West Nile, Dead Birds and Pesticides
NJAS Opinion: August, 2001

The appearance of West Nile Virus in the NY-NJ area in 1999 raises several issues: human health threat, impact on birds and the role of pesticides.

How West Nile got here is unknown, and the pathways to the US at this point are purely speculative. Possibilities include the small number of transatlantic bird migrants (certain ducks like Eurasian Wigeon and a few pelagic birds); storm driven birds from the Old World where West Nile originated; and legal or illegal imports of exotic bird species infected with West Nile.

Where the disease might go is also at this time purely speculative. There isn't much data. In the Old World, as one biologist put it, "the birds show up, the disease shows up." The disease may migrate; after all the NY-NJ is on an important flyway to the southeast. However, long-distance migrants may not be important in the spread of West Nile. The disease hasn't gone too far yet.

Viremic birds are reservoirs for the disease and the source for mosquitoes. Humans are not good reservoirs and most birds so far appear not to be good reservoirs. Only a few birds are known to be competent reservoirs. Lab work is the only thing that will tell the tale. But it will be a long time before lab work will show what species are effective reservoirs.

House Sparrow appears at this time to be the best reservoir. House Sparrows remain viremic for 5 days, long enough to infect a mosquito. The question is: how do they move the disease? Although House Sparrows are mainly sedentary, a percentage of the population does move south. A good project for an ornithologist would be to measure House Sparrow dispersal. That House Sparrow is the best reservoir so far is no surprise. It was a competent reservoir in Africa.

Infected dead crows have been frequently reported in the NY-NJ area. Crows and jays die quickly and are not competent reservoirs. Again no surprise here, since Hooded Crows were susceptible to the disease in the Old World. The mosquito may be a reservoir in New York City; mosquitoes may pass the disease to the next generation. Horses, which like crows are susceptible to the disease, are not competent reservoirs.

The extent of the threat to bird populations from West Nile is not yet clear. But some caution is in order, given the susceptibility of corvids to the disease. Among the dead birds sampled in NY and CONN, the infected dead crows and jays were in the hundreds while most other species were in the single digits. Both crows and jays are abundant in our area, so American Crow, Fish Crow and Blue Jay may not be at grave risk. But if West Nile(WNV) spreads, there could be a threat to endangered small populations of corvids such as Hawaiian Crow or Scrub Jay, or possibly to other species having small populations.

In CONN, of 7,000 dead birds tested, 279 were WNV positive. A large percentage of the sample were dead from pesticides. Likewise in NY, the largest percentage of deaths in the dead bird sample were caused by pesticides(28% of 86,000). We think it is vital that the states maintain a dead bird data base not only to monitor WNV, but also to track death from pesticides. A variety of pesticides are in use in New Jersey and elsewhere, especially in lawn care.

NJAS recommends a cautious policy for New Jersey in dealing with WNV:

* Avoid aerial spraying of adulticides. The cure is worse than the disease. Aerial spraying misses too many mosquitoes, is a threat to pregnant women and children, and is not cost effective.

* If spraying of larvicide becomes necessary because of imminent risk to human health, confine the choice of spray to Bti, which is relatively mild when used in targeted spraying of infected pools or storm drains.

* Targeted use of spray should be confined to cases of imminent risk. We would suggest that imminent risk be defined as either the presence of a human case of WNV; or the presence of all three of the following conditions: infected mosquito pools, dead crow density, and effective (competent) reservoirs, e.g., House Sparrow. Absent a human case of WNV, all three of the other conditions need to be present together; a dead crow without the other two does not by itself constitute imminent risk. Currently no data prove that adulticide is effective in preventing WNV in humans.

* Homeowners should recycle tires and regularly drain pool covers. Both are principal sources of mosquito breeding. Environmental commissions may want to communicate those two suggestions in their newsletters.

* Counties should make it easier for people to recycle tires, either through amnesty (no charge) or through increasing the number of recycle centers at the municipal level.

* The state of New Jersey should maintain a dead bird data base to track both WNV and mortality from pesticides. Collection of some birds may be necessary to determine which species are competent reservoirs. Tracking the spread of WNV may be crucial in assessing threats to endangered species whose populations are too small to survive transmission of the virus.

* Ornithological research should proceed on corvids and House Sparrows. In time other species may be chosen as targets for research as well. Thus far some 75+ species have been found dead from West Nile.

Let's err on the side of caution. There have been a few human fatalities from WNV, which is a threat to the elderly. Long sleeves and other normal precautions when going afield are in order. Likewise avian threats from WNV should be monitored on a continuing basis, as should the unintended effects of pesticides. Remember the canary in the coal mine??

Richard Kane, Vice President
Conservation and Stewardship