Introduction - rickettsial diseases
Introduction - rickettsia typhi
Demographics
Classification
Clinical features
Complications
Diagnosis
Differential diagnoses
Treatment
Prevention
Outcome
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.
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:
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).
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 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 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%).
Symptoms develop 7–14 days after inoculation. The classical triad of symptoms in all rickettsial diseases is:
Rash develops around 3–6 days after the onset of disease.
Complications of typhus include:
In epidemic louse-borne typhus, there is a risk of Brill–Zinsser disease, with a recurrence of disease months to years later.
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:
The differential diagnosis for rickettsia typhi includes any disease causing rash, fever and headache.
Rickettsial infections can be treated successfully with tetracycline antibiotics, especially doxycycline. Chloramphenicol is the second-line treatment option.
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:
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.
