The parasite that triggered this behavior was Pleistophora mulleri and the effect was seen with the shrimp Gammarus duebeni celticus. These shrimp are common to British waterways. These shrimp are actually in decline, facing a challenge from a different type of shrimp from mainland Europe, which is classed as an invasive species.
In general, shrimp are widespread around the world, and they can be found near the seafloor of most coasts and estuaries, as well as in rivers and lakes. There are various species, and often there is a species adapted to any particular habitat
Cannibalism with some species is not unusual, and with shrimps, adults often eat juveniles. However, for shrimps infected with the parasite these shrimps ate twice as much of their own kind compared with shrimps that were uninfected. Furthermore, infected shrimps attacked other shrimps more often and consumed greater quantities of food.
Researchers speculate that the reason for this change in behavior is because the parasites, which resemble the size of blood cells, occupy the muscles of the shrimp. The parasites rely on the shrimps for their food source and they trigger the shrimp into eating sufficient quantities to satisfy the parasites.
To add to this, the parasite debilitates the shrimp, making it weaker and drawing nutrients from its body. This serves to make the shrimp hungrier and it needs to keep on eating in order to survive.
When the shrimp dies (or when it is eaten by another shrimp) the parasites transfer to a new host. This ensures that the cycle continues.
While the effect is seen in shrimps, there is no known parasite that causes humans to engage in cannibalistic practices.
The research has been published in the journal Royal Society Open Science. The research paper is called “Eaten alive: cannibalism enhanced by parasites.”