103. Overview of parasitism.

Types of Parasites and Their Features

A. Arachnids: Mites and Ticks

• Ticks are blood-feeding parasites that attach to animals and humans. They transmit a wide range of pathogens such as bacteria, viruses, and protozoa. Found mainly in wooded or pasture environments, ticks are larger than mites and play a major role in spreading tick-borne diseases.
• Mites are smaller and often covered with fine hair. They include several important species:
• Sarcoptes scabiei causes scabies by burrowing into the skin and laying eggs, which results in intense itching and infection.
• Dermanyssus gallinae is a parasite of birds but can bite humans, causing dermatitis.
• House dust mites like Dermatophagoides produce allergens that trigger respiratory and skin reactions.
• Demodex folliculorum lives in human hair follicles, mostly harmless, but linked with skin conditions when present in high numbers.

B. Insects: Lice, Fleas, Mosquitoes, and Bedbugs

• Lice are wingless insects that live on humans:
• Head lice live in hair and do not transmit disease.
• Body lice live in clothing and can spread serious diseases like epidemic typhus and relapsing fever.
• Pubic lice infest coarse body hair and are not disease vectors.
• Bedbugs (Cimex lectularius) feed on blood and cause skin irritation but have not been proven to spread human diseases.
• Fleas are flat, wingless insects with strong jumping abilities:
• Human fleas feed mainly on people.
• Other species, like dog and cat fleas or the oriental rat flea, also bite humans and can transmit plague, murine typhus, and tapeworms.
• Mosquitoes:
• Female mosquitoes feed on blood to develop eggs.
• Anopheles species can transmit parasitic worms like Wuchereria bancrofti, causing lymphatic filariasis.
• Sand flies (Phlebotomus):
• Females are blood-feeders and can transmit leishmaniasis and pappataci fever.
• Males feed on nectar and do not bite.

C. Annelids: Leeches

• Leeches are segmented worms that attach to hosts using suckers. They release a mix of chemicals including anticoagulants and anesthetics while feeding. Medicinal leeches are used therapeutically to promote blood flow and reduce swelling.

D. Flatworms: Tapeworms and Flukes

• Tapeworms (Cestodes) are flat, ribbon-like worms made of multiple segments. They lack a digestive system and absorb nutrients through their skin. Common examples include Taenia saginata, Taenia solium, and Diphyllobothrium latum.
• Flukes (Trematodes) have flat, leaf-like bodies. They usually require mollusks and often other intermediate hosts to complete their life cycle. Examples include liver flukes like Fasciola hepatica and blood flukes like Schistosoma.

E. Roundworms: Nematodes