The susceptibility of muskrats and snowshoe hares to experimental infection with a chlamydial agent.

Muskrats (Ondatra zibethicus) and snowshoe hares (Lepus americanus) were exposed experimentally by various routes to a chlamydial agent (designated strain M56) originally isolated during a die-off of muskrats and snowshoe hares which occurred in Saskatchewan during 1961. Both species were susceptible to experimental infection. Whereas M56 was highly lethal for snowshoe hares (18 deaths/19 exposed), it was less virulent for muskrats (6 deaths/20 exposed). The degree of susceptibility of muskrats to induced infections with M56 was influenced by the presence or absence of specific antibodies at the time of exposure. A febrile illness was observed in 11 of 20 muskrats. In the six that died, widespread focal necrosis was found in the liver. Following intraperitoneal or oral exposures, chronic infections were established and the agent was recovered from the brain and the small intestine up to 96 days post-infection. Specific antibodies were found in 11.8% of 127 sera of muskrats trapped form the wild in Saskatchewan, the Canadian Arctic, and Wisconsin. In snowshoe hares, M56 induced an acute, febrile, emaciating illness, and the almost invariable fatal course was short with terminal signs of opisthotonos, convulsions, and hypoglycemia. Snowshoe hares succumbed with intravenous doses of less than ten mouse intracerebral LD(50) of M56. The same syndrome was produced by intravenous, subcutaneous, and oral infections. M56 was found in high titers in all tissues examined. The highest titers were found in the liver and spleen which correlated with the pathology observed. M56 was recovered from female rabbit ticks (Haemaphysalis leporispalustris) engorging on experimentally infected snowshoe hares.