Rickettsiae are obligate intracellular parasites, of which 5 diseases are of clinical importance: louse-borne typhus fever, Murine typhus, botoneuse fever, Rocky Mountain Spotted fever, and Scrub typhus. Rickettsiae are gram-negative bacterial parasites. They stain poorly with gram stain. The size of the rickettsial genome is around 40% of that of E. coli.
Typhus refers to two diseases that are similar clinically but differ markedly in epidemiological features. The louse-borne typhus complex is caused by R. prowazekii and murine typhus is caused by R. typhi (R. mooseri). The organisms show strong serological cross-reactions, bearing group-specific as well as species-specific epitopes on a major envelope protein. Convalescence from either infection is accompained by strong and lasting immunity against the infecting species and substantial cross-immunity to the other.
Louse-borne typus fever usually begins abruptly, after a 9 to 12 day incubation period, with fever, headache, myalgia, hypotension and a maculopapular rash. The untreated patient either dies or recovers in 2 to 3 weeks. The mortality rate is low for children but may reach 50% in persons over 50 years of age. Classic louse-borne typhus is a disease of poverty or natural disasters. The transmission cycle is maintained between human and louse. The louse itself dies of the infection without transmitting it to the next generation. Hence then the disease is interrupted, the disease dies out until the organism is reintroduced into the louse population. Humans are thus the primary interepidemic reservoir. The human-louse-human transmission cycle can give rise to local outbreaks, massive outbreaks or just remain endemic. Louse-borne typhus is now confined to Africa, and the mountainous regions of S. America and Asia. Louse-borne typhus may be treated by a single dose of doxycylcine. Control of classic louse-borne typhus involves control of the louse vector and immunisation of the population at risk.
Murine typhus resembles louse-brone typhus clinically and pathologically but is milder in nature. The untreated disease has a low mortality rate (<5%) and responds promptly to chemotherapy. Murine typhus is a zoonosis and exists in nature as an infection of rats and other mammals. It is transmitted among rats and to man by fleas. Endemic foci are often concentrated in rat-infested buildings where man becomes infected from fleas. Control measures involves reducing the flea and the rodent population.
The several species of tick-borne spotted fever group rickettsias are transmitted by the bite of various species of ticks. The infection is maintained transovarially or through a tick-mammal-tick cycle. Rocky mountain spotted fever (RMSF) is a dangerous acute infectious disease of the Western hemisphere caused by R. rickettsii and is transmitted by various species of ticks. Its clinical manifestations are similar to that of typhus and scrub typhus. However, its incubation period is shorter (7 to 7 days). The rash usually begins in the extremities and then involves the trunk. The disease will respond favorably to early antibiotic treatment. However, in sever cases, mycocarditis, DIC, and other physiological derangements may develop leading to death. The mortality rate is 5%. Because the disease may present as an undifferentiated fever, therapy should be started on a presumptive diagnosis based on the history of tick bites or potential exposure in an endemic area. An early presumptive diagnosis may be verified by finding the organisms in skin punch biopsy sections stained with specific fluorescent Ab.
R. conorii infections occur under numerous local names e.g. botenneuse fever, Meditteraena spotted fever, Kenya tick typhus, Indian tick typhus etc. are widely distributed in S. Europe, Africa, and Asia. The disease resembles RMSF with some differences. There is often ulcerated primary lesion (eschar) at the site of the infected tick bite. The rash tends to be more nodular and the disease milder. It responds readily to anti-rickettsial antibiotics
Scrub typhus is an acute typhus-like disease of Asia caused by multiple serotypes of R. tsutsugamushi. It is transmitted by the bite of the larvae (chiggers) of certain mites. Primary scrub typhus resembles louse-borne typhus in many ways. However, an eschar frequently develops at the site of the inoculation. Response to chemotherapy is rapid. The mortality of the disease ranges from the very low to very high.