Leishmania (diphasic protozoan)

Leishmaniasis is a worldwide infectious disease. Leishmania spp. are found in the tropical and subtropical areas of Asia, Africa, and South America.

There are 30 species of Leishmania, 20 of which are zoonotic. Dogs can serve as reservoirs for the disease for people in areas where Leishmania is endemic. Different species of the parasite predominate in different parts of the world. In the past in the U.S., most cases in dogs were associated with travel by the dog to endemic areas in the world. Recently, visceral leishmaniasis has been diagnosed here, in 21 different states. Therefore, the organism may be present locally here in the United States. The organism is maintained in wild canid reservoirs with spillover into domestic dogs via the sandfly vector.

All species of Leishmania are found in the macrophages of the vertebrate host as an amastigote and in the gut of the sandfly as a promastigote. All forms of leishmaniasis are transmitted by phlebotomine sandflies. Less commonly, transmission of the disease can also occur from direct transmission between dogs and from dogs to humans. Therefore, direct contact with open wounds or exudates of dogs with leishmaniasis and contaminated objects should be avoided to prevent potential contact transmission of Leishmania. Veterinarians need to be cautious when treating any dog with possible signs of Leishmania infection.

In companion animals, dogs are the principal species affected causing chronic systemic disease. Incubation can be from 3 months to 7 years before clinical signs appear. The most common clinical signs of leishmaniasis are cutaneous lesions, lymphadenopathy, weight loss and muscle atrophy. Cutaneous lesions include alopecia, desquamation of outer layers of epidermis, depigmentation, pustules and ulcerations.

Most dogs with dermal clinical signs also have visceral involvement because the protozoans disseminate throughout the body. chronic antigenemia results in the deposition of immune complexes, with vasculitis, uveitis, and glomerulopathy. The immune complex deposition in the kidneys results in renal failure, which is the main cause of death of dogs with leishmaniasis. The infection can also cause immunosupression via an infection-induced suppression of T cells, allowing subsequent infections by other organisms.

Diagnosis: Leishmania infection in the dog is diagnosed with various serologic tests including complement fixation, indirect FA, and ELISA methods for serology. Serology verifies the presence of antibodies but not if the infection is active. Other diagnostic methods are cytological or histological identification of amastigotes free or in macrophages of stained smears of lymph nodes or bone marrow.

Treatment of leishmaniasis in dogs is more difficult than in humans and is rarely eliminated by current methods. Drug options available include pentavalent antimonials (meglumine antimonite and sodium stibogluconate which inhibit protozoal enzymes but must be injected daily) are expensive, are not readily available and have severe side effects. Allopurinol inhibits multiplication of the parasite, is administered orally, is readily available and has few side effects. However, complete recovery is rare and relapses can occur with discontinuation of drug therapy.

Prevention in dogs is very important for the health of the dogs themselves and to prevent them serving as a reservoir of the disease for humans.

Humans

In people, leishmaniasis occurs in three clinical forms: cutaneous, visceral and mucosal. The form of disease is determined by the individual Leishmania species, geographical location and the immune response of the host.

	Cutaneous leishmaniasis is characterized by one or more cutaneous lesions, which may or may not be painful with swollen lymph nodesnear the sores. A variety of Leishmania skin lesions in humans

People with visceral leishmaniasis have fever, hepatosplenomegaly, lymphadenopathy, thrombocytopenia, progressive emaciation, anemia, and leukocytopenia. In the mucosal form, skin lesions extend into mucous membranes, often disfiguring nasal or oral passages.

Diagnosis in humans is made by histological examination of the lymph nodes and bone marrow for Leishmania amastigotes. Antibody detection is useful for visceral leishmaniasis but a significant antibody response does not develop with cutaneous leishmaniasis.

Treatment: Humans should consult a physician for treatment.

Prevention: All forms of leishmaniasis are transmitted by phlebotomine sandflies. Less commonly, transmission of the disease can also occur from direct transmission between dogs and from dogs to humans. Therefore, direct contact with open wounds or exudates of dogs with leishmaniasis and contaminated objects should be avoided to prevent zoonotic contact transmission of Leishmania. Veterinarians need to be cautious when treating any dog with possible signs of Leishmania infection.

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