Vector of Leishmaniasis, A Sand fly

Pakistan falls in one of the south Asian country that shares the highest burden of major vector borne diseases such as Malaria, Dengue, and Leishmaniasis.

Vector of Leishmaniasis, A Sand flyIn above, leishmaniasis is the destructive parasitic disease that is caused by a primary vector sand fly. According to WHO, around one billion people at risk in endemic areas, with an estimation of 600,000-10,00,000 cases each year, worldwide. In Pakistan, around 21,000–35,000 cases of both anthroponotic (ACL), feeding on humans, and zoonotic (ZCL), feeding on animals, forms of cutaneous leishmaniasis (CL) were reported last year. In Pakistan, around 20 species belong to genus Phlebotomus and Lutzomyio are causative agent of intracellular protozoan parasite of the genus Leishmania that resulted into three forms of diseases; cutaneous, visceral and mucocutaneous leishmaniasis.

Leishmaniasis is a disease caused by intracellular protozoan parasite transmitted by the bite of a female Phlebotomine sandflies, which feed on blood to produce eggs. Over 90 sand fly species are known to transmit Leishmania parasites. Rodents, dogs and wild cat may be reservoir hosts.  There are three main forms of the disease:

Visceral Leishmaniasis (VL), also known as kala-azar. Over 95% cases can be fatal, if untreated. It is characterized by irregular spells of fever, weight loss, enlargement of spleen and liver, and anaemia. Most cases occur in Brazil, East Africa and in South-East Asia. An estimated 50,000 to 80,000 new cases of Visceral Leishmaniasis occur worldwide each year. In 2017, more than 90% new cases reported to WHO occurred in Bangladesh, Brazil, China, Ethiopia, India, Kenya, Nepal, Somalia, Sudan and South Sudan. In Pakistan, human visceral leishmaniasis was first reported in 1960 and 1962 in Gilgit Baltistan Province of Pakistan.

Cutaneous Leishmaniasis (CL) is the most common form of leishmaniasis and cause skin lesions and ulcer on exposed parts of the body, leaving life-long scars and serious disability. In 2017 over 95% new cases of cutaneous leishmaniasis occurred in countries: Afghanistan, Algeria, Brazil, Colombia, Iran and estimated that between 50,000 to 1 million new cases occur worldwide each year.

Mucocutaneous Leishmaniasis is very serious disease that leads to partial or complete destruction of mucous membrane of nose, mouth and throat. Around 90% of mucocutaneous leishmaniasis cases occur in Bolivia, Brazil, Ethiopia and Peru. Both cutaneous and mucocutaneous leishmaniasis occur in Sindh, Baluchistan and Khyber Pakhtunkhwa provinces of Pakistan.

Life Cycle of Leishmania

The infected female sand fly species transfer the parasite into the body of humans, while taking the blood meal, within few days the symptoms of any specific leishmaniasis will appear. During this time of infection, if the female sand fly will take infected blood meal that may transfer to the healthy person.

Habitat of Sand fly species

The complete cycle (egg, larvae, pupae, adult) of sand fly based on terrestrial habitat. Sand flies are nocturnal and sensitive to dehydration. They shelter in caves, rocks, animal burrows, tree holes and human vicinities. They fly close to the ground in short hops (jumps) therefore they are called weak flyers. Their flying range is 300 m but in deserted environments some species can travel up to 2300 m.

Feeding behavior

Male and female sand fly feed on nectar from fruits, flowers and plant juices. Carbohydrates are the source of energy. Only the female flies suck a blood meal to complete the development of egg batches. Female sand flies tend to seek blood meals every 5 to 6 days as it essential for egg development. On average, a sand fly will digest full blood meal in 2 to 3 days, depending on sand fly species, environmental conditions and breeding habit. Some species of sand fly are autogenous, these species lay batch of eggs without first feeding on blood, female species of sand fly are the disease-causing agents. Most anthropophilic (blood sucking arthropods) sand flies bite people outside their tents, houses and accommodation. Feeding activity is influenced by temperature, humidity and air movement. Sandflies are weak fliers and even light wind inhibit their flight and reduce biting. Mostly sand fly species feed at dusk to dawn, when temperature falls and humidity rise. They commonly bite the face, hands or scalp of their victims, but they will also bite any area of exposed skin.

Reproduction and Oviposition

Adult sandflies copulate after emergence and males locate females at resting sites or at vertebrate hosts body by following the sex pheromones release by females. Generally, one blood meal requires to produce a single batch of eggs. Sand fly oviposit in soil rich in organic matter, eggs hatch after 4-20 days in favorable environment and complete its life cycle in 30-40 days. Larvae are scavengers and feeding on organic matter such as fungi, decaying leaves, animal feces and decomposing arthropods. Female sandflies lay around 30-70 eggs during a single cycle and deposited in ground holes, animal burrows and among tree roots. Eggs are not laid in water but require a microhabitat with high humidity in order to survive.

Socioeconomic conditions

Poverty increases the risk of disease, poor housing and sanitary conditions such as lack of waste management and open sewerages. This may increase breeding and resting sites of sand flies, as well as their access to humans. Risk may increase due to human behavior such as sleeping outside or on the grounds. Other major risk factors are malnutrition, changing land use, increasing human population and urban isolation are important factors that influence the distribution of leishmaniasis and the sand fly–man relationship.

Prevention and Control

Prevention and control of leishmaniasis requires a multidisciplinary approach because transmission occur in a complex biological system involving the human and animal reservoir hosts, parasite and sand fly vector. Some important strategies for prevention are listed below:

  • Early diagnosis and effective prompt treatment reduce the prevalence of disease and prevent death and disabilities by reduction in transmission, and to monitor the spread and burden of disease.
  • Social awareness among people and educate the community with effective behavioral change must be locally adapted.
  • Control of animal reservoir hosts is a key strategy to keep the non-humans in a controlled environment. It will help to break the host-vector relationship.
  • Vector control helps to reduce transmission of disease by decreasing number of sandflies. Mass trapping devices, like pheromone / semio-chemicals traps, insecticide treated bed nets can be used.
  • Chemical Control, in the last chemical control can be adapted.

By Shahid Malik

Entomologist, Vector ecologist, Medical Entomologist