Leishmaniasis

Leishmaniasis is a chronic, systemic, zoonotic protozoal disease caused by intracellular parasites of the genus Leishmania, transmitted by phlebotomine sand flies. In dogs, the disease is most commonly caused by Leishmania infantum (syn. L. chagasi in the Americas). The clinical syndrome ranges from subclinical infection to severe multi-organ involvement, with characteristic dermatological, renal, ocular, and hematological manifestations. The disease is endemic in the Mediterranean basin, South and Central America, Asia, and parts of Africa, and is emerging in non-endemic areas due to travel and climate change. In veterinary medicine, canine leishmaniasis is a major public health concern due to its zoonotic potential, and it serves as a model for human visceral leishmaniasis. The disease is characterized by a complex immune response, with a predominant cell-mediated immunity (Th1) conferring resistance, while a humoral (Th2) response is associated with susceptibility and clinical disease. The clinical presentation is highly variable, ranging from asymptomatic infection to severe systemic illness, with renal failure being the most common cause of death.

The causative agent is Leishmania infantum (syn. L. chagasi) in the Old World and New World, respectively. Other species, such as L. braziliensis and L. mexicana, can cause cutaneous disease in dogs in the Americas, but L. infantum is the primary species causing visceral and systemic disease. The parasite exists in two forms: the promastigote, which is flagellated and extracellular, found in the sand fly vector, and the amastigote, which is non-flagellated and intracellular, residing within macrophages of the mammalian host. Transmission occurs primarily through the bite of infected female phlebotomine sand flies (Phlebotomus spp. in the Old World, Lutzomyia spp. in the New World). Other less common routes include vertical transmission (transplacental), venereal transmission, blood transfusion, and direct contact through contaminated needles. The parasite's virulence factors include lipophosphoglycan (LPG), which facilitates complement resistance and macrophage adhesion, and gp63, a metalloprotease that degrades complement and promotes invasion. The parasite also modulates host immune responses by downregulating Th1 cytokines and promoting a Th2 response, leading to uncontrolled parasite replication and dissemination.

Canine leishmaniasis is endemic in more than 70 countries, with a high prevalence in the Mediterranean basin (Spain, Italy, Greece, Portugal, France, and North Africa), the Middle East, Central and South America (Brazil, Venezuela, Colombia), and parts of Asia (India, China). In endemic areas, seroprevalence in dogs can range from 10% to 40%, with some foci exceeding 60%. The disease is more common in rural and suburban areas where sand fly vectors are abundant. All breeds and ages are susceptible, but certain breeds, such as the Boxer, Rottweiler, German Shepherd, and Cocker Spaniel, are reported to have a higher risk of developing clinical disease, likely due to genetic susceptibility and immune response variations. Young dogs (under 3 years) and older dogs (over 8 years) are more likely to develop clinical signs. The disease is seasonal, with transmission peaking during warm months (May to October in the Northern Hemisphere) when sand flies are active. Climate change and increased travel have led to the emergence of leishmaniasis in previously non-endemic areas, such as northern Europe and the United States. The disease is zoonotic, with dogs serving as the primary reservoir for human visceral leishmaniasis in urban and peri-urban areas.

The pathophysiology of canine leishmaniasis is complex and involves a delicate balance between the host's immune response and the parasite's evasion mechanisms. After inoculation of promastigotes into the skin, the parasites are phagocytosed by macrophages and dendritic cells. Inside the phagolysosome, promastigotes transform into amastigotes and replicate. The outcome of infection depends on the host's T-helper cell response. A protective response is characterized by a Th1-dominant immune profile, with production of interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α), which activate macrophages to kill intracellular amastigotes via nitric oxide and reactive oxygen species. In contrast, a non-protective response is dominated by Th2 cytokines (IL-4, IL-10, IL-13), which suppress macrophage activation and promote antibody production, leading to uncontrolled parasite replication and dissemination. The excessive production of non-neutralizing antibodies leads to immune complex formation, which deposits in various tissues, particularly the kidneys, joints, and eyes, causing type III hypersensitivity reactions. Glomerulonephritis is a hallmark of canine leishmaniasis, often leading to protein-losing nephropathy and chronic renal failure. The parasite also induces a systemic inflammatory response, with elevated acute-phase proteins (e.g., C-reactive protein, haptoglobin) and hyperglobulinemia (polyclonal gammopathy). Hematological abnormalities, such as anemia, leukopenia, and thrombocytopenia, result from bone marrow infiltration, immune-mediated destruction, and hypersplenism. The skin lesions are due to granulomatous inflammation and vasculitis, often with secondary bacterial infections. The disease can affect virtually any organ, including the liver, spleen, lymph nodes, bone marrow, and eyes, leading to a wide range of clinical signs.

