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July 2000Outbreak (Updated to November 4)
West Nile Virus in Birds, Eastern USA; Israel
August 30 report from Israel: 2000 geese have been slaughtered following the death of an 84-year-old man.
Avian morbidity and mortality surveillance in the eastern United States has identified over 3600 WNV-infected dead birds this year (the majority since August 1) in Connecticut, District of Columbia, Maryland, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, Rhode Island, Vermont, and Virginia. The sparrow is now suspected to be the amplifier of the virus.
Infected species reported include American crow, blue jay, red-tailed hawk, fish crow, house sparrow, American robin, merlin, song sparrow, Canada goose, great blue heron, northern mockingbird, eastern bluebird, cockatiel, mute swan, and yellow-rumped warbler.
WNV infection detected in mosquitoes now includes the following species:
- Culex pipiens
- Culex restuans
- Culex salinarius (new, as of August 12)
- Aedes japonicus
- Aedes albopictus (Asian tiger mosquito; new, as of November 4)
From early July, a number of wild birds found dead in New York and New Jersey have tested positive for West Nile Virus in a new outbreak of the disease that appeared last summer for the first time in North America. Reactions from public health authorities focus on the potential for human cases, given the 7 deaths and 61cases of related encephalitis in 1999 that confirmed the presence of another emerging infectious disease in the US. Thus far, the virus-positive tests have prompted response that ranges from increased pesticide spraying in locales where the dead birds have been found to netting operations in far-distant states for determining possible spread of the virus via migrating birds.
The impact of the virus on bird populations is predictably a lesser concern than the prospect of more human cases. Even so, there are serious implications for wild, as well as exotic pet birds. Thus far, the birds found this year and thought to have been killed by the virus include crows, blue jays, sparrows and one red-tailed hawk. The last is believed to have been infected in last year's outbreak
Concerns for US bird populations result from the following facts:
- The virus resides harmlessly in some bird species but kills others.
- Short of an all-out sampling of wild birds, it is impossible to determine the extent of benign populations that carry the virus.
- Some experts believe that the virus exists in bird populations throughout the US, due to migratory processes, thus increasing the potential for mosquito transmission to susceptible species.
- The number of dead birds found, reported and tested will always be only a fraction of the total mortality figure. Even in rural areas, dead birds are not ordinarily observed.
- Crows appear more susceptible to the disease, while others may be able to resist the viral infection for a longer period of time, i.e., the red-tailed hawk. However, this "appearance" may be the result of the US experience with the virus, i.e., the unusual discovery of dead crows in urban locales.
- Fear of the disease may lead people to kill crows indiscriminately. And if the disease continues to reappear annually in conjunction with the mosquito season, the fear could cause general fear of all birds.
West Nile expert Zdanek Hubalek of the Czech Academy of Sciences has suggested that the virus may have originally escaped from the Sloan-Kettering Institute in New York, where in the 1950s a strain of the virus was given to 95 terminal cancer patients as an experimental treatment. While other authorities say the strain of the virus now found in birds and affecting humans in the US is different from the one used in that experiment, the current strain could have evolved within local bird populations in the past half-century.
Other scientists have posed the possibility that the virus entered the US last year with a bird imported from Israel. Still others say there is a possibility that the virus has been present in the US for many years and that changing climatic conditions and increased populations of two mosquito species (Aedes japonicus and Culex pipien), the first of which has appeared in the US only in recent years.
Genetic "archaeology" can provide the answer, just as it has in determining the evolution of human immunodeficiency virus in Africa.
Additional information is available at Beyond the Headlines.
Sources: WildlifeHealth Service, media reports and AHEAD/ILIAD project archivesUnresolved
Royal Bengal Tiger Mortalities, Orissa, India
The deaths of 12 Royal Bengal tigers, seven of which were the captive-bred white variety, began with a first mortality on June 23 at the Nandankanan Zoo in the state of Orissa, India. Thereafter, 10 more of the rare animals died within one 24-hour period. On July 16, officials at the Calcutta Pathological Laboratory run by India's federation government announced that the animals had died from "consumption of decomposed and contaminated cow meat". This announcement, however, was not sufficiently precise to completely contradict an earlier finding, widely circulated, that post mortem examinations on the animals showed they had died of trypanosomiasis, a protozoan disease affecting the central nervous system – a disease that could have been generated by consumption of meat contaminated by insect feces containing typanosomes.
Moreover, the July 16 announcement did not clarify a major problem in the diagnosis: the fact that carcasses of the dead animals had been cremated by zoo officials before federal investigators arrived on the scene, making it impossible to conduct tests on either the tigers or the meat they ate.
To add to the confusion, zoo officials (prior to the finding of the federal officials) had admitted blame for the mortalities, saying that they had in error given the animals a general antibiotic instead of a trypanocide. The only drug identified as having been administered is Berenil (Diminazene aceturate), which IS a trypanocide, albeit its curative power is more reliable than its prophylactic effect, which means that it could not have been efficacious in preventing the disease from spreading to other members of the unique Royal Bengal tiger community, now reduced to 44 animals.
Conditions in Indian zoos are troublesome. The Nandankanan Zoo, alone, has lost over 900 animals in the past five years.
Sources: ProMED/AHEAD-mail, WildlifeHealth Service, Animal Net (Canada), media reports and "Infectious Diseases of Livestock", Oxford University Press.