The term "ehrlichiosis" refers to a group of tickborne diseases caused by obligate, intracellular bacteria that infect their host's neutrophils (Ehrlichia ewingii) or monocytes (Ehrlichia canis and Ehrlichia chaffeensis). These infections lead to clinical signs that include fever, bleeding tendencies, lymphadenomegaly, lameness, and neurological signs. Diagnosis is based on cytological examination, serological testing, and PCR assay. Doxycycline is the treatment of choice.
Etiology and Epidemiology of Ehrlichiosis in Dogs
Canine monocytic ehrlichiosis is caused by Ehrlichia canis. This organism appears to be transmitted mainly by the brown dog tick (Rhipicephalus sanguineus) and infects the host's monocytes.
Ehrlichiosis is reported on all continents, with the exception of Australia and Antarctica. In North America, most cases occur within the southeastern and southwestern US.
Ehrlichia ewingii causes granulocytic ehrlichiosis in both humans and dogs. The organism is transmitted by the lone star tick (Amblyomma americanum), resulting in infection of the host's monocytes. The distribution of the disease closely parallels the distribution of the tick, affecting mainly the southeastern and central southern US.
Although there are some conflicting data, infection with E ewingii appears to be more common in summer than at other times of year. Human monocytic ehrlichiosis is caused by Ehrlichia chaffeensis and transmitted by A americanum.
Dogs naturally infected by E chaffeensis can develop systemic signs of infection but also be healthy carriers. The disease occurs predominantly in humans, and dogs are suspected to be a possible infection reservoir.
As with all other bloodborne infections, ehrlichiosis can also be transmitted via blood transfusions.
Clinical Findings of Ehrlichiosis in Dogs
Infection with E canis can result in subclinical, acute, or, in rare cases, chronic ehrlichiosis. The acute form of disease occurs within 1–3 weeks after infection and can manifest with fever, bleeding tendencies (petechiation, ecchymosis, melena, epistaxis), splenomegaly, lymphadenomegaly, and neurological signs.
Most dogs clear the infection within 2–4 weeks; however, some dogs develop the chronic form of ehrlichiosis, which is characterized by bone marrow hypoplasia and deposition of immune complexes in various organs, resulting in clinical signs that include anterior uveitis, polymyositis, vasculitis, and glomerulonephritis. Consequently, physical examination findings for acute and chronic forms of ehrlichiosis might include fever, splenomegaly, lymphadenomegaly, evidence of bleeding, pitting edema, pain, and neurological abnormalities.
The chronic form of ehrlichiosis appears to occur more commonly in German Shepherd Dogs than in other breeds. The bleeding tendencies observed in Ehrlichia infections appear to be secondary to altered platelet function, because bleeding occurs in dogs with only mild thrombocytopenia.
Dogs infected with E ewingii show clinical signs similar to those of E canis infection, with the exception that E ewingii causes acute disease, and inflammatory arthropathy has been reported in dogs with E ewingii.
CBCs of dogs infected with E canis commonly show thrombocytopenia accompanied by mild nonregenerative anemia. Patients with chronic ehrlichiosis show pancytopenia; however, leukocytosis due to either neutrophilia or lymphocytosis can occur. In addition, increased nucleated RBC counts can occur.
Blood smears from dogs with ehrlichiosis can reveal the presence of intramonocytic morulas. However, blood smear evaluation is insensitive and cannot distinguish E canis from E chaffeensis. Serum biochemical analysis most commonly reveals hyperglobulinemia that can be monoclonal.
Review of blood smears from dogs infected with E ewingii can reveal the presence of morulas within the neutrophils, as well as thrombocytopenia and nonregenerative anemia. The finding of intraneutrophilic morulas does not distinguish ehrlichiosis from anaplasmosis.
Lesions of Ehrlichiosis
During the acute or self-limiting phase of E canis infections, lesions generally are nonspecific, but splenomegaly is common. Histologically, there is lymphoreticular hyperplasia and lymphocytic and plasmacytic perivascular cuffing.
In chronic cases, these lesions can be accompanied by widespread hemorrhage and increased mononuclear cell infiltration in perivascular regions of many organs. In dogs with chronic ehrlichiosis, bone marrow analysis commonly shows hypoplasia with plasmacytosis. Similarly, cytological examination of lymph nodes can reveal plasmacytosis.
Diagnosis of Ehrlichiosis in Dogs
Cytological evaluation
Serological testing
PCR assay
Because hematologic abnormalities are common with many vector-borne infections, additional testing is required to obtain a diagnosis of ehrlichiosis.
Serological tests for ehrlichiosis are available based on point-of-care enzyme immunoassays and immunofluorescence assays (IFAs). In acute cases, dogs might develop clinical signs before seroconversion. This is especially true for E ewingii infections, which can take up to 4 weeks before demonstrating substantial seroconversion.
