The guinea pig is susceptible to the virus as well as the monkey but only shows temperature rise. Nicolle has shown that lice do not become infective until about the tenth day after feeding on typhus blood. The virus is found in the blood of man and in the spleen and blood of monkeys and guinea pigs.
Plotz has isolated a Gram-positive pleomorphic bacillus from the blood of typhus patients which has been named B. typhi exanthematici. This organism is of historical interest only and apparently has nothing to do with the causation of typhus fever.
Hort states that only blood recently taken from typhus patients will cause the disease in monkeys while the same blood which has been incubated several hours or days fails to produce the disease. Others, as well as Hort, doubt the etiological relation of the organism of Plotz to typhus fever or to the mild form of the disease as seen in New York City and there known as Brill’s disease. Tabardillo or Mexican typhus is the same as typhus.
Rocha-Lima insists upon the etiological importance of short oval bodies, often showing polar staining with Giemsa preparations, and found in the epithelial cells of the alimentary tract of lice which have fed on the blood of typhus patients.
Ricketts noted similar bodies in such lice. They differ from the Plotz organism in that they are Gram-negative and apparently cannot be cultivated. Lice feeding on blood other than that of typhus patients fail to show these bodies and furthermore the blood of typhus cases during the period of convalescence fails to infect lice. When guinea pigs are inoculated with emulsions of lice containing such bodies they show the temperature reaction of typhus fever. Plotz states that his organism may be Gram-negative at first and believes these organisms to be the same. Wolbach and Todd take the view that the bodies described by Rocha-Lima and called Rickettsia prowazeki are the cause of typhus fever. Similar bodies have been found in trench fever and even in certain normal lice. These bodies, however, are extracellular and are more oval and stain more intensely than the typhus bodies. These Rocha-Lima bodies require a Romanowsky stain to bring them out. In man these bodies are found in the endothelial cells of the small blood vessels, occurring singly or in clumps. In the louse the epithelial cells of the intestine may be found distended with masses of these organisms.
It is not certain whether the virus is transmitted by the bite of the louse or by inoculation of faeces. Monkeys and guinea pigs can be infected by injection of emulsions made from infected lice.
Loewe and others have succeeded in cultivating the virus of typhus, using deep tubes containing 10 cc. of a rich ascitic fluid and a piece of sterile rabbit kidney. The medium was inoculated with 2 cc. of typhus blood, after which 0.3 cc. of a 20% dextrose solution was added and the culture sealed with liquid petrolatum. Its reaction was pH 7 to 7.4. Cultures were incubated at both room and body temperature.
As proving the culturing of the virus, it was noted that 1 cc. of a fourth-generation culture would infect a guinea pig. This would represent 0.00000016 cc. of the original typhus blood. The bodies cultivated differ in morphology from the bacillus of Plotz in that they are even more minute, of slight hazy outline and do not assume polymorphous involution forms. They vary also in cultural characteristics. Concerning the question as to the identity of the bodies with Rickettsia prowazeki, no definite decision is as yet permissible, nor were the authors able to decide whether the bodies are of bacterial or of protozoan nature.
Epidemiology.—Until recently authorities stated that typhus fever was the most contagious of all diseases. We now know that in the absence of body or possibly head lice the disease is only slightly, if at all contagious.