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Fourteen albino rabbits were inoculated intravenously with 10(3.5)-10(4.0) mouse ICLD(50) of Chlamydia psittaci (strain M56) of mammalian origin. Ocular lesions accompanied the chlamydial infection in the rabbits. Bilateral anterior uveitis, a common occurrence, began on the second or third day and subsided by the tenth day whereas keratoconjunctivitis was observed infrequently. After 15 days the most prominent microscopic lesion was iritis. Accumulations of inflammatory cells, mainly plasma cells, were observed in the iris and ciliary body and elementary bodies were found infrequently in macrophages. Chlamydiae were recovered consistently by conjunctival swabbing from the fifth to the twenty-fourth day. The agent was present within the eye (viz. iris-ciliary body) in three of four rabbits killed at 15 days and in five of ten rabbits killed 60 days after inoculation. Chlamydiae had persisted in the cerebrum and joints as well. Although neutralizing antibody was consistently present in sera at 60 days none of the samples of aqueous humor were capable of neutralizing the agent. It is suggested that systemic chlamydial infections in the rabbit provide a model for the study of endogenous uveitis, a common ophthalmological problem.

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.