Lesser-known rickettsias infecting livestock

Lesser-known rickettsias infecting livestock


General introduction

Only a few of the rickettsias infecting livestock are well known because of their pathogenicity and the economic importance of the diseases that they cause. Other rickettsias, equally or even more ubiquitous, have achieved a high degree of equilibrium with their livestock hosts and so are less well known, and there are probably some that are unknown because their presence is unsuspected. Since 1990, however, the ehrlichias have been increasingly recognized as a cause of human disease, and also have an importance in the interference in trade, since the screening of animals for freedom from infections such as heartwater, is affected by crossreactions occurring in serological tests. Neitz65 described some of them as benign rickettsias but the adjective misleads because some of these organisms may give rise to overt disease and mortality under appropriate circumstances. For example, bovine and ovine ehrlichiosis in West and Central Africa appears more often to be a severe disease than elsewhere; in 1983 Pierre71 described an epidemic of a disease called ‘nofel’ in which 2 615 cattle sickened and 256 died after Ehrlichia bovis infection was introduced into a previously rickettsia-free region of the Ivory Coast by cattle and their ticks imported from Mali and Burkina Faso.

The lesser-known rickettsias infecting livestock comprise a group of Gram-negative cocco-bacilli in the alpha subdivision of the Proteobacteria, order Rickettsiales, family Rickettsiaceae, tribe Ehrlichieae.108, 51, 82

The species in the Ehrlichia genus infect leukocytes.81 Species in the Ehrlichieae tribe differ fundamentally from the human pathogens in the Rickettsia genus by their general lack of pathogenicity for humans; by multiplying only inside membrane-lined vacuoles in the cytoplasm of host cells (Figure 41.1), by not retaining the red colour of modified modified Ziehl-Neelsen or Macchiavello stains, by not inducing Weil-Felix antibodies, and by their inability to grow in the yolk sacs of embryonated hens’ eggs.

They resemble the human rickettsias in their response to tetracyclines, formulations of which are the drugs of choice in treating diseased animals. In order to advance our knowledge concerning the lesser-known members of the Ehrlichia genus, those encountering infections caused by them are encouraged to try to isolate them by storage of infected blood samples below –70 °C for subsequent reference purposes and by passage of blood into susceptible hosts. New isolates will undoubtedly be of assistance in better characterization of the agents and for elucidation of their epidemiology, and for improvement of diagnostic tests.

Phylogenetic analyses based on 16s rRNA gene sequences1, 24 support the recognition of three genogroups within the tribe Ehrlichieae. The formation of the genogroups is also supported by analysis of the GroESL (HSP60) heat shock protein genes.116 Genogroup I members include Ehrlichia sennetsu, Neorickettsia helminthoeca and Ehrlichia risticii. Although only definitely proven for N. helminthoeca and Ehrlichia risticii, it is likely that all of the genogroup have a transmission cycle involving trematodes of fish or aquatic snails. Genogroup II members include Ehrlichia (Cytoecetes) phagocytophila, and closely related granulocytotropic ehrlichias, namely Ehrlichia equi and the agent of human granulocytic ehrlichiosis, Ehrlichia platys, which infects platelets and can cause thrombocytopaenia in dogs, and Anaplasma spp. Genogroup III includes mainly— but not exclusively — monocytotropic ehrlichias, such as Ehrlichia canis, Ehrlichia chaffeensis and Cowdria ruminantium. It has recently been proposed that the three genogroups be recognized as clades, with constituent species of each clade given the genus of the first recognized agent in the clade,24 but retaining the species name.

* For recent changes in the nomenclature of these organisms, refer to the introduction to Section 3, Rickettsial and chlamydial diseases

Figure 41.1 Electron micrograph of Ehrlichia (Cytoecetes) phagocytophila within an intracytoplasmic vacuole in a granulocyte of a sheep

Under this proposal the three genogroups would become the genera Neorickettsia, Anaplasma and Ehrlichia. Arguably, agents in genogroup III should be Cowdria species since the discovery of the agent of heartwater by Cowdry in 1925 predates that of canine ehrlichiosis. In this chapter, the genus names used are those in use by the overwhelming proportion of authors during the period 1995 to 2001.

Proposed revisions based on genogroups do not support a genus with a single member (C. ruminantium), which would be replaced with Ehrlichia ruminantium; but broadly support Foggie’s29 previous proposal for a division of the tick-borne ehrlichias into two, with a genus containing rickettsias parasitizing circulating mononuclear cells, and the other, a Cytoecetes genus, comprising rickettsias which primarily parasitize granulocytes.

Considerable attention has been paid since the early 1990's to tick-borne ehrlichioses since the recognition of human infections with the monocytotropic E. chaffeensis and with a granulocytotropic agent closely related to E. phagocytophila. The unparalleled level of study has provided clear indications for revised...

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