Elsevier

Clinics in Dermatology

Volume 24, Issue 3, Mayā€“June 2006, Pages 191-200
Clinics in Dermatology

Tropical rickettsioses

https://doi.org/10.1016/j.clindermatol.2005.11.007Get rights and content

Abstract

In recent years, a bewildering array of emerging rickettsial pathogens have been described throughout the world, including in the tropics. Here we present an updated overview of scrub typhus, murine typhus, and epidemic typhus. We also present an update on the emerging spotted fever group rickettsioses described in the tropics through 2005, focusing on epidemiologic and clinical data and management.

Introduction

Rickettsioses (also called typhus) are infectious diseases caused by obligate intracellular bacteria of the order Rickettsiales.1, 2 These organisms have long been described simply as short, Gram-negative rods that retain basic fuchsin when stained by the method of Gimenez. In recent years, however, the rickettsial field has undergone a significant evolution due to technological advances in molecular genetics and genomics. The taxonomy of rickettsiae has been reorganized and continues to be modified as new data become available.2 Four groups of diseases, however, are still commonly called rickettsioses: (1) diseases due to bacteria of the genus Rickettsia including the spotted fever group and the typhus group of the family Rickettsiaceae, (2) scrub typhus due to Orientia tsutsugamushi (formerly named R tsutsugamushi), (3) human ehrlichioses and anaplasmosis due to bacteria within the family Anaplasmataceae, and (4) the ubiquitous Q fever due to Coxiella burnetii, which was, however, recently removed from the Rickettsiales. Here, we review tropical rickettsioses, focusing on clinical and epidemiologic aspects and management. The reader is therefore referred to recent reviews of ehrlichioses and anaplasmosis (which have not been properly demonstrated to occur in the tropics)3, 4 and Q fever.5

Section snippets

Exposure

The agents of tropical rickettsioses are associated with arthropods including ticks, mites, fleas, and lice, which may act as vectors, reservoirs, and/or amplifiers of the bacteria.1, 3 Thus, exposure to the disease is closely linked to exposure to the arthropod vectors. Most of them favor specific optimal environmental conditions, biotopes, and hosts. These factors determine the geographic repartition of the vector and consequently the risk area for the diseases. This is particularly true when

Tick-borne spotted fever group rickettsioses

Tick-borne rickettsioses are among the oldest known vector-borne diseases. Both Rocky Mountain spotted fever (RMSF) due to R rickettsii and Mediterranean spotted fever (MSF) due to R conorii conorii have been described since the beginning of the 20th century. During most of the 20th century, the epidemiology of tick-borne rickettsioses could be summarized as the occurrence of a single pathogenic Rickettsia on each continent, with the addition of R sibirica (in the former USSR and China) and R

Flea-borne murine typhus

Murine typhus (also called endemic typhus) is one of the oldest recognized arthropod-borne zoonoses. It was probably reported in Mexico in 1570 by Bravo and described clinically in grain silo workers in Australia in 1923. The disease was recognized to be distinct from epidemic typhus in the 1920s and the causative organism named R mooseri and, thereafter, R typhi. It is a Gram-negative obligate intracellular bacterium belonging to the typhus group of the genus Rickettsia.1

The main vector of

Louse-borne rickettsiosis: epidemic typhus

Epidemic typhus is caused by R prowazekii, a typhus group Rickettsia whose entire genome has been sequenced, and it is considered one of several prototypical reduced genome bacteria.30 The disease is suspected to have been responsible for the ā€œAthenian plagueā€ in the fifth century bc. Typhus was apparently widespread in Italy during the 16th century, a period when war ravaged both France and Spain.

Huxham first made the distinction between the typhus and typhoid in 1739. Boissier de Sauvages

Scrub typhus

Orientia tsutsugamushi (formerly named R tsutsugamushi) is the agent of scrub typhus. These bacteria had been classified for a long time in the order Rickettsiales. In the recent years, however, with the advent of molecular taxonomic methods in phylogenetic studies, the position of R tsutsugamushi has been shown to be sufficiently distinct to justify the creation of a new genus, Orientia.40 Orientia tsutsugamushi has enormous genetic and antigenic variability, but seems genetically stable.24

Routine laboratory investigation

Routine laboratory abnormalities in rickettsioses include leukocyte count abnormalities, anemia, thrombocytopenia, hyponatremia, hypoalbuminemia, and hepatic and renal abnormalities. In an attempt to distinguish between scrub typhus and dengue in HIV-negative patients presenting with fever of unclear etiology from northern Thailand, Watt et al51 identified three features that were found to be more common in dengue as compared with scrub typhus: bleeding from the gums, a low platelet count, and

Management of tropical rickettsioses

Empirical treatment of rickettsioses is usually initiated before laboratory confirmation of the diagnosis, which is seldom available in most tropical settings.

Doxycycline remains the treatment of choice for all rickettsioses and for all patients, including young children, with potentially life-threatening rickettsioses such as RMSF.2 In general, the risk of dental staining by doxycycline is negligible when a single, relatively short (eg, 5-10 days) course of therapy is administered. Parenteral

Prevention

Currently, there are no vaccines against scrub typhus and other tropical rickettsioses. Prevention is therefore based on avoiding contact with the arthropod vector. Topical DEET (N,N-diethyl-m-toluamide) applied to exposed skin will prevent tick, flea, and chigger bite. Furthermore, clothing (or army uniforms) should be treated with permethrin that is neurotoxic to insects and has ā€œknockdownā€ capability, killing arthropods on contact.66, 67, 68 Once applied, permethrin is stable through a

Conclusions

In conclusion, little is known about the rickettsioses in the tropics. Much of the recent information on the epidemiology and clinical particularities of the diseases have been obtained in first-world laboratories that promote international co-operation with tropical countries. Recent data on the clinical aspects of the diseases have been derived from infected visitors returning from the tropics. An increase in persons traveling to tropical climates to participate in recreational activities

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