Rickettsia prowazekii is a species of intracellular, gram-negative coccobacillus. An obligate parasite, R. prowazekii, belongs to the order Rickettsiales and is the causative agent of the epidemic typhus.
The genus Rickettsiae is composed of gram-negative bacteria and is the closest known relatives of mitochondria in eukaryotic cells.
Rickettsia prowazekii is a species that is majorly maintained in human populations and is transmitted through human body lice.
People who have been infected develop an acute, mild to severe illness that is sometimes accompanied by neurological symptoms such as gangrene of the fingers and toes, and shock.
It has been estimated that about 10-30% of untreated clinical cases are fatal, with even higher mortality rates in poor communities and the elderly.
The organism continues to reside in the body long after recovery and may cause a similar, though milder, sickness called Brill-Zinsser disease.
The bacteria is known for regularly causing extensive outbreaks in the past, killing people in their thousands and even millions.
Thus giving the common name for the disease, epidemic typhus. Although the disease no longer occurs in developed areas, the few cases that have occurred in recent years were acquired while traveling to endemic areas or those that have carried it for years without showing any symptoms.
The flying squirrel maintains the disease in North America, resulting in sporadic zoonotic cases (animal to human transmission).
However, severe outbreaks of the disease still occur in some low-income countries, especially densely populated areas with poor hygiene and sanitation.
Epidemic typhus has the potential to emerge anywhere social conditions disintegrate, and human body lice spread unchecked.
Mode of Transmission
R. prowazekii is transmitted by the bite of an infected louse, or contact with louse feces or crushed louse tissue.
The disease can also be transmitted by inhaling infectious aerosols, including louse fecal dust, which poses a potential risk for healthcare workers and laboratory personnel.
The body louse gets infected when feeding on an infected human, and the flying squirrel may be infected by lice and fleas.
The incubation period of the bacteria is usually between 10-14 days. While human-to-human transmission of the disease does not occur, proximity to an infected individual increases the chances of exposure to infected lice.
Humans can remain infective for life and can pass the disease to lice. The lice tend to leave sick individuals and move towards healthier hosts.
The infected lice become infective 5-7 days after exposure and remain so for life; however, they tend to die one week after contracting the bacteria. The lice feces remain infective for 100 days as aerosols, and this is the proposed mode of transmission between flying squirrels and humans.
Signs and Symptoms
Epidemic typhus is typically characterized by 1-3 days of illness before the sudden onset of severe headaches and fever as high as 40 ºC.
Symptoms of epidemic typhus include:
- Stupor and seeming out of touch with reality
- Hypotension (low blood pressure)
- Eye sensitivity to bright lights
- Rash that begins on the back or chest and spreads
- Abdominal pain
- Muscle pain
- Rash that starts on the back or chest and spreads
Up to 80% of cases of epidemic typhus have complications of the central nervous system such as coma, confusion, delirium, seizures, drowsiness, and hearing loss.
Cough has been reported in 38- 70% of cases, and gangrene and necrosis of fingers and toes have occurred. The disease typically lasts for two weeks; however, it may take months to recuperate fully.
Epidemic typhus has a low mortality rate if the patient is treated with the appropriate antibiotics, but the mortality rate is higher in people over the age of 60.
A milder recurrent form of typhus known as Brill Zinsser disease may occur in a patient that has previously had epidemic typhus.
The diagnosis of rickettsia prowazekii is based upon serology. Antibodies of the bacteria are detected only after ten days from the onset of systemic symptoms.
Thus appropriate antibiotics should be administered upon suspicion of an infection from the bacteria without waiting for diagnostic confirmation using specific serological tests.
This would prevent the occurrence of wide-scale pandemics.
Treatment of rickettsia prowazekii is by administering oral doses of doxycycline 100 mg twice daily until the patient improves and has been without fever for 24 to 48 hours.
The total duration of treatment is typically between seven and ten days. Chloramphenicol 500 mg by mouth or intravenously four times daily for seven to ten days can also be used to treat the disease.
In severe cases, additional therapy using intravenous fluids is recommended. Critically ill patients may have marked capillary permeability and can easily succumb to cerebral and pulmonary edema.