Background
Rickettsioses refers to a group of infectious diseases that are caused by rickettsial organisms and that result in an acute febrile illness. Arthropod vectors such as ticks, mites, lice, and fleas transmit the etiologic agents to humans. [1] Within this group there is a subgroup transmitted by fleas; these include the following: epidemic or louse-borne typhus and its recrudescent form known as Brill-Zinsser disease, murine typhus, and scrub typhus. (For more information on pediatric scrub typhus, see the Medscape Reference article Pediatric Scrub Typhus.
Pathophysiology
Epidemic typhus is the prototypical infection of the flea-borne rickettsioses group of diseases, and the pathophysiology of this illness is representative of the entire category. The arthropod vector of epidemic typhus is the body louse (Pediculus humanus humanus). This is the only vector of the group in which humans are the usual host. Rickettsia prowazekii, which is the etiologic agent of epidemic typhus, lives in the alimentary tract of the louse. A Rickettsia- harboring louse bites a human to engage in a blood meal and causes a pruritic reaction on the host's skin. The louse defecates as it eats; when the host scratches the site, the lice are crushed, and the Rickettsia- laden excrement is inoculated into the bite wound. The Rickettsia travel to the bloodstream and rickettsemia develops.
R prowazekii also is thought to be transmitted in a sylvatic cycle by fleas associated with flying squirrels in the eastern half north of North America. Here, infection is related to inhaling dried louse feces, or by rubbing Rickettsia -containing louse feces inadvertently into eyes, mucous membranes, or in insect bite–associated wounds. [1, 2]
Rickettsia sp parasitize the endothelial cells of the small vessels and proliferate, causing endothelial damage, leading to increased vascular permeability, thrombosis, and inflammation. Finally, hypovolemia, organ failure, and on occasions, death, result.
Some people with a history of epidemic typhus may develop a recrudescent type of epidemic typhus known as Brill-Zinsser disease. After a patient with typhus is treated with antibiotics and the disease appears to be cured, Rickettsia may linger in the body tissues. Months, years, or even decades after treatment, organisms may reemerge and cause a recurrence of typhus. How the Rickettsia organisms linger silently in a person and by what mechanism recrudescence is mediated are unknown. The presentation of Brill-Zinsser disease is less severe than epidemic typhus, and the associated mortality rate is much lower. Risk factors that may predispose to recrudescent typhus include improper or incomplete antibiotic therapy and decreased immunity (eg, malnutrition, alcoholism, and advanced age). [1]
Murine typhus and scrub typhus have the same physiopathology as epidemic typhus, however murine typhus usually is milder. Scrub typhus severity usually depends on time to start antibiotics, and number of recurrences. It is more severe after the first episode.
Epidemiology
United States
Sporadic cases of active infection with R prowazekii, the etiologic agent of epidemic typhus, have been reported. These occurred in the central and eastern portions of the United States and have been linked with exposure to flying squirrels (Glaucomys volans). [2] The flying squirrel acts as the host for R prowazekii, and transmission to humans is believed to occur via squirrel fleas or lice. Murine typhus caused by infection with Rickettsia typhi is associated with exposure to rats, opossums, [3] cats, and their fleas, and it occurs in southern California and southern Texas. Most cases of murine typhus in Texas occur in spring and summer, whereas in California, the illness is most common in the summer and fall.
In the last decades, there has been an increase in murine typhus cases in South Texas, which may reflect a re-emergence of R typhi in the rat population and/or a cycle involving opossums and cats. [4] Murine typhus is most common in adults, but infection may occur in any age group. [5]
International
Epidemic typhus occurs in Central and South America, Africa, northern China, and certain regions of the Himalayas. Outbreaks may occur when conditions arise that favor the propagation and transmission of lice (crowding and lack of sanitation). Brill-Zinsser disease develops in approximately 15% of people with a history of primary epidemic typhus.
Murine typhus occurs in most parts of the world, particularly in tropical and subtropical and temperate coastal port regions where rats, mice, and cats, hosts of the disease, commonly are found. Murine typhus occurs mainly in sporadic cases, and incidence probably is greatly underestimated in the more endemic regions. Populations of the flea vector may rise during the summer months in temperate climates, subsequently increasing the incidence of murine typhus. The homeless are particularly vulnerable. [6]
Scrub typhus occurs in the western Pacific region, northern Australia, and the Indian subcontinent. The incidence of scrub typhus is largely unknown, although it is estimated to be 1 million cases per year worldwide. Many cases are undiagnosed because of its nonspecific manifestations and the lack of laboratory diagnostic testing in endemic areas. It is important to identify scrub typhus as an important cause of undifferentiated fever in the returning traveler. Finally, experts believe there will be a re-emergence of disease in relation to antibiotic resistance, deforestation and global warming. [7]
Mortality/Morbidity
Epidemic typhus causes the most severe clinical presentation among the flea-borne rickettsioses group infections. Patients with severe epidemic typhus may develop gangrene, leading to a loss of digits, limbs, or other appendages. The vasculitis of epidemic typhus process also may lead to CNS dysfunction, ranging from dullness of mentation to coma, multiorgan system failure, and death. Untreated epidemic typhus carries a mortality rate of as low as 13% [8] in otherwise healthy individuals, and as high as 50% in elderly or debillitated persons. [9]
Since the advent of widely available antibiotic treatment, the mortality rates associated with epidemic typhus have fallen to approximately 3-4%.
The mortality rate among patients varies according to different reports between 0.4-4%. [10, 11]
Scrub typhus median mortality is around 6%, although if untreated cases could be as high as 70%. [12, 13]
Sex
The flea-borne rickettsioses group of infections has no sexual predilection.
Age
The flea-borne rickettsioses group of infections has no age predilection. However, in the United States, murine typhus and sporadic cases of epidemic typhus have mainly occurred in adults.
Prognosis
Uncomplicated cases of flea-borne rickettsioses that are diagnosed promptly, and antibiotic therapy initiated early generally carry an excellent prognosis. Mortality rates are greatly reduced when appropriate antibiotics are initiated promptly (see Mortality/Morbidity).
Complicated cases of flea-borne rickettsioses generally carry a good prognosis, but this varies depending on the severity of the specific complications and the health status of the patient at the time of disease onset.
Patient Education
Education concerning typhus should be focused on the preventive measures (see Deterrence/Prevention).
