Comprehensive description and the medical importance of Arthropods

General morphology

As a group, arthropods vary markedly in size-form large scorpions to nits or mites that can barely be seen without a magnifying lens. Specifically, they lack an internal skeleton, being supported instead by a tough, chitinous outer shell. Their bodies are symmetric, and the most distinguishing characteristic is the presence of jointed appendages. The prototype arthropod possesses three distinct segments: head, thorax, and abdomen. Head appendages may include a sensory organ (antenna), specialized mouth parts, and an organ for piercing (proboscis). The thorax may give rise to paired, jointed legs (or/swimming organs for aquatic species) ventrally and wings, either rudimentary or well-developed, dorsally. The abdomen is commonly divided into multiple metameric (similar-appearing) segments, with reproductive apparatus and sometimes a stinger appendages determine the classes and orders by which arthropods are sub-classified.

Medical Importance

The parasitic relation of an arthropod to its host ranges from a continuous, lifelong dependence (lice and hair-follicle mites) to a mealtime-only attack (flies and mosquitoes). Adult ticks and flea, which stay with the host most of the time, are considered rather strict parasites. Bedbugs and triatomids, normally found in the nests or habitations of their host, are considered parasites of homes. Certain midges, and flies are parasitic only as larvae, As blood suckers, arthropods pose no serious threat to their human hosts. As disease transmitters. they are more dangerous. Man in his travels has disseminated to all corners of the world parasites carried on himself. on his domestic animals, and on the rats that infested his ships and wagons. Fast, modern transportation now makes possible the introduction of more efficient vectors of diseases endemic in a given area. New diseases can be easily introduced through infected reservoir, swelling and arthropods. Bites of arthropods sometimes produce redness, swelling and varying degrees of itching and pain. These effects represent a direct or indirect reaction by the victim to the bug's salivary secretions, which are formulated as anticoagulants to maintain fluidity of the hosts's blood during the feeding period. In a severe reaction, local discomfort is accompanied by fever, headache, nausea, and fatigue, probably on an allergic basis.

Modes by which arthropods transmit diseases

1. Indirect mechanical- Arthropods act as passive carrier of disease agent, depositing adherent microbes on food or water (roaches and houseflies). Yaws, trachoma, and Oriental sore are spread in this manner.

2. Direct mechanical- Arthropods acquire microbes from one host and inoculate them directly into the skin, wound, or blood of another. Anthrax and viral encephalomyelitis are so transmitted.

3. Biologic- The parasite does more than just hitchhike. Further development, cyclic change, or multiplication takes place within the body of the arthropod. The sporogony cycle of plasmoda in the mosquito is a prime example.

4. Hereditary- The disease agent is transmitted to the offspring of the arthropod through invasion of the ovary or infection of eggs. Ticks and mites, for example, transovarially transmit protozoa, tularemia, bacilli, rickettsiae, relapsing fever spirochetes, and viruses.

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