Adaptation of Colorado Tick Fever Virus to Mouse and Developing Chick Embryo

Abstract

Colorado tick fever is a human infection, probably of tick-borne viral etiology, which runs a mild course and is characterized by intermittent nonexanthematic fever. The disease has been recognized as a separate clinical entity by Becker under the name of Colorado tick fever 1 and by Toomey under the name of American Mountain Fever. 2 Topping. Cully-ford and Davis 3 conducting a clinical and epidemiological survey of Colorado tick fever were unsuccessful in their attempts to establish the causative agent of the disease in laboratory animals or developing chick embryo. Recently, however, Florio and his colleagues 4 reported transmission of Colorado tick fever virus to the Syrian hamster and found that the virus passes gradocol collodion membrane filters with calculated pore diameters of 24 milli-microns. 5

The present observation deals with adaptation of Colorado tick fever virus to the dba (dilute brown agouti) mouse, the white mouse and developing chick embryo.

Experimental. Infected hamster serum representing the 30th hamster passage of the virus was obtained through the courtesy of Dr. Lloyd Florio, of the School of Medicine and Hospitals, University of Colorado. The serum was diluted 5 times in saline and inoculated, respectively into: 5 hamsters by intraabdominal route, 6 dba mice by intra-nasal route and 12 dba mice by intracerebral route. The hamsters weighed 90-120 g and the mice 10-12 g. Two hamsters were found dead and 2 sick on the fourth day after inoculation. A low WBC count, characteristic for Colorado tick fever 4 was observed on blood smears obtained from the 2 sick animals.

None of the intranasally-inoculated mice showed any signs of illness. Neverthelessone animal was sacrificed on the fifth day after inoculation and a bacteriologically sterile suq)ension of its lungs was instilled intranasally into a second group of 6 dba mice.