West Nile Virus

An emerging zoonosis is one that has not previously been recognized or is recognized in only a few populations, is swiftly expanding its territorial reach, or is rapidly increasing in incidence. It may be difficult to distinguish whether a zoonosis is emergent or resurgent. A new zoonosis is characterized by signs and symptoms that are distinct from those of previously recognized zoonoses: Existing tolerance to the zoonosis decreases, occurrence goes from rare to common, the nature of the illness changes from mild to severe, or cases cluster in a location or social group. A new zoonosis may be identified when a population is examined using more effective surveillance capabilities and improved diagnostic techniques. New zoonoses are often identified because of better health records, rapid global communication systems (the Web, e-mail, and television, for example), and better diagnostic methods (such as monoclonal and polyclonal antibody techniques, enzyme-linked immunosorbent assays [ELISA], and polymerase chain reaction [PCR] testing).

This article focuses on 3 zoonoses at the forefront of emergence or spread that primary care physicians must be able to recognize: Lyme disease, infection with West Nile virus, and Avian flu.

Lyme disease

First identified approximately 30 years ago in Lyme, a small town in Connecticut, Lyme disease is now found throughout New England and the Mid-Atlantic states, as well as Wisconsin, Minnesota, and northern California. It is caused by the spirochete Borrelia burgdorferi transmitted by the deer tick (Ixodes scapularis in the Northeast and Midwest and Ixodes pacificus in the West). By now, most primary care physicians who practice in endemic areas have seen patients concerned about a tick bite or presenting with erythema migrans (EM)—a bull's-eye rash—the most common presentation of early localized disease.

Bites from nymphal deer ticks, which are usually prevalent in early summer, are largely responsible for transmission of the disease. The tick must be attached to the skin for 24 to 36 hours before the spirochete is transmitted; the probability that a nymphal tick bite will lead to Lyme disease is low in patients who have removed the deer tick soon after it has attached. In cases in which a tick has fed longer and is engorged, a single, 200-mg dose of doxycycline is appropriate prophylaxis for children older than 8 years and for adults.1

Lyme disease should be considered in a patient who lives in or who has recently visited the Lyme belt and who presents with EM. The rash is often accompanied by arthralgia, fatigue, fever, headache, malaise, or myalgia. Some patients may present with these symptoms during summer, without EM. Because serologic testing is often negative in early disease, a 2- to 3-week course of amoxicillin or doxycycline is recommended, without serologic testing being performed.2

Early disseminated disease may include multiple EM lesions, cranial nerve palsy, lymphocytic meningitis, migratory joint