On the Path of Bird Flu
If Robert Gill gets his way, he will soon be on the sandy beach of a southern island — but that decision is ultimately up to the birds. Gill, a wildlife biologist at the U.S. Geological Survey’s Alaska Science Center in Anchorage, is betting on the day the first bar-tailed godwit will take off for its annual migration from New Zealand to southeast Asia and then on to Alaska. Information about the godwits’ migration, collected this spring via satellite transmitters, could result in more than just an exotic trip for one lucky researcher at the science center, however; it could also forward the understanding of how and where avian influenza, or bird flu, is likely to spread.
Uncovering the path by which bird flu could enter the United States is a task that has gained new urgency since the outbreak of the virus in Hong Kong in 1996 and 1997, and its subsequent spread to parts of Asia, Europe, the Middle East and Africa. That the virus is on the move has U.S. researchers moving quickly to improve monitoring and testing efforts. Stomping out the virus or at least minimizing its effects, however, will require the collaboration of researchers around the world sharing information gleaned from newly adapted tracking technologies.
High-path H5N1 is of particular concern, in that this strain can be transmitted to humans coming into close contact with infected poultry. While H5N1 is not currently known to readily spread from human to human, the potential exists should the strain mutate. It is also possible for some low-path strains to mutate into high-path strains.
A total of 25 countries and regions have reported cases of the high-path H5N1 virus in birds, according to USDA. And as of March 12, the virus had infected 278 people in 13 countries, from Asia, the Middle East and Africa, according to the World Health Organization. More than half of the cases resulted in death, and the severity of the situation has been told in numerous broadcast and print media stories.
But the story is not over. Although humans and birds in the United States have not turned up any cases of high-path bird flu, USDA is “doing all [they] can to keep it that way,” said agriculture secretary Mike Johanns at a March 14 press conference. That effort includes the continued monitoring of the three most likely entry routes: through poultry trade, pet bird trade and wild migratory birds.
The northern pintail duck is one of the species that migrated between Asia and Alaska, and Hupp wants to find out where Asian pintails might be coming into contact with North American pintails during that migration. Previous tracking experiments tagged the birds with coded bands — a method that showed pintails move across continental boundaries, between North America and Japan.
But banding experiments do not provide details about the birds’ exact path and time spent in particular locations, whether or not they nest in Siberia and Russia, or if Asian pintails mingle with American pintails. That’s because band studies depend on someone finding the band and reporting it, which is not likely to occur in remote regions such as Siberia.
In February, Hupp and colleagues in Japan set out to deploy 18-gram satellite transmitters on pintails wintering in Japan. They hope to uncover specifics about where pintails migrate and nest, and for how long, which will help them find out if birds migrating back to North America mingle with potential carriers of high-path bird flu. “Satellite telemetry really helps fill in the gaps,” Hupp says.
Other USGS researchers are also employing the satellite telemetry tracking technique. Gill, for example, wants to learn about the potential for godwits to bring the virus to Alaska, by measuring their proximity to H5N1 “hotspots” when they stop in Asia during their northward migration from New Zealand.
Gill and colleagues in New Zealand marked 16 birds, half with 25-gram implantable transmitters that will provide a location for the birds every 36 hours, from the time of attachment in February through their migration and arrival in Alaska in May. The reason for using implants, rather than external transmitters, is that godwits migrate for extremely long distances, flying eight days nonstop and covering one end of the globe to the other, Gill says. “We were reluctant to put anything external on them that might interfere with wind resistance, and is just not compatible aerodynamically to a bird that engages in such flights.” The technology has confirmed the great lengths that godwits travel and has “opened our eyes to just what marathoners these birds are,” Gill says.
Despite the accuracy and near real-time data that can be gleaned from tracking birds via satellites, researchers are still not about to give up the tried and true banding technique. Transmitters typically cost more than $2,000 each, in addition to the cost of the data collection service, Hupp says. As such, researchers use transmitters only when leg bands are unlikely to paint a complete enough picture of bird migration.
