

A study of infected crows found that an experimental DNA vaccine reduced mortality rates by 60%. Manufacture of an oral vaccine for use on wild birds is being investigated by government researchers in collaboration with scientists at several universities.

Oct. 8, 2003