What is Leishmaniasis?
Leishmaniasis is a disease caused by infection with Leishmania parasites, which are spread to people by the bite of infected female phlebotomine sandflies. There are several different forms of leishmaniasis in people. The most common forms are cutaneous leishmaniasis, which causes skin sores, and visceral leishmaniasis, which affects some of the internal organs of the body (CDC, 2022).
Leishmaniasis is classified as a neglected tropical disease (NTD). NTDs affect populations living in poverty, especially in low-income countries in the tropics and subtropics. Leishmaniasis occurs in parts of about 98 countries and 3 territories on 5 continents (WHO, 2023). Approximately 0.7–1 million new cases and 20,000–30,000 deaths occur annually (WHO, 2023).
The Leishmania parasites that cause leishmaniasis are transmitted to humans by the bite of infected female phlebotomine sandflies. There are about 30 different species of Leishmania that can cause disease in humans. The parasites have a complex life cycle, inhabiting certain species of sandflies as well as humans and some other mammals like rodents and dogs (CDC, 2022).
Types of Leishmaniasis
There are three main forms of leishmaniasis caused by different species of Leishmania parasites:1
Visceral Leishmaniasis
Visceral leishmaniasis, also known as kala-azar, is the most serious form of leishmaniasis. It’s caused by the parasite Leishmania donovani and transmitted by the bite of infected female sandflies. Visceral leishmaniasis affects internal organs, usually the spleen, liver, and bone marrow.2
Symptoms include fever, weight loss, enlargement of the spleen and liver, and anemia. If left untreated, visceral leishmaniasis is almost always fatal. Treatment involves medications like pentavalent antimonials, amphotericin B, miltefosine, and liposomal amphotericin B.
Cutaneous Leishmaniasis
Cutaneous leishmaniasis causes skin sores and is the most common form of leishmaniasis. It’s caused by several Leishmania species like L. major, L. tropica, and L. mexicana transmitted by sandflies. The sores typically form on exposed areas like the face, arms, and legs.1
Treatment options include local heat therapy, cryotherapy, and medications like pentavalent antimonials, amphotericin B, miltefosine, and paromomycin. Lesions usually self-heal over months to years, resulting in scarring.
Leishmaniasis in Dogs
Dogs are one of the most common animal hosts for Leishmania parasites (VCA Animal Hospitals). The disease affects dogs in many countries around the world, especially in the Mediterranean region, South America, and parts of Asia and Africa. Dogs get infected through the bites of certain types of sandflies that carry the parasite.
The most common signs and symptoms of leishmaniasis in dogs include (Merck Veterinary Manual):
- Skin lesions, ulcers, and fur loss, especially around the eyes, ears, muzzle, and nose
- Kidney failure
- Weight loss and muscle wasting
- Fever
- Swollen lymph nodes
- Eye inflammation
- Lameness
- Bleeding disorders
However, some dogs may be asymptomatic carriers, showing no signs of disease for months or even years after infection (Vet Specialists). When clinical disease does develop, it can range from mild cutaneous signs to severe systemic illness.
Transmission
Leishmaniasis is spread through the bites of infected female sandflies (Phlebotomus species in the Old World and Lutzomyia species in the New World). Sandflies become infected when they feed on an infected host, such as rodents, dogs, and sometimes humans. The parasites multiply in the sandfly’s gut and migrate to the proboscis. When the infected sandfly bites a dog, it injects the parasite promastigotes into the skin. Promastigotes are then taken up by macrophages and transform into amastigotes that multiply in infected cells and spread to other tissues, like lymph nodes, skin, spleen, liver, and bone marrow.
Dogs are considered a major reservoir for human infection. The disease cycle involves sandflies being infected when feeding on infected dogs and then transmitting the parasites when subsequently feeding on humans. Limiting sandfly populations and preventing transmission between dogs and sandflies are important disease control measures.
Diagnosis
Leishmaniasis is diagnosed in dogs based on clinical signs, history of possible exposure, and diagnostic testing. The most common clinical signs that may lead a veterinarian to suspect leishmaniasis include:
- Enlarged lymph nodes
- Skin lesions, dermatitis, alopecia
- Loss of appetite and weight loss
- Lethargy, exercise intolerance
- Ocular lesions
- Lameness
A travel history to endemic areas like parts of southern Europe, Africa, Asia, Central and South America may also increase suspicion. Diagnostic tests include:
Physical exam: May reveal enlarged lymph nodes, skin lesions, ocular issues, and other abnormalities.[1]
Complete blood count (CBC): Often shows increased globulins, anemia, and abnormalities in white blood cells.[2]
Biochemical profile: May indicate kidney and liver impairment.[3]
Bone marrow aspiration: Can reveal the presence of the Leishmania parasite via microscopic examination.[1]
Treatment
Treatment for leishmaniasis in dogs focuses on medications and supportive care. The main medications used are antimonials such as meglumine antimoniate and sodium stibogluconate. These are given for 4-8 weeks at a dosage of 50-100 mg/kg. Antimonials can have toxic side effects so regular bloodwork is needed during treatment. Other medications like miltefosine and allopurinol may also be used.
