Medical Entomology — Mosquito, Flea and Louse Vectors and the Diseases They Spread

WHO defines vectors as living organisms that transmit infectious pathogens between humans, or from animals to humans. Many are blood-sucking insects that ingest a disease-producing micro-organism during a blood meal from an infected host and later pass it to a new host after the pathogen has replicated. Once a vector is infectious it can usually transmit the pathogen for the rest of its life at each subsequent blood meal.

WHO's 2024 fact sheet states that vector-borne diseases account for more than 17% of all infectious diseases and cause over 700 000 deaths a year. They can be caused by parasites, bacteria or viruses, and the burden is highest in tropical and subtropical areas. Many are preventable through protective measures and community mobilisation, which is why vector control is a core public-health topic.

WHO lists vector-borne diseases according to the vector. The table below follows that list, and adds the pathogen type.

Mosquito-borne questions usually ask for the genus. Only the female feeds on blood. The following genus-level points come from WHO fact sheets.

• Anopheles — malaria spreads mostly through the bites of infected female Anopheles mosquitoes. WHO warns that emerging insecticide resistance among Anopheles threatens malaria control, and that Anopheles stephensi thrives in urban settings, tolerates high temperatures and is resistant to many insecticides used in public health.
• Aedes — dengue is spread chiefly by Aedes aegypti; Aedes albopictus and other Aedes species can also be vectors but are secondary. The mosquitoes that spread dengue are active during the day, so avoiding bites during the day matters. Exposure is closely linked to water storage, plant-keeping and self-protection behaviours.
• Culex — Japanese encephalitis is transmitted mainly by Culex tritaeniorhynchus, in an enzootic cycle between mosquitoes, pigs and/or water birds; it is predominantly rural and periurban, where people live close to domestic pigs.
• Lymphatic filariasis — transmitted by different mosquitoes: Culex is widespread in urban and semi-urban areas, Anopheles mainly in rural areas, and Aedes mainly in endemic islands of the Pacific.

Plague is caused by Yersinia pestis, a zoonotic bacterium usually found in small mammals and their fleas. StatPearls states that the oriental rat flea, Xenopsylla cheopis, is the most efficient and common vector, although many other flea species are competent vectors. WHO lists three routes of transmission between animals and humans: the bite of an infected flea, direct contact with infected tissues, and inhalation of infected respiratory particles.

WHO gives an incubation period of one to seven days, and notes that antibiotic treatment is effective, so early diagnosis and treatment save lives.

The same flea also carries murine (endemic) typhus. StatPearls states that Rickettsia typhi is maintained in a rat-flea cycle (Rattus rattus and Rattus norvegicus) with X. cheopis as the classic vector. In parts of the southern United States and Mexico, opossums and cat fleas (Ctenocephalides felis) are more important. Humans are accidental hosts and become infected when infected flea faeces are inoculated into the skin by scratching or minor abrasions, or reach the conjunctiva or mucous membranes.

WHO lists two diseases transmitted by lice: typhus and louse-borne relapsing fever, both bacterial. Epidemic typhus is caused by Rickettsia prowazekii and spread by the human body louse, *Pediculus humanus corporis*. It occurs where louse infestation is common — war, natural disasters, homelessness, refugee camps, overcrowding and poor sanitation.

StatPearls stresses that treatment, usually doxycycline, should be started as soon as typhus is suspected and not delayed for laboratory confirmation. See also scabies and pediculosis.

Leishmaniasis is caused by protozoan parasites transmitted by the bite of infected female phlebotomine sandflies; over 90 sandfly species are known to transmit Leishmania. Visceral leishmaniasis (kala-azar) is fatal in over 95% of cases if left untreated. Post-kala-azar dermal leishmaniasis (PKDL), a macular, papular or nodular rash on the face, upper arms and trunk, occurs in east Africa and on the Indian subcontinent, where 5–10% of kala-azar patients are reported to develop it.

WHO's South-East Asia Region has the only initiative for elimination of visceral leishmaniasis as a public-health problem (2022–2026), defined as fewer than one case per 10 000 inhabitants at district level in Nepal and at sub-district level in Bangladesh and India. WHO also reports high Leishmania-HIV coinfection rates in Brazil, Ethiopia and the state of Bihar, India.

• Ticks — Crimean-Congo haemorrhagic fever, Lyme disease, relapsing fever, spotted fever and Q fever, tick-borne encephalitis, tularaemia.
• Blackflies — onchocerciasis (river blindness).
• Tsetse flies — African sleeping sickness; triatome bugs — Chagas disease.
• Culicoides flies — Oropouche fever.

WHO's Global Vector Control Response (GVCR) 2017–2030, approved by the World Health Assembly in 2017, gives strategic guidance for strengthening vector control as a fundamental approach to preventing disease and responding to outbreaks. It calls for realigned vector-control programmes, more technical capacity, better infrastructure, stronger surveillance and greater community mobilisation.

• Dengue — apply appropriate insecticides to water storage and outdoor containers, and avoid bites during the day.
• Malaria — avoid mosquito bites and take preventive medicines; insecticide resistance in Anopheles is a growing threat.
• Lymphatic filariasis — where Anopheles is the main vector, insecticide-treated nets enhance the impact of mass drug administration.
• Japanese encephalitis — WHO says vaccination of humans should be prioritised over vaccination of pigs and mosquito-control measures.