At first blush, the idea that hibernation and treatment of HIV infection have common characteristics seems unconventional at best. But a collaboration between scientists Carol Meteyer from the US Geological Survey (Madison, WI) and Daniel Barber and Judith Mandl of the US National Institute of Allergy and Infectious Diseases (Bethesda, MD) has produced evidence that both phenomena can result in a type of immune hyperactivity called immune reconstitution inflammatory syndrome (IRIS).

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People with HIV may be treated with anti-retroviral medications to suppress replication of the virus. This also results in immune suppression, which may leave the patient vulnerable to opportunistic bacterial or fungal infections. Once anti-retroviral therapy is complete, the immune system is slowly restored and may over-react to an existing infection, triggering severe tissue damage and even death. This over-reaction and its clinical consequences—IRIS—can also occur after recovery from other forms of immune suppression in humans.

Hibernation is a strategy by which homeothermic mammals can reduce their energy requirements during the cold season when food is scarce. During hibernation, body temperature is substantially reduced and the animal enters a state of torpor characterized by reductions in physical activity and in metabolic, heart and respiratory rates. Immune function may also be downregulated during hibernation, as it is energy-intensive and temperature-sensitive.

In the past few years, mortality among several species of cave-hibernating bats in the US has climbed steadily. An estimated 5–7 million bats in North America have died from white nose syndrome, an infectious disease that is now known to be caused by the cave-dwelling fungus Geomyces destructans. The optimal temperature range for its growth closely corresponds with the body temperature of hibernating bats, and the fungus rapidly colonizes the skin of their wings, ears and muzzles. Despite extensive fungal colonization, however, affected bats show minor gross pathology and minimal immune response. Upon emergence from hibernation, however, the bats' immune systems mount an aggressive response to the fungal infection, causing serious tissue damage that can lead to death. Meteyer, Barber and Mandl propose that this process is a form of IRIS (Virulence 3, 1–7; 2012). A better understanding of this immune response is crucial both for people with IRIS and for bats with white nose syndrome. Bats have essential roles in the ecosystem as pollinators and as controllers of insect populations. Hence, declines in bat populations have serious economic and environmental implications for crop production and pesticide use.