The study found that fevers didn't last as long in winter daylight conditions, the hamsters resumed normal eating patterns sooner after infection, and cytokine production - one measure of immune function - was reduced.
The researchers believe that illness symptoms are reduced in winter so the animals can save energy to survive during a season when food is scarce and conditions are harsh.
"Some of the body's defenses against infection, such as fever, take a lot of energy," said Randy Nelson, co-author of the study and a professor of psychology and neuroscience at Ohio State University.
"If animals have to maintain a fever for too long during the winter, it is going to strain their ability to survive."
The changes in symptom response are tied to the changing length of daylight from summer to winter, said Staci Bilbo, a doctoral student in psychology at The Johns Hopkins University, currently working in Nelson's lab. "Siberian hamsters respond to day length to orchestrate their response to infections," she said.
Bilbo and Nelson conducted the study with Ning Quan, assistant professor, and Lingli Hi, a research associate, both in oral biology at Ohio State; and Deborah Drazen, a post-doctoral fellow in psychiatry at the University of Cincinnati.
Their research was published in the February 22 issue of the journal Proceedings of the Royal Society of London: Biological Sciences.
The study involved 64 adult male Siberian hamsters. Using artificial light, the researchers had half of them live in long days (14 hours of light per day) to simulate summer conditions. The other half lived in short days (10 hours of light per day) to simulate winter daylight.
The hamsters were injected with lipopolysaccharide (LPS) - the dead, outer cell wall of bacteria that stimulates an immune reaction in animals.
The researchers wanted to find out if the hamsters reacted differently to the LPS in the short days versus the long days.
In general, the hamsters reacted less vigorously to the simulated infection in the winter-like short days than they did in the long days.
Results showed that, after the LPS injection, both groups started a fever at about the same time and their temperatures rose about the same amount. However, the length of the fever was significantly reduced from 11 hours in the long-day hamsters to about 6.5 hours in the short-day animals.
Hamsters, as do other animals, lose their appetite when sick and they eat less. However, the short-day animals in this study resumed normal food intake within 24 hours of infection and normal milk intake within 48 hours. In contrast, the long-day animals did not have normal milk or food intake within 48 hours after infection.
"Normally, Siberian hamsters weigh less in the winter than they do in the summer, so they can't afford to stop eating for very long," Nelson said. "They need to get back to normal caloric intake as soon as possible if they're going to survive."
The researchers examined in particular the iron intake of the hamsters. Iron is a vital nutrient for the growth of bacteria, so animals normally avoid iron-rich foods when they are sick. In this study, the short-day hamsters avoided iron-rich milk more so than did the long-day hamsters.
"The short-day hamsters drank less of the iron-rich concentrations of milk, maybe because they had more to lose," Bilbo said. "They didn't want to consume anything that would help their infection."
The study also examined levels of two key cytokines - interleukin-6 and interleukin-13 - that are involved in the immune response. Results showed lower levels of both cytokines in the short-day hamsters when compared to those living in long days, indicating that immune response was not as vigorous.
Overall, the results show that "the immune response is suppressed during winter in the short-day hamsters," Bilbo said. While a less-intensive immune response may help hamsters save much-needed energy during winter, it is not known if this hurts the animals by making them less able to fight infection. Nelson said new research in their lab will examine this question.
"It may be that the hamsters in winter are somehow able to fight infections just as well as they do in the summer, but in a shorter amount of time. We just don't know," he said.
Added Bilbo, "It's a balancing act. The hamsters have to fight the infection, but they can't spend too much energy."
While these results aren't directly applicable to humans, Nelson said it is worth exploring how changes in season and day length may affect the occurrence and susceptibility to various diseases, as well as the severity and duration of illness in humans. "We need to have a better understanding of the mechanisms that underlie the interaction of the environment and human reaction to sickness," he said.
The study was supported by grants from the National Institute of Mental Health and the National Science Foundation.
Additional Contact: Staci Bilbo, (614) 538-9540; Bilbo.1@osu.edu
Written by Jeff Grabmeier, (614) 292-8457; Grabmeier.1@osu.edu