Climate change could make bird flu even more common among birds at a US hotspot for the disease. That’s what mathematical models of bird flu levels in Delaware Bay developed by biologists Pej Rohani and Vicki Brown at the University of Michigan in Ann Arbor suggest. “We’re not suggesting that our findings necessarily indicate an increased risk to human health,” said Pej. “But every single pandemic influenza virus that has been studied has included gene segments from avian influenza viruses. So from that perspective, understanding avian influenza transmission in its natural reservoir is, in itself, very important.”
Bird flu levels in Delaware Bay are at least ten times as high any other site in the world watched by scientists. That’s thanks in large part to ruddy turnstones, birds that pause there each year as they migrate between South America and the Arctic. Once in the Arctic, ruddy turnstones who spend their winters in America can meet – and share diseases – with others spending winter all over the world. During their Delaware stop-off they feed on horseshoe crab eggs, but fisherman harvest the crabs for fishing bait, while development in the area destroys the crabs’ nesting sites. This has meant fewer eggs, and in turn fewer shorebirds.
Global warming adds an extra layer to the problems facing these birds, by changing when important natural events happen. For example, birds that migrate long distances have started to make their spring journeys earlier. That could mean that ruddy turnstones arrive in Delaware Bay before all the horseshoe crabs have laid their eggs. Faced with an even more limited food supply, the birds must pack together yet closer in hunting for those that are available, creating an ideal scenario for disease transmission.
Bird flu? Who knew?
Scientists don’t yet know how important all this is for spreading flu, Pej explained. “One of the challenges is that, despite much effort, the actual data that’s available in places like Delaware Bay are somewhat limited,” he told Simple Climate. “For the past few years we’ve been developing a mathematical model for influenza transmission at Delaware Bay. We believe our model is the best available, but there are going to be aspects of that system that our model doesn’t capture, so we’re not going to make any outlandish claims.”
Vicki and Pej’s model considers three different bird classes: ducks that always live in the area, ducks that migrate from Canada and long distance migrants – particularly ruddy turnstones. “The model combines the arrival times of the migrant birds and assumes there is potential transmission between individuals of different species while they’re mixing,” Pej said. “There is also an overall impact of the availability of horseshoe crab eggs for reproduction and survival of ruddy turnstones. We can use it to do a ‘what if’ type analysis: what happens if they arrive a couple of weeks earlier or later?”
In a paper published in research journal Biology Letters on Wednesday, the scientists used the model to look at two such scenarios. The first scenario looked at different dates for when ruddy turnstones arrived, while the second changed the dates that the horseshoe crabs laid their eggs. Pej and Vicki find that when ruddy turnstones arrive either earlier or later than the current May date, flu levels are higher in resident ducks when they get there. That raises the levels in ruddy turnstones, which feeds back to the ducks’ flu levels, pushing them higher than currently seen. If climate change made the horseshoe crabs lay eggs slightly earlier, there was little effect. But if egg-laying dates were much earlier, both ruddy turnstone bird numbers and the flu level in those remaining were much lower.
Understanding the risks
The researchers stressed that the factors that make a flu virus successful in birds can be quite different to those needed to infect humans. This means that flu seldom passes to us from birds, but when it has done, many of those infected have died. Since 2003 more than 600 cases of human infection with avian influenza A H5N1 have been reported worldwide, leading to more than 300 deaths.
Pej and his team are therefore looking to understand more about what their model of Delaware Bay birds can say about temporary flu immunity birds get after being infected. But while they are not look directly at how bird flu gets into humans, Pej pointed out many other researchers are. “The process of emergence of pandemic influenza viruses is complicated and poorly understood,” he said. “There are a number of important ways that needs to be studied. That involves better understanding influenza virus genetics and the processes that go on in their transmission, especially in poultry. That work is being carried out – there are a large number of groups working on such issues.”
V. L. Brown and Pejman Rohani (2012). The consequences of climate change at an avian influenza ‘hotspot’ Biology Letters DOI: 10.1098/rsbl.2012.0635