September 1998

Ehrlichiosis

Introduction

There has been increasing awareness of disease states caused by Ehrlichia species, both in veterinary and human medicine. This newsletter reviews canine and feline ehrlichiosis.

Ehrlichia species are gram negative pleomorphic bacteria of the family Rickettseaceae. Many genera of ticks are vectors for Ehrlichia species, which are transmitted within salivary secretions of infected ticks. Organisms are phagocytized by circulating leukocytes, or enter platelets by endocytosis. In the acute stages of disease, organisms may be visualized within membrane-lined intracytoplasmic vacuoles, called morulae.

Canine ehrlichiosis is classified by the cellular trophism of the infecting organisms. Monocytic morulae represent infection with E. canis, and produce the disease called canine monocytic ehrlichiosis. Neutrophilic morulae are seen with E. equi and E. ewingii infections, and the resulting disease is called canine granulocytic ehrlichiosis. Morulae found within platelets represent infection with E. platys, which causes canine cyclic thrombocytopenia.

Feline ehrlichiosis can occur naturally or be induced experimentally with E. risticii and E. equi., but not E. canis.

Canine Monocytic Ehrlichiosis (CME)

CME, caused by infection with E. canis, is transmitted by the brown dog tick Ripicephalus sanguineus. Although this vector is found primarily in the southern United States, CME can be found in all states due to the chronic nature of the disease, and transportation of dogs from endemic to nonendemic areas. Concurrent infection with Babesia canis, Hepatozoon canis, and other Ehrlichia species is possible.

CME is divided into 3 phases of infection: acute, subclinical, and chronic. The acute phase occurs 1-3 weeks after infection, and clinical signs last for 2-4 weeks. During this phase, organisms multiply in circulating lymphocytes and monocytes, and in fixed tissue macrophases within lymph nodes, liver, and spleen. Infected monocytes adhere to the vascular endothelium in multiple organs, inducing a vasculitis. Clinical signs during the acute phase include anorexia, fever, weight loss, dyspnea, lymphadenopathy, splenomegaly, and occasional central nervous system ( CNS) signs. Clinical signs here can mimic Rocky Mountain Spotted Fever (RMSF). Dogs typically recover from the acute phase and become subclinical for months to years. Immunocompetent dogs eliminate the parasite during this phase, and so do not develop the chronic phase. Dogs with ineffective immune responses become chronically infected. Clinical signs during the chronic stage can be mild to severe, and include depression, weight loss, abdominal pain; bleeding episodes such as epistaxis, petechiation and ecchymoses, melena, and hematemesis; ocular lesions of anterior uveitis, chorioretinitis, and retinal hemorrhages; and CNS signs compatible with meningitis. Polyarthritis can also occur in dogs with chronic CME. Death of these dogs is often due to secondary infections from severe leukopenia.

Cytopenias and hyperglobulinemia are the major laboratory abnormalities seen in CME. Thrombocytopenia is the most consistent abnormality, and is present in most dogs through all phases of the disease. Transient mild leukopenia is present during the acute phase, but will often normalize during the subclinical phase. Nonregenerative anemia is encountered in all phases, and Coomb's positive hemolytic anemia can be found.

Hyperglobulinemia and hypoalbuminemia may be evident during the subclinical and chronic phases. Polyclonal gammopathies are most common, but monoclonal gammopathies have also been reported. Lymphocytosis (up to 20,000/µl) has been reported in subclinical and chronic CME.

Bone marrow plasmacytosis is commonly present in all phases of disease. The bone marrow is hypercellular during the acute and subclinical phases of disease, but becomes hypocellular during the chronic stage. Severe pancytopenia or aplastic anemia may be seen in advanced chronic disease, and may not reverse with therapy. Less frequently reported laboratory findings include protein-losing nephropathy, azotemia, and elevated liver enzyme activities. Dogs with the CNS form may have mononuclear pleocytosis and increased protein in CSF. Synovial fluid in dogs with polyarthritis contains high numbers of neutrophils and elevated protein content, consistent with immune-mediated polyarthritis.

Canine Granulocytic Ehrlichiosis (CGE)

E. ewingii and E. equi are the etiologic agents of CGE. Tick transmission is suspected, but a definite vector has not been confirmed. Nonerosive polyarthritis and fever are the most common clinical signs. Laboratory abnormalities associated with CGE include mild anemia, neutropenia, thrombocytopenia, lymphocytosis, monocytosis, and eosinophilia. Synovial fluid analysis reveals nonseptic, suppurative inflammation.

