Pathogens Transmitted by Migratory Birds: Threat Perceptions to Poultry Health and Production

The role of migrating wild birds in transmitting diseases of poultry or zoonoses is a contentious issue as the researchers and naturalists stands divided regarding their capability to disperse pathogens over continents. Recently, migratory birds got world wide attention during the bird flu outbreaks, as they were found capable to disseminate the deadly H5N1 avian influenza (bird flu) virus, without themselves getting affected. However, the death of migratory birds due to H5N1, reported from Asia, has fuelled anxiety and concern over the whole issue. Apart from avian influenza, migratory birds are also thought to play role in the transmission of avian viruses like Newcastle disease virus, avian pneumovirus and duck plague virus. Similarly, bacterial pathogens like Chlamydophila psittaci and Pasteurella multocida can be transmitted to domestic poultry via migratory birds. They are also known to spread West Nile virus, equine encephalitis virus, Borrelia burgdorferi and enteropathogens like Campylobacter and Salmonella, which could affect animals as will as human beings. To prevent such etiological agents from entering poultry premises, strict biosecurity and constant surveillance are of paramount importance. Hence, in the scenario of migratory birds contributing significantly to the global spread of infectious diseases, a better understanding of their role in the disease epidemiology has to be gained by implementing superior surveillance and tracking strategies. Key word: Avian influenza, biosecurity, migratory birds, poultry diseases, surveillance, wild birds Introduction Migratory birds can disperse microorganisms across international borders and myriad pathogens harmful to poultry or other vertebrates have been associated with such birds (Hubalek, 1994, 2004). They circulate avian influenza virus (AIV), avian pneumovirus, Newcastle disease virus, duck plague virus, Chlamydophila psittaci, Campylobacter and Salmonella (Hubalek, 1994; Palmgren et al., 1997; Wobeser, 1997; Alexander, 2000; Hubalek, 2004). The pathogen disseminating potential of migrating birds has become a major concern during the spread of West Nile virus (WNV) in North America during the late 1990’s (Rappole and Hubalek, 2000; Reed et al., 2003). During the last few years, migrating birds have introduced H5N1 influenza virus to many countries, choosing specific flyways in different continents (Brand, 1984; Rappole et al., 2000; Stroud et al., 2004). Migrating birds generally include species that cyclically cross one or more national boundaries and use a variety of habitats including wetlands, marshes and other water bodies (UNEP, 2005). Such migrants make series of shorter flights, traveling more at night and during stop-over, disseminate the harbored pathogens (Reed et al., 2003). Migration, a remarkable biological phenomenon having epizootic implications depends on factors like susceptibility of birds, pathogen viability, vectors and factors such as temperature and humidity (Keymer, 1958; Page, 1976). The stress associated with migration can increase the bird’s susceptibility to pathogens or enhance their shedding rate. Migration is highly demanding and if the birds become infected, they may spread the pathogens, for shorter distances rather than distant destinations (Weber and Stilianakis, 2007; Feare, 2007). For the efficient dispersal of pathogens, they serve as biological carriers of microbes or mechanical dispersers of vectors that harbor pathogens (Hubalek, 1994; Singh et al., 2003; Hubalek, 2004). As biological carriers, the infection in migratory birds can be acute (Newcastle disease, Duck plague, Pasteurellosis) chronic (Pox) or asymptomatic (influenza, salmonellosis). Feces and nasal and respiratory exudates of infected migrants transmit AIV, paramyxovirus, herpes virus, C. psittaci, Campylobacter, Salmonella, Mycobacterium avium, P. multocida and Clostridium spp. Likewise, they transport ticks along with pathogens, from one site to another (Hubalek, 1994, 2004). Even though such birds carry influenza virus and other pathogens, as per the suggestions of Food and Agricultural Organization (FAO), eliminating these birds is not a sagacious Dhama et al.: Pathogens Transmitted by Migratory Birds 517 measure. Instead, biosecurity and surveillance system geese, gulls and cormorants were the migratory birds has to be enhanced to ensure that the poultry dwelling that were mostly affected (Sturm-Ramirez et al., 2004; zones are not contaminated by migrating birds. Youling, 2005; Sanjaatogtokh, 2005). The H5N1 virus in Pathogens transmitted by migratory birds: head tilt and staggering gait and pathology in multiple Avian influenza virus (AIV): Avian influenza virus (AIV) of organs, including the brain resulting in severe genus Influenzavirus (type A), family Orthomyxoviridae, neurological dysfunction and was transmitted between is the causative agent of avian influenza (bird flu), that migratory birds (Sturm-Ramirez et al., 2004; Chen et al., causes high flock mortality in poultry. The H5N1 subtype 2006). These isolates from migratory birds were found of highly pathogenic avian influenza (HPAI) virus has to be distinct from Southeast Asian H5N1 virus, but caused severe outbreaks in poultry worldwide, capable of inflicting highly pathogenic infection in accounting for huge economic losses (Alexander, 2000; chickens (Lei et al., 2007). Besides occurrence of HPAI Swayne and Halvorson, 2003; Dhama et al., 2005; in such birds, it is a matter of concern that the LPAI Burgos and Burgos, 2007a). It is a highly contagious, viruses in migratory birds can mutate within them or get zoonotic and multi-systemic disease placed under ‘List into the poultry population which later act as precursor A’ disease of OIE, having potential of rapid spread, for the generation of deadly HPAI viruses (Swayne and irrespective of national borders. The H5N1 virus, earlier Halvorson, 2003; Dhama et al., 2005; Mathew et al., limited to poultry, is now capable of infecting migratory 2006; Mehrabanpour et al., 2007). Geospatial analyses birds, animals and human beings (Chen et al., 2006; clearly prove that the distribution of HPAI outbreaks in Kataria et al., 2006; Burgos and Burgos, 2007b). During domestic poultry has been strongly associated with free the past few years, the H5N1 virus has led to losses of grazing water fowls present in the region (Gilbert et al., more than 250 million birds besides claiming 21