Covid-19 as well as Ebola and HIV all originated in animals. The current pandemic, in particular, has caused us to refocus our attention to the infections of wild animals. Professor Mark Viney from the Department of Evolution, Ecology and Behaviour at the University of Liverpool has shared his research on the ‘zoo of infections’ living in our pest rodents and what it means for vector-borne diseases.
Covid-19, Ebola and HIV originated in animals.
The Covid-19 pandemic has rightly focused our attention back to the infections caused by wildlife. Professor Mark Viney from the Department of Evolution, Ecology and Behaviour at the University of Liverpool published his research on the so-called ‘zoo of infections’ living in pest rodents and what it could mean for vector control diseases. All around us, there are pest species that carry parasites and pathogens. These pests are in a constant battle against infection.
In its lifetime, an animal is typically exposed to a myriad of infections – bacteria, viruses, fungi, protozoa, worms and arthropods (ticks, fleas and lice). These infections are present both inside the animal (the gut, mouth, nose and genital tract) and on its skin. Infections that affect humans and livestock are well researched, for obvious reasons, but we know much less about the infections of wild animals.
Animals fight against these infections with their immune systems. This system plays a vital role, as can be seen with severely immune-deficient people who have to live in sterile bubbles to avoid life-threatening infections. Animal immune systems protect and control the zoo of infecting organisms; however, we know very little about the immune responses of wild animals.
Professor Mark Viney recently conducted a study into the immune responses of wild mice, which has been a long-overlooked field of study. His work revealed some surprising information about the lives of these wild animals.
Immune responses in wild mice
Viney conducted this study on wild house mice (Mus musculus domesticus) - the same species as the laboratory mice used - which allowed for the comparison of the immune responses in the wild and the lab. The findings were remarkable. Wild mice have short lives, with an average age of about seven weeks, so they have a lot to achieve and endure in a very short time.
An examination of antibodies and immune cells allowed Viney to investigate the immune responses of the mice. Wild mice have many more antibodies in their circulation than their lab counterparts. Antibodies are made in response to an infection, so these high levels of antibodies are a sign of the very high levels of infections in wild mice. Laboratory mice are deliberately protected from infections, and so have much lower antibody levels.
Another area of study was immune cells, specifically ‘T cells’ that act directly against infections. Wild mice had many active T cells, many more than laboratory mice, which is another sign of a higher infection load in wild animals. Together the amount of antibodies and T cells in wild mice tell us that they have lots of infections against which they create immune responses.
While antibodies and T cells attack infections, an excessive immune response can be harmful to the individual; this is called immunopathology. The immune system uses molecules (called cytokines) to remain in homeostasis by regulating - turning-up or turning-down - immune responses. The study showed that wild mice have lower concentrations of cytokines than laboratory mice.
This finding may seem surprising; after all, shouldn’t wild mice with many infections and high immune responses also have more cytokines? The answer is no. What the experts say on this matter is that because wild mice have so many infections they have to decrease their immune response, achieving a balance whereby they can deal with the infection without harming themselves; a low level of cytokines is an indication of their immune response reducing.
While wild mice carefully balance making their immune response strong enough to fight against infections without causing themselves harm in the process, laboratory mice have essentially no infections and therefore don’t have to turn-down their immune responses.
Wild and laboratory mice have different immune responses because of the very different environments in which they live, resulting in contrasting levels of infections. While lab mice have taught us a lot about the basic functioning of immune systems, they probably aren’t such good models for understanding the immune systems of wild animals.
Wild animal infections
Wild animals are an increasingly significant source of human infection. Covid-19 came from bats, as did Ebola, and HIV came from primates; all of which establish the importance to better understand wild animal infections. The infections of wild animals also depend on their immune responses, which is why it is vital to study these responses.
Experts are now studying the infections of wild urban rodents, using the Covid-19 pandemic as a natural experiment. Covid-19 is a coronavirus, a group of viruses of which many infect rodents. As a matter of fact, the wild mice whose immune responses were studied were infected with certain coronaviruses.
The Covid-19 pandemic has drawn attention towards animal viruses that spread to people, but we are yet to ask if the opposite process happens, whereby humans transmit Covid-19 to wild animals. Scientists say that with many people infected with Covid-19, many virus particles are being produced in aerosols and in faeces, which might then infect rodents. Cities that have dense populations of people and of pests might provide the perfect conditions for a virus to move from people to rodents. To be clear, this is just a hypothesis, and there is currently no evidence that this is the case.
The testing of urban rodents for Covid-19 and other coronaviruses is now taking place, looking for these viruses in lung and gut tissue as well as in blood where tests for antibodies are being conducted. This work is one of only a few studies that have looked for these viruses in rodents, which is perhaps surprising given how commonly large populations of rodents are found in cities.
Viruses alter and adapt all the time. One way that they do this is by recombining, which is when two different viruses merge their genes to make a new virus. This process might have happened when the animal virus moved into people. In urban pest rodents, studies are being conducted to see if different coronaviruses are recombining with each other.
Wild animals are present in our environments; they live in our towns and cities and come into contact with us. It is fascinating to think of the daily battle they are fighting against a whole series of infections that constantly attack them. These infections and how they respond to them shape the ecology and biology of these species. It is becoming ever clearer that our own lives are intimately connected with those of wild animals, which emphasises the importance of studying their infections and immune responses.
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BPCA (2020) Infection and immunity in wild rodents - the zoo of infections. [online] BPCA. Available at: https://bpca.org.uk/News-and-Blog/infection-and-immunity-in-wild-rodents-rats-mice-and-covid-19/265329
Sky News (2020) Coronavirus: Reports of rat and mouse infestations rocket during lockdown. [online] Sky News. Available at: https://news.sky.com/story/coronavirus-reports-of-rat-and-mouse-infestations-rocket-during-lockdown-12028983