Medical entomology, Arthropods types, Toxaemia, Scorpion sting, Mosquito dermatitis and scabies
Arthropods are highly organized invertebrate animals. They have a bilaterally symmetrical body and possess jointed appendages in the form of legs, mouth parts, antennae, and wings. They are covered externally with a chitinous exoskeleton which serves to the protection of the internal organs. In the body cavity of arthropods, the haemocoele is filled with a clear colourless fluid representing the blood called the haemolymph.
Arthropods are differentiated sexually into males and females. After fertilization, females deposit eggs or larvae. Two different types of development then take place namely:
- Incomplete Metamorphosis: The newly hatched larvae in this type of development resemble the adults but are smaller in size and sexually immature, Then they gradually increase in size by successive molting e.g. scorpion, ticks, mites, and lice.
- Complete Metamorphosis: The newly hatched larvae are totally different morphologically from the adults. They may be worm-like, and they transform into the inactive pupal stage from which the adults emerge e.g, fleas, flies, and mosquitoes.
Classification of Arthropods
Arthropods of medical importance are included in the following classes:
- Class Crustacea e.g. cyclops.
- Class Arachnida e.g. scorpions, spiders, ticks, and mites.
- Class Insecta or Hexapoda e.g. fleas, lice, bugs, mosquitoes, and flies.
Criteria of arthropods
- Crustacea: Fused cephalothorax and segmented abdomen, 5 pairs of legs, 2 pairs of antenna, No wings. Ex: Cyclops.
- Arachnida: Either used, cephalothorax and segmented abdomen or fused unsegmented body, 4 pairs of legs, No antenna, No wings. Ex: Ticks, Mites and Scorpion, Spiders.
- Insecta (Hexapoda): The body is segmented into a head thorax and abdomen, 3 pairs of legs, 1 pair antenna, Winged or not. Ex: Mosquitoes, Files, Fleas Lice, and Bugs.
Medical Importance of Arthropods
Arthropods are agents transmitting and /or causing diseases to man.
Arthropods as Vectors of Pathogens (Disease Transmitters)
They may act either as mechanical vectors of the etiologic agent or as biological vectors taking an essential role in the life cycle.
1. Mechanical Transmission
When the arthropods transport the various pathogens in or on their bodies (their legs, hairs, and antennae) and drop them unchanged on human food, drink or tissues e.g. the role of house flies in the transmission of enteric pathogens as polio virus and cholera cysts of protozoa and egg of helminths.
2. Biological Transmission
This type of transmission occurs when the arthropod takes an active role in the transmission of the organism, This type of transmission can be divided into:
- Propagative: The organism simply multiplies in the arthropod without any cyclical development like in a culture tube, e.g., multiplication of Yersinia pestis (causing plague) in the fleas.
- Cyclodevelopmental: The organism undergoes developmental or morphological changes only without multiplication in the arthropod, e.g., development of microfilaria of filarial worms within the mosquitoes.
- Cyclopropagative: Organisms undergo both developmental changes and multiplication in the arthropod, e.g. Plasmodia in the female Anopheles mosquito and Leishmania species in the sand fly.
Arthropods can convey the organisms to man by different methods
- Vomit-drop: Non-blood-sucking flies may deposit a vomit drop containing the pathogens on human food or drink, e.g. cysts of Entamoeba histolytica, Other arthropod species may obtain the pathogenic organisms in a blood meal, from an infected person and deposit them in a vomit drop in the puncture wound made in the skin of an uninfected person, eg. Transmission of leishmaniasis by sandflies and plague by fleas.
- Faeces: Non-blood-sucking flies may deposit their faeces containing the pathogens on human food or drink e.g., (cysts of Entamoeba histolytica.
- Salivary Secretion: Pathogens are discharged in minute droplets with the salivary secretion at the time the arthropod is taking a blood meal. e.g. Plasmodia by female Anopheles mosquitoes.
- Crushing: Contamination from the infected haemolymph may occur when the arthropod is crushed on the skin.
- Coxal Fluid: During the act of feeding, soft ticks produce infective fluids from their coxal glands that contaminate the bite wound or skin.