For such a tiny worm, the schistosome parasite causes a great deal of suffering. Infecting more than 200 million people worldwide and causing liver damage, kidney failure, infertility and cancer, the parasite could be responsible for up to 200,000 deaths annually. ‘Some estimates suggest that human schistosomiasis imposes a greater global disease burden than either malaria or tuberculosis’, says Susanne Sokolow from Stanford University. Snails carry the parasite and release its larvae into fresh water; the larvae then burrow into the skin of humans contacting the water before migrating through the victim's body and transforming into adult worms, ready to release eggs back into the water to infect the next snail generation. And other factors in the environment could alter schistosome infection rates. Sokolow explains that the exotic Louisiana crayfish may have reduced schistosome infection rates in Egypt by consuming the parasite's intermediate snail hosts. Knowing this, Sokolow wondered whether the parasite could contribute to its own demise. Could the infection alter the snail's behaviour, in a bid to improve the chances of schistosome transmission while inadvertently placing the snail at greater risk?
Teaming up with Scott Schwarz, Giulio De Leo and Chelsea Wood, Sokolow collected two species of snail that are known to transmit schistosome infections from the Schistosomiasis Resource Center in the US and obtained a colony of predatory Macrobrachium vollenhovenii prawns, originally from Cameroon, to find out how infected and parasite-free snails reacted to the threatening crustaceans. First, the team tested the prawns’ preferences by isolating individuals with two infected and eight uninfected snails, and found that the prawns were twice as likely to devour the infected snails as the parasite-free animals.
Then the team tested whether the parasite impacted the snails' mobility and found that the infected snails were less mobile and moved much more slowly (18.9 mm s−1) than the uninfected snails (25.2 mm s−1). Finally, the team tested the snails’ reactions to a threat by placing them in an enclosure with a caged prawn and some crushed snails. After recording how much evasive action the snails took, the team could see that the uninfected snails were much more cautious than the infected animals – heading for high ground out of the water, avoiding open water and seeking shelter where possible – while the infected snails left themselves much more vulnerable to attack in open water.
Sokolow suggests that the parasite is either deliberately manipulating the snails’ behaviour in some way – to improve its chances of being passed on to its human hosts – or the parasite debilitates the snail – restricting the snails’ mobility – despite the increased risk of predation. She also suspects that the infected snails may be more reluctant to evade predation by climbing out of the water in order to improve their chances of releasing infectious schistosome larvae ready to infect the first bather they encounter.
Either way, schistosome-infected snails suffer a double whammy: not only are they less mobile than uninfected animals, but predatory prawns also find them more appealing. And the aggressive crustaceans could play a significant role in breaking the insidious Schistosoma infection cycle by controlling infected snail populations.