Powassan Virus: Field Investigations during the Summer of 1963

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During summer 1963, examination of sera from 496 forest mammals collected within 30 miles of North Bay, Ontario, Canada, revealed the presence of neutralizing antibody to Powassan virus in 22 of 194 red squirrels, 16 of 29 woodchucks, 13 of 34 porcupines, none of 107 snowshoe hares and 7 of 132 other mammals. Ixodes sp. ticks were removed from 54 squirrels, woodchucks and porcupines, 12 of whose sera neutralized Powassan virus. The prevalence of antibody-positive animals was highest during May and September, and these animals were captured almost exclusively in small forested areas of Nipissing, South Himsworth and Bonfield Townships. No virus was isolated from any ticks removed from 59 animals apart from snowshoe hares, or from 325 animal blood clots, by inoculation of suckling mice. One strain of Silverwater virus was isolated from Haemaphysalis leporis-palustris ticks taken from a snowshoe hare in North Himsworth Township on 5th May 1963, and complement-fixing antibody to Silverwater virus was detected in 21 of 107 showshoe hare sera, obtained principally in that Township. Antibody to California encephalitis virus was detected both by complement-fixation and neutralization tests in sera from 9 of 107 snowshoe hares.

Author Notes

Virology Department, The Research Institute of The Hospital for Sick Children, Toronto, Canada.

Zoology Department, Ontario Agricultural College, Guelph, Canada.