New information sheds more awareness on canine ehrlichiosis

New information sheds more awareness on canine ehrlichiosis

Jun 01, 2003

Photo1: Morula in monocyte.
The incidence and awareness of canine ehrlichiosis have increased dramatically during the last few years. Some ehrli-chia organisms have been reclassified with new nomenclature using highly specific molecular testing procedures. Diagnosis, treatment and prevention options have been improved for this chronic and occasionally fatal disease complex. Because veterinarians understand the basic biology of tick vectors, the nature of the organisms and are able to prescribe several safe and effective acarides, we can help educate our clients and thus minimize tick exposure and disease in their pets.

Incidence and awareness Ehrlichia species have been found in tropical and subtropical regions on four continents - Asia, Africa, Europe and the Americas. These organisms were first recognized in Algeria in 1935 and in the United States in 1963. Ehrlichia species infection was once thought to be host specific; however, E. canis has caused disease in dogs, wolves, jackals, cats and lemurs while E. chaffeensis has produced disease in humans, deer, dogs and coyotes. The human sylvan interface has become blurred as homes are built in once forested environments and more families venture forth with their canine companions into the woodlands. The mobility of vectors and infected hosts has further disseminated the disease. White-tailed deer populations have migrated into new areas and some herds have dramatically increased in size, thereby expanding and increasing those ticks associated with deer. Dogs are frequently shipped out of endemic areas and into non-endemic areas or moved from kennels into private homes (retired racing Greyhounds).

Photo 2: Engorged adult Rhipicephalus female.
Several ehrlichia species have been shown to cause disease in dogs, E. canis, E. ewingii and E. chaffeensis. Other tick borne pathogens that may infect dogs include Anaplasma phagocytophila (E. phagocytophila, E. equi), Anaplasma platys (E. platys), Rickettsia spp., Borrellia sp. and Babesia spp. Frequently dogs are infected simultaneously or sequentially by more than one of these pathogens. In one study, approximately 30 percent of dogs exposed to Ehrlichia had positive titers to Bartonella sp. (Table 1, p. 4).

Organism The family Anaplasmataceae is compromised of four genera of complex organisms: ehrlichia, anaplasma, neorickettsia and wolbachia. Based on new genetic analysis of 16S rRNA genes, heat shock genes and surface protein genes, the classification and nomenclature of members of this family have been changed (Table 1, p. 4).

Ehrlichia organisms are small, pleomorphic, Gram-negative, intracellular bacteria. The intracellular nature of these bacteria allows them to multiply and thrive where they are protected from the humoral immune system. E. canis organisms multiply in clusters called morulae in monocytes and lymphocytes (Photo 1, p. 3). Transmission of infective organisms is usually through ticks, but may also be transmitted via blood transfusions.

Table 1: Family Anaplasmataceae
Disease Canine ehrlichiosis has been known by many different names including canine rickettsiosis, canine typhus, canine hemorrhagic fever, tracker dog disease and tropical canine pancytopenia. These names may represent the many different clinical presentations of the disease in dogs. The three most common disease-causing ehrlichia species in dogs are E. canis, E. ewingii and E. chaffeensis.

The presentation as well as the course of the disease varies with the infecting Ehrlichia spp., other concurrent infections and the individual animal. Granulocytic ehrlichosis resulting from E. ewingii may be less pathogenic in dogs and cause vomiting, diarrhea, polyarthritis and meningitis. When Bartonella sp. and E. canis co-infect dogs, epistaxis is more likely to occur than with either pathogen alone. In some areas, 50 percent of dogs infected with E. canis also have titers to A. platys. The clinical relationship of these two species is uncertain. German Shepards have a breed predilection for depressed cell-mediated immunity and may develop more severe pancytopenia than other breeds when infected with E. canis.

Canine ehrlichosis has three phases, acute, subclinical and chronic. When cases are presented clinically, the onset and duration of the infection is usually unknown. E. canis undergoes an incubation and multiplication period of eight to 20 days. During this acute phase, clinical signs will resemble other tick-borne rickettsial diseases. Thrombocytopenia and leukocytopenia develop 10 to 20 days after infection and may be the result of an autoimmune destruction of platelets. Platelet bound and serum antiplatelet antibodies appear with the occurrence of severe thrombocytopenia.

Photo 3: Amblyomma nymph.
Presentation with lameness and stiffness in the acute phase is the most common indication for ehrlichia testing. Other presenting signs include fever, lethargy, depression, anorexia, ocular and nasal discharges, generalized lymphadenopathy, edema, weight loss and splenomegaly. A variety of CNS signs have been noted due to inflammation and bleeding into the meninges. Death is rare during the acute phase and spontaneous recovery may occur.

The subclinical phase of E. canis begins five to eight weeks after infection. Animals then enter into a chronic phase that may last for a few months to several years. Clinical signs range from mild to very severe. The bone marrow of chronically infected dogs may become hypoplastic. Platelet numbers may be normal but functionally defective, allowing bleeding to occur.