Rickettsiales and Coxiella burnetii

The members of the Rickettsiales are minute, non-motile, pleomorphic, obligate intracellular Gram-negative bacteria. They are rods or coccobacilli, visible under the light microscope and varying in size from 0.3–0.6 µm in length. They are aerobic and divide by binary fission. The order comprises two families of veterinary significance. The members of the Rickettsiaceae (Family) have cell walls similar to those of other Gram-negative bacteria, but those belonging to members of the Anaplasmataceae lack a pepditoglycan layer. They all stain poorly with basic aniline dyes, such as those of the Gram stain, but they stain well with Giemsa and other Romanowsky stains. A few of the Rickettsiaceae will grow in conventional inert media, but most require living cells for propagation and are cultured in the yolk sac of fertile eggs or in tissue culture.

Significant re-classification of the order has occurred in recent years (Dumler et al. 2001), much of it based on DNA sequencing information and in particular 16S and 23S rRNA gene sequence comparisons. The original tribe structure, the Rickettsiaceae family, has been eliminated, while members of the former tribe Ehrlichieae have been placed in the family Anaplasmataceae. The family Rickettsiaceae now comprises two genera, Rickettsia and Orienta while the family Anaplasmataceae comprises five genera; Anaplasma, Ehrlichia, Neorickettsia, Aegyptianella and Wolbachia. The status of the genus Aegyptianella is somewhat uncertain with recent data suggesting a close relationship to the genus Anaplasma. Several species have been re-assigned including Cowdria ruminantium to the genus Ehrlichia and Ehrlichia (Cytoecetes) phagocytophila to the genus Anaplasma. The genera Haemobartonella and Eperythrozoon, formerly assigned within the Anaplasmataceae, contain haemotrophic bacteria that lack a cell wall which have been shown to be closely aligned with species in the Mycoplasma pneumoniae group. These microorganisms now represent a new clade within the genus Mycoplasma (Neimark et al. 2001, 2002). The bacteria formerly known as HGE (human granulocytic ehrlichiosis) agent, Ehrlichia equi and Ehrlichia phagocytophila have been unified under the single species name of Anaplasma phagocytophilum. Coxiella burnetii, the cause of Q fever, is now known to be closely related to Legionella species and Francisella tularensis and has been re-classified to the gamma subgroup of the proteobacteria within the order Legionellales. For convenience, Coxiella burnetii is dealt with in this chapter as a separate section.

Natural Habitat

Members of the Rickettsiales are essentially parasites of arthropods, replicating in the cells of the gut before spreading to other organs such as salivary glands and ovaries. This requirement for an invertebrate vector distinguishes these organisms from most other bacteria. Some can be passed transovarially in ticks (R. rickettsii) and mites but others, such as Ehrlichia canis, Anaplasma phagocytophilum and Ehrlichia ruminantium are passed transstadially but not transovarially. Neorickettsia species infect a number of fluke species and all stages of the life cycle of the fluke are infectious. A number of pathogenic species, such as Ehrlichia canis and Anaplasma marginale persist in the host in a latent form.

Pathogenesis

Infection typically occurs as a result of the bite of an affected arthropod or following ingestion of infected flukes in the case of salmon poisoning and Potomac horse fever. The types of cells parasitized and disease pathogenesis varies with the rickettsial species (Table 34.1). Toxins, haemolysins and endotoxin-like lipopolysaccharides are known to be produced by some members of the group. Rickettsia rickettsii produces a phospholipase which aids escape from the phagosome soon after engulfment. Once in the cytosol nutrient acquisition and replication by binary fission is possible. The clinical signs of Rocky Mountain spotted fever are due to damage to the capillary endothelial cells, resulting in a necrotizing vasculitis and perivasculitis. Haemorrhages, thrombosis, oedema, loss of plasma and shock may follow. Ehrlichia species have a predilection for leukocytes and manage to replicate inside phagosomes by inhibiting fusion of the phagosome with a lysosome. When mammalian erythrocytes are parasitized, as in anaplasmosis or aegyptianellosis, anaemia probably results from the immune clearance of the altered erythrocytes. Anaplasma phagocytophilum and A. bovis are capable of surviving and multiplying within granulocytes while A. platys targets platelets. The principal diseases, hosts, vectors and geographical distribution of pathogens of veterinary importance in the Rickettsiales are summarized in Table 34.2.

Table 34.1

Families and genera of the order Rickettsiales of veterinary importance and cell types targeted