Medical Entomology: Arthropods & Disease

• Medical entomology is the study of arthropods of medical importance and diseases they cause or transmit.
• Arthropods are living organisms with segmented bodies and jointed legs.

Classification of Arthropods

• Major classes of arthropods of medical importance include insecta and arachnida, broken down respectively as follows:
  • Insecta: Mosquitoes, Flies, Fleas, Lice, Bugs
  • Arachnida: Mites, House dust mites, Sarcoptes, Ticks (hard and soft), Demodex

Characteristics of Arthropods Classes

• Insecta
  • Bodies have a head, thorax and abdomen
  • Have 3 pairs of legs (hexapods)
  • Have 2 or 4 wings
• Arachnida
  • Bodies have a cephalothorax and abdomen
  • Have 4 pairs of legs (octapods)
  • Do not have wings

Medical Importance of Arthropods

• Arthropods can act as vectors of pathogens, transmitting diseases.
• Arthropods can cause disease directly.

Methods of Transmission

• Mechanical transmission occurs when arthropods transport pathogens on their body parts and deposit them on food, drinks, or tissues.
  • Arthropods transport pathogens unchanged.
  • Direct mechanical transmission involves immediate transfer, using Stomoxys in transmission of West Nile, and Rift Valley viruses, and Trypanosoma.
  • Indirect mechanical transmission involves a more roundabout transfer, using Musca in transmission of typhoid, amoeba, cholera and viral hepatitis.
• Biological transmission occurs when the arthropod plays a role in the pathogens life cycle.
  • Propagative transmission, the organism multiplies in the vector without any cyclical development, e.g. multiplication of bacterial and viral diseases.
  • Cyclo-developmental transmission, the organism undergoes developmental changes only without multiplication in the arthropod, e.g. development of microfilaria in the female mosquito.
  • Cyclo-propagative transmission, the organism undergoes both developmental changes and multiplication in the arthropod, e.g. malaria in female Anopheles.

Arthropods as Etiological Agents of Disease

• Specific lesions can form on tissue from certain arthropods, like flies causing myiasis, and sarcoptes scabiei causing scabies.
• Toxins and venoms can be introduced through bites of spiders, scorpions, and ticks (paralysis).
• Allergic reactions can be caused by house dust mites (rhinitis or asthma), and lice, fleas & mosquitoes (dermatitis).
• Entomophobia is the fear of arthropods that can cause neurosis.

Class Insecta: Mosquitoes

• Only females bite and take blood meals.
• Culex, Anopheles, Aedes are three common types of mosquito.

Morphology of adult mosquitos

• Mosquitoes have a head, thorax and abdomen.

Medical Importance of Mosquitoes

• Culex transmits non-human malaria and Wuchereria bancrofti.
• Anopheles transmits human malaria and Wuchereria bancrofti.
• Aedes transmits yellow fever, dengue fever, and Wuchereria bancrofti.
• Mosquitoes may also cause annoyance, itching, and dermatitis.

Class Insecta: Sand Flies (Phlebotomous)

• Sand flies hold their hairy wings in a vertical V-shape at rest.
• The six legs of adults are extremely long.
• Only females suck blood.
• Sandflies bite humans at dusk and dawn.
• Sand flies causes pappataci fever and biological transmission of leishmaniasis.

Class Insecta: Flies (2)

• Flies include Musca, Lucilia, Sarcophaga, Glossina, and Stomoxys.
• Some species of fly take blood meals; both sexes bite.

Life Cycle of Musca

Medical Importance of Flies

• Medical importance of flies are causative agents of annoyance, and myiasis.
• Musca transmits viruses, bacterial and parasitic diseases via mechanical transmission.
• Glossina transmits sleeping sickness disease (African trypanosomiasis) via biological transmission.
• Control of flies include wire screening, usage of insecticides as DDT, and spraying repellents as DEET and permethrin

Class Insecta: Fleas (3)

• All adult fleas (male and females) bite and take blood meals.
• Ctenocephalus canis and cati, Xenopsyella cheopis, and Pulex irritans are common flea species.

Medical Importance of Fleas

• Fleas are causative agents of flea dermatitis.
• Rats carry plague bacteria, and fleas may also spread endemic typhus, Hymenolepis diminuta worm, and Dipylidium caninum worm

Control of Fleas

• Flea population can be controlled by: sanitation (regular vacuuming), boiling of clothes, application of insecticides (pyrethrins or insect growth regulators sprays).
• Spraying repellents as DEET and permethrin may also help avoid fleas.

Class Insecta: Lice (4)

• All stages of lice bite and take blood meals.
• Types of lice include Pediculus humanus capitis (head lice), Pediculus humanus corporis (body lice), Phthirus pubis (pubic /crab lice).

Medical Importance of Lice

• Pediculus humanus (Lice): Is responsible for the transmission of; epidemic relapsing fever, endemic typhus, and trench fever.
  • They can also cause pediculosis (vagabond's disease):severe itching, dermatitis, secondary bacterial infection, pustule formation and deep pigmentation of skin, or blepharitis
• Phthirus pubis: Does not transmit disease, but also may cause pediculosis with blue maculae, blepharitis

Treatment and control of Lice

• Control and treatment of lice come from: Health education, frequent bathing, washing clothes in boiled water, wet combing of hair, or topical pediculicides such as: permethrin lotion 1%, benzyl alcohol 5%, malathion 0.5% or lindane 1% or oral ivermectin

Class Insecta (5) Bugs

• All stages of bugs bite and their host to have a blood meal

Medical Importance of Bugs

• Cimex lectularius (Bed bugs): Mechanical transmission of Hepatitis B, may cause bed bugs dermatitis, Urticarial rash and insomnia
• Triatoma megista: Biological transmission of Trypansoma cruzi causing chagas disease, with itching and dermatitis

Control of Bugs

• Use of Insecticides such as malathion, or spraying infected beds

Importance of Vector-Borne Infections to Public Health

• About 200 million insects are alive per human, in which 14,000 humans feed on blood
• Vector-borne diseases account for 17% of all infectious diseases, causing 700,000 deaths per year
• The bulk of diseases effect individuals in poor countries of the tropics & subtropics
• The deadliest neglected tropical disases from leishmaniasis are associated with significant incidence, morbidity & mortality In 2022, 249 million malaria cases (not a NTD) caused 608,000 deaths