What are the rickettsial diseases?
Rickettsiae are small, obligate intracellular, Gram-negative bacteria that spend part of their life cycle in an arthropod host (eg, a 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 diverse 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 three arthropod-borne rickettsial infections that cause typhus fever. These infections are also called typhus group rickettsial disease. These infections are:
- Epidemic louse-borne typhus
- Sylvatic typhus
- Murine typhus.
Who gets rickettsia typhi?
As the rickettsial diseases are transmitted via arthropods, people 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 (eg, in refugee camps).
Vectors for rickettsia typhi
Classification of rickettsia typhi
Epidemic louse-borne typhus
Epidemic louse-borne typhus is distributed worldwide but is associated with emergency situations. The bacterium Rickettsia prowazekii is carried and transmitted to humans by the human body louse (Pediculus humanuscorporis).
The infection tends to be moderate to severe and a generalised maculopapular eruption is seen. The systemic features of epidemic louse-borne typhus include neurological symptoms (eg, deafness and delirium) and abdominal pain. Its complications include multisystem involvement and Brill–Zinsser disease — with a recurrence of disease months to years later.
There is a significant mortality risk of 20–50% if left untreated.
Sylvatic typhus
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.
Murine typhus
Murine typhus is caused by R. typhi and is distributed worldwide. Rodents are the main animal hosts for fleas (Xenopsylla cheopis or Ctenocephalides felis) carrying the infection.
Murine typhus is typically mild in severity, with a maculopapular rash affecting the trunk and limbs, while sparing the palms and soles. Its systemic features are similar to louse-borne typhus but more mild.
Murine typhus occurs more commonly in warmer months and the mortality risk is 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:
- Fever
- Severe headache
- Myalgia (muscle pain).
Rash develops around 3–6 days after the onset of disease.
- The rash is typically maculopapular (erythematous macules and papules).
- It spares the 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
- Meningoencephalitis
- Acute renal failure
- Multiple organ failure.
In epidemic louse-borne typhus, there is a risk of Brill–Zinsser disease, with a recurrence of disease months to years later.
How are rickettsia typhi diagnosed?
The 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:
- Serology
- 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
There are no existing vaccines for rickettsial infections.
Rickettsial infections can be prevented by taking measures to reduce exposure to lice and fleas. These measures include:
- Reducing domestic rodent populations (to prevent murine typhus)
- Reducing exposure to flying squirrels and their nests (to prevent sylvatic typhus)
- Applying an effective insect repellent, such as N,N diethyl-3-methylbenzamide (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 recurrence of the disease months to years later; this is typically milder than the original disease.