By David N. Leff

A pride of lions, a school of fish, a sloth of bears, a pod of seals, a kindle of kittens, an exaltation of larks, a murder of crows.

These ¿terms of venery¿ are typical of the monikers that old-time hunters attached to bunches of their prey. The ¿murder of crows¿ was particularly topical last summer, when large numbers of these glossy black, noisy birds dropped dead in Connecticut. What murdered them?

The short answer ¿ mosquitoes. These biting insects were the smoking gun; the bullets were a species of arbovirus identified as West Nile flavivirus. Besides the crows, and a hawk, its victims included several exotic species of birds in New York City¿s Bronx Zoo, and seven human New Yorkers, dead of viral encephalitis. Also, a few horses on Long Island.

¿This unprecedented outbreak of an arbovirus infection,¿ said entomologist John Anderson, ¿developed in New York City in the late summer and early fall of 1999. The particular virus is one that is transmitted among birds by mosquitoes. It is West Nile virus, which occurs naturally in Africa, and parts of Asia and Europe. It previously had not been found in the New World,¿ Anderson observed. ¿Besides killing seven people in New York City and affecting dozens more, it killed relatively large numbers of birds in the city and state of New York, and extended up into Connecticut.¿ Anderson is director of the Connecticut Agricultural Experiment Station in New Haven.

¿Once we learned of the outbreak in New York City last summer, we had our mosquito traps up the following day,¿ he recounted. ¿We¿ve isolated this virus from the brain tissues of 28 of 31 crows that died, from two species of mosquitoes, and from brain tissues of a magnificent Cooper¿s hawk that was observed on a lawn while it was still alive, but close to death.

A Virus Strictly For The Birds

¿This is a virus that was initially isolated in Africa in 1937,¿ Anderson went on, ¿from a native woman in Uganda. The name of the virus, West Nile, comes from that area where the first isolation was made. In terms of its effect on birds, it was first reported in the early 1950s in Egypt that this virus could cause disease and death in Egyptian birds. The West Nile virus was isolated from brain tissues and serum from naturally infected rock pigeons, and lab experiments found that it had a severe effect on carrion crows as well as house sparrows.

¿To my knowledge,¿ Anderson said, ¿large die-offs of birds have not been reported in Africa, Europe or Asia. And I know of no report in the Old World similar to what we experienced here in the U.S.¿ He made the added point, ¿What we have here are birds that have never in their evolutionary history been exposed to the West Nile virus, which clearly does tend to end up in brain tissues and have a severe effect.¿

Anderson is lead author of a report in the forthcoming issue of Science, dated Dec.17, 1999, titled: ¿Isolation of West Nile virus from mosquitoes, crows and a Cooper¿s hawk in Connecticut.¿ Back to back with this paper is a companion article by scientists at the Centers for Disease Control (CDC) bearing the title: ¿Origin of the West Nile virus responsible for an outbreak of encephalitis in the Northeastern United States.¿

Science advanced its press embargo on these two papers by two days, BioWorld Today has learned, to accommodate a public hearing called by Connecticut¿s junior U.S. senator, Joseph Lieberman, on Tuesday morning, Dec. 14.

Anderson¿s co-author, Theodore Andreadis, who testified at the hearing, briefed the senator on measures to avoid being taken by surprise next summer, if and when the West Nile virus returns. ¿We certainly here in Connecticut are doing all we can to see if we can locate this virus here during the wintertime, and will continue doing that during the spring and summer,¿ Anderson told BioWorld Today. ¿We¿re doing that by collecting adult mosquitoes and birds on a weekly basis, and testing their blood. From the standpoint of mosquito control,¿ he concluded, ¿we intend to have a good surveillance system in place in Connecticut for the year 2000.¿

One of the exotic birds laid low at the Bronx Zoo was a Chilean flamingo (Phoenicopterus chilensis). Analysis of its brain tissue, plus partial DNA sequencing of the virus, enabled the CDC scientists to distinguish it from the half dozen or more West Nile viruses known in other countries of the Eastern hemisphere, including Egypt, Romania, Australia, and Israel. The analysis revealed that the New York virus came closest to genomic identity with a goose that died last year in Israel.

Surveillance For Y2K All Year Long

Virologist and microbiologist Robert Lanciotti is lead author of the CDC paper in Science. ¿In terms of where do we go from here,¿ he told BioWorld Today, ¿probably the most significant event was a meeting sponsored by the CDC¿s Division of Vector-Borne Infectious Diseases, here in Ft. Collins, Colo., late in October. We had about 150 folks invited, to discuss all aspects of the epidemic, and how do we plan for next year. We had virtually everybody in any way shape or form involved in disciplines ranging from mosquito control to medical doctors who saw some of the actual patients, epidemiologists, microbiologists, entomologists ¿ among others.

¿There¿s going to be a lot of surveillance in the spring and summer of 2000,¿ Lanciotti observed. ¿We¿ll be looking aggressively for this virus both in mosquitoes and birds. One infected bird was found in Baltimore. So Maryland would be a part of the active surveillance ¿ setting mosquito and bird traps ¿ as well as New Jersey, Delaware, New York, Connecticut and that general area. Add to this passive surveillance, where all wildlife people will be on alert. In an ideal situation, ¿Lanciotti concluded, ¿we¿d find the West Nile virus before it found us humans.¿

He suggested, ¿One contribution from the biotech world would a good diagnostic test. The testing at this point is limited to CDC and the New York State Health Dept. Obviously, if every reference and government lab had access to good commercial test kits for West Nile, that would be a help.¿