What are the rickettsial diseases?
Rickettsiae are small, obligate intracellular, Gram negative bacteria that spend part of their life cycle in an arthropod host (tick, flea, body louse or mite). Humans are infected with the rickettsial organism either via a bite or contact with faeces from an infected arthropod. The disease that develops depends upon the specific bacterium transmitted. Many different bacterial species are classified within the rickettsial group and a variety of different clinical manifestations are seen which vary greatly in terms of severity. The majority of rickettsial diseases result in a rash and some will also result in a cutaneous eschar, aiding diagnosis.
What are rickettsia typhi?
Rickettsial typhi refers to a group of 3 arthropod-borne rickettsial infections that cause typhus fever. They are also called typhus group rickettsial disease.
- Epidemic louse-borne typhus
- Sylvatic typhus
- Murine typhus
Who gets rickettsia typhi?
As the rickettsial diseases are transmitted via arthropods, humans at risk of infection are those who come into contact with the specific arthropod vectors. In the case of rickettsial typhi, the vectors are lice and fleas. The bacteria are transmitted via infected faeces that are either inhaled or rubbed into the skin/mucous membranes.
Louse-borne epidemic typhus is seen in emergency situations where there is overcrowding and washing facilities are limited, for example in refugee camps.
Classification of rickettsia typhi
Epidemic louse-borne typhus
- Distribution: worldwide but associated with emergency situations
- Bacteria: Rickettsia prowazekii
- Vector: human body louse (Pediculus humanus var. corporis)
- Hosts: humans
- Severity: moderate to severe
- Rash: generalised maculopapular eruption
- Systemic features: neurological (deafness, delirium), abdominal pain
- Complications: multisystem involvement, Brill-Zinsser disease – recrudescence months to years later
- Mortality risk: significant (20–50%) if untreated
- Sylvatic typhus is a rare and milder form of epidemic typhus occurring the eastern United States.
- The infection is associated with close contact with flying squirrels, although the mode of transmission is not yet clear.
- Distribution: worldwide
- Bacteria: R typhi
- Vector: fleas (Xenopsylla cheopis, Ctenocephalides felis)
- Hosts: rodents
- Severity: mild
- Rash: maculopapular; affecting trunk, then limbs, with sparing of palms and soles
- Systemic features: similar to louse-borne typhus but milder
- Seasonality: more common in warmer months
- Mortality risk: low (< 1%)
What are the clinical features of rickettsia typhi?
Symptoms develop 7–14 days after inoculation.
The classical triad of symptoms in all rickettsial diseases is:
- Severe headache
- Myalgia (muscle pain).
Rash develops around 3–6 days after the onset of disease.
- It is typically maculopapular (erythematous macules and papules).
- It spares face, palms and soles.
- Eschars are NOT seen in rickettsial typhus (unlike scrub typhus).
What are the complications of rickettsia typhi?
Complications of typhus include:
- Inflammation of the lungs or liver
- Acute renal failure
- Multiple organ failure.
In epidemic louse-borne typhus there is a risk of Brill-Zinsser disease – recrudescence months to years later.
How are rickettsia typhi diagnosed?
Diagnosis of a rickettsial typhus infection is based on the clinical presentation and risk factors for exposure to body lice or fleas. Laboratory confirmation can be difficult and includes:
- Polymerase chain reaction (PCR)
- Cell culture.
What is the differential diagnosis for rickettsia typhi?
The differential diagnosis for rickettsia typhi includes any disease causing rash, fever and headache.
What is the treatment for rickettsia typhi?
Rickettsial infections can be treated successfully with tetracycline antibiotics, especially doxycycline. Chloramphenicol is the second-line treatment option.
Prevention of rickettsia typhi
No vaccines exist for rickettsial infections.
Rickettsial infections can be prevented by taking measures to reduce exposure to lice and fleas.
- Reducing domestic rodent populations (murine typhus)
- Reducing exposure to flying squirrels and their nests (sylvatic typhus)
- Applying an effective insect repellent, such as DEET
- Treating clothes and equipment with permethrin
- Washing clothes and bed linen regularly (epidemic typhus)
What is the outcome of rickettsia typhi?
All forms of rickettsial typhus infection can be life-threatening. Murine and sylvatic typhus tend to be much less severe than epidemic typhus.
Brill-Zinsser disease is a potential complication of epidemic typhus, where there is a recrudescence of the disease months to years later; this is typically milder than the original disease.