CHOLERA, PLAGUE, SMALLPOX, TYPHUS FEVER, AND YELLOW

FEVER-Continued.

Reports Received from December 30, 1922, to February 9, 1923-Continued. SMALLPOX-Continued.

CHOLERA, PLAGUE, SMALLPOX, TYPHUS FEVER, AND YELLOW

FEVER-Continued.

Reports Received from December 30, 1922, to February 9, 1923—Continued. TYPHUS FEVER-Continued.

PUBLIC HEALTH REPORTS

VOL. 38

FEBRUARY 23, 1923

No. 8

ROCKY MOUNTAIN SPOTTED FEVER: INFECTIVITY OF FASTING AND RECENTLY FED TICKS.

By R. R. SPENCER, Passed Assistant Surgeon, and R. R. PARKER, Special Expert, United States Public Health Service.

During the spring and summer of 1921 certain experiments were conducted at the field laboratory of the United States Public Health Service at Hamilton, Mont., the results of which have added somewhat to our knowledge of the virus of Rocky Mountain spotted fever as it occurs in the tick. Some of these experiments involved the testing for infectivity of large numbers of adult ticks secured in the field; and, since it was believed that the usual method of feeding on guinea pigs was not dependable, a preliminary experiment was carried out in order to compare the results of feeding with the inoculation of macerated tick contents, our criterion of infectivity being either the development of spotted fever or the development of an immunity to spotted fever. Following is an outline of this experiment:

March 16 to 20: Twenty-five adult ticks were fed on infected guinea pig No. 420.

March 22: Twelve of these ticks were selected for testing; six were permitted to feed on a guinea pig each for three days; the remaining six were dissected and the entire contents of each emulsified in salt solution and inoculated into six pigs, a separate guinea pig being used for each tick.

Results of tick-content inoculation. Four of the six pigs inoculated with the contents of infected ticks developed spotted fever and two were rendered immune.1

Results of tick feeding.-None of the six pigs upon which infected ticks were fed developed spotted fever; one died of pneumonia. After an afebrile period of 10 days the five remaining pigs each received intraperitoneally the contents of the tick which had previously fed upon it. Three of these pigs developed spotted fever as a result of this inoculation; the other two died of an intercurrent infection.

The above experiments, if taken at face value, indicate that the tick-content inoculation method is the more reliable. However, a possible basis for misinterpretation lay in the fact that the ticks used were but recently infected with the virus. Indeed, it was known

Animals are regarded as immune when spotted fever does not develop following an intraperitoneal injection of 1 c. c. of citrated heart's blood of a guinea pig at the height of infection, while it develops in control animals.

that many failures have resulted in our own experiences, and in those of others, in attempting to transmit the virus by the feeding of recently infected adult ticks (infected as adults). Such failures may be due to the existence of an incubation period of unknown duration necessary for the development and distribution of the organism in the tick before it can become infective. Similar experiments are, therefore, being carried out with this possible time element in mind. At the time, however, it was thought that the results of this preliminary experiment were sufficiently suggestive to justify the use of the inoculation method; and although the results of the sum total of our subsequent tests indicate its value, yet it is considered that a combined feeding and inoculation procedure (Tables I, II, and III) is more dependable in indicating infection. The following observations are our basis for this opinion:

Over 100 lots of drag ticks (drag ticks are unfed males and females secured by dragging a white outing flannel "flag" over vegetation), using 15 to 25 ticks from each lot, were tested by inoculating the tick contents into guinea pigs without once resulting in spotted fever, although many of these tick lots were collected in areas known to be infected. When immunity tests were given to these pigs, many of them were found to be immune.

In view of the results of the preliminary experiment, these results were not easy to interpret until three similarly conducted tests with ticks secured from a mountain goat resulted in spotted fever. The essential difference between the drag ticks and the goat ticks was that the latter had recently ingested blood, whereas the former had not fed since engorging as nymphs not later than during the fall of 1921. The possibility suggested itself that the ingestion of fresh blood was sufficient to reactivate the virus in the tick, since goats are not, as far as known, susceptible to the infection. (Our use of the term "reactivation" is discussed later.)

Our procedure was then changed as follows: Ticks to be tested were first fed on guinea pigs for 48 hours, and if no fever developed within 10 days following the removal of the ticks the contents of the same ticks were then inoculated into the pig upon which they had fed; if fever did not develop in another 10 days the pig was given an immunity test. In order to check the results previously secured by inoculation alone, ticks were first tested from some of the same lots of drag ticks which had failed to give spotted fever on immediate inoculation of their contents. These tests included some of those lots which had conferred immunity but failed to give spotted fever. Spotted fever was secured from the first lots thus retested. In some pigs it developed after feeding (Table I, pig 1100, and Table II, pig 1095), and in some after the subsequent inoculation of tick contents (Table III, pig 897); still others were rendered immune, as shown by