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The pathogenesis of Powassan virus in mice after airborne infection Gaunt, Roderick Allan


Although arboviruses usually infect humans by bites of infected ticks or mosquitoes accidental infection with several agents has been encountered following inhalation of virus-laden aerosols generated accidentally during spillage of material in the laboratory. This project provides a model of accidental airborne infection of man by Powassan virus. The specific aims were: 1. to determine if mice could be infected with Powassan virus, a member of the Russian spring-summer complex of tick-borne group B arboviruses by a means which did not introduce the virus directly into the bloodstream; 2. to provide information on the dose-response relationships using various routes of inoculation; 3. to determine the pathogenesis of Powassan virus in mice after airborne infection. Intranasal instillation of Powassan virus suspensions into anaesthetized mice induced a fatal infection by a route which did not introduce virus directly into the bloodstream. Although mice developed fatal encephalitis following inoculation of Powassan virus by several routes, the smallest quantity of virus which induced an overt infection was injected either intracerebrally or intravenously. Fatal infections followed the subcutaneous injection of 20 times the minimum amount of virus. Mice could not be infected by the instillation of more than one million intracerebral doses of virus directly into the gastrointestinal tract. Infection by inhalation of aerosols required about 600 times the minimum virus dose, and infection was initiated by the intranasal instillation of 10,000 or more minimum intracerebral doses of virus. The infectivity of Powassan virus suspensions and aerosols decayed rapidly under ultraviolet irradiation. Aerosolized virus was slightly more stable under conditions of low relative humidity in contrast to high and intermediate relative humidities, when aerosols were aged up to five hours in the dark at 21°C. After aerosol infection of mice with Powassan virus the following sequence of events occurred. Powassan virus invaded and multiplied within the epithelial cells and/or macrophages of the mouse lung. Subsequently, virus appeared in the blood. When the virus titre within the blood attained approximately 3 log₁₀ mouse intracerebral LD₅₀ per ml. at two days post exposure it appeared in other tissues including the brain where it multiplied to titres as high as 9 log₁₀ mouse intracerebral LD₅₀ per gram by the seventh day. Mice died with encephalitis seven to eight days after aerosol inhalation. Using fluorescent antibody techniques, it was determined that virus present in the blood of the mouse invaded the cuboidal epithelium of the choroid plexus and later spread either from this tissue to the cerebrospinal fluid or directly by way of the vascular endothelium to the tissues of the brain. Electron micrographs of nasal epithelium infected with Powassan virus presented very preliminary information on the possible morphogenesis of the virus within this tissue. Cytoplasmic vacuoles were observed which could serve as the site for synthesis of virus particles.

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