Granulocytic ehrlichiosis and anaplasmosis are caused by organisms that have a tropism for neutrophils. There are two main organisms that are known to infect dogs, Ehrlichia ewingii and Anaplasma phagocytophilum. These organisms are morphologically indistinguishable and produce very similar clinical disease in the dog.
Anaplasma phagocytophilum (AP) is a tick-transmitted, obligate intracellular bacterium. Infection with AP has been recognized in a wide variety of hosts including, humans, dogs, cats, horses, ruminants and many wildlife species such as white-tailed deer, dusky-footed woodrat, mountain lions and bears. E. ewingii has been reported in dogs in central, eastern and southeastern U.S. Human infections have been reported from Missouri, Oklahoma, and Tennessee. The only confirmed tick vector is Amblyomma americanum. The White-tailed deer suspected to be an important reservoir for infection, similar to E. chaffeensis.
Infections, as with most tick-borne disease go through an acute, a subclinical and a chronic phase of the disease. Clinical features of the infection with A. phagocytophilum and E. ewingii are identical.
Acute Phase of Infection
Most clinical cases, typically 1 to 3 weeks after tick inoculation. Subclinical carrier states do exist in dogs, but there is debate whether or not these subclinical carriers may progress to chronic disease or not. Clinical signs, include fever, lethargy, malaise, anorexia, reluctance to move. The predominant characteristic clinical finding is polyarthritis, although occasionally, CNS disease and respiratory disease, cough and labored breathing, has also been reported with canine anaplasmosis. These diseases produce clinical findings indistinguishable from those seen in Lyme disease.
Laboratory findings include a mild to moderate nonregenerative anemia, thrombocytopenia, and lymphopenia +/ neutropenia. The most frequent hematological abnormality is thrombocytopenia and is seen if over 80% of clinical cases.
Diagnosis of Acute Infection
Diagnosis can often be made by microscopic identification of AP or EW morulae in circulating neutrophils in the peripheral blood or synovial fluid. These are most often found during acute phase of the disease (1 to 10 wks Pl), and the y may be seen in 1–15% or more of circulating neutrophils. In animals presenting with polyarthritis, organisms can usually be found in a small percent of neutrophils in synovial fluid.
Acute cases of AP infection may be negative on serological assays, and PCR assays have been designed to amplify various specific genes of this organism. The SNAP 4Dx Plus assay can be used to identify antibodies in infected animals. It is the only commercially available to diagnose infection with Ehrlichia ewingii.
The treatment for the granulocytic forms of ehrlichiosis/anaplasmosis is the same as for Ehrlichia canis. Resolution of clinical signs typically occurs shortly after institution of therapy.
Ehrlichia canis and Anaplasma phagocytophilum may also infect cats and cause clinical disease (see above for details).
Mycoplasma haemofelis is the causative agent of hemotropic mycoplasma in the cat.
Transmission is primarily through the bite of infected fleas. Blood transfusions may also be a source of infection. The organism can also be transmitted from mother to offspring, but the exact mechanism is not clearly understood.
Clinical signs may appear in adult or young immunocompetent animals. Fever, weakness and anemia are predominant findings. Splenomegaly is common. The anemia in uncomplicated infections is typically associated with extravascular destruction of erythrocytes and icterus may be seen. However, nonregenerative anemia may be seen in animals co-infected with FeLV or FIV.
Routine Laboratory Findings
Parasites may or may not be visualized in blood even when animals are markedly anemic. Organisms appear during cyclic parasitemic episodes and then rapidly disappear from circulation. When visualized on the surface of infected erythrocytes they appear as small dots, chains or ringlets.
The anemia may appear nonregenerative if a precipitous decrease in hematocrit has occurred early in the disease or if other concurrent disorders (e.g., FeLV or FIV infections in cats) are present. Many infected animals are Coombs’ test positive.
Diagnosis may be made by microscopic visualization of organisms on the surface of infected erythrocytes but cyclic parasitemias may result in circulating numbers of organisms that are too low for microscopic visualization. PCR analysis can be used to detect organism DNA in these animals or to identify carrier cats.
The recommended treatment is doxycycline (5 mg/kg PO q12h) or tetracycline/oxytetracycline (20 mg/kg PO q8h) antibiotics administered for a 3-week period.