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Spring is here, that’s encouraging. Mosquitoes are on their way and that’s discouraging, as is what may also be on the way - West Nile Virus (WNV). In 1999 WNV was noted mostly within a 50 mile radius of New York City. In 2000 it was noted in several north Atlantic states, and in 2001 it was detected in Florida and sporadically in states east of the Mississippi. By 2002 it was present in most counties and all states east of the Rockies.
WNV (virus) is transmitted to humans (victim) by the bite of a mosquito (vector) that have previously bitten an infected bird (victim - more on bird victims later). In 2002 fewer than 1% of people infected became seriously ill, yet 254 did die. Horses, mules and burros can be infected. Although numbers are sketchy, there were 5,380 horses reported infected in 2002 with an estimated 1 out of 3 dying or being euthanized.
Studies in the Old World outline the close link between the virus and migratory birds. WNV does not pass from an adult female mosquito to her eggs, so the only way WNV can move from one generation to the next is through the blood of a bird or other host Thus, either by a migratory bird, an introduced bird (legal or illegally), transport of an infected mosquito, or, by another host was WNV introduced in North America.
Although WNV does make birds sick, and has been confirmed in 111 species, for most it poses only a minor health risk. However, large species such as hawks and owls, and especially members of the family Corvidae (crows, jays, ravens) are more susceptible to the virus, suffering high rates of infection and mortality. When laboratory crows were infected, 100% died within 7 days. House Finches may also be vulnerable.
An inconsistent pattern was evident with crows in some 2001 Christmas Bird Counts. A 90% drop in crows would be noted in an area, while an adjacent circle had historic numbers. Perhaps there is a connection to a particular mosquito species? There have been antidotal reports of “thousands of dead hawks and Great Horned Owls”. However, with few exceptions, hard facts on the effects of WNV on avian populations is lacking.
Most dead birds are never found, and until recent years, those that were found, except for critical species (such as California Condors, Bald Eagles, Peregrine Falcons), are not routinely tested for cause of death. Since 1999 New York State’s Wildlife Pathology Unit has examined over 12,500 dead birds representing 213 species. Mortality seems to vary greatly among species, and outbreaks seem sporadic and localized.
Although the sample of House Finches was small, 23% tested positive for WNV, as did 14% of House Sparrows. Among corvids, 34% of Fish Crows, and 29% of Blue Jays tested positive. Of all the remaining dead song birds, WNV was found to be the cause of death in only 4%. As for wild raptors, high percentages (33%) were found in Merlins, Kestrels, and 14 % for Great Horned Owls. Dead hawks tested 12% positive for WNV.
As a result of this testing, important statistics on the causes of non-WNV mortality among wild raptors have been compiled. Most die from traumatic injuries such as gunshots, electrocution, collisions (vehicles, trains). Other causes of death were poisonings (lead, rodenticides, pesticides), as well as starvation, other diseases, fungus, and parasites. These causes and numbers plus WNV mortality rates among raptors may reveal the outlook for raptors over the long term.
The biology of small populations of a single bird species is deserving of a whole column (or even two). However, for now, let us mention just three small populations of endangered species; the Florida Scrub Jay (a susceptible corvid), Whooping Crane and California Condor (both large birds). A high infection rate among these localized, small populations might very well bring about the extinction of the species.
Also of concern are captive populations in zoos, and raptor rehabilitation centers where injured and crippled birds are held for demonstration, study and education. In the Midwest WNV deaths have occurred at such sites. Nine of thirty dead raptors were positive for WNV. The California Condor breeding populations in San Diego, Los Angeles and Boise, and birds at facilities such as our own local CALM would be at risk.
Bird populations in northeast Africa do show immunities to WNV. Unlike small, isolated populations, most resident specie populations are spread across North American. Thus it is possible for immunities to evolve within these species as long as outbreaks remain sporadic. As for migrating birds from the tropics, prospects are unknown.
There is a vaccination for horses. Horses and burros in the BLM Adopt-a-Horse or Burro program are being vaccinated. New vaccinations may provide protection for birds in captivity. Although investigation of oral vaccines is underway, wild birds present a different challenge, after all, most birds do not feed at feeders nor out of your hand. It will probably be well into the mosquito season before we know what progress the Center for Disease Control has made in respect to vaccines for humans.
Horses and people are dead end hosts. They can become infected, but it does not spread from there. Infected birds are not a threat to humans. Only birds can infect mosquitoes and only mosquitoes can spread the disease. West Nile Virus presents a knotty ecological problem for birds and people. Identifying hot spots where migratory birds, humans, and mosquitoes converge will be important in formulating successful prevention measures. References – Living Bird, Birdscope (publications of the Cornell Ornithology Laboratory) and the Center for Disease Control (CDC).
Good birding, gone birding.
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