A white-footed deermouse. Image credit: John White
Lyme disease is an illness caused by bacteria that spread from infected animals to humans through tick bites. While most people fully recover after a week or two of antibiotic treatments, some will continue to experience debilitating symptoms due, potentially, to the way their immune system responded to the infection.
In North America, the white-footed deermouse is one of the most common hosts of the Lyme disease bacteria. Despite its name, this rodent is more closely related to hamsters than to the mice or rats most often used in laboratory studies. Unlike mice and humans, however, deermice carrying Lyme disease bacteria do not get sick; in fact, most deermice living in a Lyme disease region will acquire the infection during their lifetimes, but it has little apparent effect on population numbers. These animals can also better tolerate infection from other microbes.
To investigate why this is the case, Milovic et al. exposed mice, rats and deermice to a bacterial toxin that triggers inflammation common to encounters with many kinds of microbes. While all species exhibited physical symptoms as a result, blood samples revealed that mice and rats, but not deermice, reacted as if they were infected with viruses as well as bacteria. This was particularly the case for interferons, a group of hormone-like proteins that protect against viruses but can also lead to harmful long-term inflammatory effects. The deermice controlled their interferon responses to the bacterial substance in a way that mice and rats could not.
Milovic et al. also checked which genes each species switched on after exposure to the toxin. This revealed that, unlike deer mice, rats and mice turned on some DNA sequences called endogenous retroviruses, which have no role in fighting infection from bacteria but can lead to harmful persistent inflammation.
These results provide elements to better understand why recovery from Lyme disease may differ between people, with some patients retaining symptoms long after their infection has abated. They could also help to better grasp why other diseases, such as COVID-19, can be followed by fatigue and other symptoms of ongoing inflammation.
