Parasite interactions increase risk of infection
2 November 2010, by Adele Walker
The way that viruses and bacteria interact in host animals can have a significant influence on the risk of infection, according to new research.
Field vole (Microtus agrestis).
After studying parasites in natural vole populations scientists found a web of interactions that could both increase and decrease the likelihood of a host contracting a disease.
'Even parasites that induce few clinical signs of disease in a host have a significant impact on an individual's susceptibility to other parasites,' says Dr Sandra Telfer of the University of Aberdeen, lead author of the research published in Science.
The results have important consequences for predicting and managing disease in animals, and could help further our understanding of infection in people.
It's likely that most animal and human hosts are infected with more than one parasite species most of the time.
And while single infections by worms and insects (macroparasites) and viruses and bacteria (microparasites) have been relatively well studied, until now studies of how these different parasites interact have been limited.
So a team of researchers, from the University of Liverpool and the University of Aberdeen, and also involving scientists now based at the Centre for Ecology & Hydrology and the Universidad Nacional del Litoral in Argentina, studied microparasites in four populations of wild field voles (Microtus agrestis) over five years.
Their aim was to see whether a vole was at greater or lesser risk of infection by one microparasite if it was already infected by others.
The voles were ideal subjects as they are not only abundant but have short life spans. The length of the study meant the scientists could see when the voles became infected and identify patterns of interaction between different parasites that would not be evident if they had just looked at the populations at one point in time.
The researchers periodically captured and examined 5981 individual voles to monitor their infection with four microparasites: cowpox virus (transmitted by direct contact); Bartonella spp. (transmitted by fleas), Babesia microti and Anaplasma phagocytophilum (both carried by ticks).
They used statistical models of infection risk to account for variables like the time of year that fleas and ticks are around, and the sex and health of the vole. As well as these effects, they found considerable interactions between all the parasites.
'We expected we might pick up a few interactions between different pairs of parasites,' says Telfer, 'but the thing that surprised me most was the widespread nature and strength of these interactions.'
Interestingly the interaction could have negative as well as positive effects. So infection by one parasite might increase the likelihood of infection by some other parasites but reduce the chance of infection by others.
These differences are likely to depend on the interactions between the vole's immune system and the parasites. In some cases, a parasite may weaken the immune system, enabling further infections to take hold (for example, human immunodeficiency virus (HIV) infections are thought to be contributing to tuberculosis epidemics in Africa). In others, an immune response triggered by one parasite may also be effective against a second parasite, allowing the vole to fight off further infection.
The researchers found that both positive and negative effects could be considerable. Infected voles could be up to 5.5 times more likely to contract a further infection; or to have 15 per cent less risk of becoming infected, depending on the nature of the interaction.
'Interactions between parasite species had similar effects on infection risk as the time of year – which affects risk of exposure,' explains Telfer.
These findings highlight the importance of considering parasite communities, rather different species in isolation, when lookiing at infection.
This study focused on the risk of infection; the next step will be to look at its effects. Laboratory research suggests that the effects of parasites in a host may be more intense when they are combined – causing more damage to things like reproductive ability.
And the implications for humans? Studies like this will help to increase awareness of parasite interactions within the medical research community. But, cautions Telfer, 'there is a limit to the research possible in humans – both because of our long lifespan and because medical interventions will affect the interactions.'
Sandra Telfer, Xavier Lambin, Richard Birtles, Pablo Beldomenico, Sarah Burthe, Steve Paterson, Mike Begon. (2010) Species interactions in a parasite community drive infection risk in a wildlife population. Science doi: 10.1126/science.1190333
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