The findings offer another example of how evolution in response to one pathogen may lead to resistance to others—an effect that has been seen with other parasites and is an area of active research with implications for human health.
Scientists have known for some time that bighorn sheep (Ovis canadensis) that have coevolved with a species of gut worm known as Protostrongylus columbianus are better able to tolerate the damage that those worms cause to the lining of their intestines. But little is known about how those adaptations affect the sheep’s susceptibility to other kinds of infection, says evolutionary biologist Sonia Altizer of the University of Georgia in Athens.
To find out, Altizer and her colleagues experimentally infected bighorn sheep from two different populations with two common respiratory pathogens: a bacterium known as Mannheimia haemolytica and a virus known as parainfluenza virus type 3 (PI-3).
The researchers chose those pathogens because previous work had shown that sheep infected with either were likely to develop pneumonia, they write in the 13 June issue of Proceedings of the Royal Society B. Pneumonia is also a common cause of death in wild bighorn sheep, Altizer says.
The team infected 10 sheep from each population with both respiratory pathogens and monitored them daily for signs of pneumonia over the course of 2 months. One group of sheep had coevolved with P. columbianus; the other group had not.
The sheep that had coevolved with P. columbianus developed significantly less pneumonia than those that had not, Altizer’s team found. Furthermore, the amount of gut worm infection each sheep had was negatively correlated with the severity of pneumonia caused by PI-3.
The researchers also measured the cellular immune response of white blood cells isolated from the two sheep populations to both respiratory pathogens. Bighorn sheep that had been exposed to P. columbianus had white blood cells that responded more strongly to both the bacterium and the virus.
Those results “provide evidence for an immunological mechanism underlying cross-resistance between parasites,” the authors write.
Overall, the findings “suggest that coevolution between hosts and their gastrointestinal parasites may drive a suite of correlated resistance traits that extend across parasite taxa and influence the outcome of other infectious diseases,” the report says.
Altizer says the results fit into a growing body of research showing how evolution in response to one pathogen may lead to resistance to others. Scientists have observed that phenomenon in other types of parasites, such as malaria parasites that protect against other infections that cause fever, she says.
But Altizer says that most research has focused on how coevolution with one parasite affects resistance to other parasites, not other types of infections.
Understanding the mechanisms underlying such “cross-resistance” is important for human health, she says, because it may lead to new ways to treat infectious disease and to design new vaccines.
For instance, there has been some research exploring whether the body’s immune response to a hookworm infection could be used to develop a vaccine against HIV, Altizer says.
“We are only just beginning to get our heads around these ideas,” she says, but “I think it’s a very exciting direction for research.”
