Gregory C. Troy,     Blacksburg, Virginia

Epidemiology

CVL is a worldwide zoonotic disease. Domestic and wild canids are important primary reservoir hosts for human visceral leishmaniasis in geographic regions of the world where it is endemic. In North America, risk to humans from infected dogs has not been identified because individuals who have worked intimately with dogs have not been infected.

The primary mode of transmission of the North American isolate of L. infantum still is not known definitively at this time (Breitschwerdt and Schantz, 2006; Petersen and Barr, 2009). Transmission of CVL has occurred by whole and packed red cell blood transfusions and by horizontal and vertical means. Experimental infection of mice and dogs with L. infantum (strain L. infantum Virginia Tech-1) has shown low levels of vertical and sexual transmission. These findings may provide a plausible explanation for transmission of CVL in kennel situations resulting in low-grade maintenance of infection. These modes of transmission of L. infantum may play a more significant role in North American CVL than in CVL in other worldwide geographic regions.

Monitoring of seropositivity in kennels in the United States over time has revealed an increase in seropositivity and in positive results on quantitative polymerase chain reaction (PCR) assay. Approximately 44.8% of dogs in high-risk kennels had a positive quantitative PCR result and were asymptomatic at the time of testing (Petersen and Barr, 2009).

Clinical Signs

Dogs infected with L. infantum do not always show clinical signs of disease, and subclinical infections are common. The predominant clinical sign of CVL is a generalized, chronic, debilitating systemic disease process affecting cutaneous and visceral tissues. In dogs the presence of cutaneous lesions implies concurrent visceral involvement.

Most dogs exhibit serologic conversion within 6 to 7 months after exposure. Clinical manifestations of CVL in the United States are variable and depend on the phase of disease and the state of immunity in the animal, but they are similar to those in dogs with CVL in other geographic locations. Genetic susceptibility in certain breeds of dogs also plays a role in infection. Common clinical and historical findings included lethargy, exercise intolerance, weight loss, inappetence, epistaxis, and lameness. Physical abnormalities included weight loss, generalized lymphadenopathy, abdominal distention, lameness, and periocular, facial, and auricular dermatitis with exfoliation and development of cutaneous ulcers over bony protuberances with extension to the trunk and extremities.

Diagnosis

Leishmaniasis should be suspected in dogs exhibiting chronic skin lesions, ocular discharge, generalized lymphadenopathy, splenomegaly, and weight loss. Laboratory abnormalities that should heighten clinical suspicion of leishmaniasis include a nonregenerative anemia, thrombocytopenia, hyperproteinemia, hypergammaglobulinemia, hypoalbuminemia, azotemia, and proteinuria.

A definitive diagnosis of leishmaniasis can be made by identification of amastigotes in tissue samples or by growth of the organism in culture. Splenic, lymph node, and bone marrow aspirates and imprints from skin lesions can be examined in animals with suspected leishmaniasis in an attempt to identify the organism. Rates of microscopic detection of amastigotes range from 20% to 60% and depend on the quality of samples and preparations and the time allotted to evaluate samples. Lymph nodes, spleen, and bone marrow are tissues that can be sampled readily. The immunofluorescent antibody test (IFAT) is considered the standard to which other serologic methods are compared and is used for evaluating dogs with suspected infection. A titer of 1 : 64 usually is considered positive. Titers fluctuate during the course of the disease, and dogs with titers of more than 1 : 16 and less than 1 : 64 should be evaluated more comprehensively. Other serologic methods to evaluate dogs with suspected leishmaniasis include complement fixation, direct agglutination, indirect hemagglutination, antigen-specific enzyme-linked immunosorbent assay, Western blot, and recombinant antigen (rK39) immunoassay. The rK39 assay is a more rapid testing method with fewer technical limitations than IFAT or PCR testing (Scalone et al, 2002). The recombinant antigen rK39 is a repetitive immunodominant epitope that mimics a kinesin-related protein that is highly conserved among viscerotropic Leishmania spp. In addition, the rK39 test does not cross-react with antibodies to Trypanosoma cruzi, which is a problem with the IFAT test in areas in which this parasite may be endemic. The rK39 assay is available commercially.

Molecular diagnostic techniques like PCR (single, nested, and real time) have been used to amplify Leishmania DNA. PCR using primers that amplify a conserved minicircle region of the kinetoplast (extranuclear DNA structure of parasitic flagellates) that is common to all Leishmania spp. is available at several diagnostic laboratories (Iowa State Diagnostic Laboratory, Centers for Disease Control and Prevention Division of Parasitic Diseases, IDEXX Laboratories). The sensitivity of PCR testing of blood, bone marrow, and lymph node aspirates is approximately 90% to 95% and is higher for bone marrow and lymph node samples than for blood (Baneth, 2006; Breitschwerdt and Schantz, 2006). These differences in sensitivity of PCR assays appear to be caused by the different numbers of parasites present in different tissues. PCR assay results also support the observation that a greater number of dogs have subclinical infection than show clinical signs of disease.