The rickettsiae are aerobic, Gram-negative, coccoid bacteria. They propagate obligatorily in the plasma or nucleus of predominantly intestinal cells of arthropods and in endothelial cells of small blood vessels of mammals. The vectors are lice, ticks, mites and sometimes fleas. The pathogens are transmitted to mammals by the bite of vectors or by scratching the skin or by contamination of mucous membranes or the lungs (tab. 4.1, page 209).
Epidemic typhus, typhus exanthematicus (Rickettsia prowazeki). The pathogen is highly infective and, without treatment, lethality of the disease reaches 50%. In the human louse Pediculus humanus, R. prowazeki attacks and destroys the cells of the midgut. Man is infected by inhalation of the dust of louse faeces (fig. 4.1, page 210). After an incubation time of 10 – 14 days, acute fever and multiple haemorrhages appear. Mice, rats, hamsters and guinea pigs are susceptible but without epidemiological significance. The disease is an anthroponosis. Acquired immunity protects for 10 – 20 years. - At moderate humidity, R. prowazeki exhibits reduced survival. De-infestation of living quarters can be carried out by sprinkling followed by quiting the abode for 5 days (at +2 4 °C) or for 1 day (+18 °C). Clothes can be disinfected by steam-treatment for 2 hours (at 80 0C). Survival times of Rickettsiae are generally short under humid conditions (fig. 4.2, page 211). Control is possible by insecticides and >70 % relative humidity.
Rocky Mountain spotted fever, RMSF (Rickettsia rickettsi). Human cases occur sporadically; in spite of the pathogenesis, the course of the disease is similar to that of epidemic typhus. There are strains of low and high virulence. Accordingly, incubation varies from 3 to 14 days and the fatality rate from 5 to 80 % without treatment by antibiotics. Children are not as badly affected as adults, as a rule. Vectors are all stages of the hard ticks Dermacentor spec., Amblyomma spec., Rhipicephalus spec. Propagation of R. rickettsi takes place by intraplasmic binary division in cells of the midgut, haemolymph, ovaries, muscles, nerves and salivary glands. They are transmitted via the saliva of the tick but tick faeces are not infectious. In adult ticks, generative (transovarial) transmission takes place. The disease is brought into communities by forest rangers, hunters, collectors and their dogs. The protective acquired immunity is strain-specific and is directed against rickettsial toxins. Mice, squirrels, rabbits and perhaps birds and dogs establish a natural reservoir. RMSF is a Zoonosis.
Biology of the human louse Pediculus humanus capitis (head louse), P.h. corporis (body louse) and Phthirus pubis (pubic louse): Embryogenesis within the egg lasts 8 days (29 32 °C); juvenile development is pauro(hemi-) metabolic, passing through two nymphal stages until the imago and lasting 9 19 days. The lifetime of adults is 48 days, during which 10 eggs per day are laid. A blood meal is taken 1 3 times per day. The starving potential is 10 days at 0 6 °C. - Anatomical features: the thorax is non-segmented and wingless and the tibia extends to a thumb-like process opposing the claw of the pretrarsus. The enclosed opening corresponds to the cross section of the hair of the host and prevents the louse slipping off. The abdomen has 9 segments and the midgut disc contains a mycetoma for endosymbionts that migrate via the haemolymph to the oviduct for the infection of the passing egg via its micropyle (fig. 4.3, page 213). - The mouth parts are of the biting-sucking type, viz. the hypopharynx forms a dorsal styletto and the labium a ventral styletto, both being completely entognathous (fig. 4.5, page 215). On extension of the louse at feeding, the endocranial volume changes are compensated for hydrostatically (fig. 4.6, page 216). - P. pubis, the pubic louse, lives a more sedentary existence (fig. 4.4, page 213).