A study published on Wednesday (Sept. 7) found that ducks exposed to mercury pollution are substantially more likely to contract bird flu. The results indicate another way in which disruptions to the natural environment brought about by people raise the likelihood of virus transmission.

According to the U.S. Department of Agriculture, avian influenza, also known as bird flu, can infect poultry including chickens and turkeys as well as free-flying waterfowl like ducks, geese, and shorebirds.

In California's San Francisco Bay, which is on a migratory route that extends from Alaska to Patagonia, researchers shot down over 750 wild ducks from 11 distinct species for the new study.

The ducks were then examined for mercury contamination as well as if they were infected with bird flu or had antibodies to the virus in their systems.

Wild waterfowl, such as ducks, are thought to be superspreaders of the virus, in part because they migrate so far and may infect other birds along the trip.

The findings, published in the journal Proceedings of the Royal Society B, revealed that ducks exposed to mercury were up to 3.5 times more likely to have contracted bird flu in the last year.

A quantitative ecologist at the USGS Eastern Ecological Science Center and the study's lead author, Claire Teitelbaum, said that mercury contamination "can suppress the immune system, and that might make infection with anything - including influenza - more likely."

Additionally, she told AFP, the San Francisco Bay is a "significant hotspot for mercury contamination in North America ... largely from historical gold mining, where mercury was part of that process"

The highly virulent H5N1 avian flu virus, which has been found in several regions of the world, was not detected in the ducks, however.

Because many wild birds are up on their nesting grounds further north, Teitelbaum claimed that bird flu epidemics in the U.S. had slowed down over the summer.

She cautioned, however, that once they begin to descend again, there would likely be an increase in activity.

Researchers are increasingly warning that causes including deforestation, livestock husbandry, climate change, and other human-caused phenomena increase the likelihood of viruses spreading from animals to people. This spread coincides with their alarm.

Teitelbaum observed that "there are just so many ways in which humans have historically altered and are continuing to alter the natural environment."

How pollution and contamination increase the risk of disease spread is "just another link that we need to add to our more holistic view of what's going on in the world," she concluded.