Leishmaniosis is a disease caused by protozoan parasites of the genus Leishmania and transmitted through the bites of female phlebotomine sand flies. More than 23 species of Leishmania have been described, most of which are zoonotic. The most important Leishmania parasite to affect domestic animals is L infantum, also known as L chagasi in Latin America. Dogs are the main reservoir host for human visceral leishmaniosis caused by L infantum, and the disease is potentially fatal in dogs and people. Because the internal organs and skin of the dog are affected, the canine disease is termed viscerocutaneous or canine leishmaniosis. Cats, horses, and other mammals can be infected by L infantum or other Leishmania species. The disease in cats is rarer than in dogs and may manifest in cutaneous or visceral organs. L braziliensis, the cause of tegumentary canine leishmaniosis, is widespread in regions of South America and may geographically overlap with L chagasi.
Canine leishmaniosis is a major zoonosis endemic in >89 countries. It is prevalent in Europe, Africa, Asia, and South and Central America and vertically transmitted from dog to dog in the USA. It is also of concern in nonendemic countries where imported disease constitutes a veterinary and public health problem.
Leishmania is a diphasic parasite that completes its life cycle in two hosts: a sand fly that harbors the flagellated extracellular promastigote form and a mammal in which the intracellular amastigote parasite form develops.
Transmission has been confirmed by two principal routes, vector-borne by phlebotomine sand flies and vertically from dam to pup. Sand fly transmission is a complex process that requires special adaptation between the sand fly host and the particular Leishmania species transmitted. There are numerous species of sand flies, only a minority of which are competent vectors of Leishmania. Dogs with or without clinical signs are infectious to sand flies and may transmit Leishmania parasites. Congenital vertical transmission of canine leishmaniosis from an infected dam to its offspring has been reported. Transmission by transfusion of blood products from infected dogs has been shown to cause infection in recipients. Direct dog-to-dog transmission by contact has been reported as a mode of disease transmission in several case reports where household fighting or contact with diseased dogs has been the only known contact.
Dogs are thought to be most commonly infected by L infantum promastigotes deposited into the skin via the bites of infected sand flies. Promastigotes invade host macrophages and replicate as intracellular amastigotes. Immune responses mounted at the time of infection and thereafter appear to be the most important factor in determining whether a persistent infection will develop and progress from subclinical to clinical disease. The incubation period may last months to years, during which the parasite disseminates throughout the host’s body, primarily to the hemolymphatic system organs. Age, breed, host genetics, nutrition, concurrent infectious and noninfectious diseases, and other factors may also influence the progression from infection to clinical disease.
Canine leishmaniosis is a multisystemic disease with a highly variable spectrum of immune responses and clinical manifestations. In endemic areas, the prevalence of dogs carrying infection is much higher than those demonstrating clinical disease. Clinical disease is associated with a marked antibody response that does not confer protection. In fact, immune-mediated mechanisms are responsible for much of the pathology in canine leishmaniosis.
The typical history reported by owners of dogs with clinical disease due to L infantum includes the appearance of skin lesions, ocular abnormalities, or epistaxis. These are frequently accompanied by weight loss, exercise intolerance, and lethargy. The main physical examination findings are dermal abnormalities in 80%–90% of the dogs, lymphadenomegaly in 62%–90%, ocular disease in 16%–81%, splenomegaly in 10%–53%, and abnormal nail growth (onychogryphosis) in 20%–31%. Other clinical findings may include polyuria and polydipsia due to kidney disease, vomiting, colitis, melena, and lameness due to joint, muscle, or bone lesions.
The sole presenting signs of disease could be epistaxis, ocular abnormalities, or manifestations of kidney disease without dermal abnormalities. The dermal lesions associated with canine leishmaniosis include exfoliative dermatitis, which can be generalized or localized over the face, ears, and limbs. Ulcerative, nodular, or mucocutaneous dermatitis are also seen. Cutaneous ulcers over the ears or other locations may be associated with considerable bleeding. A mild form of papular dermatitis has been reported in dogs with no other signs of disease. Ocular or periocular lesions include keratoconjunctivitis and uveitis.
Clinical laboratory findings include mild to moderate nonregenerative or rarely regenerative anemia in 60%–73% of dogs; thrombocytopenia is less common. The most consistent serum biochemistry findings in dogs with clinical canine leishmaniosis are serum hyperproteinemia with hyperglobulinemia and hypoalbuminemia, frequently expressed by a decreased albumin:globulin ratio. Marked hyperglobulinemia with no apparent cause in dogs from Leishmania-endemic regions should suggest canine leishmaniosis. Some degree of renal pathology is present in most dogs with canine leishmaniosis, but may not be clinically apparent. Grossly increased liver enzyme activity or azotemia are found in only a minority of infected dogs. Subsequent renal failure due to immune-complex glomerulonephritis may eventually develop and is believed to be the main natural cause of death. The presence of proteinuria should be evaluated and kidney disease staged with measurement of the urine protein:creatinine ratio.
The typical histopathologic finding in canine leishmaniosis is granulomatous inflammation associated with a variable number of Leishmania amastigotes with macrophages. Protective immunity against Leishmania parasites is mediated through CD4+T helper 1 type cells and the activation of a complex cascade of cytokine mediators. In contrast, T regulatory correlated high levels of circulating immune complexes and antinuclear antibodies can be detected in animals with canine leishmaniosis, and deposition of immune complexes in the kidneys, blood vessels, and joints occurs as infection progresses. Glomerulonephritis is a hallmark of this disease.
