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.

    19/150 page 7
    In an attempt to cultivate the virus, Fricks (Fricks 1916-a) in 1915 inoculated infected tick eggs which had been incubated with a special culture medium for 25 days and produced immunity in the inoculated pig. In 1921, one of the writers (Parker) produced immunity by the inoculation of comparatively fresh eggs from an infected rabbit tick, the immunity test being given three months after inoculation. 3. The term “ reactivation ” is used for lack of a better descriptive word to designate the transition of the virus from a nonvirulent immunity-producing phase to a virulent fever-producing phase. It is not known whether this transition is due to multiplication of the virus, to development of a possible distinct stage in its life cycle, to renewal of virulence following a period of attenuation, or, perhaps to some other unrecognized condition initiated by the ingestion of fresh mammalian blood. This phenomenon provides an explanation of the fact first dem- onstrated by Ricketts, that ticks usually do not infect unless at- tached for some hours. In a single instance he secured infection in one hour and three-fourths, but found that 10 to 20 hours’ feeding was usually required. In our studies we have repeatedly fed ticks for 48 hours without securing infection; whereas the presence of the virus was afterwards demonstrated, either by infection following inoculation of the contents of the ticks concerned or by the subse- quent immunity test. (Table 3, pigs No. 897 and 819.) It seems probable, therefore, that when unfed infected adult ticks have hiber- nated, the virus must be reactivated by fresh blood before such ticks become infective. Furthermore, it is possible that this apparent need of blood furnishes an explanation of the comparatively small number of human cases which occur. Many ticks which attach them- selves to human beings are doubtless removed before “ reactivation ” has taken place. The study of this phenomenon suggests the possibility that the virus may sometimes die out in the tick. Observations made by King * have shown that unfed adult ticks frequently live for two and three years and occasionally into the fourth season. If the ingestion of fresh blood is essential for the “ reactivation ” of the virus, it seems probable that in instances when infected ticks do not secure hosts for extended periods of time the virus may die or its vitality be so re- duced that “reactivation” can not occur. The results shown by Table 3 may be indicative of different periods of fasting on the part of ticks infected in nature; although immunity which could not be ascribed to such cause has occasionally been unexpectedly produced. 3’ King, W. V.: Unpublished experiments. 76599—30——2
    digitised image 1
    digitised image 2
    digitised image 3