Have You Heard? Getting a leash on visceral leishmaniasis (script)

Leishmania infantum—you might think it's just another parasite studied and then forgotten in veterinary school. But revisiting this intracellular parasite of dogs and humans may be a good idea.

Endemic to the Mediterranean region, including portions of Europe, Asia, and Africa and some areas of South America, the parasite has recently been on the rise in North America. A patient with a suspicious travel history is not the only one to be concerned about. In endemic areas, the disease caused by Leishmania infantum infection, known as visceral leishmaniasis, is predominantly transmitted by the sand fly. However, in North America, the infection appears to be passed to puppies during gestation, birth, or nursing. Direct contact with infected blood has also been shown to be a means of transmission.

While sand flies are found in regions of North America where Leishmania species has been documented, no vector-borne transmission has been seen to date. However, dogs are the main reservoir for this disease-causing parasite, and it may only be a matter of time before vector transmission is recognized. Given its zoonotic potential, watching this emerging parasite is warranted. A recent Veterinary Clinics of North America article provided an overview of this expanding threat, including the results of ongoing investigations.

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The foxhound is the breed most affected in North America. This may be because of a genetic predisposition. Additionally, proximity to wooded areas puts these dogs in close contact with sand flies and climates similar to those in endemic areas. Infected sand flies have not been reported in the United States, but they should be considered an able vector. The distribution of this protozoan parasite has been steadily spreading to include many states, ranging from New Jersey to Kansas.

Between 2000 and 2003, the Centers for Disease Control and Prevention looked at serum samples from more than 12,000 foxhounds as well as other dogs and wild canids. Results indicated an 8.9% seroprevalence of Leishmania species in foxhounds but not in other randomly selected dogs or wild canids. In an ongoing study, the researchers are finding 9.8% of the dogs are now seropositive in participating foxhound kennels. They are also finding that among high-risk kennels, 13.5% of dogs are seropositive and have clinical disease; 44.8% of dogs in these high-risk kennels have positive results on qualitative polymerase chain reaction assays. In endemic countries, a higher percentage of positive results are seen in breeds from southern Europe including Spinones and Neapolitan mastiffs. The origin of the infection found in the North American foxhounds studied by the CDC has been linked to infected hounds from southern France that were imported to Great Britain before arriving in the United States.

Visceral leishmaniasis does not develop in all infected animals. In fact, in the same ongoing study, about half of the infected individuals did not exhibit clinical signs, which typically include depression, loss of condition, muscle wasting, a dull coat, mild abdominal distention, serosanguineous nasal discharge, splenomegaly, generalized lymphadenopathy, and fever.

Clinical findings of the disease can include

• A decreased hematocrit

• Thrombocytopenia

• Signs of renal failure, including azotemia, hyperphosphatemia, and hypermagnesemia

• Signs of hepatic compromise, including elevated alkaline phosphatase activity, elevated alanine transferase activity, and hypercholesterolemia

• Hyperproteinemia in the form of hyperglobulinemia with hypoalbuminemia

• And proteinuria—glomerular nephritis is the leading cause of death in untreated dogs.

Diagnosis remains difficult in both people and dogs since infected individuals can remain seronegative for years. People can be screened through the CDC by indirect fluorescent antibody assay. But this test can cross react with Trypanosoma cruzi, the infective agent of Chagas disease. In the United States, an ELISA test and a K39-antigen assay are available for diagnosing canine leishmaniasis. However, the use of qualitative polymerase chain reaction assays appears to be the best predictor of clinical disease.

Treatment of canine leishmaniasis is not considered curative, and the prognosis for severely affected animals is poor. Owners should be informed that the organism is never completely removed and that it has zoonotic potential, especially if there are immunocompromised individuals in the household. Relapses are common and require ongoing treatment. In the United States, infected dogs without evidence of renal insufficiency are maintained on allopurinol. This treatment, along with a high-quality diet to reduce the wasting, results in an 80% survival rate. Other medical treatment options exist, but they do not appear to provide additional benefit, and several are not licensed for use in the United States. Brazil is the first country to license a vaccine for use in dogs, and sand fly vector control efforts including permethrin and deltamethrin collars have helped to prevent disease in endemic areas.

Thus, in North America, canine leishmaniasis appears to only be endemic in foxhounds at this time, but finding a genetic link will require further investigation. Likewise, while sand flies are not confirmed vectors here, they are likely to play a role in transmission as this agent proliferates.

Source: Petersen CA, Barr SC. Canine leishmaniasis in North America: emerging or newly recognized? Vet Clin North Am Small Anim Pract 2009;39(6):1065-1074.