Abstract

Toxoplasma gondii, a coccidian parasite, has been reported in marine mammals for almost 50 years. Initially it was reported from marine mammals either born in captivity, captured from the wild or undergoing rehabilitation and it was believed that somehow they were exposed to ooycts of T. gondii in food or water contaminated by infected cat faeces. Lately, there are increasing reports of T. gondii in histologic sections of tissue from dead stranded marine mammals, or in live stranded marine mammals with clinical toxoplasmosis. Marine mammals reported infected with T. gondii include otariids, phocids, West Indian manatee, sea otter, dolphins and beluga. To date reports of T. gondii in marine mammals are primarily from North America but there are a few reports from Europe. A serological study of over 600 marine mammals (harp, ringed and hooded seals and minke whale) from the northeast Atlantic ocean did not detect any seropositive cases.

The present serological study examined over 300 phocids from the east coast of Canada including harp, hooded, grey and harbour seals. Sera from seals were obtained from hunted seals or live-captured seals. Using a modified agglutination test 327 sera were diluted 1:25, 1:50, 1:500 and tested for antibodies to T. gondii. Titers greater than 1:25 were considered evidence of exposure. Grey seals (11/122 or 9%), harbour seal (3/34 or 9%), and hooded seals (1/59 or 2%) were seropositive with titers of 1:25 and 1:50. Harp seals were seronegative.

How marine mammals are exposed to or acquire infections of T. gondii is presently unknown. It has been suggested that human sewage containing oocysts from cats or natural runoff of oocysts in the terrestrial environment enters marine waters, contaminating marine organisms. The presence of T. gondii in marine mammals may be an example of pathogen pollution whereby human activities contaminate or facilitate the contamination of the marine environment with viruses, bacteria or parasites not endemic to this environment. Non-felid hosts such as seals and whales appear to act as intermediate hosts and it could be that the life cycle of, T. gondii in the marine environment differs from that in the terrestrial environment. Further research on the pathogenesis of T. gondii in marine mammals and its transmission in the marine environment is needed to understand fully the significance of this parasite in marine mammals.