STUDIES ON INFECTIOUS MONONUCLEOSIS INDUCED IN THE MONKEY BY EXPERIMENTAL INFECTION WITH RICKETTSIA SENNETSU. II. PATHOLOGICAL FINDINGS

Neorickettsial endosymbionts of the digenea: diversity, transmission and distribution

Digeneans are endoparasitic flatworms with complex life cycles and distinct life stages that parasitize different host species. Some digenean species harbour bacterial endosymbionts known as Neorickettsia (Order Rickettsiales, Family Anaplasmataceae). Neorickettsia occur in all life stages and are maintained by vertical transmission. Far from benign however, Neorickettsia may also be transmitted horizontally by digenean parasites to their vertebrate definitive hosts. Once inside, Neorickettsia can infect macrophages and other cell types. In some vertebrate species (e.g. dogs, horses and humans), neorickettsial infections cause severe disease. Taken from a mostly parasitological perspective, this article summarizes our current knowledge on the transmission ecology of neorickettsiae, both for pathogenic species and for neorickettsiae of unknown pathogenicity. In addition, we discuss the diversity, phylogeny and geographical distribution of neorickettsiae, as well as their possible evolutionary associations with various groups of digeneans. Our understanding of neorickettsiae is at an early stage and there are undoubtedly many more neorickettsial endosymbioses with digeneans waiting to be discovered. Because neorickettsiae can infect vertebrates, it is particularly important to examine digenean species that regularly infect humans. Rapid advances in molecular tools and their application towards bacterial identification bode well for our future progress in understanding the biology of Neorickettsia.

The tribe Ehrlichieae and ehrlichial diseases

The tribe Ehrlichieae consists of gram-negative minute cocci that are obligate intracellular parasites classified in the family Rickettsiaceae. Although ehrlichial organisms have been observed in leukocytes for many years, only a few species have been cultured in quantities sufficient for biochemical and molecular analyses. Recents studies on 16S-rRNA sequence analysis and energy metabolism showed that the genus Ehrlichia is closely related to the genus Rickettsia. There is, however, no antigenic cross-reactivity between these genera. Ehrlichial organisms cause a disease called "ehrlichiosis," a noncontagious infectious disease known to be transmitted by a tick in several cases and by a fluke in one case. Ehrlichia spp. infect dogs, ruminants, horses, and humans. Recently, two new ehrlichial diseases, Potomac horse fever and human ehrlichiosis, were discovered in the United States. The etiologic agent of Potomac horse fever, Ehrlichia risticii, is closely related to the known human pathogen Ehrlichia sennetsu. The etiologic agent of human ehrlichiosis is related to Ehrlichia canis, a canine pathogen. In contrast to the genus Rickettsia, members of the tribe Ehrlichieae reside primarily in the cytoplasmic vacuoles of monocytes or granulocytes and cause hematologic abnormalities, lymphadenopathy, and other pathologic changes in the host. However, the actual mechanisms whereby Ehrlichia spp. infect leukocytes, multiply in them, and produce various forms of systemic disease have not been defined. Depending on the ehrlichial species involved, serologic or direct microscopic observation of stained blood smears is currently used to diagnose ehrlichial disease.

Effect of cyclophosphamide on the growth of Rickettsia sennetsu in experimentally infected mice

The growth rate of Rickettsia sennetsu, the etiological agent of sennetsu rickettsiosis, a special type of infectious mononucleosis found in western Japan, is very low when the ordinary experimental methods used for other members of the rickettsiae are employed. Attempts have been made to increase its growth the rickettsiae are employed. Attempts have been made to increase its growth rate in cells and animals. By treating mice with cyclophosphamide, considerable enhancement of growth of the rickettsia was observed. In the drug-treated mice, infective titers expressed as mean lethal dose in the spleens reached 10(8,5) in animals treated by the intraperitoneal route three times at 5-day intervals with 0.33 mg/g (body weight). Infective titers in drug-treated mice were more than 100-fold above those of the infected non-drug-treated mice. In peritoneal cells of drug-treated mice rickettsial particles were observed in great abundance. The numbers of cells containing rickettsiae almost paralleled the infective titers of the spleens in each group in mice. Lymph nodes and spleens of the drug-treated mice diminished in size during infection.