Leishmaniasis is an infectious disease endemic in nearly 100 countries around East Africa, the Americas, Southeast Asia and the Mediterranean. It belongs to the list of neglected tropical diseases together with rabies and dengue, despite the fact that the World Health Organization estimates that over 1 million humans are infected each year, making it one of the most dangerous and deadly tropical parasitic diseases, after malaria. The agent is a single-cell parasite named Leishmania and is spread via the bite of infected female sandflies.

Learn more about one of the oldest and more mysterious diseases of mankind, responsible for thousands of deaths around the world yearly.

By Ricardo Ribas

Leishmaniasis is an endemic disease that affect humans and animals in tropical and subtropical regions around the world, particularly in countries affected by malnutrition, poor housing conditions and a lack of financial resources. Not every human infected develops symptoms, but children, the elderly and people with a weakened immune system, are the more vulnerable groups.

WHO IS THE AGENT?

Leishmaniasis is caused by a microscopic single-cell parasite named Leishmania and is spread by the bite of infected female sandflies. There are over 90 species of sandflies around the world known to transmit the disease to animals and humans. Adult sandflies are small measuring around 3mm, yellow, with black eyes and hairy all over their body, wings and legs.

In nature, Leishmania can be found in two different forms according to the stage of their life-cycle and the host they infect. The promastigote form corresponds to the phase of the cycle when the parasite is living inside the sandfly, whilst the amastigote form is present in the vertebrate (humans or animals). In order to complete the life cycle, the leishmania needs to pass through both hosts, hence the transmission between vertebrates is only possible via the sandfly’s bite.

HOW IS IT TRANSMITTED?

Leishmaniasis is transmitted to humans and animals through a bite of an infected female sandfly while feeding on blood required for their eggs production. After biting the animal, the sandfly ingests the infected blood containing the amastigote parasites allowing them to lodge in their intestine, multiply and convert into the promastigote form. When the sandfly bites the animal or the human again, it deposits the parasite back into their blood. The parasites penetrate the vertebrate’s defence cells, multiply and are transported to the various parts of the body causing damage to the different organs. Fortunately, not all individuals get sick and hosts with a good immune system are able to destroy the parasite and prevent damage.

Transmission is more likely to occur in the warmer months when there are an increased number of sandflies, and usually during dusk and dawn, when the insects are most likely to be looking for food. Once infected, it may take months or even years for the host to develop symptoms.

HOW DOES IT AFFECT ANIMALS?

Over 70 species of animals can be natural reservoir for the Leishmania parasites. These include cats, goats, wolfs, rabbits, rodents and even birds, but dogs seem to be the main reservoir of the disease. Leishmaniasis can affect any breed of dog, yet middle-aged German Shepherds, Boxers and Dobermanns are more susceptible. Males and females are equally affected, but the disease is rare in animals with less than six months. The disease is slow and progressive, and can lead to a wide variety of symptoms such as: painless skin lesions and hair loss in the biting site; fever; lack of appetite; weight loss; diarrhoea and vomiting; swelling of the lymph nodes; conjunctivitis and blindness due to the presence of the parasite in the eyes; lameness and joint inflammation; enlargement of spleen and liver as well as serious kidney problems that can lead to the animal’s death. Cats can also develop the disease and suffer from symptoms similar to the dogs. Due to the presence of nonspecific symptoms, diagnosis can be challenging, often requiring specific blood tests. At times animals may show no symptoms, making it important to perform periodic screening tests particularly to animals inhabiting endemic regions.

AND HOW DOES IT MANIFESTS IN HUMANS?

Humans are generally more resistant to leishmaniasis infection than dogs, however immunocompromised individuals, as well as children and elderly people tend to be more susceptible. Direct transmission between animals and humans is not possible unless via the insect vector.

Humans can be affected with three forms of the disease:

1) Cutaneous is the most common form of the disease, characterised by the presence of skin lesions in the insect biting site, as well as the presence of painless ulcers leaving life-long scars and serious disability;

2) Visceral or Kala-azar is the most serious form of the disease affecting internal organs. If left untreated, it can be fatal in over 95% of the cases. Symptoms are similar to the ones observed in animals, including fever, weight loss, anaemia and enlargement of the spleen and liver. Most cases occur in Brazil, East Africa and in India.

