“The distance from Los Angeles to Honolulu is half that between the Middle East and New York City, and it made that jump,” U.S. Fish and Wildlife Service biologist Jeff Burgett says, describing the leaps made by the West Nile Virus.
The mosquito-borne virus, first discovered in the West Nile district of Uganda in 1937, had by the 1990s spread thousands of miles, causing outbreaks in Bucharest and South Africa. In 1999, WNV finally hit the United States, infecting birds and people in the South Queens borough of New York City. Sixty-two humans were infected (seven died) that first year, and the virus has spread across the country since then. To date, the federal Centers for Disease Control reports 8,977 documented human cases in the United States, with 218 deaths.
West Nile Virus finally reached California last year. As Burgett and other speakers at a WNV workshop last month put it, the virus is at our doorstep. To ensure that Hawai’i isn’t caught flat-footed when WNV finally does arrive, the Hawai’i Conservation Alliance, the Fish and Wildlife Service, the state Department of Health, and the U.S. Geological Survey’s Biological Resources Division sponsored the workshop, “Protecting Hawai’i and the Pacific from West Nile Virus.” Over two days, at the Ala Moana Hotel and the University of Hawai’i at Manoa, local, national and international scientists worked with government officials and other experts to prepare a plan to respond to, track, and limit the spread of West Nile Virus in Hawai’i.
The Threat
West Nile Virus can kill humans, but such a result is the exception, not the rule. For Hawai’i’s native birds, however, the arrival of WNV might well be devastating.
Most people infected with WNV show no symptoms at all, but about 20 percent come down with a fever (which, in Colorado, had an average duration of two weeks), and about one percent suffer severe disease, either West Nile meningitis (inflammation of the membrane around the brain and spinal cord), encephalitis (brain inflammation) or meningoencephalitis (inflammation of the brain and the membrane surrounding it).
For humans, the death rate for those infected with WNV seems to be around 2 percent. For some bird species, however, it approaches 100 percent. WNV is primarily a bird virus. As with humans, the virus’s effect on birds varies. Chickens may exhibit mild symptoms, while corvids (the family that includes crows and ravens), have high fatalities. Robert McLean, research manager for wildlife diseases with the U.S. Department of Agriculture’s Wildlife Services Division, told the group that WNV kills nearly 100 percent of the corvids it infects. (The virus is known to infect mammals and reptiles as well, although it is widely believed that the virus can no longer be transmitted once it infects them.) In the four years that WNV has been in North America, McLean said, 203 species have become infected, covering 20 orders and 55 families of birds. Countless birds have died from the infection.
In Connecticut, 80 percent of crows tested were infected, McLean said, and in the Midwest, hawks and owls died off in significant numbers, while in Montana and Wyoming, entire populations of the threatened greater sage grouse succumbed to the virus. Because infection among birds spreads so rapidly, “Once a county gets a positive bird test, they don’t test birds anymore,” he says.
While it is unclear how the virus has affected overall populations of North American birds, FWS biologist Jack Jeffrey said there is great potential for losses of Hawaiian birds. The remoteness of the archipelago filtered out a lot of the diseases and natural threats to the plants and animals that settled here, Jeffrey said, adding: “That can be overridden by us.” Speaker Peter Daszak of the Consortium for Conservation Medicine underscored that point, noting that the most common cause of the spread of human, wildlife, and livestock diseases is “pathogen pollution”- the human movement of pathogens around the planet.
Many species here have, through evolution, lost defense mechanisms, such as virus resistance. If the native forest birds go, Jeffrey says, the native plants may follow, since many are bird-pollinated.
Sixty-five percent of the native bird species have already gone extinct since Captain Cook’s arrival in 1776. Of those that remain, half are endangered, Jeffrey noted. Hawai’i already has avian malaria and pox, he said, and many Hawaiian bird species don’t have the populations to resist another disease. The native birds most likely to be first and hardest hit by WNV, he predicts, will be the lowland birds like the nene, the O’ahu elepaio, and the ‘io, all of which are endangered.
“Prevention is our best bet at this point,’ he told the group.
Pathways
The key to prevention is determining how WNV will come to Hawai’i. Although the virus has yet to be detected in Oregon, Washington, and Alaska, the conference’s keynote speaker, Colorado state epidemiologist John Pape, says it’s only a matter of time. Hawai’i will be last state to get it, he predicted, but it will arrive – most probably coming in on planes or boats carrying infected mosquitoes or birds.
Daszak and his associate, Marm Kilpatrick, agreed.
“We value air travel twice as much as we can afford it. We can use this to predict that WNV will be knocking on our door again and again,” Daszak said. Kilpatrick discounted the notion that infected humans or wind-blown mosquitoes could bring the virus to Hawai`i: the viremia, or concentration of the virus in the blood, of an infected human is not enough to infect a mosquito, he said, and winds have never brought mosquitoes to Hawai’i. However, he noted, once the virus is on one island, mosquitoes could carry it to other islands in the chain.
Kilpatrick admitted that no one knows whether an infected bird capable of passing on the virus to mosquitoes might survive migration to Hawai’i.
