Medical Entomology: Arthropod Classification

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

Classification of Arthropods

• Insecta and Arachnida are the two major classes of arthropods
• Insecta includes mosquitoes, flies, fleas, lice, and bugs
• Arachnida includes mites and ticks

General Characteristics

• Insecta have a body with 3 parts: head, thorax, and abdomen
• Insecta have 3 pairs of legs (hexapods)
• Insecta have 2 or 4 wings
• Arachnida have a body with 2 parts: cephalothorax and abdomen
• Arachnida have 4 pairs of legs (octapods)
• Arachnida have no wings

Medical Importance of Arthropods

• Arthropods can act as vectors of pathogens, transmitting diseases

Mechanical Transmission

• Arthropods transport pathogens on their body without changes to the pathogens
• Direct mechanical transmission involves arthropods like Stomoxys, transmitting West Nile, Rift Valley viruses, and Trypanosoma
• Indirect mechanical transmission involves arthropods like Musca, transmitting typhoid, amoeba, cholera, and viral hepatitis

Biological Transmission

• Arthropods play an important role in the life cycle of the transmitted organism
• Propagative transmission involves the organism multiplying in the vector without cyclical development, e.g., multiplication of bacterial and viral diseases
• Cyclo-developmental transmission involves the organism undergoing developmental changes only without multiplication in the arthropod, e.g., development of microfilaria in the female mosquito
• Cyclo-propagative transmission involves the organism undergoing both developmental changes and multiplication in the arthropod, e.g., malaria in female Anopheles

Arthropods as Etiological Agents of Disease

• Arthropods can cause specific lesions in human tissue
• Flies cause myiasis, and Sarcoptes scabiei causes scabies
• Arthropods can introduce toxins and venoms through bites
• Spiders, scorpions, and ticks can cause paralysis
• Allergic reactions can occur due to arthropods
• House dust mites can cause rhinitis or asthma
• Lice, fleas, and mosquitoes can cause dermatitis
• Entomophobia is the fear of arthropods, which can cause severe neurosis

Class Insecta: Mosquitoes

• Only female mosquitoes bite and take blood meals
• Culex, Anopheles, and Aedes are included

Mosquitoes: Medical Importance

• Cause annoyance, itching, and dermatitis
• Anopheles transmits human malaria and Wucheraria bancrofti
• Culex transmits non-human malaria and Wucheraria bancrofti
• Aedes transmits yellow fever, dengue fever, and Wucheraria bancrofti

Phlebotomous (Sand Fly)

• Sand flies hold their hairy wings in a vertical V-shape at rest
• Adults have six legs that are extremely long
• Only females suck blood
• Sandflies bite humans at dusk and dawn
• Sand flies cause pappataci fever and biologically transmit leishmaniasis

Mosquito Control

• Larval stage control includes eliminating breeding places
• Biological control uses natural enemies like frogs and Gambusia fish
• Chemical control uses Paris green and Dichlorodiphenyltrichloroethane (DDT) on the water surface
• Adult stage control includes wire screening of inlets and outlets of houses
• Biological control uses natural enemies like frogs and other insects
• Chemical control uses repellents like citronella oil, DEET (diethyl toluamide) and insecticides like DDT and gammaxene

Class Insecta: Flies

• Musca, Lucilia, Sarcophaga, Glossina, Stomoxys are examples
• Some species take blood meals; both sexes can bite

Medical Importance of Flies

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

Fly Control

• Wire screening of doors and windows
• Usage of insecticides like DDT
• Spraying repellents as DEET and permethrin

Class Insecta: Fleas

• Adults bite and take blood meals, both male and female
• Included are Ctenocephalus canis and cati, Xenopsyella cheopis, and Pulex irritans

Medical Importance of Fleas

• Fleas cause flea dermatitis
• Fleas transmit plague, endemic typhus, Hymenolepis diminuta worm, Dipylidium caninum worm

Flea Control

• Sanitation involves regular vacuuming of carpets and furniture
• Boiling clothes and linings
• Application of insecticides like pyrethrins or insect growth regulators for indoor and infested pets
• Spraying repellents as DEET and permethrin

Class Insecta: Lice

• All stages bite and take blood meals
• Included are Pediculus humanus capitis (head lice), Pediculus humanus corporis (body lice), Phthirus pubis (pubic/crab lice)*

Lice Transmissions

• Pediculus humanus: Transmits epidemic relapsing fever, epidemic typhus, and trench fever
• Phthirus pubis: Doesn't transmit any disease agents

Diseases Caused by Lice

• Pediculus humanus: Causes pediculosis (vagabond's disease) and blepharitis
• Phthirus pubis: Causes pediculosis with blue maculae and blepharitis

Lice Control

• Health education and good hygiene
• Frequent bathing
• Washing clothes and bed lining in boiled water
• Wet combing of hair
• Topical pediculicides like permethrin lotion 1%, benzyl alcohol 5%, malathion 0.5%, and lindane 1%
• Oral ivermectin

Class Insecta: Bugs

• All stages bite and take blood meals
• Cimex lectularius and Triatoma megista exist

Diseases Transmitted by Bugs

• Cimex lectularius mechanically transmits Hepatitis B virus and cause bed bug dermatitis, urticarial rash, and insomnia
• Triatoma megista biologically transmits Trypansoma cruzi, causing Chagas disease; causes itching and dermatitis

Bugs Control

• Applying insecticides like malathion
• Spray infected beds with 5% DDT

Importance of Vector-Borne Infections

• There are about 200 million insects alive per human at any point; 14,000 species feed on blood
• Vector-borne diseases account for >17% of all infectious diseases and cause at least 700,000 deaths annually
• Most diseases affect individuals in poor countries of the tropics and subtropics, therefore these are considered Neglected Tropical Diseases (NTDs)
• Leishmaniasis is associated with significant incidence, morbidity, and mortality and is the deadliest NTD
• In 2022, there were 249 million cases of malaria with an estimated 608,000 deaths