Can't Stand the Heat? Get Out of the Host!
Behavioural fever reduces ranaviral infection in toads (2019) Sauer et al, Functional Ecology, https://doi.org/10.1111/1365-2435.13427
Being infected with a pathogen such as a bacteria or virus can be bad for whatever organism is unfortunate enough to suffer the infection, and sometimes it’s bad enough to kill the host. Because of that, there is a strong pressure to engage in behaviors that reduce the chances of becoming infected in the first place. While these behaviors can be inherited and evolve over time, others take place within the lifetime of the infected individual itself, making it a ‘plastic’ response (see the “Did You Know” from our previous breakdown for the difference between plasticity and evolution).
One plastic response is that of a behavioral fever. In organisms that cannot regulate their own body temperature, like reptiles and amphibians, this behavior involves moving from an area with low temperature to one with a higher temperature, ideally limiting the damage that a pathogen can do or even killing it outright. Because this behavioral fever is so dependent on temperature, it is important to know how climate change may impact emerging infectious disease.
Today’s authors used toads and the deadly Ranavirus to test if these ectothermic hosts adjust their temperature after infection, and if that change in temperature reduced the viral infection.
What They Did
The researchers used both adult toads and larval tadpoles in their tests. For both groups, they first measured the baseline temperature preference. This allowed them to know what temperature the toads normally preferred. The researchers then split the groups into two smaller groups, infecting half with ranavirus and the other half with a sham-infection as a control.
After infection, the adult toads and tadpoles were monitored to see what temperature they preferred. In addition, after monitoring the temperature preference of individual toads the authors measured the viral load of infected toads.
Did You Know: Fever
The behavioral fever in this study is something that ectothermic organisms engage in, but what about endothermic organisms (basically, species that control their own body heat from within, like us)? Turns out that getting a fever is one way that our bodies are able to fight off an infection, by increasing our temperatures (much like the toads in this study) we not only make it harder for the infecting bacteria or viruses to survive and reproduce, but it can also even help own immune system fight more efficiently.
What They Found
Infected toads, both adults and tadpoles, exhibited behavioral fever by moving to warmer areas to help fight the virus. The increase in temperature was also associated with a decrease in viral load in the adults (not tadpoles), such that toads changing their preferred temperature (behavioral fever) were able to reduce viral loads.
The researchers infected the smaller tadpoles with the same amount of virus that they gave to the larger adult toads, which could have affected the responses of the two groups as the tadpoles were getting more virus per unit body mass compared to the adults. Additionally, the tadpoles cleared their infection before the researchers could sample their viral loads, thus it is hard to draw conclusions about the generality of these results in this system, as each life stage may perform differently.
Today’s researchers showed that not only do toads engage in behavioral fever, but that this fever helps to fight infection (in adults). This is an important proof-of-concept study that provides evidence for a widely-known theory, and it opens the door for further research into the exact mechanisms of how this fever helps to fight infection. Looking at these results through the lens of climate change, it will be interesting to see how these host-parasite relationships change when ectothermic hosts like these toads are forced to not only tackle an infection, but to also balance that against the rising temperatures they are experiencing.