New information sheds more awareness on canine ehrlichiosis

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.

Photo1: Morula in monocyte.

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).

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).

Photo 2: Engorged adult Rhipicephalus female.

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.

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.

Table 1: Family Anaplasmataceae

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.

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.

Photo 3: Amblyomma nymph.

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.

Diagnosis

There isn't any one direct path to a definitive diagnosis of canine ehrlichiosis. The accurate diagnosis of this disease is dependent on the veterinarian's ability to combine a heightened index of suspicion, one or more of the typical clinical signs, appropriate testing and accurate interpretation of test results.

Photo 4: Adult females, Ixodes scapularis and Ammbylomma americanium.

Cytology is usually not reliable but is more likely to be diagnostic during the acute stage of the disease. Low numbers of lymphocytes containing morulae may be seen on blood or buffy coat smears or lymph node aspirates 12 to 14 days after infection.

Because thrombocytopenia is a relatively consistent finding, an EDTA-preserved blood sample along with a good quality blood smear should be sent to the laboratory as rapidly as possible. Platelet concentrations should be stable for five hours at room temperature and 24 hours when refrigerated (4oC). The chemical profile is usually normal or near normal with mild to moderate elevations in ALT and/or the BUN, and other non-specific changes possible.

Ehrlichia titers indicate the patient's ability to generate antibodies specific for the intracellular parasite. Seroconversion normally occurs 13 to 19 days after infection but the antibody response may be delayed for up to 28 days. Positive titers denote exposure; high titers are not an indication of severe infection. Antigenic diversity may exist among strains of E. canis and cross-reactivity may occur between E. canis, E. chaffeensis and E. ewingii.

The indirect immunofluoresence antibody test (IFA) has been the gold standard for many years. There has been difficulty in the interpretation of results and titers from one laboratory may not be the same at other facilities.

The in-clinic ELISA based SNAP® 3DX test has actually redefined the canine ehrlichia seroprevalence data. It has also caused a great deal of confusion and diagnostic dilemmas. This convenient and easy-to-use format allows veterinarians to evaluate a dog for Dirofilaria immitis antigen, Borrelia burgdorferi antibodies and Ehrlichia canis (just this species) antibodies in less than 10 minutes. Positive results for any of these tests become problematic when the patient is clinically normal. This test is positive for E. canis titers of 1:100; other test formats may be positive at 1:10, 1:20 or 1:40. A negative ELISA test does not rule out ehrlichia exposure.

Polymerase chain reaction (PCR) testing detects the genetic footprints of the organism. It is highly specific and is becoming more available, affordable and more reliable. A positive ehrlichia PCR test means the organism is present. PCR will be positive during the acute phase of the disease. It may also be useful in dogs that are chronically infected with persistent antibody titers to determine if re-exposure had occured.

Kansas State University has recently developed a molecular test that has the ability to diagnose three Ehrlichia species (E. canis, E. chaffeensis and E. ewingii) and two Anaplasma species (A. phagocytophila and A. platys) all five of which are known to cause disease in dogs. This rapid test uses the novel molecular biology methods and is complete in four hours. Ehrlichia /Anaplasma species RNA is captured from the test sample and used to detect the presence of specific-pathogen RNA by a real time RT-PCR assay.

Treatment

Doxycycline has been used clinically with good success for ehrlichia infection at 5 mg/kg PO BID for three to four weeks. Clinical response is usually noticed within 48 to 72 hours. However, complete clinical recovery may take several weeks. A 30 percent rise in the platelet count is indicative of a good response to treatment (10 percent is not adequate). Injections of imidocarb dipropronate, 5 mg/kg IM, twice two weeks apart, has been used successfully for treatment of ehrlichiosis but is substantially more expensive than doxycycline.

The decision to treat clinically normal antibody positive dogs is difficult. It may be helpful to check a CBC and platelet count and affirm the possibility of tick exposure. Patients testing negative that create a high degree of suspicion for ehrlichia should undergo a complete CBC and be rechecked for antibodies in two weeks.

