40       Rickettsiales and Coxiella burnetii

 

Organisms in the order Rickettsiales form a diverse group of non-motile Gram-negative bacteria which replicate only in host cells. In addition to host-cell dependence and poor affinity for basic dyes, a requirement for an invertebrate vector distinguishes them from most conventional bacteria.

At present, two families, Rickettsiaceae and Anaplasmataceae, comprise the Rickettsiales. The genera Rickettsia and Orienta belong to the family Rickettsiaceae, with Rickettsia rickettsii the principal pathogen of veterinary importance. Members of the Anaplasmataceae parasitize cells of the haematopoietic system and possess cytoplasmic membranes but lack cell walls (Table 40.1). The genera Haemobartonella and Eperythrozoon, previously classified in the Anaplasmataceae, have been transferred to the genus Mycoplasma. Coxiella burnetii is now classified within the order Legionellales and is dealt with in a separate section of this chapter.

Table 40.1 Rickettsial pathogens of veterinary importance.

Pathogen	Hosts / Vectors	Disease	Geographical distribution
Rickettsia rickettsii	Humans, dogs / Ticks	Rocky Mountain spotted fever	Western hemisphere, principally the Americas
Aegyptianella pullorum	Poultry / Ticks	Aegyptianellosis	Africa, Asia, Mediterranean region
Anaplasma bovis	Cattle / Ticks	Bovine ehrlichiosis	Africa, Middle East, Asia, South America
A. marginale	Ruminants / Ticks	Anaplasmosis	Tropical and subtropical regions
A. ovis	Sheep, goats / Ticks	Anaplasmosis	Asia, Africa, Europe, USA
A. phagocytophilum	Ruminants, horses, humans / Ticks	Tick-borne fever, equine and human granulocytotropic anaplasmosis	Worldwide
A. platys	Dogs / Ticks suspected	Canine cyclic thrombocytopenia	Americas, Middle East, Mediterranean
Ehrlichia canis	Dogs / Ticks	Canine monocytic ehrlichiosis	Tropical and subtropical regions
E. ewingii	Dogs / Ticks	Canine granulocytic ehrlichiosis	USA
E. ruminantium	Ruminants / Ticks	Heartwater	Sub-Saharan Africa, Caribbean islands
‘E. ondiri’a	Cattle / Ticks suspected	Bovine petechial fever	Highlands of East Africa
‘E. ovina’a	Sheep / Ticks	Ovine ehrlichiosis	Africa, Asia, Middle East
‘Neorickettsia elokominica’a	Dogs, bears, racoons / Flukes	Elokomin fluke fever	West coast of North America
N. helminthoeca	Dogs, bears / Flukes	Salmon poisoning disease	West coast of North America
N. risticii	Horses / Flukes	Potomac horse fever	North America, Europe

^aThese organisms do not have fully approved taxonomic status.

Animal hosts and arthropod vectors, frequently ticks, are the reservoirs of most rickettsias. In arthropods, rickettsias replicate in epithelial cells of the gut before spreading to the salivary glands and ovaries where further replication may occur. Organisms are transmitted when the arthropod feeds on the animal host. Some organisms are maintained in tick populations by transovarial transmission; trans-stadial but not transovarial transmission of Ehrlichia canis and Anaplasma phagocytophilum occurs in ticks. Transmission by flukes has been confirmed for Neorickettsia species.

Rickettsial organisms are relatively host-specific. Because definitive arthropod or fluke vectors are involved in the transmission of most rickettsias, diseases associated with these organisms tend to occur in defined geographical regions. The clinical signs frequently reflect the targeting of a particular cell type by the causal rickettsial agent.

Rocky Mountain spotted fever, caused by Rickettsia rickettsii, is a common rickettsial disease of humans in the Americas, and also affects dogs. These highly pathogenic organisms have a predilection for endothelial cells of small blood vessels. Ehrlichia species have a predilection for leukocytes. Members of the Anaplasmataceae have an affinity for erythrocytes and neutrophils. Tick-borne fever, caused by Anaplasma phagocytophilum, affects domestic and wild ruminants in many European countries. The organism can presist for months or years in a proportion of infected animals. Wildlife may constitute an important reservoir of infection also. Fever, inappetence and a reduced growth rate may be evident in young animals, while abortions or stillbirths may occur in susceptible, pregnant animals. Transient immunosuppression is a feature of the disease. Variants of A. phagocytophilum infecting different species of animals are reported, but the epidemiological significance of these variants is not determined.

Members of the Rickettsiales can be recognized and differentiated by the species of animals affected, cell predilection, microscopic appearance and molecular techniques. Blood or tissue smears stained by the Giemsa technique can be used to demonstrate the morphology of many rickettsial organisms. They occur as purplish-blue, small individual organisms, sometimes in clusters. Fluorescent antibody techniques can be used to identify specific rickettsial organisms in smears. Some rickettsias can be isolated in the yolk sac of embryonated eggs or in defined tissue culture cell lines. However, Ehrlichia and Anaplasma species which target granulocytes and Anaplasma species which affect erythrocytes have not been cultured in vitro. Molecular methods, including nucleic acid probes and PCR techniques, have been developed to detect most rickettsial pathogens.

A limited number of vaccines are available for rickettsial pathogens. In many instances, arthropod vectors such as ticks are involved in pathogen transmission. For diseases transmitted in this manner, tick control is an essential part of disease prevention. Tetracyclines administered early in the disease, may be effective. For some rickettsial diseases, such as Rocky Mountain spotted fever, treatment should be continued for up to two weeks.

Coxiella burnetii

Although the biological features and reproductive strategies of this unusual organism are similar to those of rickettsial organisms, analysis of the 16S rRNA gene has resulted in its reclassification within the Legionellales order. This obligate intracellular pathogen produces endospore-like forms, which can survive for long periods in the environment. Aerosol transmission of C. burnetii commonly occurs in domestic animals and humans. Coxiella burnetii localizes in cells of the female reproductive tract and mammary glands of ruminants and replicates in the acidic environment of the phagolysosome. Infection may cause abortion in many species of ruminants, including small ruminants. Q fever, caused by C. burnetii, is an influenza-like occupational disease of humans in contact with farm animals and their contaminated products. Most infections are acquired by inhalation of aerosols from parturient sheep, goats or cattle. The largest outbreak of Q fever recorded to date occurred in humans in the Netherlands between 2007 and 2010, with more than 4000 cases documented. Infection can be diagnosed using serology, isolation in embryonated eggs or tissue culture, or by PCR-based methods. Control measures include careful disposal of birth products from ruminants to prevent aerosol transmission of the organisms and attention to disinfection procedures. Vaccines are available for use in ruminants. A vaccine has also been developed for use in humans at high risk of infection.