Several factors predispose dogs to the development of clinical leishmaniasis. Intrinsic factors include genetic susceptibility, with certain breeds (e.g., Boxer, Rottweiler, German Shepherd) having a higher risk of developing severe disease. Age is also a factor, with young dogs (under 3 years) and older dogs (over 8 years) being more susceptible. Immunosuppression, whether due to concurrent infections (e.g., ehrlichiosis, babesiosis), chronic stress, or immunosuppressive drug therapy (e.g., corticosteroids, cyclosporine), can exacerbate the disease. Extrinsic factors include environmental conditions that favor sand fly breeding, such as warm climates, high humidity, and the presence of organic matter. Dogs living outdoors in endemic areas are at higher risk. Poor nutrition and concurrent diseases can also impair the immune response. Additionally, dogs that are subclinically infected may develop clinical disease when they become immunosuppressed or when they are moved to non-endemic areas where they are no longer exposed to sand flies, leading to a loss of natural boosting of immunity.

The clinical signs of canine leishmaniasis are highly variable and depend on the stage of the disease and the organs involved. The incubation period can range from a few months to several years. The disease can be classified into subclinical, oligosymptomatic, and symptomatic forms. Subclinical dogs are infected but show no clinical signs, although they may have laboratory abnormalities. Oligosymptomatic dogs have mild, non-specific signs such as lymphadenomegaly, weight loss, and poor coat condition. Symptomatic dogs present with a combination of the following signs: Cutaneous lesions are the most common, occurring in up to 90% of cases, and include exfoliative dermatitis (scaling, dandruff), ulcerative dermatitis (especially on the ears, nose, footpads, and pressure points), nodular dermatitis, and onychogryphosis (abnormal overgrowth of claws). Alopecia, particularly around the eyes (periocular alopecia), is also common. Systemic signs include chronic weight loss, muscle atrophy, lethargy, and anorexia. Lymphadenomegaly is a frequent finding, with peripheral lymph nodes being enlarged and sometimes painful. Splenomegaly and hepatomegaly may be present. Renal involvement is a major concern, with signs of chronic kidney disease such as polyuria, polydipsia, vomiting, and uremic halitosis. Ocular signs include blepharitis, conjunctivitis, keratitis, uveitis, and glaucoma. Epistaxis (nosebleeds) is a characteristic sign, often due to immune-mediated vasculitis. Other signs include lameness due to polyarthritis, diarrhea, and respiratory signs. In advanced stages, dogs may develop renal failure, which is the most common cause of death.

The differential diagnoses for canine leishmaniasis are numerous due to the non-specific nature of many clinical signs. Key differentials include: 1) Ehrlichiosis (Ehrlichia canis) - causes similar hematological abnormalities, epistaxis, and lymphadenomegaly; differentiation via serology and PCR. 2) Babesiosis (Babesia spp.) - presents with fever, anemia, and thrombocytopenia; blood smear and PCR are diagnostic. 3) Systemic lupus erythematosus (SLE) - can cause polyarthritis, glomerulonephritis, and skin lesions; antinuclear antibody (ANA) testing is helpful. 4) Demodicosis (Demodex spp.) - causes alopecia and dermatitis, but skin scrapings reveal mites. 5) Dermatophytosis - fungal skin infection with scaling and alopecia; fungal culture is diagnostic. 6) Neoplasia (e.g., lymphoma, multiple myeloma) - can cause lymphadenomegaly, hyperglobulinemia, and cytopenias; cytology and histopathology are needed. 7) Chronic bacterial dermatitis or deep pyoderma - may mimic skin lesions; bacterial culture and response to antibiotics help. 8) Other infectious diseases such as brucellosis, hepatozoonosis, and toxoplasmosis should also be considered. A thorough diagnostic workup, including serology, PCR, and cytology, is essential to differentiate these conditions.

