1. Pleomorphic, bacterium-like, Gram-negative, intracellular microorganisms, which stained much less intensely with ordinary dyes than most bacteria were found in sixteen species of ticks comprising examples of the Argasidæ and the Ixodidæ. 2. In six of these species studied intensively slight differences in the microorganisms were detected, sufficient to permit identification of the vectors by microscopic examination of the microorganism alone. 3. No evidence was seen of injury to the tissues of the arachnid hosts of the microorganisms other than that incident to mechanical distention of the cells containing them. 4. The detection of the microorganisms in the eggs of ten species, in the unfed larvæ of eight species, and at nearly related stages throughout the life cycle of three others leads to the conclusion that they are transmitted hereditarily.
In the absence of a satisfactory definition of Rickettsia the observations herein recorded were arbitrarily limited to bacterium-like organisms which are intracellular and Gram-negative. Rickettsia of this type were found in the following species: Amblyomma americana, Amblyomma hebræum, Boophilus decoloratus, Atomus sp., Casinaria infesta, Chrysopa oculata, Ctenocephalus canis, Dermacentor variabilis, Lepisma saccharina, Lucoppia curviseta, Margaropus annulatus, Margaropus annulatus australis, Ornithodoros turicata, Pulex irritans, Rhipicephalus sanguineus, Rhipicephalus evertsi, and Salticus scenicus. Since intracellular, Gram-negative Rickettsia have been recorded in the literature as existing in Cimex lectularius, Dermacentor venustus, Melophagus ovinus, and Pediculus humanus, the occasional occurrence of such bodies must be conceded in the following groups not closely related phylogenetically: Attidæ, Trombidiidæ, Argasidæ, lxodidæ, Cinura, Acanthiidæ, Pediculidæ, Hippoboscidæ, Chrysopidæ, Pulicidæ, and Ichneumonidæ. The species which harbor Rickettsia differ widely in diet and habitat. One such species is insectivorous throughout life, two are insectivorous in larval stages, becoming vegetarian in the adult condition, one is chiefly vegetarian but partakes of some animal products, and two are usually entirely vegetarian; while the remainder subsist wholly upon a diet of mammalian blood. Rickettsia are associated, in only a few cases, with diseases in mammals. The evidence at hand does not lead beyond the conclusion that the Rickettsia mentioned above are true Gram-negative microorganisms, easily distinguishable from mitochondria and all other cytoplasmic and nuclear granulations, rather completely adapted to an intracellular existence, exhibiting in some cases a remarkable degree of host specificity, and often inherited through the eggs.
A Gram-negative, intracellular, coccus-like microorganism was found in cases of heartwater in the three species which are susceptible to the disease; namely, goats, sheep, and cattle. It was absent in the case of control animals, both normal ones and those dying of some. other diseases. The presence of this microorganism was definitely related to the febrile reaction. It was most easily detected in the renal glomeruli and in the small capillaries of the cerebral cortex but probably occurred throughout the body. The microorganism was a typical endothelial parasite, being restricted in distribution to the endothelial cells of the smaller blood vessels and to portions of such elements which had broken off into the blood stream. It was never observed to cause injury to the cells other than those incident to mechanical distention through accumulation within them of many individuals in large densely packed masses which were characteristically spherical. A typical attribute was the presence of several of these masses within the cytoplasm of a single endothelial cell. In view of the association of this microorganism with heartwater, a disease of ruminants, and thus far the only one in which microorganisms resembling Rickettsiœ have been reported, the designation Rickettsia ruminantium is proposed.
No abstract
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