Medical Entomology: Arthropod Classification

• Medical entomology studies arthropods of medical importance and the diseases that they cause or transmit.
• Arthropods are living organisms characterized by segmented bodies and jointed legs.

Classification of Arthropods

• Insecta includes flies, fleas, lice, bugs and mosquitoes.
• Arachnida includes mites and ticks.

General Characteristics of Arthropods

• Insecta bodies have three parts: head, thorax, and abdomen.
• Arachnida bodies have two parts: cephalothorax and abdomen.
• Insecta have three pairs of legs (hexapods).
• Arachnida have four pairs of legs (octapods).
• Insecta have two or four wings; Arachnida have no wings.

Medical Importance - Vectors of Pathogens

• Arthropods can act as vectors of pathogens, transmitting diseases.
• There are two methods of transmission: mechanical and biological.

Mechanical Transmission

• Pathogens are transported on the arthropod's body (legs, hair, wings) and dropped unchanged onto food, drinks, or tissues.
• Direct mechanical transmission involves immediate transfer, such as Stomoxys transmitting West Nile, Rift Valley viruses, and Trypanosoma.
• Indirect mechanical transmission involves intermediary steps, such as Musca transmitting typhoid, amoeba, cholera and viral hepatitis.

Biological Transmission

• The arthropod plays a role in the pathogen's life cycle.
• There are three types: propagative, cyclo-developmental, and cyclo-propagative.
  • Propagative: The organism multiplies in the vector without cyclical development, like bacterial and viral diseases.
  • Cyclo-developmental: The organism undergoes developmental changes only, without multiplication, such as microfilaria in the female mosquito.
  • Cyclo-propagative: The organism undergoes both developmental changes and multiplication, such as malaria in female Anopheles.

Medical Importance - Etiological Agents of Disease

• Arthropods can directly cause disease through specific lesions, toxins/venoms, allergic reactions, or entomophobia.
  • Specific lesions can be caused by flies that cause myiasis or Sarcoptes scabiei, which causes scabies.
  • Toxins and venoms: Spiders, scorpions, and ticks introduce venoms, sometimes causing paralysis.
  • Allergic reactions: House dust mites can cause rhinitis or asthma and lice, fleas, and mosquitoes can cause dermatitis from bites.
  • Entomophobia: fear of arthropods can cause severe neurosis.

Mosquitoes

• Mosquitoes belong to class Insecta.
• Only females bite and take blood meals.
• Examples include Culex, Anopheles, and Aedes.

Medical importance of mosquitos

• Anopheles mosquitoes transmit human malaria and Wucheraria bancrofti.
• Culex mosquitoes transmit non-human malaria and Wucheraria bancrofti.
• Aedes mosquitoes transmit yellow fever, dengue fever and Wucheraria bancrofti.

Phlebotomous (Sand fly)

• Sand flies hold their hairy wings in a vertical V-shape
• Six legs of adults are extremely longer than the insect's body.
• Only females suck blood.
• Sandflies bite humans at dusk and dawn.
  • Causes pappataci fever.
  • Biological transmission of leishmaniasis.

Control of Mosquitoes

• Control of larval stages includes physical elimination of breeding places, biological control using natural enemies like frogs and gambusia fish, and chemical control using Paris green or DDT on water surfaces.
• Control of adult stages includes physical barriers like wire screens, biological control using natural enemies, and chemical control using repellants like citronella oil or DEET, and insecticides like DDT and gammaxene.

Flies

• Flies belong to Class Insecta.
• Examples: Musca, Lucilia, Sarcophaga, Glossina, and Stomoxys.
• Some species take blood meals, with both sexes biting.

Medical importance of flies

• Flies can cause annoyance and myiasis.
• Musca flies mechanically transmit viruses, bacterial, and parasitic diseases.
• Glossina flies biologically transmit sleeping sickness (African trypanosomiasis).

Fly control

• Wire screening of doors and windows can help control flies
• Insecticides, such as DDT, can be used
• Repellents like DEET and permethrin can be sprayed – these can act as both insecticide and repellent.

Fleas

• Fleas belong to Class Insecta
• Adults (male and females) bite and take blood meal
  • Ctenocephalus canis and cati
  • Xenopsyella cheopis
  • Pulex irritans

Medical importance of fleas

• Fleas can cause Flea dermatitis Disease Transmission
  • Plague
  • Endemic typhus
  • Hymenolepis diminuta worm
  • Dipylidium caninum worm

Flea control

• Regular vacuuming of carpets and furniture can help control fleas
• Boiling clothes and linings can also help
• Insecticides such as pyrethrins or insect growth regulators sprays for indoor and infested pets can be used
• Spraying repellents as DEET and permethrin

Lice

• Lice belong to Class Insecta
• All stages bite and take blood meal
  • Pediculus humanus capitis (head lice)
  • Pediculus humanus corporis (body lice)
  • Phthirus pubis (pubic /crab lice)

Medical importance of Lice (Pediculus humanus)

• They can transmit Epidemic relapsing fever, Epidemic typhus, and Trench fever
• They are Causative agents for Pediculosis (vagabond’s disease) and Blepharitis

The Pubic Louse (Phthirus pubis)

• Does not transmit any disease agents
• Can cause Pediculosis with blue maculae, blepharitis

Treatment and control of lice

• Health education and good hygiene.
• Frequent bathing.
• Washing clothes and bed lining in boiled water.
• Wet combing of hair.
• Topical pediculicides (Permethrin lotion 1%, Benzyl alcohol 5%, Malathion 0.5%, Lindane 1%).
• Oral ivermectin.

Bugs

• Bugs belong to Class Insecta
• All stages bite and take blood meal
  • Cimex lectularius
    • Mechanical transmission of Hepatitis B virus
    • Bed bugs dermatitis
    • Urticarial rash and insomnia
  • Triatoma megista
    • Biological transmission of Trypansoma cruzi causing chagas disease
    • Itching and dermatitis.

Bug control

• Application of insecticides as malathion.
• Infected beds are sprayed by 5% D.D.T.

Summary

• Arthropods include two major classes: Insecta and Arachnida.
• Arthropods are causative agents or disease transmitters (vectors).
• Class insecta includes many types of arthropods

Importance of vector-borne infections to public health

• About 200 million insects alive/ human at any point; among them ~ 14,000 species feed on blood
• Vector-borne diseases account for > 17% of all infectious diseases, and cause at least 700,000 deaths annually (WHO, 2020).
• Most of these diseases affect individuals in poor countries of the tropics & subtropics, so they are considered Neglected Tropical Diseases (NTDs).
• Among NTDs, leishmaniasis is associated with significant incidence, morbidity & mortality (the deadliest NTD). According to the World malaria report in 2022, there were 249 million cases of malaria (not a NTD) with an estimated number of malaria deaths of 608 000.