The diagnostic approach to canine leishmaniasis should be systematic and include both direct and indirect methods. The algorithm begins with a thorough history and physical examination, with special attention to endemic exposure and clinical signs. If leishmaniasis is suspected, initial screening tests include a complete blood count (CBC), serum biochemistry profile, urinalysis, and serum protein electrophoresis. Common findings include non-regenerative anemia, leukopenia, thrombocytopenia, hyperglobulinemia (polyclonal gammopathy), hypoalbuminemia, and proteinuria. If these findings are present, specific testing for Leishmania is indicated. The most commonly used serological tests are the immunofluorescence antibody test (IFAT) and enzyme-linked immunosorbent assay (ELISA), which detect anti-Leishmania antibodies. A positive serology in a dog with compatible clinical signs is highly suggestive of the disease. However, serology can be negative in early or subclinical infections, and false positives can occur due to cross-reactivity with other pathogens. Therefore, confirmation by direct detection of the parasite is recommended. This can be achieved by cytological examination of lymph node, bone marrow, or spleen aspirates, which may reveal amastigotes within macrophages. PCR on blood, bone marrow, lymph node, or skin samples is highly sensitive and specific and can be used to confirm infection. Quantitative PCR (qPCR) can also be used to monitor response to treatment. In cases with renal involvement, a renal biopsy may be indicated to assess the severity of glomerulonephritis. The diagnostic algorithm should also include screening for concurrent infections, especially ehrlichiosis and babesiosis, which are common in endemic areas.

Laboratory findings in canine leishmaniasis are variable but often include: Hematology: Normocytic, normochromic non-regenerative anemia (due to chronic disease and renal failure), leukopenia (neutropenia, lymphopenia), thrombocytopenia (immune-mediated or due to bone marrow suppression). Serum Biochemistry: Hyperglobulinemia (polyclonal gammopathy) is a hallmark, with a marked increase in beta and gamma globulins on protein electrophoresis. Hypoalbuminemia is common, leading to a decreased albumin/globulin ratio. Elevated liver enzymes (ALT, ALP) may be seen due to hepatic involvement. Renal parameters (BUN, creatinine) are elevated in cases of renal failure. Hypercalcemia may occur in some dogs. Urinalysis: Proteinuria is a common finding, and the urine protein-to-creatinine ratio (UPC) is often elevated (>0.5). The urine sediment may show casts and red blood cells. Blood Gas Analysis: May reveal metabolic acidosis in advanced renal failure. Specific Biomarkers: Serum amyloid A (SAA) and C-reactive protein (CRP) are elevated in active disease. SDMA (symmetric dimethylarginine) may be elevated earlier than creatinine in renal dysfunction. Serology: IFAT and ELISA are used to detect antibodies; titers correlate with disease activity but can be positive in subclinical infections. PCR: Qualitative and quantitative PCR on blood, bone marrow, or tissue samples are highly sensitive and specific for detecting Leishmania DNA. Quantitative PCR can be used to monitor treatment efficacy, with a decrease in parasite load over time.

Imaging findings in canine leishmaniasis are non-specific but can support the diagnosis and assess organ involvement. Radiography: Thoracic radiographs may show a diffuse interstitial pattern due to pulmonary infiltration, but this is uncommon. Abdominal radiographs may reveal hepatosplenomegaly. In cases of renal failure, radiographs may show small, irregular kidneys. Ultrasonography: Abdominal ultrasound often reveals splenomegaly, hepatomegaly, and lymphadenomegaly. The kidneys may show increased cortical echogenicity, loss of corticomedullary distinction, and irregular contours in chronic renal disease. Doppler ultrasound can assess renal blood flow. Echocardiography: May be indicated if cardiac involvement is suspected, but this is rare. Computed Tomography (CT) and Magnetic Resonance Imaging (MRI): These advanced imaging modalities are rarely needed but may be useful for evaluating central nervous system involvement or for surgical planning in cases of severe organomegaly. Endoscopy: Can be used to obtain biopsies of the gastrointestinal tract if there is chronic diarrhea, but this is not a primary diagnostic tool for leishmaniasis.