Adequate supportive care may be necessary in most cases, but the new alternative treatments (granulocyte colony stimulating factors or human recombinant erythropoiten) are expensive and may not be beneficial to most canine patients. The effects of these new compounds are short lived and may only be useful in acutely ill patients.

Ticks

The parasite/host interface is a dynamic interaction of tick, pathogen and host. Ticks are not just the mechanical vectors of ehrlichia organisms. There is both a cellular and a humoral immune response generated to the parasite. Tick saliva contains bioactive components that affect the host's hemostatic, inflammatory and immunologic systems.

Ticks can obtain E. canis organisms when they attach to an infected host and engorge during all phases of the disease (Photo 2, p. 3). E. canis is not transmitted transovarially in the tick. Therefore, tick larva will not transmit ehrlichia to dogs. Larvae or nymphs that feed on acutely infected dogs or other infected hosts will drop off the host, molt and transmit infective organisms to their next host (Photo 3, p. 5). In the tick, ehrlichia organisms are thought to multiply in the gut and salivary glands. When infected nymphs and adult ticks feed, ehrlichia organisms are injected into the host along with tick salivary secretions. The longer the tick is attached and feeding, the more bacteria and bioactive components will be injected. The time needed for enough organisms to be injected to cause disease transmission is assumed to be about 24 to 48 hours for most tick-borne pathogens. However, precise transmission data in dogs is lacking.

The likelihood of disease transmission is dependent upon the abundance of appropriate tick vectors and infected mammalian hosts in the same geographic region. The species of ticks involved in the transmission of ehrlichia organisms to dogs include Amblyomma, Ixodes and Rhipicephalus (Photo 4). The three-life stages of these ticks feed on separate hosts.

Prevention

Ehrlichia vaccines for dogs are not available but may be in the near future. Continuous low-dose tetracycline has been reported to be successful in the prevention of E. canis infection in an endemic area of Africa. The incidence of disease in treated dogs in this study was less than 1 percent. This is not a practical or wise treatment regime because of the potential selection of resistant strains of bacteria.

As simple as it may sound, avoidance of tick-infested areas during peak tick questing seasons is a highly effective strategy. Clients should realize that walking their dogs through tall grassy or wooded areas in the spring and the fall might lead to multiple tick infestations. Keeping lawns and brush mowed in and around the dog's yard will reduce tick numbers.

If exposure to questing ticks cannot be avoided, their prompt removal from the animal before attachment will benefit the animal. Ticks that have started the attachment process should be grasped with fine forceps, pulled straight out and placed into 70 percent alcohol. No twisting, heat, nail polish or other household compound has proven to be useful in tick removal. Once the tick is removed, disinfection of the attachment site is recommended. Some ticks will survive the flush of a toilet. Clients should be reminded of the rare, but possible, disease transmission hazard of crushing engorged ticks. Repellants containing DEET are effective for adult clients but should never be used on children, cats or dogs. Environmental control of ticks is challenging even for professional pest control operators (PCOs). Identification of the tick species involved will help direct the prevention effort. Rhipicepalus spp. infestations in kennels are very difficult to control because all life stages will feed on dogs. Rodent control will help, but will not eliminate the infestation. Most attempts to eliminate deer from backyards are relatively futile.

Tick control can usually be accomplished with the regular topical application of residual insecticides such as amitraz, fipronil and permethrin. Duration of activity varies considerably based upon formulation, dog activity, and acaricide tolerance or resistance in a given region. All permethrin products must never be used on cats and only with extreme caution on dogs that interact with cats.

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Conclusion

Veterinarians play a major role in the prevention, diagnosis and treatment of ehrlichia infections in dogs. There is a very fine line for creating awareness of tick-borne diseases without generating panic and concern in the pet-owning public. Even though the zoonotic role of dogs as a reservoir for human infection has never been shown to be a threat, it is still a great concern for many canine patients.

Editing credits to M.W. Dryden, R.R. Ganta.

Dr. Payne is an assistant professor at Kansas State University in the Department of Diagnostic Medicine/Pathobiology. She received her DVM from Kansas State University in 1971 and worked in small animal clinical practice in Virginia for 23 years before returning to the university. Her Ph.D. was earned in 2000 for research in insecticide resistance of the cat flea.