Each animal species has an optimal temperature at which it can metabolise food and its immune system can best fight off pathogens.

As our recent research shows, temperature directly affects the immune systems of vertebrates – regardless of how they moderate their own body temperatures. At first, slightly hotter temperatures actually give many animal immune systems a boost. But when temperatures get still hotter, conditions favour pathogens – organisms which cause disease.

This is a real problem, given many pathogens found in warmer areas are likely to expand their range as the climate changes.

The good news: learning more about how temperatures affect animal immune systems gives us new options, such as using “frog saunas” to help frogs fight off the lethal chytrid fungus.

How do animals maintain body temperatures?

Different types of vertebrates have very different ways of maintaining an optimal body temperature.

Mammals and birds are endotherms. In cold conditions, they can keep their body temperature close to optimal by burning energy stored as fat. Animals such as reindeer are able to live in temperatures as low as -40°C while keeping their core body temperature at 38-40°C.

At the other extreme are snakes, lizards and other poikilotherms – so-called “cold blooded” animals who rely on the environment to modify their temperature. If they’re too cold, they seek the sun. If too hot, they seek the shade.

Regardless of the method, the goal is the same: keep body temperature as close to optimal as possible.

Pathogens have temperature preferences too

Pathogens are very diverse. Some prefer hotter conditions and others cooler. For some, high temperatures can stop them replicating. But for others, heat is great. The lethal Ebola virus replicates best at 41°C.

The rhinoviruses which cause the common cold prefer the slightly cooler temperatures (33°C) found in human airways.

In birds, outbreaks of lethal H5N1 avian influenza have been shown to come shortly after a large sudden drop in temperatures.

The fungus causing devastating white-nose syndrome in bats likes colder temperatures of 12-16°C. When bats hibernate, their body temperatures drop and their immune response isn’t as strong. This is when the fungus can invade.

Most fish species are poikilotherms. If they move into water colder than their optimal, their immune defences are lowered and they’re more susceptible to pathogens such as viral haemorrhagic septicaemia virus or the bacteria Flavobacterium psychrophilum causing coldwater disease.

Frogs and other amphibians are now declining globally. A major cause is the disease chytridiomycosis caused by the chytrid fungus. The disease is implicated in at least 90 extinctions. This fungus lives in water or damp soil and prefers the cold. As the world heats up, the fungus will likely gain access to new water bodies – and amphibian hosts.

Researchers found leopard frogs (Rana yavapaiensis) living in warmer water were infected less than those in colder water. Australian researchers are now building “frog saunas” which let infected frogs kill off the infection.

How does temperature affect animal immune systems?

When an animal’s body temperature is lower than optimal, it can’t mount as strong an immune defence against specific pathogens. Interestingly, we found this effect only seems to impair specific defences, while the animal’s innate defences aren’t affected.

Ground squirrels and many other species can go into short hibernation periods known as torpor. In this state, their metabolism slows down, body temperature drops and reduces numbers of cells and molecules responsible for specific immune defences circulating. In most cases, the lower body temperature also stops pathogens from replicating. Once an animal leaves the torpor state and its body warms up, its specific immune responses bounce back.

How does this work? When temperatures fall, changes take place in the physical structure of the molecules necessary to mount a specific defence against a pathogen, making an immune response impossible. For instance, the major histocompatibility complex, a key immune molecule found in almost all vertebrates, loses the ability to bind to other immune system molecules in the cold.

Heat acts differently. Humans and all other endotherms can induce a fever, which means the immune system raises the body temperature to stop an invading bacterium, virus or other pathogen from replicating. Fevers put most pathogens at a disadvantage and triggers specific immune responses. But too much heat is a problem, as it can stress the body or even kill. Luckily, special molecules called heat shock proteins can buffer cells against heat and help restore the proteins needed to induce a specific immune response.

Lizards, fish and other poikilotherms can’t increase their own body temperature. Instead, when they get an infection, they employ “behavioural fever” – moving to warmer environments to boost their immune response.

Can we use this to protect species?

Knowledge of how temperature affects animal immune systems lets us plan new ways of protecting threatened species.

We can use heat or cold to change body temperatures and trigger immune responses, or to stop pathogens replicating.

But as climate change intensifies, rapid temperature changes will bring many unwelcome changes for animals. Heat-loving pathogens such as malaria will expand their range, as will cold-hating parasites such as ticks. Milder winters in Canada and the United States, for instance, are letting winter ticks survive the cold. These blood-sucking parasites are now killing many young moose.

The more we understand about how temperatures and animal immune systems intersect, the better we are placed to help animals whatever is to come.

Brian Dixon receives funding from the Natural Sciences and Engineering Research Council of Canada

Julie Old does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.