Growing evidence suggests that for avian influenza viruses, prolonged persistence in the environment plays an important role in transmission among aquatic birds, the viruses’ natural host.

Since temperatures within the environment and within a host can be markedly different, researchers at the University of Georgia and colleagues wondered if influenza virus strains might face a trade-off, specializing in survival either inside or outside a bird.

Their work, published online at Proceedings of the Royal Society B: Biological Sciences, revealed that while flu virus strains did face a trade-off when adapting to high or low temperatures in the environment, little evidence was found for a temperature-dependent trade-off “between scales.”

“The full ‘life cycle’ of a pathogen involves multiple layers, or scales: virus replication within cells, spread inside a host, and proliferation between hosts,” said Dr. Andreas Handel, assistant professor of epidemiology and biostatistics at the UGA College of Public Health and the study’s lead author. “If one wants to fully understand the dynamics and potential evolutionary trajectory of any pathogen, one needs to understand the full cycle across all scales.”

To be successful at spreading disease, a pathogen needs to be good at replicating and evading the immune system within an infected host and it needs to be good at transmitting to the next host.

“I assumed there would be trade-offs because to some extent they make intuitive sense. If you are good at one thing, you might not be good at the other,” Handel said.

In the case of avian influenza, Handel assumed the virus strains would either optimize their ability to persist outside of a host at low temperatures or optimize their ability to replicate to high viral load in a host at higher temperatures.

To determine if such a trade-off actually occurred, Handel analyzed a unique dataset reporting the environmental decay rates of seven avian influenza strains together with strain-specific viral load estimates from influenza-infected ducks.

The results were not as expected.

“Basically, there was no-cross scale interaction,” Handel said. “How well these influenza strains persisted in the environment at either low or high temperatures did not affect the viral load during infections inside the host.”

The study was able to find evidence for a trade-off at the environmental scale, confirming findings from a previous study by Handel and colleagues.

“As temperature goes up, all pathogens decay faster,” Handel said. “But what we found that was interesting was that strains that persisted the longest at low temperatures compared to other strains had on average the shortest persistence at high temperatures, and vice versa. This suggests potential adaptation of different avian influenza strains to ecological niches.”

The dynamics of avian flu are rather complex and there are many strains circulating and infecting different aquatic bird species, Handel explained.

“We still know relatively little about the ecological and infection dynamics of avian flu,” he said. “The more we understand about the dynamics and processes of these avian strains, the better we can prepare for any new strains that may emerge and cause a pandemic.”

Contributing authors for the study include Dr. Camille Lebarbenchon from the University of Reunion Island; Dr. David Stallknecht from the UGA College of Veterinary Medicine; and Dr. Pejman Rohani from the University of Michigan.

The full article is available at https://dx.doi.org/10.1098/rspb.2013.3051.

Coverage also at ASPPH Member Research and Reports.

Posted June 20, 2014.