RICKETTSIAL FEVERS

Rickettsial diseases are primarily zoonotic infections, with humans infected accidentally. These infections are more common in adults, specially during the warm and humid season.

Incidence: While rarely diagnosed, four rickettesial infections, i.e. scrub typhus, murine typhus, Indian tick typhus and Q fever seem to be significantly prevalent in India (Table 10.31). Scrub typhus is prevalent all over India; murine typhus and Q fever in north India; and Indian tick typhus fever in central India. Focal outbreaks of murine typhus occur in coastal cities, due to high rodent population at ports/docks.

Other rickettsial infections, e.g. epidemic typhus (R. prowazekii), Rocky mountain-spotted fever (R. rickettsii), rickettesial pox (R. akari) are rare in India, though prevalent in other parts of world.

Epidemiology: Rickettsiae are obligate intracellular bacteria that usually infect arthropods, e.g. ticks and mites, or other animals e.g. cattle and sheep. Human infection is acquired by: (a) bite of infected mite or tick (scrub typhus, Indian tick typhus), or (b) inhalation or contact with vector/animal excreta (murine typhus) or (c) ingestion of infected cattle products, e.g. milk (Q fever). Rickettsial diseases are more common in rainy and winter seasons and in rural population.

Pathogenesis of most rickettsial infections involves diffuse vasculitis leading to microvascular leakage and vascular lumen obstruction.

Clinically, most rickettsial diseases present after an incubation period of 1-2 weeks with: (a) sudden onset of high-grade fever with severe myalgia an headache; (b) Rash on day 2-5 of illness, which may be pruritic and petechial, usually involving palms and soles; (c) Eschar formation - a crusty necrotic lesion at the site of vector bite with or without regional lymphadenopathy. Q fever typically presents as acute pneumonia or influenza-like illness.

More serious manifestations may include aseptic meningitis or encephalitis, pneumonia, acute renal failure and acute gastroenteritis or surgical abdomen. Complications, e.g. DIC, ARDS, hemophagocytic lymphohistiocytosis, purpura fulminans, gangrene and myocarditis are not uncommon.

Diagnosis of rickettsial fever should be considered in any fever of gt; 5 days with suggestive epidemiological, clinical and laboratory indicators (Table 10.32), after exclusion of other causes.

Diagnosis may be confirmed by polymerase chain reaction (PCR) or positive IgM ELISA test. Weil Felix reaction is a simple test with low sensitivity but good specificity, specially when higher titer cut-offs of gt; 1:320 are used. However, a single titre of gt; 1:80 also indicates possibility of infection.

Epidemiological confirmation is also possible on ELISA or immunofluorescence assay suggesting gt;4-fold rise in antibody titers after 2-4 weeks.

TABLE 10.32: When to suspect Rickettsial infections

Undifferentiated fever for gt; 5 days with..

• Suggestive epidemiological features

- Residence in endemic area

- Presence of rodents/ticks in/around home

- Visit to forests and farmlands

- Animal sheds in proximity of homes

- Contact with pet/stray dog infested with ticks

- Similar cases in family, neighbourhood

• Suggestive epidemiological features

- Dengue-like illness with rash, headache, myalgia

- Eschar formation

- Hepatosplenomegaly/lymphadenopathy

- Acute menigoencephalitis

- Community acquired pneumonia

- Presence of GIT, liver or kidney involvement

- Sepsis of unclear etiology

• Suggestive laboratory features

- Normal/low leukocyte count with thrombocytopenia

- Raised ESR and CRP

- Others e.g. elevated hepatic transaminases

TABLE 10.31: Important Rickettsial infections in India

¹contact or inhalation of rat-flea excreta.

²or contact with dust contaminated with animal excreta or ingestion of infected products, e.g. milk.

Treatment: PO/IV Doxycycline 100 mg BD (2.2. mg/kg in children lt;40 kg) for three days after defervescence or total 7 days, is the drug of choice for all rickettesial infections, which may be initiated without waiting for laboratory confirmation.

Doxycycline can be used in younger children as well since a short course will not lead to dental staining. Alternative drugs include Azithromycin (PO 10 mg/ kg/d x 5 days), or Rifampicin (only in refractory cases). Response to treatment is dramatic and persistence of fever beyond 48 hours of therapy should alert about possibility of alternative or additional diagnosis. Treatment also includes other supportive measures including treatment of organ dysfunction.

Prevention of Rickettsial diseases depend on:

• Control of reservoir/vector population, e.g. Rodents (scrub typhus), insecticide sprays (Murine typhus) and disinfection of pet-dogs (Indian tick typhus), etc. and

• Avoidance of personal exposure, e.g. protective clothings (scrub typhus), handling of tick-infested dogs (Indian tick-typhus) and use of pasteurized/ boiled milk (Q fever).

• Short-term weekly Doxycycline prophylaxis for up to 6 weeks may be recommended for travellers in setting of high-risk exposure.

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