Cytological examination of aspirates from lymph nodes, bone marrow, spleen, or skin lesions can reveal Leishmania amastigotes, which are small (2-4 μm) oval or round organisms with a nucleus and a kinetoplast, often seen within macrophages. The sensitivity of cytology is variable, ranging from 30% to 90% depending on the sample site and the chronicity of the disease. Bone marrow and spleen aspirates have higher sensitivity than lymph node aspirates. Histopathological examination of skin biopsies may show granulomatous dermatitis with macrophages containing amastigotes, as well as vasculitis and perifollicular inflammation. Renal biopsies typically reveal glomerulonephritis, which may be membranoproliferative, membranous, or focal segmental, with immune complex deposition. Immunohistochemistry (IHC) using anti-Leishmania antibodies can enhance the detection of amastigotes in tissue sections. Special stains such as Giemsa or Wright's stain are used for cytology and histopathology to visualize the parasites.

The treatment of canine leishmaniasis aims to control clinical signs, reduce parasite load, and prevent relapse, but complete elimination of the parasite is difficult. The current standard of care involves the use of meglumine antimoniate (Glucantime) at a dose of 75-100 mg/kg subcutaneously or intravenously every 24 hours for 4-6 weeks, often combined with allopurinol (10 mg/kg orally every 12 hours) for at least 6-12 months. Allopurinol is a xanthine oxidase inhibitor that is leishmaniostatic and is used to maintain remission. Alternative protocols include miltefosine (2 mg/kg orally every 24 hours for 28 days) combined with allopurinol, or aminosidine (paromomycin) (18-20 mg/kg subcutaneously every 24 hours for 3-4 weeks) combined with allopurinol. In cases of severe renal disease, treatment should be adjusted, and nephrotoxic drugs should be avoided. Supportive care includes fluid therapy for dehydration and renal support, a renal diet for proteinuria, and treatment of secondary bacterial infections. Immunomodulatory therapy with domperidone (1 mg/kg orally every 24 hours for 30 days) has been used as an adjunct to enhance cell-mediated immunity. In endemic areas, vector control (insecticides, insect repellents, and indoor housing) is essential to prevent transmission. The use of vaccines (e.g., Leish-Tec, CaniLeish) is available in some countries and may reduce the risk of clinical disease but does not prevent infection.

The prognosis for canine leishmaniasis is guarded to good, depending on the stage of the disease and the presence of renal involvement. Dogs with mild to moderate clinical signs and no significant proteinuria or renal failure have a good prognosis with appropriate treatment, with clinical remission achieved in 70-90% of cases. However, relapse is common, and long-term maintenance therapy with allopurinol is often required. Dogs with severe renal disease (IRIS stage III or IV) have a poor prognosis, with a median survival time of less than 6 months despite treatment. Negative prognostic indicators include marked proteinuria (UPC > 2.0), elevated creatinine, hypoalbuminemia, and severe anemia. The response to treatment is monitored by clinical improvement, reduction in antibody titers, and decrease in parasite load as assessed by qPCR. Dogs that achieve clinical remission and have stable renal function can have a good quality of life for several years.

Follow-up of dogs with leishmaniasis is essential to monitor treatment response and detect relapses. During the initial treatment phase (first 4-6 weeks), dogs should be re-examined every 2-4 weeks to assess clinical improvement and monitor for adverse drug effects. After completion of the initial treatment, re-evaluation should be performed every 3-6 months. Each re-check should include a complete physical examination, CBC, serum biochemistry profile, urinalysis, and UPC ratio. Serology (IFAT or ELISA) can be repeated every 6-12 months to monitor antibody titers, which should decrease with successful treatment. Quantitative PCR can be used to monitor parasite load, with a goal of reducing or eliminating detectable DNA. In dogs with renal involvement, more frequent monitoring of renal parameters (creatinine, SDMA, UPC) is necessary. Long-term maintenance therapy with allopurinol should be continued for at least 6-12 months, and some dogs may require lifelong therapy. Owners should be educated about the zoonotic risk and the importance of vector control measures to prevent transmission to humans and other dogs.

