You may already have read, in one of the many magazines, news services and blogs that have picked up this story (see here, for example), about some findings presented the other week at the annual meeting of the American Society for Microbiology, and more recently at the annual meeting of the Pacific division of the American Association for the Advancement of Science. It is suggested that the parasite Toxoplasma gondii is spread to marine mammals by anchovies. It appears that the research has not yet been formally published in a journal, but searching the CAB Abstracts database and the Internet revealed some more information about the background to the study.
The Californian population of sea otters (Enhydra lutris) recovered only slowly (compared to populations elsewhere) during the 20th century after protection from the fur trade1, and high mortality rates still appear to be restricting the population2. A study3 published in 2002 found a high prevalence of antibodies to Toxoplasma in Californian sea otters, especially those living near areas with high freshwater runoff; few Alaskan otters had such antibodies4. Several other studies also suggested that Toxoplasma infection might be affecting Californian sea otter numbers, although it was not the only infectious cause of death among otters found dead on beaches5. Interestingly, given the possible effects of latent toxoplasmosis on host behaviour (see my blog entry from last August), otters with fatal shark bites were much more likely than others to have Toxoplasma encephalitis, although this is thought to be because of aberrant behaviour attracting sharks to diseased otters, rather than latent infection making otters less able to escape.
As for how the parasite reaches sea otters, the suggestion is that it is a result of faeces from domestic or feral cats (the commonest definitive host of T. gondii) being washed into rivers or disposed of via the sewage system6; the oocysts survive treatment plants and are filtered out of the water by the bivalve molluscs that sea otters eat. (For more on the problems facing Californian sea otters, see the Sea Otter Alliance website).
Although many reports of Toxoplasma infection in marine mammals are about sea otters in California, there are a couple of dozen reports from the past 10 years or so, and a few more from earlier years, of Toxoplasma in other species in many parts of the world. It was not clear how animals became infected if they did not live in areas with high freshwater runoff and did not eat filter-feeding shellfish; the recently reported research suggests that infection might be transmitted by anchovies.
The researchers, Gloeta Massie and Michael Black of California Polytechnic State University, studied the Californian anchovy, Engraulis mordax, chosen because it moves between estuaries and the open ocean, is an important food source for many marine mammals, and eats small particles the size of Toxoplasma oocysts. Although this species is confined to the north-eastern Pacific, closely related species are found in many other parts of the world. Anchovies exposed to the oocysts did indeed accumulate them in their guts, and retained them for at least a few hours (further research is needed to see whether they retain them for long enough to be eaten by whales or seals in distant parts of the ocean).
Little previous research seems to have been done on this subject; a study7 from 2005 suggested that T. gondii could infect goldfish (Carassius auratus) but that infection did not persist beyond a few days; the few other articles that I could find about Toxoplasma in fishes did not seem relevant. Massie and Black point out that oocysts are destroyed by cooking, but fish, including herring which belong to the same family as anchovies, are eaten raw in some parts of the world (see also another of my previous blog entries), so their work is relevant to human health too. It will be interesting to see whether more research is done on the subject as a result.
2: Research reported in the Mail Tribune of Medford, Oregon, USA, 15 June 2007.
3: Miller, M. A. et al.: Coastal freshwater runoff is a risk factor for Toxoplasma gondii infection of southern sea otters Enhydra lutris nereis). International Journal for Parasitology (2002) 32 (8) pp. 997-1006. DOI: 10.1016/S0020-7519(02)00069-3.
4: Hanni, K.D. et al.: Clinical pathology and assessment of pathogen exposure in Southern and Alaskan sea otters. Journal of Wildlife Diseases (2003) 39 (4) pp. 837-850.
5: Kreuder, C. et al.: Patterns of mortality in Southern Sea otters (Enhydra lutris nereis) from 1998-2001. Journal of Wildlife Diseases (2003) 39 (4) pp. 495-509.
6: Kelsey, E.: Toxic tide. New Scientist (2002) issue 2358, 31 August. p. 42.
7: Omata, Y. et al.: Toxoplasma gondii does not persist in goldfish Carassius auratus). Journal of Parasitology (2005) 91 (6) pp. 1496-1499. DOI: 10.1645/GE-3503RN.1.