West Nile virus infection: Essential facts for small-animal practitioners
The media buzz may have quieted considerably, but West Nile virus continues to cause illness and death nationwide and is here to stay. Within six years of the initial detection of this exotic mosquito-borne virus in New York, it has spread to all continental states, through Canada and Mexico, and into Central America. What's more, West Nile virus continues to surprise infectious disease experts and epidemiologists. It was predicted that West Nile virus cases would wane dramatically after initial outbreaks in naïve regions, but flare-ups have occurred in some parts of the country.
Morbidity and mortality from West Nile virus infection are certainly most prevalent in birds, followed by horses and people, but the virus has an unusual ability to infect myriad species, including dogs and cats. Fortunately, West Nile virus infection appears to rarely cause disease in dogs and has been documented even less frequently in cats. The Centers for Disease Control and Prevention (CDC) has fielded a staggering number of inquiries about the virus in pets. Do you know the correct answers to your clients' questions about West Nile virus?
THE STORY OF WEST NILE VIRUSThis flavivirus was historically known to cause sporadic outbreaks of febrile disease in Africa and the Middle East. It was first identified in the West Nile district of Uganda in 1937.1 Few cases of neurologic involvement or of West Nile virus-induced disease in species other than humans were reported. However, in the mid- to late 1990s, a more aggressive, neurovirulent strain of West Nile virus appeared in the Mediterranean, causing morbidity and mortality not only in people but also in horses and birds.2-5 Genetic analyses suggest that the strain of West Nile virus that was brought to New York and first detected in the fall of 1999 was a descendant of an avian West Nile virus isolate from Israel.6,7 Despite aggressive mosquito control efforts and hopes that the cold northeastern winter would eradicate West Nile virus, never before seen on the American continents, it has become established in the environment and has spread rapidly. It is thought to spread through multiple means, such as insects blowing in the wind and migratory birds, although no one knows exactly how it spreads.
ECOLOGY AND EPIDEMIOLOGY
The ability of West Nile virus to infect a wide variety of species is probably the biggest factor not only in its successful establishment but also in the patchy, unpredictable character of West Nile virus activity, which is explosive in some regions and nearly silent in others. West Nile virus is maintained in nature in a bird-mosquito cycle, but thousands of species of birds and mosquitoes exist, and their distribution varies with local ecology, season, and climate fluctuations such as drought. Birds vary dramatically in their ability to replicate West Nile virus (and, thus, to pass it on to feeding mosquitoes),8 and some mosquitoes are much more efficient at transmitting West Nile virus.9-11 Furthermore, mosquito species vary in their feeding habits, which range from specific to comparatively unselective. Mosquitoes that feed on both birds and mammals are referred to as bridging vectors, since they provide a link for mammalian exposure to the bird-mosquito cycle of West Nile virus. Variations in feeding preferences of local bridging vector mosquitoes may explain why some areas of the nation have seen a high intensity of both equine and human cases, while in other regions, there have been sizeable outbreaks in one species yet relatively few cases in the other.
WEST NILE VIRUS IN CATS AND DOGS
Many viruses encountered in small-animal veterinary practice can infect only a few or even a single mammalian species. As demonstrated by laboratory and field data, West Nile virus has the unusual ability to establish infection in a variety of animals, ranging from alligators to wolves—and including dogs and cats.
Infection and contagion
Studies performed at Colorado State University revealed that both dogs and cats readily become infected after being bitten by West Nile virus-infected mosquitoes.12,13 Fortunately, it appears that the risk of the virus spreading to owners, other animals, or veterinarians working with infected dogs and cats is minimal; West Nile virus replicates to fairly low levels in the tissues of these animals, and no virus was detected in saliva.12 Of course, proper precautions should be taken with patients that potentially have zoonotic diseases. In cases of West Nile virus, particular care should be used in handling blood from affected animals.