Mini review
Vectors of rickettsiae in Africa

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Abstract

Vector-borne diseases are caused by parasites, bacteria, or viruses transmitted by the bites of hematophagous arthropods. In Africa, there has been a recent emergence of new diseases and the re-emergence of existing diseases, usually with changes in disease epidemiology (e.g., geographical distribution, prevalence, and pathogenicity). In Africa, rickettsioses are recognized as important emerging vector-borne infections in humans. Rickettsial diseases are transmitted by different types of arthropods, ticks, fleas, lice, and mites. This review will examine the roles of these different arthropod vectors and their geographical distributions.

Introduction

Members of the genus Rickettsia are divided into 2 main groups: the spotted fever group (SFG) and the typhus group (TG). SFG rickettsiae are mainly associated with ticks, but also with fleas (Rickettsia felis) and mites (Ri. akari). Within ticks, transmission can be transovarial and transstadial. SFG rickettsiae have an optimal growth temperature of 32 °C and a G/C content between 32 and 33%. They can polymerize actin and thus can move into the nuclei of host cells (Heinzen et al., 1993, Teysseire et al., 1992, Teysseire et al., 1995). SFG rickettsiae can cause spotted fevers in humans. In contrast, TG rickettsiae are associated with human body lice (Ri. prowazekii) or fleas (Ri. typhi). TG rickettsiae have an optimal growth temperature of 35 °C and a G/C content of 29%. They cannot polymerize actin and thus cannot enter the nuclei of host cells and are thereby only found in the cytoplasms of host cells (Heinzen et al., 1993, Teysseire et al., 1995). TG rickettsiae cause epidemic (Ri. prowazekii) and murine (Ri. typhi) typhus in humans.

The distribution of vector-borne rickettsioses is summarized in Table 1. In this review, we have analyzed 15 species of the Rickettsia genus that have been detected or isolated in arthropods in Africa.

Section snippets

Flea-borne rickettsioses

Siphonaptera (fleas) is a highly specialized holometabolous insect order, currently comprising 246 genera and approximately 2575 described taxa (including subspecies – modified from Lewis, 1999). Fleas are laterally compressed, wingless insects that range from 1 to 10 mm in length. Their head is usually small and shield-shaped (Dunnet and Mardon, 1999). Fleas are obligate endo- or ectoparasites of birds and mammals. Fleas occurring on birds are only represented by 5 families: Ceratophyllidae (68

Murine typhus and Rickettsia typhi

Murine typhus is a worldwide zoonosis and is also known as flea-borne, rat, urban and endemic typhus. The etiological agent, Ri. typhi, is transferred from a rodent reservoir by an arthropod (often X. cheopis) to humans (Houhamdi and Raoult, 2007). Many recent reports state that this is a re-emerging disease that spreads via travelers returning from endemic regions.

Occasionally, other flea species or arthropod vectors have been reported to transmit Ri. typhi including the cat flea C. felis, the

Tick-borne rickettsioses

Ticks are currently considered to be second only to mosquitoes as vectors of human infectious diseases worldwide. All of the nearly 900 known species of ticks require blood for their development and reproduction, and they parasitize animals in almost every region of the world. Two families of ticks are of medical significance: Ixodidae (hard ticks) and Argasidae (soft ticks). To date, most ticks infected with SFG rickettsiae belong to the Ixodidae family. Ixodid ticks feed once within each

Mite-borne rickettsioses

Mites are one of the most common groups of animals, with 45,000 recognized species. The house mouse mite (Liponyssoides sanguineus) is the vector for rickettsialpox due to infection with Ri. akari. The relatively mild disease, characterized by fever and exanthema, was first described in the neighborhood of Kew Garden in the Bronx, New York, during the 1940s (Huebner et al., 1946). This disease has been reported from the Republic of Central Africa (Le Gac and Giroud, 1951). Although this

Louse-borne diseases

The body louse, Pediculus humanus corporis, is a strictly hematophagous ectoparasite that lives in clothing throughout its life cycle. It is specific for human beings in natural conditions. Body lice are distributed in worldwide (Bechah et al., 2008).

The role of the body louse P. humanus corporis in the transmission of epidemic typhus was demonstrated by Charles Nicolle in 1909 (Gross, 1996). Nicolle observed that patients with epidemic typhus were no longer contagious after being admitted to

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