"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

August 21, 2014

Sarcocystis cernae

This is the fifth post in a series of blog posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2014. This particular post was written by Reece Dalais that he had titled "A fuzzy shuttle bus to a feathery airport" about what the parasite Sarcocystis does to its vole host (you can read the previous post about a midge that sucks blood from the belly of mosquitoes here).
Photo from here

Many protozoan parasites make use of one or more hosts before finally infecting the host species with suitable real estate for sexual reproduction (e.g. Sarcocystis dispersa and S. putorii). These ‘intermediate’ hosts act as temporary living quarters, in which the parasite accumulates resources, multiplies and then prepares for the trip to the next neighbourhood. In the Netherlands, the protozoan parasite Sarcocystis cernae, uses its intermediate host, the common vole (Microtus arvalis), to multiply itself and then as a vehicle to its honeymoon suite – the small intestine of the common kestrel (Falco tinnunculus). In the lining of the kestrel’s intestine, S. cernae lays its sporocysts, (which are equivalent to eggs) which leave the intestine with the stool of the bird.

Voles forage daily at regular intervals before scurrying back underground. During this time, they can accidentally consume kestrel faeces as they eat vegetation. Once inside the common vole, S. cernae develop in the rodent’s liver before entering its bloodstream and then declaring war on its muscles. In the vole’s musculature the parasite sits tight, and multiplies (asexually) to form large cysts – known as statocysts – which contain numerous bodies capable of sexual reproduction – or cystozoites. These cystozoites break free to reproduce (sexually) once the vole is torn apart and ingested by an adult kestrel or its young – which become the future protozoan distributors. In the mid to late 1980s, it was been discovered by a pair of scientists (Hoogenboom and Dijkstra) that infection with S. cernae makes the vole twice as likely to be taken in aerial attacks. The reason for this is still under question, and has oddly been ignored by researchers since 1987. Could it be due to some form of host manipulation whereby S. cernae forces a change in the behaviour in the vole? Or is it merely a helpful side effect caused by the protozoan running amuck inside the vole’s muscles?
Photo by Małgorzata Miłaszewska

The researchers collected vole samples by snap trapping and from nest boxes during the breeding season. Voles brought to the kestrel nestboxes for their young were taken and replaced them with lab mice of a similar weight – so feeding could continue as usual. Once these voles were dissected, the results revealed that 92% of infected voles had cysts present in the locomotory muscles (the biceps, triceps and quadriceps) – the muscles responsible for movement. Hence it is likely that infected voles were slower to escape the kestrels than their Sarcocystis-free pals. However, it was also proposed that once a vole becomes infected with S. cernae they may be forced to find food at dangerous times. Without infection, voles forage at the same time as other voles and, as a group, are more aware of predators. So if these inbuilt rhythms were to be interrupted by a parasite, the vole would become an easier target. This would be an example of host manipulation, as S. cernae, would be forcing the vole to change its foraging behaviour.

Although the effect of S. cernae on the common vole is not completely understood, it is without doubt that the cunning protozoan helps to drive its furry rodent host towards a feathery final destination.

Hoogenboom, I., Dijkstra, C. (1987) Sarcocystis cernae: A parasite increasing the risk of predation of its intermediate host, Microtis arvalis. Oecologia 74: 86-92

This post was written by Reece Dalais



  2. When I first saw this post, it caught my eye because of the intermediate host – the vole – seemed like a very odd choice, due to their time underground. What I was even more interested to learn was that the parasites definitive host is the kestrel. Previously, I thought that parasites could not infect birds at all. One very interesting thing I learned was that the parasite makes the voles 2x more likely to be attacked by the kestrels, which makes me think that there is definitely some type of host manipulation at work (behavioural change). I just find it so interesting how such a small parasite has evolved to be so efficient in infecting these two hosts!