Canine Cyclic Thrombocytopenia (CCT)

E. platys is the causative agent of CCT. Tick transmission is presumed, but an actual vector has not been identified. E. platys organisms enter platelets and reproduce by binary fission, forming morulae within parasitized platelets. Platelets counts decline precipitously to about 20,000-50,000/µl over a 3-4 day period, parasitemia diminishes, and platelet counts return to normal within another 3-4 days. Subsequent parasitemic and thrombocytopenic episodes occur at 10-14 day intervals. The degree of parasitemia and thrombocytopenia diminishes with time, and eventually hematologic changes resolve. Naturally occurring and experimental infections result in few clinical signs, unless hemostasis is compromised by some other problem. Mild nonregenerative anemia, leukopenia, hypoalbuminemia, and hyperglobulinemia have been reported with experimental E. platys infections. Coinfections with E. canis and Babesia canis are common, which may aggravate clinical signs.

Feline Ehrlichiosis

Feline ehrlichiosis is an emerging infectious disease recognized in Europe, Africa and North America. It has been documented with E. risticii, the etiologic agent of Potomac Horse Fever, and E. equi. Clinical signs of disease include anorexia, depression, fever, weight loss, lymphadenopathy, and diarrhea. Laboratory abnormalities include anemia, neutropenia, thrombocytopenia, and hyperglobulinemia. Serologic testing of cats with clinical signs of ehrlichiosis over a 7 year period at Colorado State University revealed antibodies to E. canis and E. risticii in 69 of 340 (20%). Of 12 cats given a presumptive diagnosis of ehrlichiosis, E. canis titers were consistently higher than E. risticii titers, leading to speculation that the organism involved was closely related to E. canis.

Serologic Assays

The historical, clinical and clinical pathologic findings of canine ehrlichiosis must be differentiated from other infectious diseases (eg., RMSF, gram negative sepsis), immune-mediated diseases (eg., ITP, IMHA, immune-mediated polyarthritis, aplastic anemia), and neoplastic diseases (eg., myeloma, chronic lymphocytic leukemia, myelophthisis). Serologic testing using the indirect immunofluorescent assay (IFA), is an important adjunct to the diagnosis of ehrlichiosis. The IFA test for E. canis is very sensitive and specific, though cross reactivity has been reported between E. canis and E. ewingii, and between E. canis and Neorickettsia helminthoeca, the etiologic agent of salmon poisoning disease. Serologic cross-reaction has not been reported with Rickettsia rickettsii or Babesia canis. In acute ehrlichiosis, dogs will be seronegative during the initial 3 weeks of disease. Documentation of seroconversion is necessary to confirm a diagnosis. Serum samples from initial presentation, and from about 3 weeks later, should demonstrate a 4-fold increase in titer. Positive titers indicate infection, since these persist until treatment is given. Infection with E. canis does not confer protective immunity.

Therapy

Antirickettsial agents recommended for the treatment of ehrlichiosis include tetracycline (22 mg/kg tid for 14-21 days), doxycycline (10 mg/kd sid or divided bid for 10-14 days), chloramphenicol (20 mg/kg tid for 14 days) and imidocarb diproprionate (5 mg/kg IM, 2 injections at 14 day intervals). In CME, there is controversy regarding length of therapy, with some authors advising 6-12 weeks of antibiotic therapy. Additional supportive therapy, including fluids, blood transfusions, and short-term immunosuppressive glucocorticoids, may be required. In dogs with severe chronic CME, response to treatment may be minimal, and establishing success can be challenging. Rapid clinical improvement is frequently noted, but antibody titers may persist for months to years. Complete hematologic recovery may take up to 1 year. Progressive decrease in gamma globulin level coincides with elimination of infection. Detection of E. canis DNA in peripheral blood samples using PCR has also been used to evaluate treatment success.

Ehrlichiosis Serology

Dog and Cat Panels (E. canis, E. equi, E. risticii)

Test codes

Note: E. platys and Babesia canis can be ordered separately.

References: Lappin, Proc 11th ACVIM Forum, Washington, DC, 1993, p 447; Kordick et al, In Current Vet Therapy XII (Kirk and Bonagura, eds.), WB Saunders, Philadelphia, 1995, p 287; Stubbs et al, Proc 16th ACVIM Forum, San Diego, CA 1998, p 720.

LAB TIPS

Blood Ammonia Level

Antech Diagnostics will no longer offer an Ammonia Level as a test of hepatic function. The necessity of careful sample preparation, combined with transportation time, limits the usefulness of this test in most clinical settings. Accurate results depend upon the use of ammonia-free heparin as the anticoagulant, continuous icing of the sample once drawn, immediate plasma separation (red cells contain ammonia and can create a false elevation), and immediate assay (within one hour of collection), as the ammonia is volatile. Clearly, these restrictions make it a cumbersome test of questionable diagnostic usefulness.

The Bile Acids Assay provides similar diagnostic information and is the test of choice, when evaluation of hepatic function flow and function is required.