Transmission and Tissue Tropism

Little is known about the routes of transmission of cowpox in endemic rodent populations. Voles and mice can be infected experimentally by both skin and oronasal inoculation, and arthropod vectors may play a role in mechanical transmission in wild colonies. Inoculation by either route can cause viremia and systemic infection, with virus detected particularly at the sites of inoculation, in lymphoid tissue and in lung. Rodent infections may persist for several weeks, although the role of possible persistent infection in the epidemiology of cowpox is unknown.

Among the occasional, or 'accidental', hosts of cowpox virus, the most frequent route of infection appears to be through the skin, probably through a cut or abrasion. Domestic cats, however, can be infected experimentally by oronasal inoculation, and respiratory spread may have been involved in some outbreaks of cowpox in big cat collections. Virus replication in cattle and humans is mainly limited to the epidermis at the site of entry, and possibly also to draining lymph nodes. In cats, virus can be isolated not only from skin lesions but also from lymphoid, lung and turbinate tissue. Skin inoculation of cats is followed by virus replication both at the site of entry and in draining lymph nodes, there is development of a viremia, and virus can be isolated from the white cell fraction of blood, from the spleen and other lymphoid organs. After about 7-10 days, virus can be

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