4. Microbial epidemics transmitted by arthropods

Microbial pathogens are outside the scope of books covering primarily eucaryotic protozoan or metazoan parasites. However, the selected examples of the following chapters demonstrate epidemiological modifications determined by obligatory arthropod vectors of microbes.

Rickettsia prowazeki is transmitted by dust contaminated with the faeces of the human louse, a permanent ectoparasite. The result is the epidemic spotted fever, an anthroponosis that spreads quickly through crowded human populations.

R. rickettsi is transmitted by the saliva of hard ticks and causes sporadic cases of Rocky Mountain Spotted Fever, which is a zoonosis in spite of the same pathogenesis.

R. tsutsugamushi is exclusively transmitted by the larva of its mite vector, which is otherwise phytophagous or entomophagous. The larva is the only stage that feeds on the lymph of mammals and birds. The cycle of the pathogen is obligatory generative, namely at first transstadially through the nymphs and transovarially to the larvae. Epidemics are markedly seasonal because of the biology of the mite.

Borellia duttoni, the bacterial pathogen of African relapsing fever, is transmitted by a soft tick. In contrast to hard ticks, it feeds repeatedly during the adult stage. The pathogen attacks all organs of its vector. Thereby, B. duttoni changes its shape and antigenic specifity. It results in an endemic anthroponosis with strain specific immunity.

Yersinia pestis, the bacterial pathogen of plague, is transmitted primarily by fleas, which are linked to their specific blood hosts. The pathogen propagates particularly well in the intestine of the flea but this is, however, dependent on the external temperature. The capability of fleas to endure starvation for several months but to attempt to feed on the blood of non-specific hosts occasionally gives a transmission chain from wild animals to man. Poor sanitary conditions and crowded human populations encourage pandemics, which start as a zoonosis and end in an anthroponosis.

The pathogen of yellow fever is a virus, the transmission of which is stricctly linked with Aedes spec., a mosquito genus with specific breeding habits, namely in small amounts of water. Various species link silvatic animal reservoir cycles to rural human cycles, from which the pathogen spreads by human traffic to large cities, where it gains a foothold in regions of poorly regulated economies, thereby causing urban human cycles.

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