Vectorborne Rickettsial Diseases Typhus and spotted fever

The typhus group of rickettsiae contains two important human pathogens, Rickettsia prowazekii (the agent of louse-borne typhus) and Rickettsia typhi (the agent of flea-borne murine typhus). The former disease can be spread between humans by lice and does not require an alternative vertebrate host reservoir. It is usually found amongst displaced, crowded populations in which thousands of cases can occur in epidemics with about a 10 % fatality rate. The disease is transmitted by the bite of an infected louse or merely by inhaling aerosols of infectious louse faeces. The primary targets of infection by rickettsiae are vascular endothelial cells, and depending on the extent of vascular damage and inflammation, vasculitis may result clinically in meningo-encephalitis, pneumonitis, myocarditis or focal necrosis of the liver or kidneys. In the louse, which becomes infected by ingesting infected blood, the rickettsiae multiply rapidly within the gut cells and are then discharged in the louse faeces. The lice are killed by the infection, but survive long enough (about 2 weeks) to carry the infection to other individuals (Dasch & Weiss, 1998).

Although modern standards of hygiene have largely eliminated louse-borne typhus from most populations, outbreaks have occurred in refugee populations in Africa, and the disease is endemic in the highlands of Peru and Ethiopia (McDade, 1998). In addition, a sylvatic cycle is maintained in the Eastern USA involving the flying squirrel, Glaucomys volans, and its lice and fleas, and occasionally humans may become infected from this reservoir (Reynolds et al. 2003).

Murine typhus is a true zoonosis, and the disease is found in areas in which rats are living in close proximity to humans. Infections may cause severe disease but are rarely fatal. Although rat lice may transmit the disease between rats, transmission to humans usually involves the oriental rat flea, Xenopsylla cheopis. The rickettsiae grow in the midgut cells of the flea and are excreted in the faeces, but the fleas are not killed by the infection. The distribution of murine typhus is worldwide, wherever the host rats, Rattus norvegicus or Rattus rattus, are found.

The spotted fever group of rickettsiae contains at least nine pathogens of humans, and probably a few more which are emerging causes of disease. One of the most important of these is Rickettsia rickettsii, the cause of Rocky Mountain spotted fever, the aetiology of which was elucidated by Howard Taylor Ricketts in a series of experiments at the beginning of the 20th century (Ricketts, 1909).

The disease is an acute febrile illness, with sudden onset of headache and chills followed by a fever that persists for 2-3 weeks, and a characteristic rash appearing on the trunk and extremities on about the 4th day of the disease. Delirium, shock and renal failure may occur in severe cases, and despite the use of antibiotics, the case-fatality rate has remained at about 5 %. The disease occurs widely throughout South, Central and North America, where it is transmitted by several genera of ixodid ticks (Derma-centor, Ixodes, Rhipicephalus and Amblyomma) depending upon the locality. The ticks serve both as reservoirs and vectors of spotted fever group rickettsiae. The infection is maintained in the tick during all stages of the developmental cycle, and most tissues are infected, including the salivary glands. Humans coming into contact with ticks become infected during the feeding process.

Other spotted fever group rickettsiae of note include Rickettsia conorii, causing Mediterranean spotted fever, Rickettsia japonica, causing Oriental spotted fever, and Rickettsia sibirica, causing Siberian tick typhus. Also included in this group is Rickettsia akari, the cause of rickettsialpox, but notably the vector for this is a mite, Liponysoides sanguinus, that feeds on the house mouse, Mus muculus (Kass et al., 1994).

Finally, an important disease of the Asia-Pacific region, spread by chigger mites, particularly of the genus Leptotrombidium, is scrub typhus caused by a single species, Orientia tsutsugamushi. Transmission of this disease, which generally presents with a fever, rash and eschar at the site of the bite, is limited to the larval stage (chigger) of the mite, which occurs in vegetation and usually feeds on rodents but will also bite humans who come into contact with it. Occasionally the disease can be fatal, with generalized organ failure. Treatment with doxycycline is usually effective, however.

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