Supportive care is important for treating the symptoms of leishmaniasis. Intravenous fluids help manage dehydration and kidney issues. Therapeutic diets provide optimal nutrition. Antibiotics may be prescribed if skin lesions become infected. Specific treatments can be used for individual symptoms – for example, eye drops for uveitis.
With aggressive treatment, the long-term prognosis for leishmaniasis depends on the extent of infection and damage to the dog’s organs. Mild to moderate cases have a fair to good prognosis with months to years of survival. Severe disseminated disease that affects multiple organs has a poorer prognosis. Relapses are possible even after treatment.
Prevention
There are several ways to help prevent leishmaniasis in dogs:
Insect Repellents: Topical insect repellents containing ingredients like permethrin or pyrethroids can be applied to the dog’s coat to help repel sandflies. These should be used according to label instructions. Some collars containing repellents are also available. However, repellents do not provide 100% protection and should be used along with other preventative measures.
Reducing Sandfly Breeding Areas: Sandflies breed in humid, organic material like compost piles or manure. Cleaning areas around the home and removing sources of damp organic matter can help reduce sandfly populations. Trimming vegetation and trees can also reduce shady resting areas.
Vaccines: Vaccines for leishmaniasis in dogs are available in some countries. Consult with your veterinarian to see if vaccination is recommended for your dog based on risk factors like geographic location, travel plans, age and breed. Vaccines do not provide 100% protection but can reduce disease severity if infection occurs. Multiple doses over time are required for best immunity.
Geographic Distribution
Leishmaniasis is found in parts of the tropics, subtropics, and Mediterranean regions. According to the CDC, it is endemic in 98 countries across these regions, including Mediterranean countries like Italy, Portugal, and Spain. The disease also extends from Central Asia to India and China in the east as well as from Mexico to Argentina in the Americas. Some of the most affected countries include Afghanistan, Algeria, Brazil, Ethiopia, Iran, Sudan, Syria, Peru, Saudi Arabia and Pakistan (CDC, 2021).
Leishmania parasites are found in parts of southern Europe, Africa, Asia, South and Central America, and southern parts of North America like Texas and Oklahoma. Different Leishmania species have adapted to specific mammalian hosts and sandfly species, leading to varied geographic distributions. For example, Leishmania infantum is found around the Mediterranean basin while Leishmania donovani is predominant on the Indian subcontinent (Morales-Yuste et al., 2022). Environmental and climatic factors like temperature, rainfall and humidity also influence the distribution and transmission of the disease.
Risk Factors
There are several risk factors that can increase a dog’s chance of developing leishmaniasis:
Outdoor dogs – Dogs that spend most of their time outdoors have a higher risk of being bitten by sand flies and exposed to the Leishmania parasite. Sand flies are most active from dusk to dawn, so dogs that sleep outside overnight are especially vulnerable.
Weakened immune systems – Dogs with weakened or compromised immune systems are less able to fight off infection if bitten by an infected sand fly. Conditions like cancer, autoimmune disease, or co-infections can suppress the immune system.
Co-infections – Dogs co-infected with other vector-borne diseases like heartworm are more susceptible to developing leishmaniasis if bitten by a sand fly. The additional disease burden taxes the immune system.
According to research, other factors that increase risk include dogs over 2 years old, dogs with short fur, and lack of preventative veterinary care (https://pubmed.ncbi.nlm.nih.gov/22575278/). Prevalence is also higher in areas where the sand fly vector is abundant.
Public Health Significance
Leishmaniasis is considered a zoonotic disease, meaning it can spread between animals and people. Dogs serve as a major reservoir for the Leishmania parasite and are considered a significant risk factor for transmitting the infection to humans (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2805016/). When a sandfly bites an infected dog, it can pick up the parasite and then transmit it to humans through subsequent bites.
According to research from the University of Iowa, dogs play a key role as the reservoir of Leishmania infantum. As the number of infected dogs increases, so does the risk of human infection (https://petersen.lab.uiowa.edu/about-leishmaniasis-research-dogs). Controlling the disease in the canine population is therefore critically important from a public health perspective to prevent further spread to humans.
