Rickettsial infections

Rickettsial infections include scrub typhus and Queensland tick typhus (spotted fever).

Notification requirement for rickettsial infections

Notification is not required.

Primary school and children’s services centres exclusion for rickettsial infections

Exclusion is not required.

Infectious agents of rickettsial infections

Rickettsiae are obligate intracellular organisms, and there are numerous species of concern to humans. They are divided into three groups: spotted fever, typhus and scrub typhus. Rickettsiae (and their associated diseases) of particular importance in Australia are Rickettsia australis (Queensland tick typhus, spotted fever), Orientia tsutsugamushi (scrub typhus), R. honei (Flinders Island spotted fever) and R. typhi (murine typhus).

Identification of rickettsial infections

Clinical features

There is great variation in the severity of illness produced by each organism. Infection most commonly begins with formation of a papule at the site of the bite where the infection was introduced. This usually becomes necrotic and forms a typical black eschar (scab). Four days to 2 weeks after the bite, symptoms begin with fever and malaise, followed by adenitis in the lymph nodes draining the bite site. As the organisms spread throughout the body, fever, malaise and headache increase, and general lymphadenopathy occurs in most cases. About a week after onset, the main features are continuous fever, signs of bronchitis or pneumonia, photophobia, conjunctivitis, generalised adenopathy, delirium, deafness and a maculopapular rash, most commonly over the trunk and proximal limb parts. Splenomegaly occurs in some cases.

Fever may persist for 14 days without antibiotic treatment. The fatality rate in untreated cases is 1–40 per cent. This increases with age and depends on the infection site, the type of Rickettsia involved and previous exposure.

Diagnosis

In endemic areas, the clinical picture is sufficiently distinctive for a clinical diagnosis, and empirical treatment should be instigated because of the frequent delay in laboratory diagnosis. Definitive diagnosis can be made by isolation of Rickettsia after inoculation of the patient’s blood into mice, although this requires a specialised laboratory and is not used routinely. Serological methods are the current mainstay of diagnosis; however, the results need to be interpreted with caution because of cross-reactivity between strains. A biopsy of the eschar can be used to demonstrate Rickettsia by immunofluorescence or polymerase chain reaction (PCR). PCR may also be run on serum specimens.

Incubation period of rickettsial infections

The incubation period is 2–14 days. The variation in incubation period may in part be related to the inoculum size.

Public health significance and occurrence of rickettsial infections

The epidemiology varies in different parts of the world. Disease occurrence is often associated with the modification of natural habitats by humans, such as felling of a forest and its replacement by a secondary growth of scrub.

R. australis occurs along the eastern side of Australia, R. honei has been recognised on Flinders Island near Tasmania, and R. typhi occurs throughout many states of Australia. Scrub typhus occurs in Queensland, and parts of the Northern Territory and Kimberley region. The public health impact on lives or productivity lost is largely unmeasured, but is suspected to be high.

Reservoir of rickettsial infections

Humans are incidental hosts and are not useful in propagating the organism in nature. Many rats, mice and other small mammals act as reservoirs, as does transovarial transmission within ticks and mites. An exception is louse-borne typhus (R. prowazekii), which does not occur in Australia. Humans are the principal reservoir for louse-borne typhus, and the human body louse (Pediculus humanus var. humanus) is the vector.

Mode of transmission of rickettsial infections

The disease is not directly transmitted from person to person. Humans are infected by the bite of an infected larval tick, flea, louse or chigger mite.

Period of communicability of rickettsial infections

People are at risk of infection for as long as they remain in infected areas. In the case of louse-borne typhus, a person is infective for lice during the febrile illness and probably 2–3 days after their temperature returns to normal.

Susceptibility and resistance to rickettsial infections

All nonimmune people are susceptible to infection, depending on environmental exposure. Long-lasting immunity probably follows infection.

Control measures for rickettsial infections

Preventive measures

There is no vaccine available. People who enter infected areas can be protected by impregnating their clothing with pesticide and renewing personal insect repellent frequently. Chemoprophylaxis can be successfully used short term; for this, a consultation with an infectious diseases specialist is recommended. Campers can also help to prevent tick bites by using camp beds for elevation from the floor and wearing clothing that minimises exposed skin.

Control of case

Treatment is generally oral doxycycline. Consult the current version of Therapeutic guidelines: antibiotic.

In severe disease, consultation with an infectious diseases specialist is recommended.

Control of contacts

Consider active case finding if other people were exposed to the same setting as the case, such as a camping holiday or military exercise.

Control of environment

Not applicable. The mites themselves act as reservoirs, so no immediate effect is achieved by rodent control.

Special settings

Not applicable.

Outbreak measures for rickettsial infections

Except in the case of an epidemic of louse-borne typhus, no outbreak measures are necessary.

International measures

In the event of an epidemic of louse-borne typhus in Australia, the department will notify the World Health Organization and neighbouring countries of this occurrence in an area previously free from the disease.

Further information

Bell D 1995, Tropical medicine, 4th edn, Blackwell Science.