Thus, in cases with a high index of suspicion for acute ehrlichiosis, convalescent titers should be tested with IFA 2–4 weeks after the initial testing. A 4-fold or greater increase in titers is consistent with a diagnosis of ehrlichiosis. Likewise, because antibody titers can remain increased for months after infection, the use of paired serological titers can help discriminate acute from chronic forms of the disease.
Pearls & Pitfalls
|
Point-of-care tests cannot be used for paired titers, because they do not provide a quantitative result. Serological cross-reactivity is high, so serological testing does not distinguish E canis, E ewingii, and E chaffeensis.
PCR assay can sometimes lack sensitivity if the number of organisms is low or the patient has already received antimicrobials. Nonetheless, this technology enables speciation of the organisms, as well as earlier diagnosis because seroconversion is not necessary.
Samples appropriate for PCR assay include blood, tissue aspirates, or biopsy specimens of lymph nodes, spleen, liver, or bone marrow. PCR assay can also be used to detect the effectiveness of treatment in clearing infection; however, false-positive results can occur briefly after treatment because the target for the test is DNA, which might remain at a site even after the pathogen has been killed.
Differential diagnoses for ehrlichiosis include the following:
Acute stage:
Chronic stage:
drug toxicosis (from estrogen, fenbendazole, chloramphenicol, etc)
myelophthisis
multisystemic diseases associated with specific organ dysfunction (eg, glomerulonephritis)
Treatment of Ehrlichiosis in Dogs
Doxycycline or minocycline
Supportive care
To treat infection with Ehrlichia, the drug of choice is doxycycline because of its superior intracellular penetration and bacteriostatic properties against rickettsiae. Doxycycline (5 mg/kg every 12 hours or 10 mg/kg every 24 hours, PO or IV, for 28 days) is recommended for dogs of all ages. If doxycycline is not available, minocycline (5–10 mg/kg, PO, every 12 hours for 28 days) can be substituted.
If infection is suspected, dogs should be treated pending diagnostic results. Early seronegative tests should not be considered a reason to stop treatment, because antibodies can take ≥ 1 week to develop in acute cases.
In acute cases of ehrlichiosis treated with appropriate antimicrobials, fever is expected to resolve within 24–48 hours after initiation of treatment. In chronic cases associated with E canis infection, hematologic abnormalities can persist for 3–6 months; however, clinical response to treatment often occurs much sooner.
If a dog is thought to have chronic ehrlichiosis, the E canis antibody titer and whole blood PCR assay should be repeated after 6 months of illness to confirm successful treatment. Serum titers that persist at lower but positive levels after 6 months should be rechecked in another 6 months to confirm that they are not increasing.
Supportive care might be necessary for ehrlichiosis patients with fever or bleeding tendencies, or in patients with chronic disease complicated by wasting and specific organ dysfunction. Patients with blood loss anemia or severe thrombocytopenia should be treated with platelet or whole blood transfusions as appropriate, particularly if hemorrhage is extensive.
Prevention of Ehrlichiosis in Dogs
Ehrlichiosis can be prevented by controlling tick exposure of dogs. Dogs in areas infested by ticks should be restricted from walking off-trail, in certain grassy areas, and where there might be ticks in leaf litter. Dogs should be examined after walks, and any ticks should be removed carefully (to prevent human exposure). It is unrealistic, however, to expect that owners will find all ticks in the coat of a dog.
Multiple topical medications are available to prevent tick bites; these should be used in compliance with labeling before dogs are brought into areas infested with ticks.Formulations containing fipronil, isoxazoline, pyrethroid, or amitraz are approved for this indication and are the most effective against ticks when used as labeled.
Transfusion-associated E canis transmission can be decreased by using seronegative screened blood donors; however, new donors with a negative screen cannot be presumed free of infection for several weeks, because they could be incubating infection.
Zoonotic Risk of Ehrlichiosis in Dogs
E chaffeensis and E ewingii are considered zoonotic agents. Even though these infections occur in both animals and humans, the involvement of a required intermediate tick vector for transmission means that dogs and other infected animals do not pose a direct transmission risk to humans in normal circumstances.
Infection in dogs can indicate a heightened risk of infection in humans from tick exposure in a given area.
Key Points
Multiple ehrlichioses can be transmitted by ticks from wildlife to pets and humans (or from dog to dog in the case of canine monocytic ehrlichiosis).
Ehrlichioses have similar clinical signs, with acute fever and hematologic abnormalities commonly occurring.
Ehrlichial infection can be prevented by tick preventatives or can be treated with doxycycline.
For More Information
Canine ehrlichiosis prevalence map. Companion Animal Parasite Council.
Ehrlichia spp. and Anaplasma spp. Companion Animal Parasite Council.
Diniz PPVP, de Aguiar DM. Ehrlichiosis and anaplasmosis: an update. Vet Clin North Am Small Anim Pract. 2022;52:1225-1266.
Sykes JE. Canine and Feline Infectious Diseases. Elsevier/Saunders; 2014.
Also see pet owner content regarding ehrlichiosis and related infections in dogs.
