ETIOLOGY 



1345 



and in a few instances can pass through the eggs into a second 

 generation of ticks. According to Ricketts, the Idaho disease is 

 spread by Dermatocentor modestus, and the Montana by D. venustus 

 Banks, 1908, nec Marx, 1897 { = D. andersoni Stiles, 1905). He is 

 inclined to think that there is a difference between the two forms 

 of fever, especially as the former has a death-rate of some 5 per 

 cent, and the latter of about 90 per cent. 



Infected ticks are found but sparingly in Nature. Thus, of 

 513 ticks found on animals, 296 or more were allowed to attack 

 guinea-pigs, with the result that only one of the animals took the 

 disease after an incubation of seven days. The infected guinea- 

 pig was found to have thirty-six male ticks upon it, all of which 

 had come from a horse. 



A tick fed on a human being suffering from the disease com- 

 municated it first to a man and afterwards to a woman by its 

 bites. Ricketts, examining the blood of patients and the eggs of 

 infected ticks, has observed peculiar bacillary-like structures, 

 which show bi-polar staining. He has not succeeded in cultivating 

 the germ. Wolbach in 1916 and 1918 has confirmed Ricketts' 

 observations. Arkwright, Bacot and Duncan consider the Rickett- 

 sia bodies found in Rocky Mountain Fever to be slightly larger and 

 usually longer and more lancet-shaped than those observed in typhus 

 and trench fever. Michie and Parsons have no doubt that the 

 infective agent is in the salivary glands of the tick. Transmission to 

 a tick requires twenty-four hours, and infection of a guinea-pig one 

 hour and forty- five minutes at least. The tick is the natural 

 reservoir of the parasite of the disease, and lives upon domestic 

 animals, horses, and cattle, as well as upon six varieties of wild 

 rodents, including Citellus columbianus and Marmota jiaviventes, and 

 the jack- rabbit ; but according to Pricks sh^ep are unsuitable as hosts, 

 but this has failed to be confirmed. There is no doubt that the 

 parasite of the fever multiplies in the tick, and there seems to be an 

 opinion that Dermatocentor andersoni {=D. venustus) in Montana 

 produces a fever with a 90 per cent, mortality, while D. maturatus 

 in Idaho one with a 5 per cent, mortality. See also Chapter XXXV. 



With regard to predisposing causes, there are sex and age in- 

 fluences to be noted. Men are more frequently attacked than 

 women, and the most common age is from fifteen to fifty 

 years, both of which merely signify that persons performing out- 

 door work run a greater risk of infection than those otherwise 

 employed. 



Pathology. — During the fever the virus can be found in the red 

 and white cells as well as in the serum. It also exists in the liver and 

 spleen. 



Morbid Anatomy. — Rigor mortis is well marked, and the skin shows 

 lividity in dependent and petechiae in non-dependent parts, and at 

 times the marks of the tick-bites may still be visible. 



The pleura, lungs, and pericardium, and most of the organs, are 

 normal, but petechiae may be seen on the epicardium; while the 



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