Birds, migration and emerging zoonoses: west nile virus, lyme disease, influenza A and enteropathogens

Abstract

Wild birds are important to public health because they carry emerging zoonotic pathogens, either as a reservoir host or by dispersing infected arthropod vectors. In addition, bird migration provides a mechanism for the establishment of new endemic foci of disease at great distances from where an infection was acquired. Birds are central to the epidemiology of West Nile virus (WNV) because they are the main amplifying host of the virus in nature. The initial spread of WNV in the U.S. along the eastern seaboard coincided with a major bird migration corridor. The subsequent rapid movement of the virus inland could have been facilitated by the elliptical migration routes used by many songbirds. A number of bird species can be infected with Borrelia burgdorferi, the etiologic agent of Lyme disease, but most are not competent to transmit the infection to Ixodes ticks. The major role birds play in the geographic expansion of Lyme disease is as dispersers of B. burgdorferi-infected ticks. Aquatic waterfowl are asymptomatic carriers of essentially all hemagglutinin and neuraminidase combinations of influenza A virus. Avian influenza strains do not usually replicate well in humans, but they can undergo genetic reassortment with human strains that co-infect pigs. This can result in new strains with a marked increase in virulence for humans. Wild birds can acquire enteropathogens, such as Salmonella and Campylobacter spp., by feeding on raw sewage and garbage, and can spread these agents to humans directly or by contaminating commercial poultry operations. Conversely, wild birds can acquire drug-resistant enteropathogens from farms and spread these strains along migration routes. Birds contribute to the global spread of emerging infectious diseases in a manner analogous to humans traveling on aircraft. A better understanding of avian migration patterns and infectious diseases of birds would be useful in helping to predict future outbreaks of infections due to emerging zoonotic pathogens.

Figure 1

Distribution and migration routes for the Arctic Tern (Sterna paradisaea). An example of inter-hemispheric travel of wild birds. The trip from the breeding ground in the high Arctic to wintering areas at the southern tip of South America and Antarctica covers up to 17,000 km and takes several months to complete. Other species of ducks and gulls regularly migrate along transatlantic route breeding sites in the Old World to wintering areas along the eastern coast of North America. Map courtesy of the U.S. Department of the Interior and the U.S. Fish and Wildlife Service, Migration of Birds, Circular 16 (Washington, D.C., 1998).

Figure 2

Elliptical migration route for the Connecticut Warbler (Oporornis agilis). This relatively common pattern of migration for songbirds concentrates them along the Atlantic seaboard during the fall migration, but more inland during the spring. This is a possible mechanism for the western extension of the range for West Nile virus in the United States. Map courtesy of the U.S. Department of the Interior and the U.S. Fish and Wildlife Service, Migration of Birds, Circular 16 (Washington, D.C., 1998).

Figure 3A

Engorged Black-legged tick (Ixodes scapularis), the vector of Lyme disease, attached to the skin just below the eye of a Dark-eyed Junco (Junco hyemalis). Ticks attach most frequently to ground-for-aging species of birds and tend to concentrate around the ears and eyes, two areas that are difficult for the bird to preen. Photograph courtesy of Dr. Thomas Nicholls

Figure 3B

Multiple engorged ticks removed from a single bird. Photograph courtesy of Dr. Thomas Nicholls