The role of cottontail rabbits (Sylvilagus spp.) in the ecology of Rickettsia rickettsii in the United States.

Rocky Mountain (Sylvilagus nuttallii) and eastern (S. floridanus) cottontails were examined for their susceptibility to virulent and avirulent strains of the spotted fever agent, Rickettsia rickettsii. Both species of rabbits responded to inoculation of yolk sac suspensions containing 500 egg LD50 of either virulent or avirulent rickettsiae, with rickettsemias detectable as early as 3 days after inoculation and lasting up to 7 days. When fed upon by infected ticks, only one of three Rocky Mountain and one of four eastern cottontails developed rickettsemias detectable in embryonated hens' eggs. Rickettsial concentrations in the peripheral blood fo Rocky Mountain cottontails, infected either by syringe or by bites of one or more Dermacentor andersoni ticks harboring virulent R. rickettsii, were sufficient to infect simultaneously feeding normal larvae of this tick species. However, infection rates wre low and did not exceed 11.7%. In a single experiment, the bites of D. variabilis, infected with an avirulent strain of R. rickettsii, did not produce rickettsemias sufficient to infect normal D. variabilis larvae. These laboratory findings suggest that cottontail rabbits, although susceptible to R. rickettsii, do not serve as efficient reservoirs for infecting ticks.

Occurrence of pox in a northern fur seal on the Pribilof Islands in 1951.

The presence of pox in a northern fur seal pup, Callorhinus ursinus, examined at necropsy on the Pribilof Islands, Alaska, 13 September 1951 was confirmed by re-examination of formalin-fixed tissues collected at that time. The disease was characterized by multiple 3 to 8 mm modules in the skin of the nose and flippers. Unlike pox lesions in other pinnipeds studied so far, the nodules were discrete dermal epitheliomas free of any direct connections with the overlying atrophic epidermis. They most likely arose from outer sheath cells of hair follicles. The tightly packed epithelial cells forming the nodules contained large, round, eosinophilic, intracytoplasmic inclusion bodies. As demonstrated by electron microscopy nearly 27 years after the tissues were fixed in formalin, poxvirus virions also were present in these cells. These findings indicate pox was present in a northern fur seal, hitherto undescribed in that species, 18 years before the disease was first reported in pinnipeds.

Yersinia philomiragia sp. n., a new member of the Pasteurella group of bacteria, naturally pathogenic for the muskrat (Ondatra zibethica).

A bacterium experimentally pathogenic for muskrats (Ondatra zibethica), white mice, mountain voles (Microtus montanus), and deer mice (Peromyscus maniculatus) was isolated from the tissues of a sick muskrat captured on the Bear River Migratory Bird Refuge (Brigham City, Utah) and from four surface water samples collected within 15 miles of that point. In culture, the cells are chiefly coccoid, but in the tissues of muskrats and voles they resemble the bizarre forms of Yersinia pestis, except for their smaller size. The characteristics of the organism are described and the name Yersinia philomiragia sp. n. is proposed.