With an average of seven out of 10,000 shipping containers holding mosquitoes, and with an even smaller number of those capable of transmitting the virus, Kilpatrick says, there is a very slim chance – one in a million – that container shipments would bring the virus to Hawai’i. Planes are another story. A survey of airplanes in Japan found that one in five had live mosquitoes, 82 percent of which were found in the cargo holds. Planes surveyed in Australia averaged 2.2 live mosquitoes per plane, Kilpatrick said. Each day, more than 50 flights from the continental United States arrive in Hawai’i. Assuming 1.5 mosquitoes per plane, with a 0.1 to 0.5 percent chance of infection and a vector competence of .22, Kilpatrick said, Hawai’i could expect somewhere between roughly one and three infected mosquitoes to arrive each month. To circumvent WNV from becoming established here, Kilpatrick recommended disinfection of planes, especially in cargo holds – a practice long used in New Zealand and Australia.
Another possible pathway is human-transported birds. Adult chickens have low viremia, so they are not a great concern, Kilpatrick says. But the pet trade is. Hawai’i receives about 5,000 birds a year through the pet trade, primarily from California, Texas, Oregon and Florida. Most birds must sit through a 7-day quarantine before they get here, but Kilpatrick said the bird could still become viremic if it is not separated from mosquitoes and other birds that may already have the virus.
Two pet species that could transmit the virus are exempt from quarantine before coming here, Kilpatrick noted: rock doves, and budgerigars (parakeets). Hundreds of these birds arrive in Hawai’i each year and could generate enough virus to infect mosquitoes, Kilpatrick said. While they are not great transmitters of the virus, these species should not only have to go through quarantine, but should be held for 10 days, not seven, he suggested. Also, he said, birds should be individually quarantined in a mosquito-proof area, to prevent the passing of the virus between birds.
At the workshop, one state Department of Agriculture official argued that the potential viremia of budgerigars was speculative. He called for experiments on those birds to determine whether or not they should be added to the quarantine list.
To this, Daszak responded: “Do we have time? Or should we put in blanket measures and do experiments?”
What to Expect
While the experience of every state is different, John Pape’s description of what happened in Colorado after WNV arrived is a window into what Hawai’i officials can expect if it become established here. In Colorado, people knew the virus was coming and they had a little time to prepare, Pape said. Even so, he said, he was surprised at the rate at which the virus swept through his state.
Surveillance started in 2001, and on August 15, 2002, the virus had hit. By October, half of the mosquito pools tested positive for WNV. By December, the number of cases had gone from three to nearly three thousand (2,264 with WNV fever, 355 with meningitis, 214 with encephalitis). By the end of the outbreak, 856 people had been hospitalized and 54 had died.
Colorado is now spending hundreds of thousands of dollars to control the virus. Before it hit, the figure was closer to $90,000 a year on surveillance, upgraded lab equipment, and other WNV-related activities, Pape said. After the initial outbreak, the state’s budget for WNV control more than doubled to $190,000 in 2002, and grew to $300,000 for 2003. Nearly half of the money goes for laboratory costs.
All totaled, Colorado has spent more than half a million dollars in personnel costs related to treatment of human WNV cases; medical costs for each infected person range from $2,000 to $242,000, with a mean of $52,000.
In just one county, mosquito control costs came to nearly $800,000, Pape said. Aerial spraying of another county cost $293,000. And Colorado’s “Fight the Bite” education program and a caller hotline cost about $100,000.
How such expenditures will affect the future impact of WNV is unknown. There are still unanswered questions about the frequency and intensity of the virus over time, Pape noted. Connecticut, New Jersey, New York, and Pennsylvania were the first states to have WNV, and they have more cases now than ever, he said. “Once the virus is here, it’s here to stay,” he observed. Even so, early detection is important. Because testing tissue from dead birds took too long, last year Colorado switched to an oral swab test for quick screening in the field. While the test isn’t perfect (the bird needs to have a lot of virus to have it show up in the oral cavity), its quick turnaround is attractive.
“If you wait weeks for a test, you miss the boat and the virus is long gone,” Pape added. A lag in shipping time delayed the diagnosis of the first infected Colorado bird for two weeks. “In Hawai’i, that time is unacceptable,” he said. By then, “you’ve lost the battle.”
Damage Control
Nicholas Komar, a microbiologist with the CDC, observed that WNV has “never been contained anywhere.” Still, he said, if it ever is to be contained, an island situation might be the best place to try. To do that, he says, Hawai’i needs to do what other states are doing: conduct good surveillance, find hot spots, and get education about the virus out to those hot spots. In Hawai’i, he says, the coastal wetlands – where mosquitoes and birds hang out – should be important areas of surveillance and control.
McLean added that Hawai’i should start surveillance at ports of entry. At present, the Department of Health monitors mosquito population density and uses some larvicide.
According to Harry Savage of the Centers for Disease Control, no good information is available on the effectiveness of a pre-emptive strike on mosquito populations. While some relationship exists between the transmission of WNV and mosquito populations, “there is no magic number,” he said, noting that every part of the country is different. He cited the case of Louisiana, where an intense WNV outbreak occurred in 2001. The CDC had worked closely with Louisiana officials, detected the virus early, was doing vector and nuisance control and had reduced the mosquito population to below nuisance thresholds, but the area still got hit with WNV.
“Even with low mosquito numbers, there can be very efficient transmission. That’s true for all arboviruses, not just West Nile,” he said. One the other hand, in Larimer County, Colorado, where parts were sprayed and others weren’t, human transmission decreased in the control area, “but the data is still being analyzed.”
— Teresa Dawson
Volume 14, Number 8 February 2004
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