Ehrlichiosis, also known as “Tropical Canine Pancytopenia” or “Canine Rickettsiosis”, is a tick-borne disease caused by obligate intracellular bacteria of the genus Ehrlichia of the family rickettsiaceae. Dogs can become naturally infected with several species of Ehrlichia including E. canis, E. equi, E. risticii, E. platys, and E. ewingii. E. canis is the most common and causes the most severe clinical disease. Dogs seropositive for E. canis have been identified throughout most of the U.S., but most cases occur in areas with an increased concentration of Rhipicephalussanguineus, the brown dog tick, such as the Southwest and the Gulf coast. Canine Ehrlichiosis is principally of importance in Africa, Asia, and India.

Ehrlichiacanis was discovered in Algeria in 1935. The first case in the United States was reported in 1963. It was not until about 1968-1970, during the Vietnam war, when the full pathologic potential of E. canis was first recognized. A severe epizootic episode of Ehrlichiosis occurred among U.S. military dogs resulting in hundreds of cases of morbidity and mortality.

Transmission: The arthropod vector of E. canis is Rhipicephalussanguineus and transmission is transtadial. Ticks acquire E. canis by feeding, as either larvae or nymphs, on infected dogs and transmit the infection as nymphs or adults. The organism can also be transmitted by blood transfusions.

Pathogenesis: The life cycle of Ehrlichiacanis is not completely understood. There are three intracellular forms. Initial bodies are small spherical structures (1-2 microns) which are believed to develop into larger multiple units known as morulae. The morula is thought to dissociate into small granules called elementary bodies.

Once the organism has been transmitted, there are three clinical phases of Ehrlichiosis: acute, subclinical, and chronic. The acute phase begins after an incubation period of 8-20 days and lasts 2-4 weeks, during which time the organisms multiply in reticuloendothelial cells, lymphocytes, and monocytes. Infected mononuclear cells marginate in the small vessels or migrate into endothelial tissues and vasculitis ensues. Immunologic and inflammatory mechanisms are involved with increased platelet consumption. Platelet-associated IgG and antibodies that recognize platelet proteins in dogs with E. canis infection may play a role in the thrombocytopenia. In addition, platelet migration-inhibition factor (PMIF) has been found to exist in dogs with Ehrlichiosis and its level is related inversely to the platelet count. The acute phase usually resolves spontaneously. The subclinical phase can persist for years. Immuno-competent dogs may be able to eliminate E. canis; however, the organism persists intracellularly in most dogs, leading to the chronic phase. This phase may be mild to severe. In the mild form, there is vague illness and weight loss. Bone marrow hypoplasia leading to pancytopenia occurs in the severe chronic form. The severity of the disease depends on the dog’s age (i.e., young dogs are more susceptible), strain of the organism, the presence of concurrent disease, and breed (e.g., German Shepherds) are more likely to be infected.

Clinical signs: Clinical findings in dogs with Ehrlichiosis vary with the phase of the infection. During the acute phase, non-specific signs such as fever, oculonasal discharge, anorexia, weight loss, dyspnea, and lymphadenopathy may occur. Clinical signs commonly seen during the chronic phase include depression, weight loss, pale mucous membranes, abdominal pain, hemorrhage, lymphadenopathy, splenomegaly, dyspnea, increased lung sounds, hepatomegaly, arrhythmias, pulse deficits, polyuria, polydipsia, and stiff, swollen, painful joints. Ocular abnormalities such as perivascular retinitis, hyphemia, retinal detachment, anterior or posterior uveitis, and corneal edema may occur. Abnormalities of the CNS, including meningeal pain, paresis, cranial nerve deficits, and seizures have been reported.

Diagnosis: Ehrlichiosis is an important differential diagnosis for pancytopenia. Haematological changes for infections caused by E. canis generally include non-regenerative anemia, thrombocytopenia, and leukopenia. Serum chemistry abnormalities include hyperproteinemia with hyperglobulinemia, and elevated alanineaminotransferase and alkaline phosphatase. Other clinicopathologic findings include proteinuria, hematuria, and prolonged bleeding time. CSF analysis in dogs with CNS signs shows an increased protein level and predominant lymphocytic pleocytosis.

A definitive diagnosis of Ehrlichiosis can be made by demonstration of morulae in leukocytes from blood smears or tissue aspirates from spleen, lung, or lymph node.; however, finding morulae on smears is often difficult and time-consuming. A diagnosis of Ehrlichiosis is usually based on positive results of the indirect FA test on serum. This test detects serum antibodies as early as 7 days post-infection. Serum antibody levels in untreated dogs peak at 80 days after infection. Most laboratories measure an IgG titer. A titer of 20 or greater is generally considered to be evidence of infection and/or exposure. An ELISA test has also been developed to detect antibodies and circulating antigen in dogs with E. canis. Cross-reactivity occurs between several of the Ehrlichia species. For academic interest, Western immunoblotting and PCR may be used to characterize different organisms.

Pathologic findings: Ehrlichiosis is not characterized by specific pathologic findings, but gross lesions may include petechial and ecchymotic hemorrhages on the serosal surfaces of the gastrointestinal and urogenital tracts and kidneys, edematous or hemorrhagic enlargement of most lymph nodes, and edema of the limbs. Dogs are generally emaciated at death and may have signs of epistaxis. Splenomegaly and/or hepatomegaly may be observed.

Histopathologic findings include widespread perivascular accumulations of lymphoreticular and plasma cells, particularly in the meninges, kidneys, liver and lymphopoietic tissues. Multiple Kupffer cell hyperplasia and degeneration and acute centrilobular necrosis of the liver may be seen. Lesions of the CNS include hemorrhage and plasma cell accumulations in the meninges and occasionally lymphocytic and plasma cell infiltrations are present in the brain parenchyma. Other microscopic findings may include crescent-shaped perifollicular hemorrhages in the spleen, bone marrow hypoplasia, interstitial pneumonia, and glomerulonephritis.

Ehrlichia organisms are difficult to detect histologically. Ultraucturally, morulae in blood monocytes are intracytoplasmic inclusions made up of numerous organisms. The organisms are round, ovoid, or elongated and are surrounded by a double membrane.

By Jeanine Peters, Class of 2000 University of Georgia
- edited by Evan Janovitz, DVM, PhD


Main Categories