An Overview of Canine Leishmania Infections
Leishmaniasis is an infectious disease transmitted by sand flies and caused by various species of Leishmania. The parasites can infect both humans and canines, and the resulting condition is known as visceral leishmaniasis. The disease is particularly common in tropical and subtropical areas with significant sand fly populations. The domestic dog seems to be the main reservoir for human visceral leishmaniasis, rendering disease control that much more vital. Unfortunately efforts to control leishmaniasis in dogs have been largely unsuccessful, though as more research is being done, progress is being and will continue to be made.
How might my dog become infected?
The natural infection cycle requires involvement of an insect vector – the sand fly. Female sand flies contract the Leishmania parasite as they feed on an infected animal and ingest the intracellular, non-flagellated form of parasite. Once ingested, the parasite transforms into a flagellated form, called promastigote. The promastigotes attach themselves to the gut wall of the sand fly and proceed to multiply and differentiate into infective promastiogotes. When the sand fly feeds on a new host, the promastigotes may be injected into the animal’s skin. Once within their new host, the promastigotes lose their flagella and multiply inside certain types of white blood cells, the macrophages, as amastigotes.
This initial multiplication occurs at or near the site of infection on the skin. The parasites will then spread from the skin to the bone marrow, spleen, and liver. Once in these organs the parasites cause a chronic, debilitating, and potentially fatal disease.
How is visceral leishmaniasis diagnosed?
The list of symptoms associated with leishmania infections in dogs is long. The most frequently reported complaints from owners of sick dogs include skin lesions, progressive weight loss, decreased appetite and an intolerance of exercise. Other common indicators of visceral leishmaniasis include depression, ocular changes, epistaxis, excessive thirst and urine production, diarrhea, vomiting, sneezing, coughing, and lameness. This is however by no means an exhaustive list – leishmaniasis affects so many parts of the body that the variety of clinical signs is nearly boundless. It should be noted that Leishmania infection should only be suspected in areas where sand flies are present as they are the major vector of infection.
Definitive diagnosis of visceral leishmaniasis is typically done by laboratory tests. Most commonly, serologic methods such as IFAT, ELISA and western blotting are used to detect anti-Leishmania antibodies. If the infection is at an advanced stage, it may be possible to isolate the Leishmania organism directly. Your veterinarian will utilize the appropriate diagnostic technique(s) should it become necessary.
What are the treatment options for dogs diagnosed as having visceral leishmaniasis?
Canine cases of leishmaniasis are unfortunately much more resistant to treatment than are human infections. For this reason a full parasitological cure in dogs is rarely achieved. Drugs such as pentavalent antimonials are frequently used to treat symptoms, and significant clinical improvement often occurs initially. Long term, however, relapses are common and the situation is complicated by the fact that frequent, long term drug use can both be harmful to the dog and create drug resistant microorganisms.
What are the public health concerns associated with visceral leishmaniasis?
Humans can and do become infected with the Leishmania bacteria. More than 90 percent of the world’s human leishmaniasis cases occur in India, Bangladesh, Nepal, Sudan, Brazil, and the countries along the Mediterranean Basin. Affected people have an irregular fever, anemia, enlarged spleen and liver, abnormal blood counts, and a high risk of mortality if untreated. Humans with normal immune function are at a minimal risk of infection, but infants, the elderly, and individuals with HIV or AIDS are more vulnerable. Thankfully, humans respond much better and more thoroughly to treatment than do dogs.
What steps can I take to minimize the risk of infection for both my dog and myself?
It should first be noted that if you live in the United States or Canada, the chances of either you or your dog contracting leishmaniasis are low. Only if you live in southern Texas might the risk be significant although leishmaniasis has recently been declared endmic in the US after a CDC survey that found positive dogs in 21 states. The sand fly distribution in the US is largely uknown. If you do live in an area where Leishmania is endemic, or if you have traveled or will soon be traveling to such an area, certain steps may be taken to minimize the risk of exposure.
As far as dogs are concerned, if you live in an area at high risk, blood screening should be done on a regular basis. This allows for an infection to be caught and treated early and avoid the risk of spreading the infection. General health status of your companion should be kept high, as lowered immune response greatly increases the chance of infection. Preventative collars that repel the sand fly are commercially available and have been used with some success. It is well known that sand flies are most active between dusk and dawn. If possible, time outdoors should be limited during these hours.
As humans are also infected only by the bite of a sand fly, avoidance of these insects is the key to prevention. Staying inside between dusk and dawn can reduce the risk of getting bitten. Insect repellent containing DEET should be used at all hours of the day, and homes should be well-screened so that flies are less likely to gain entrance to the house.
Additional resources on the web:
Centers for Disease Control
Discusses the human/pet interaction in leishmaniasis.
The Canine Leishmaniasis web site by Vetstream
Thorough and professional site of peer-reviewed material, including extensive references.
The University of Georgia College of Veterinary Medicine
Provides an intermediate level of detail on etiology, pathology, clinical signs and diagnosis, and treatment.
International Veterinary Information Service
Highly detailed discussion of canine visceral leishmaniasis for veterinary professionals.