Studies on Rocky Mountain spotted fever / U.S. Treasury Department, Public Health Service.

Date:
[1930]
Catalogue details

Licence: Public Domain Mark

Credit: Studies on Rocky Mountain spotted fever / U.S. Treasury Department, Public Health Service. Source: Wellcome Collection.

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    some partly immune. Of 6 guinea pigs injected with incubated infected ticks (fig. 2), all developed spotted fever, but survived 10) days or longer. Of 4 guinea pigs on which 2 incubated infected ticks each were allowed to feed (fig. 3), all died of spotted fever in 10 days or less, and of 8 guinea pigs into which these identical fed ticks were inoculated, all died in 4 to 9 days. These differences are observed in a lot of ticks, the progeny of a single female. They were infected on the same host at the same time and subsequently kept in the same environment until the beginning of the test. Virulence of low grade is manifested in unfed, unincubated ticks: taken directly from the ice box; virulence sufficient to produce defi- nite spotted fever but no early deaths is seen in the unfed, incu- bated ticks, and a virulence of high degree in ticks zncubated 24 hours at 87° C. and then fed for three days on a guinea pig. In brief, a progressive development of the virulence has taken place. Nothing comparable to this is seen in a strain of tissue virus, the virulence of which remains fairly constant for months. Many more tests aside from those given in Figures 1, 2, and 3 were performed. Some ticks of this infected group (2351-B) failed to give infection, but no results were obtained inconsistent with those outlined in the charts. Figure 4, however, represents the total number (82) of uninfected ticks tested as controls. Figure 5 represents the results of injecting guinea pigs on January 5, 1924, with various dilutions of a tick virus emulsion. The viscera of infected adults of lot 2351-B were ground in a mortar with a small amount of salt solution. Dilutions were made so that 1 ¢. e. of each represented a definite fractional part of a tick and each such fraction was inoculated into two guinea pigs. AJl guinea pigs receiving from 1/3 to 1/3000 of a tick developed spotted fever, ex- cept one animal that received the latter dilution. Although this. guinea pig developed fever, it was shown not to be spotted fever by the subsequent immunity test. Both 1/30000 dilutions were nega- tive. Subsequent titrations of virus from the same tick lot made on March 28, 1924, gave an infectious dose as low as 1/5000 of a tick. The contents of one adult tick after three days’ feeding (the same as used for titrations) weighs about 0.01 gram and therefore 5,000 M. I. D. (minimum infectious dose) of tick virus may be concen- trated in this amount of recently fed tick tissue. The tick has served,. therefore, as a more efficient culture media than the guinea pig,. the blood of which is infectious in minimal doses of from 1/100 to. 1/1000 c. c. On this basis, tick virus of adult ticks when reactivated by freshly ingested blood may contain, volume for volume, five hundred to five thousand times as many M. I. D. as guinea pig serum. virus.