Lesser known rickettsial infections in animals and humans

Previous authors: K J SUMPTION AND G R SCOTT

Current authors:
A L FEDROW - Assistant Professor, MS, PhD, Shippensburg University, 1871 Old Main Drive, Shippensburg, Pennsylvania, 17257, United States of America
K E MULLINS - Associate Director of Chemistry, Point of Care, and Clinical Laboratory Support, PhD, 111 Michigan Avenue, N.W., Washington DC, 20010, United States of America
M L LEHMAN - Professor, PhD, Biology Department, Shippensburg University, 1871 Old Main Drive, Shippensburg, Pennsylvania, 17257, United States of America
R L STEWART - Shippensburg University, 1871 Old Main Drive, Shippensburg, Pennsylvania, 17257, United States of America

GENERAL INTRODUCTION: RICKETTSIAL, CHLAMYDIAL AND HAEMOTROPIC MYCOPLASMAL DISEASES

General introduction

In 2001, the order Rickettsiales was subjected to a taxonomic reorganization based upon genetic analyses of 16S rRNA, groESL, and surface protein genes⁸⁷ The major changes were in the families Rickettsiaceaeand Anaplasmataceae, including:

1. the family Rickettsiaeceaewas amended to include the genera Rickettsia and Orientia; and
2. the family Anaplasmataceaewas broadened to include all species in the genera Ehrlichia, Anaplasma, Cowdria, Wolbachia, and Neorickettsia.  As such, the genus Anaplasma was modified: 1) to include the organisms originally designated as Ehrlichia phagocytophila, Ehrlichia bovis, and Ehrlichia platys; 2) to include Ehrlichia equi and the Ehrlichia HGE agent as synonymous with Anaplasma phagocytophilum; and 3) to place Cowdria ruminantium into the genus Ehrlichia (i.e. Ehrlichia ruminantium).⁸⁷  Consequently, the re-classification resulted in nomenclature changes of certain animal pathogens that are discussed in this chapter (Table 1).

Table 1 Nomenclature changes associated with reorganization of the order Rickettsiales in 2001⁸⁷

Infections of domestic animals associated with members of the Rickettsiales were reported in Europe since approximately 1780, with historical reports describing febrile diseases (sometimes lethal) associated with tick infestations in farm animals and dogs.^{110, 111, 259}  Members of the Rickettsiaceaeand the Anaplasmataceae are transmitted to humans by ticks, mites, fleas, and lice, and can result in disease(s) ranging from subclinical, mild, to severe to deadly. These include, Rickettsia prowazekii (the causative agent of endemic typhus), Rickettsia conorii (the causative agent of Mediterranean spotted fever), Ehrlichia chaffeensis (the causative agent of human monocytic ehrlichiosis), and A. phagocytophilum (the causative agent of human granulocytic anaplasmosis) The members of Rickettsiales that cause disease in both domestic animals and humans are not always mutually exclusive, occur worldwide, are often transmitted by specific vectors (e.g. ticks, fleas) in certain geographic regions, and vaccines are most often not available for the disease-causing agents.  Despite the advent of more advanced molecular and serological diagnostic techniques, cross- reactivity among the different genera/species within the order Rickettsiales still pose a problem in making a definitive diagnosis, particularly in resource-restricted countries or laboratories.

In Africa, tick-borne pathogens are responsible for many important and serious diseases^{93, 226, 227, 232} resulting in livestock productivity and economic losses and may also pose public health risks.  The ecological richness of the continent of Africa provides a variety of ecosystems that are ideal for supporting an array of tick/ectoparasite species, and consequently the maintenance of pathogen diversity.  Moreover, close interfaces between humans and livestock/wildlife on the continent has a bearing on farming systems as well as ecotourism/tourism industries.  At these interfaces there is often increased transmission of pathogens between livestock and wildlife species, resulting in the emergence of unknown pathogens or new diseases or endemicity (see Infectious diseases of animals in sub-Saharan Africa: The wildlife/livestock interface). Consequently, the need for continued and new molecular and serological surveys throughout Africa cannot be overemphasised.  In this chapter, the lesser known members of Rickettsiales (Anaplasmataceae and Rickettsiaceae) are discussed, some of which have a history of causing disease amongst animals and humans in Africa.

Order Rickettsiales

Members of the order Rickettsiales are all Gram-negative, obligate intracellular bacteria vectored by either arthropods or trematodes; within their host cell they replicate in vacuoles in the cytoplasm and/or can be found within the host nucleus.^{224, 241} Bacteria in the order Rickettsiales that are responsible for diseases in humans and animals are associated with the genera Rickettsia (rickettsioses), Orientia (scrub typhus), Anaplasma (anaplasmoses) and Ehrlichia (ehrlichioses).

Family Anaplasmataceae (genera Anaplasma and Ehrlichia)

The...