RICKETTSIAL INFECTIONS

• Murine or Endemic Typhus - Rat-Flea Typhus; Urban Typhus Of Malaya

An acute febrile disease clinically similar to, but milder than, epidemic typhus, caused by Rickettsia typhi (R. mooseri) and transmitted to humans by rat fleas.

Epidemiology

The causative agent, R. typhi (R. mooseri), resembles other rickettsiae morphologically and in intracellular parasitism. The animal reservoir includes wild rats, mice, and other rodents; rat fleas (Xenopsylla cheopis) and probably cat fleas (Ctenocephalides felis) transmit the agent to humans. The distribution is sporadic and worldwide, but the incidence is low and is more prevalent in rat-infested areas.

• Scrub Typhus - Tsutsugamushi Disease; Mite-Borne Typhus; Tropical Typhus

A mite-borne disease caused by Rickettsia tsutsugamushi and characterized by fever, a primary lesion, a macular rash, and lymphadenopathy.

Epidemiology

R. tsutsugamushi is transmitted in nature by trombiculid mites, which feed on forest and rural rodents, including rats, voles, and field mice. Human infection follows a chigger (mite larva) bite. Scrub typhus occurs in the Asiatic-Pacific area bounded by Japan, India, and Australia. Sporadic cases have occurred in Americans, particularly those who visit northern Thailand or India.

• Ehrlichiosis

A febrile illness resembling Rocky Mountain spotted fever caused by rickettsial-like bacteria of the genus Ehrlichia and transmitted to humans by ticks.

Etiology and Epidemiology

Ehrlichia are obligate, intracellular bacteria that appear as small cytoplasmic inclusions in lymphocytes and neutrophils. Infections are transmitted to humans via tick bites sometimes via contact with dogs that carry the brown tick. Most cases have been identified in the southeastern and south-central parts of the USA. Two species of Ehrlichia are human pathogens in the USA: E. chaffeensis causes human monocytic ehrlichiosis and E. phagocytophilia or a related organism causes human granulocytic ehrlichiosis. E. canis is now regarded as human monocytic ehrlichiosis.

• Rickettsialpox (Vesicular Rickettsiosis)

A mild, self-limited, febrile disease caused by Rickettsia akari with an initial local lesion and a generalized papulovesicular rash.

First observed in New York City, rickettsialpox has occurred in other U.S. areas and in Russia, Korea and Africa. The vector--a small, colorless mite, Allodermanyssus sanguineus--is widely distributed. It infects the house mouse (Mus musculus) as well as some species of wild mice and can transmit R. akari transovarially. Humans may be infected by either chigger or adult mite bites.

• Q Fever

An acute disease caused by Coxiella burnetii (Rickettsia burnetii) and characterized by sudden onset of fever, headache, malaise and interstitial pneumonitis.

Epidemiology

Worldwide in its distribution, Q fever is maintained as an inapparent infection in domestic or farm animals. Sheep, cattle, and goats are the principal reservoirs for human infection. C. burnetii persists in feces, urine, milk and tissues (especially the placenta), so that fomites and infective aerosols form easily. C. burnetii is also maintained in nature through an animal-tick cycle. Various arthropods, rodents, other mammals, and birds are naturally infected and may play a role in human infection.

Cases occur among workers whose occupations bring them in close contact with farm animals or their products. Transmission is usually by inhalation of infected aerosols, but the disease can also be contracted by ingesting infective raw milk.

• Bartonellosis

Infections caused by Bartonella sp that usually cause an acute febrile anemia, a chronic cutaneous eruption, cat-scratch disease or disseminated disease in immunocompromised hosts.

Epidemiology

The genus Bartonella, a group of small, weakly-staining, gram-negative bacteria, includes three species of human medical importance. The genus has been expanded recently by the reclassification of organisms formerly in the genus Rochalimaea. The organisms in this genus are:  B. quintana, B. henselae and B. bacilliformis.

• Epidemic Typhus (European, Classic, Or Louse-Borne Typhus; Jail Fever)

An acute, severe, febrile, louse-borne disease caused by Rickettsia prowazekii, characterized by prolonged high fever, intractable headache, and a maculopapular rash.

Epidemiology

R. prowazekii is prevalent worldwide and transmitted to humans in feces of the human body louse Pediculus humanus when a puncture wound is contaminated by scratching. Dried louse feces may also transfer the microbe to the mucous membranes of the eyes or mouth. Humans are the natural reservoir of infection. In the USA, humans may occasionally contract epidemic typhus fever that is generally milder than classic typhus after contact with flying squirrels, their ectoparasites, or aerosolized lice feces; it is identified by serologic tests.

• Eastern Tick-Borne Rickettsioses

Mild to moderately severe febrile diseases caused by various rickettsia transmitted by ixodid ticks and characterized by an initial lesion, satellite adenopathy, and an erythematous maculopapular rash.

Epidemiology

Eastern tick-borne rickettsioses include North Asian tick-borne rickettsiosis, Queensland tick typhus, African tick typhus, and Mediterranean spotted fever [fièvre boutonneuse]. The causative agents belong to the spotted fever group of rickettsiae. North Asian tick-borne rickettsiosis, caused by R. sibirica, occurs in Armenia, Central Asia, Siberia, and Mongolia; Queensland tick typhus, caused by R. australis, occurs in Australia. Fièvre boutonneuse, the prototype of the group, caused by R. conorii, occurs throughout the African continent, in India, and in areas of Europe and the Mideast adjacent to the Mediterranean, Black, and Caspian seas. It often is known by the area in which it occurs (eg, Indian tick typhus, Mediterranean spotted fever, Marseilles fever).

The epidemiology of these tick-borne rickettsioses resembles that of spotted fever in the Western Hemisphere. Ixodid ticks and wild animals maintain the rickettsiae in nature; if humans intrude accidentally into the cycle, they become infected. In certain areas, the cycle of fièvre boutonneuse involves domiciliary environments, with the brown dog tick, Rhipicephalus sanguineus, as the dominant vector. Transovarial transmission of rickettsiae occurs in various ticks.

• Rocky Mountain Spotted Fever (Spotted Fever; Tick Fever; Tick Typhus) - RMSF

An acute febrile disease caused by Rickettsia rickettsii and transmitted by ixodid ticks, producing high fever, cough, and rash.

Epidemiology & Pathology

Rocky Mountain spotted fever (RMSF) is limited to the Western Hemisphere. Initially recognized in the Rocky Mountain states, it occurs in practically all states (except Maine, Hawaii, and Alaska) in the USA, especially the Atlantic states. In humans, infection occurs mainly from May to September, when adult ticks are active and persons are most likely to be in tick-infested areas. In southern states, cases occur throughout the year. The incidence is high in children < 15 yr and in others who frequent tick-infested areas for work or recreation.

Hard-shelled ticks (family Ixodidae) harbor R. rickettsii and infected females transmit the agent to their progeny. These ticks and some mammalian hosts are the natural reservoirs; animals provide blood nourishment. Dermacentor andersoni (the wood tick) is the principal vector in the western USA; D. variabilis (dog tick) is the vector in the eastern and southern USA. The organism is also maintained in rabbits and other small mammals. 

RMSF is probably not transmitted directly from person to person even via infectious particles produced by the cough.

Source: Merck