Pearls: 1) Always consider leishmaniasis in any dog with chronic weight loss, lymphadenomegaly, skin lesions, and proteinuria, especially if there is a history of travel to endemic areas. 2) Onychogryphosis is a highly suggestive sign, though not pathognomonic. 3) A positive serology in a dog with compatible clinical signs is sufficient to initiate treatment, but confirmatory PCR or cytology is recommended for definitive diagnosis. 4) Renal disease is the most common cause of death; early detection of proteinuria and initiation of renal protective therapy (e.g., ACE inhibitors) is crucial. 5) Combination therapy with meglumine antimoniate and allopurinol is more effective than monotherapy. Pitfalls: 1) Do not rely solely on serology for diagnosis, as false negatives can occur in early infection, and false positives can occur due to cross-reactivity. 2) Avoid using corticosteroids in dogs with leishmaniasis unless absolutely necessary, as they can exacerbate the disease. 3) Do not discontinue allopurinol too early, as this can lead to relapse. 4) Be aware that some dogs may develop adverse reactions to meglumine antimoniate, such as nephrotoxicity and local pain at the injection site. 5) In non-endemic areas, a lack of awareness can lead to misdiagnosis; always ask about travel history.

Based on Plumb's Veterinary Drug Handbook, the following protocols are recommended: 1) Meglumine antimoniate (Glucantime): 75-100 mg/kg SC or IV once daily for 4-6 weeks. It is important to monitor renal function, as it can be nephrotoxic. 2) Allopurinol: 10 mg/kg PO q12h, administered for at least 6-12 months, often lifelong. It is generally well-tolerated, but xanthine urolithiasis can occur with prolonged use. 3) Miltefosine: 2 mg/kg PO q24h for 28 days. It is less nephrotoxic than antimonials but can cause gastrointestinal upset. 4) Aminosidine (paromomycin): 18-20 mg/kg SC q24h for 3-4 weeks. It is an alternative to antimonials but can be nephrotoxic. 5) Domperidone: 1 mg/kg PO q24h for 30 days, used as an immunomodulator to enhance Th1 response. 6) Supportive therapy: For proteinuria, enalapril (0.5 mg/kg PO q12h) or benazepril (0.25-0.5 mg/kg PO q24h) is recommended. For secondary bacterial infections, appropriate antibiotics should be chosen based on culture and sensitivity. In cases of severe anemia, blood transfusion may be necessary. All doses should be adjusted in patients with renal or hepatic impairment, and drug interactions should be considered. For example, allopurinol can increase the risk of xanthine stones when used with uricosuric drugs.

The evidence base for the treatment of canine leishmaniasis is supported by several clinical trials and consensus guidelines. The LeishVet group has published consensus recommendations for the diagnosis, staging, and treatment of canine leishmaniasis. A landmark study by Oliva et al. (2010) compared the efficacy of meglumine antimoniate plus allopurinol versus allopurinol alone, showing that the combination therapy resulted in faster clinical improvement and lower relapse rates. Another study by Miró et al. (2009) demonstrated that miltefosine plus allopurinol was as effective as meglumine antimoniate plus allopurinol, with fewer adverse effects. A meta-analysis by Solano-Gallego et al. (2017) concluded that combination therapy is superior to monotherapy for achieving clinical cure. The use of allopurinol as maintenance therapy is supported by long-term follow-up studies showing reduced relapse rates. The ACVIM consensus statement on leishmaniasis (2018) recommends a staged approach to treatment based on the severity of clinical signs and renal involvement. The use of vaccines is supported by field trials showing a reduction in the incidence of clinical disease, but they do not prevent infection. Overall, the evidence supports the use of combination therapy with an antileishmanial drug and allopurinol, with close monitoring of renal function and long-term follow-up.