3) Mucocutaneous: is a form of the disease that affects predominantly the mucous membranes and skin of the nose, mouth and throat. More than 90% of the cases occur in South America (Bolivia, Brazil, Peru) and Ethiopia.

WHAT ARE THE RISK FACTORS FOR THE DISEASE?

Many factors affect the likelihood of contracting the disease. Poverty particularly in developing countries, where people live in overcrowded and under bad sanitary conditions, as well as certain human behaviours such as sleeping outside, increases the risk of contracting the disease, creating more opportunities for the insects to breed, to find resting sites and to have more access to human or animal blood. Malnutrition is also a very important risk factor since it compromises the host’s immune system and its capacity to fight the parasite.

Environmental, climate and urbanisational changes are also vital. For instance, human expansion into forested areas and subsequent deforestation affects insect’s numbers, distribution and survival, creating ideal opportunities for the spread of the disease outside the endemic regions.

Treatment of leishmaniasis is possible but involves long duration therapies generally administered into the vein. Its success is also dependent on multiple factors, such as the form of the disease, the type of leishmania as well as the capacity of the host’s immune system to get rid of the parasite. Is important to remember that if left untreated, leishmaniasis can kill.

HOW CAN WE CONTROL AND PREVENT THE DISEASE?

Although several laboratories are trying to develop vaccines against the disease, these are still not effective and the reason why preventive measures are crucial. Here is some advice:

1. Early identification of the disease in humans and animals is crucial to prevent its spread and allow proper and quick treatments.

2. In endemic regions, avoid leaving pets outside the house, particularly at dawn and dusk, since insects are more likely to be looking for food.

3. Use repellents and insecticides in humans in animals and in the environment to keep kennels and homes free from insects. The use of mosquito nets is equally effective, particularly in endemic areas.

4. Perform periodic screening tests on animals inhabiting endemic regions.

6. Educate the community about the disease, prevention measures and encourage behavioural changes.

It is crucial to increase the focus on this neglected disease by helping implement control and prevention measures as well as promoting research to discover new treatments to avoid many thousands of deaths each year.

By Ricardo Ribas, Veterinary Doctor, doctorate in veterinary sciences and researcher in the area of oncology in London

References sources

1. Sumter, J & Gull, K. Shape, Form, Function and Leishmania Pathogenicity: From Textbook Descriptions to Biological Understanding. [published correction appears in Open Biol. 2018 Aug;8(8):]. Open Biol. 2017;7(9):170165. doi:10.1098/rsob.170165

2. Ribeiro, RR; Michalick, MSM et al. Canine Leishmaniasis: An Overview of the Current Status and Strategies for Control. Biomed Res Int. 2018. doi:10.1155/2018/3296893

3. Leishmaniasis. World Health Organisation. 2 March 2020. https://www.who.int/news-room/fact-sheets/detail/leishmaniasis & https://www.who.int/leishmaniasis/disease/en/. Accessed April 2020

4. WHO Report on Global Surveillance of Epidemic-prone Infectious Diseases – Leishmaniasis. World Health Organisation. https://www.who.int/csr/resources/publications/CSR_ISR_2000_1leish/en/. Accessed April 2020

5. Parasites- Leishmaniasis. Centre for Disease Control and Prevention. Last updated February 2020. https://www.cdc.gov/parasites/leishmaniasis/index.html. Accessed April 2020

6. Leishmaniasis. Wikipedia, Wikipedia Foundation. https://en.m.wikipedia.org/wiki/Leishmania. Accessed April 2020

7. Leishmaniasis in other animals. Stop leishmania.org. http://www.stopleishmania.org/leishmaniosis-animals.php. Accessed April 2020

Disclaimer

The material, ideas as well as control and prevention measures discussed on this article are for informational purposes only. For more information consult a vet or a professional in the area. Whilst every effort is made to make sure the article is accurate at the time of publication, I take no liability for any new developments on the subject as well as any errors or omissions.