Cat and mouse chase by Jeroen Moes (CC BY-SA 2.0)

Parasites play a game of cat and mouse

Wild mice produce proteins that allow them to survive infection by a deadly cat parasite.

The parasite Toxoplasma gondii is one of the most common parasites worldwide and is known for its unusual life cycle. It reproduces sexually inside its primary host — the cat — and produces eggs that are released in faeces. Other animals, most often rodents, can then become infected when they unknowingly eat the eggs while foraging. Once inside its new host, the parasite reproduces asexually until the rodent’s immune system begins to fight back. It then becomes semi-dormant and forms cysts within the brain and muscle cells of its host. In an added twist, the parasite also causes rodents to lose their fear of cats. This increases their chances of being caught and eaten, thereby helping the parasite to return to its primary host and complete its life cycle.

Previous work has shown that virulent strains of T. gondii can evade the host immune system in mice by secreting enzymes that inactivate immune-related proteins called IRG proteins. This prevents the infection being cleared and leads to death of the host within a few days. The existence of these virulent strains is intriguing because parasites that kill their host, and thus prevent their own reproduction, should be eliminated from the population. The fact that they are fairly common suggests that there must be a hitherto unknown mechanism that allows rodents to survive these virulent strains.

Jingtao Lilue and colleagues now report the existence of such a mechanism in strains of mice found in the wild. In contrast to laboratory mice, wild mice produce IRG proteins that inhibit the enzymes secreted by the virulent strains of T. gondii. Moreover, the IRG genes in wild mice are highly variable, whereas laboratory mice all have virtually identical IRG genes.

By uncovering the complexity and variability of IRG genes in wild mice — complexity that has been lost from laboratory strains — Lilue and colleagues solve the conundrum of how highly virulent T. gondii strains can persist in the mouse population, and offer an explanation for the evolution of parasitic strains with differing levels of virulence.

To find out more

Read the eLife research paper on which this eLife digest is based: Reciprocal virulence and resistance polymorphism in the relationship between Toxoplasma gondii and the house mouse(October 29, 2013).
Read a commentary on this research paper: “Co-evolution: Immune to defeat”
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