Diagnostic tests for canine leishmaniosis include a CBC, biochemical profile, urinalysis, and one or more specific tests to confirm infection. Quantitative serology is best for diagnosis and particularly sensitive when compatible clinical signs are present. High antibody titers are found in 80%–100% of dogs with clinical disease and could be conclusive of a diagnosis. Various quantitative serologic methods to detect anti-Leishmania antibodies have been developed, including indirect immunofluorescence assays, ELISA, and direct agglutination assays. Purified recombinant antigens such as rK39 are also used to detect leishmaniosis in dogs and people in several rapid lateral flow formats as screening assays. Serologic cross-reactivity with trypanosomes may be found in regions where Trypanosoma infection is prevalent, particularly with T cruzi in the Americas.
Detection of parasite-specific DNA by PCR allows sensitive and specific diagnosis of infection. Several different assays with various target sequences using genomic or kinetoplast DNA (kDNA) have been developed for canine leishmaniosis. PCR can be performed on DNA extracted from tissues, blood, or even from histopathologic specimens. Assays based on kDNA are the most sensitive for direct detection in infected tissues, but these sequences have been shown to have wobble or variability over time. Bone marrow, lymph node, or spleen samples are superior to blood with most of the current PCR techniques.
Leishmania amastigotes can be demonstrated by cytology from lymph nodes, spleen, skin impressions, bone marrow, or joint fluids stained with Giemsa stain or a quick commercial stain. As mentioned earlier, T cruzi, a similar parasite, also has an amastigote form with a kinetoplastid, so in the Americas visualization of amastigotes does not necessarily diagnose Leishmania infection. Detection of amastigotes by cytology is sometimes unrewarding because of a low number of detectable parasites, even in dogs with full-blown clinical disease. Leishmania parasites may also be viewed in histopathologic formalin-fixed, paraffin-embedded biopsy sections of the skin or other infected organs. Identification of parasites within tissue macrophages may be difficult; immunolabeling with immunohistochemical staining can verify the presence of Leishmania in the tissue.
Detection of infection in dogs with no clinical disease for purposes such as importation to nonendemic countries or use as blood donors may require quantitative PCR, which is the most sensitive diagnostic technique. Cross-sectional studies of dog populations in highly endemic areas have shown that infection rates can reach 65%–80%. Typically, only approximately 10%–13% manifest clinical signs of disease, 26% are seropositive and include sick and subclinically infected dogs, and an additional 40%–60% are carriers positive only by tissue PCR.
The main protocol used for treatment of canine leishmaniosis includes N-methylglucamine antimoniate (not approved for use in dogs in the USA) at 50–100 mg/kg, SC, for 4–6 weeks combined with allopurinol (10 mg/kg, PO, twice daily, for 6–12 months or longer as needed). Allopurinol may also be used as a single therapeutic agent at the same dose. Miltefosine at 2 mg/kg/day, PO, for 4 weeks can also be combined with allopurinol (10 mg/kg, PO, twice daily) as an available alternative to N-methylglucamine antimoniate in the USA. Treatment frequently does not provide sterilizing cure. Treated dogs can remain carriers of infection and may relapse. They may remain infectious to sand flies. Stopping treatment with allopurinol is recommended only when clinical signs disappear, hematologic and serum biochemistry abnormalities return to normal, and the animal becomes seronegative by a quantitative laboratory serologic test, which may mean lifetime treatment.
Chemotherapeutic Options with Leishmanicidal/Leishmaniostatic Effects
Specific repellent topical insecticides effectively reduce sand fly bites and transmission of leishmaniosis. A deltamethrin-impregnated collar and a spot-on formulation of permethrin and imidacloprid have been shown to confer protection against sand fly bites. The application of protective insecticides is recommended for dogs in Leishmania-endemic areas, dogs traveling to sites of infection, and infected dogs (to reduce potential transmission). Purified-fraction commercial vaccines against canine leishmaniosis are marketed in Europe and Brazil, and other vaccines are under development, but no vaccines against leishmaniosis are licensed for use in the USA and must be used under USDA permit.
Insecticides with Anti-sand Fly Efficacy
Human visceral leishmaniosis caused by L infantum is a serious public health problem in areas where canine leishmaniosis is endemic, and dogs remain the main reservoir of infection. Malnutrition and other immune-altering diseases, including human immunodeficiency virus (HIV) infection are recognized as key risk factors and may explain why this disease is an opportunistic one. HIV and leishmaniosis coinfection has been reported in >80 countries worldwide, and control of leishmaniosis returns when there is a sufficient CD4+ T cell population after treatment with combination antiretroviral therapy. Efforts to control canine leishmaniosis and human disease in endemic areas focus on disrupting transmission of infection through sand fly control measures and preventing canine infection at the population level.
Leishmaniosis remains an important zoonoses, particularly in South America and the Mediterranean basin as well as in East Africa. Dogs are still thought to be a predominant reservoir, although other companion animals, including cats and horses, have been found with clinically significant infection.
Where disease is not found to be endemic in people, dogs can still become infected by breeding infected animals imported from endemic areas or bringing dogs or other pets to endemic areas without protecting them against sand fly bites.
Prevention using appropriate insecticides and vaccination where available is best. Treatment is unlikely to lead to sterile cure and can recrudesce in months to years after treatment.