Meanwhile, the eight other godwits were marked externally with smaller, 10-gram, solar-powered transmitters that will track the godwits for two years and map their migratory patterns over multiple seasons. Time will tell if godwits indeed mingle with their Asian counterparts, and could potentially bring H5N1 to North America. “If your assumption is that flu will enter North America via Asia and Alaska, then [the godwit] is sort of the poster bird to follow because it does pass through these areas of known outbreaks of H5N1,” Gill says.
Of the nine godwits caught in Alaska and tested in 2006, three tested positive for low-path bird flu. So far, the high-path strain has not turned up in Alaska or anywhere else in the United States.
Since 1998, Winker’s team has been screening birds for bird flu in western Alaska. Here, shorebirds congregate in the water, which was considered to be a potential pathway by which contaminated birds could spread the virus. Seven years and more than 8,000 samples later, the team concluded that the risk of viruses in general entering western Alaska through migratory birds is low, they reported in April in Emerging Infectious Diseases.
Specifically, the infection rate of low-path bird flu across all birds tested by the team was less than 0.1 percent. The rate pales in comparison to infection rates found in other studies at lower latitudes: southern Minnesota at 10.8 percent, Alberta at 22.2 percent, British Columbia at 55 percent, and the Alaskan interior at 9 percent. The team attributes the Alaska coast’s low infection rate on the “Arctic effect,” which suggests that the number of birds that congregate in the water is low compared to the amount of water available to dilute the virus.
“The results of this study suggest that the risk of the introduction of high-path H5N1 [bird flu] through migratory birds in this region is relatively low,” USDA’s Johanns says. “While this is good news, we must not let our guard down. We will remain vigilant in our efforts to protect the nation from high-path H5N1.”
To determine which route — migratory birds, poultry or the pet bird trade — poses the greatest risk of transmitting bird flu into the United States, Kilpatrick’s team calculated which of the three pathways was the most likely for 52 different introduction events into countries known to have cases of H5N1 bird flu. The researchers added the numbers of birds moving by various pathways — poultry versus migratory birds versus pet birds — and then multiplied those numbers by the probability that they were infected, as well as the number of days that the species of bird is known to have the potential to transmit the virus. Chickens, for example, tend to die after a few days of infection, whereas ducks can live for a number of days.
The team found that introduction events within Asia were more likely to occur through poultry, and will therefore most likely spread to South and Central American countries through a similar route. Once there, bird flu could be carried by migratory birds northward into the United States, the team reported Dec. 19, 2006, in Proceedings of the National Academy of Sciences. “Poultry represents a huge risk, and so efforts should be spent on trying to basically address that pathway,” Kilpatrick says (see sidebar).
Overall, the research aims to identify the most likely transmission route of bird flu to try to prevent its future spread, and to try to “stomp out the virus globally, so that it doesn’t pop up again and keep wreaking havoc both in the poultry industry as well as on human health,” Kilpatrick says. Indeed, bird flu has been quelled three times in U.S. history when high-path strains emerged, although those cases involved strains other than H5N1.
Controlling the virus requires “stringent controls on poultry movement, on the interaction between wild birds and poultry, and really good veterinary care,” Kilpatrick says. “The challenge is in countries where they don’t have the resources to do that very well.”
And in April, USDA implemented the 2007 Highly Pathogenic Avian Influenza Early Detection System as a means to update the previous Wild Bird Surveillance Plan. As such, surveillance will continue to be conducted in the four major paths of bird migration, but will also take into account data collected in 2006 to better focus sampling on species and locations that pose the most risk, Johanns says. USDA is now “not only conducting surveillance throughout the United States; we are doing similar testing in Mexico and other strategically located countries,” said Ron DeHaven of USDA at the March 14 press conference.
Should H5N1 eventually make its way into the United States, however, a bird flu pandemic among humans would not necessarily result, Johanns says. A pandemic would require the virus to mutate in order to become easily transmittable between humans, which has thus far not occurred. The world is currently in a phase three of a pandemic alert, which implies “no or very limited human-to-human transmission,” out of a scale from one (low risk of human cases) to six (efficient and sustained human-to-human transmission), according to WHO. Still, to avoid a pandemic like that of the 1918 flu, which killed 40 million to 50 million people worldwide, bird experts continue trying to stay one step ahead of the virus.