This party doesn't start until the hosts arrive

Disease causing organisms can be present in some areas where their hosts are not. If their hosts arrive, novel disease outbreaks may result. In the first comprehensive genetic analysis of an invasive marine host and its parasites, researchers trace invasion pathways of snails and trematodes from Japan to North America.

Their results, published in the Proceedings of the National Academy of Sciences, have broad implications for identifying and mitigating spreading disease in a global economy. Simultaneously understanding the invasion pathways of disease-causing organisms and their hosts will be key in limiting future disease outbreaks—in humans, agriculture and wildlife.

Invasive populations of Asian mud snails, Batillaria attramentaria, probably arrived in North America with Pacific oysters, Crassostrea gigas, imported from northern Japan in the early 1900's. Genetic research by Osamu Miura, Tohoku University, and colleagues from the Smithsonian Institution and UC Santa Barbara confirmed this. “We saw a lot of genetic variation among snail populations in Japan but the North American snails are genetically most similar to those from northern Japan, the source of the imported oysters,” Miura reports.

Of the eight species of trematode parasites that plague the snails in Japan, only the most common, Cercaria batillariae, has arrived in America. Luckily for the researchers, gene sequencing showed that this single species actually consisted of several genetically distinct cryptic species in its home range in Japan. In North America, they commonly found two of the cryptic species. One parasite shows much less genetic diversity in America than in Japan, whereas the other parasite is equally diverse in both regions.

“Genetic evidence suggests that while one cryptic parasite species experienced a bottleneck and probably came with the snails, the other was probably historically dispersed by migratory birds and could only establish in North America after the snail hosts arrived,” adds Mark Torchin, of the Smithsonian Tropical Research Institute.

“This is because these trematode parasites have complex life cycles, using snails as first intermediate hosts, fishes as second intermediate hosts and birds as final hosts. As we homogenize biotas as a result of repeated species invasions through global trade, we increase the chances of reuniting infectious agents with suitable hosts,” says Torchin. Parasites which may have historically gone unnoticed as tourists in some regions may become pervasive residents after invasion of their missing hosts.