Hospitalization for trichinellosis and echinococcosis in Canada, 2001-2005: the tip of the iceberg?

OBJECTIVES: This study was undertaken to measure the incidence of echinococcosis and trichinellosis hospitalization in Canada, and to compare these incidence rates between residents of northern regions and the rest of the Canadian population. METHODS: Cases hospitalized in 2001-2005 for either echinococcosis or trichinellosis were retrieved from the hospital morbidity database (HMDB) held by the Canadian Institute for Health Information. Crude and standardized incidence rates were calculated by province and by latitude range. RESULTS: A total of 108 echinococcosis and 14 trichinellosis hospitalizations were found, yielding incidence rates of 0.72 and 0.09 per million per year, respectively. There was a clear south-north gradient in the incidence of echinococcosis hospitalization, the highest incidence (2.9 per million per year) being found north of the 55th parallel. The risk of echinococcosis hospitalization was also significantly higher in women than in men (RR 1.92, 95% CI 1.29-2.87). For trichinellosis, the highest incidence (42 per million per year) was found in Nunavut and Northern Quebec. CONCLUSION: Incidence of hospitalization for echinococcosis and trichinellosis is low at the national level. However, significantly higher rates have been measured in northern regions of Canada despite the fact that both diseases are theoretically preventable and that a Trichinella control program is in place in Nunavik. Further efforts, probably educational in nature, will be required to reduce the incidence of these infections in high-risk areas.

Patterns of infection by lungworms, Rhabdias ranae and Haematoloechus spp., in northern leopard frogs: a relationship between sex and parasitism.

We examined a population of northern leopard frogs to determine whether sex biases in investment in immunity, previously reported for this host species under controlled exposures to lung nematodes, is predictive of patterns of parasitism in nature. We examined Rhabdias ranae and Haematoloechus spp. infections in 74 breeding adult, 28 non-breeding adult, and 53 juvenile frogs. Contrary to our predictions, R. ranae prevalence and mean abundance were higher in breeding female frogs (prevalence: 39.4%, abundance: 3.05 +/- 0.85) than on breeding males (prevalence: 26.0%, abundance: 1.17 +/- 0.52), although no sex bias was observed among non-breeding adults or juvenile frogs. Female frogs also carried larger R. ranae worms, on average, than did males (females: 6407.38 microm +/- 153.80; males: 5198 microm +/- 131.09), regardless of age or breeding condition. We observed no sex-linked patterns of parasitism by Haematoloechus spp. worms in either adult or juvenile frogs. Alternative hypotheses, such as differences among sexes in the selection of thermal clines for hibernation, may explain the observed female bias in parasitism by nematode lungworms in nature and, thus, need to be considered.

Nematode lungworms of two species of anuran amphibians: evidence for co-adaptation.

Genetic studies have indicated that some parasite species formerly thought to be generalists are complexes of morphologically similar species, each appearing to specialize on different host species. Studies on such species are needed to obtain ecological and parasitological data to address whether there are fitness costs in parasitizing atypical host species. We examined whether lungworms from two anuran host species, Lithobates sylvaticus and Lithobates pipiens, differed in measures of infection success in L. pipiens recipient hosts. We also determined if the worms from the two host species were sources of genetically resolvable species of morphologically similar nematodes. Sequences of internal transcribed spacer and lsrDNA regions of adult lungworms from each host species indicated that worms from L. sylvaticus matched Rhabdias bakeri, whereas worms from L. pipiens matched Rhabdias ranae. Our work suggested that these morphologically similar species are distant non-sibling taxa. We infected male and female metamorphs experimentally with lungworm larvae of the two species. We observed higher penetration, higher prevalence and higher mean abundance of adult worms in lungs of male and female metamorphs exposed to R. ranae larvae than in lungs of metamorphs exposed to R. bakeri larvae. Furthermore, metamorphs exposed to R. ranae larvae carried larger adult female worms in their lungs. Some variation in infection measures depended on host sex, but only for one parasite species considered. Overall, the differential establishment and reproductive potential of R. ranae and R. bakeri in L. pipiens suggests co-adaptation.

Rearing density and susceptibility of Rana pipiens metamorphs to cercariae of a digenetic trematode.

Cercariae of many digenean trematodes target particular developmental stages of their hosts. For some digeneans that are parasites of amphibians, infection appears timed to host metamorphosis. The success and timing of metamorphosis is itself affected by a number of factors, including host density. We investigated the degree to which rearing density of Rana pipiens larvae influenced time to metamorphosis and snout-to-vent length and mass at metamorphosis, as well as establishment of cercariae of the trematode Manodistomum syntomentera Stafford, 1905. As expected, individuals metamorphosed later, were smaller, and weighed less at metamorphosis if they were reared under intermediate to high densities compared with low densities, in experimental outdoor mesocosms. Cercariae establishment was higher in smaller metamorphs that took longer to metamorphose within the low-density treatment. Additionally, cercariae establishment was lower in larvae from the low-density tanks compared with larvae from the intermediate- to high-density tanks. However, more tadpoles had failed to metamorphose in the intermediate to high rearing densities by the time cercariae were no longer available from natural collections of first intermediate hosts, Physa spp. Larval amphibians under crowded conditions should experience increased susceptibility to trematode establishment in nature, but only if they metamorphose within the time period when cercariae are still available.