92. Phylum Nematoda. Ascaris lumbricoides.

Phylum Nematoda (Roundworms)

General Characteristics:

• Nematodes are also known as roundworms
• They were the first major animal group to successfully adapt to life on land
• Nematodes are extremely abundant, especially in soil, where they play important ecological roles
• While many nematodes are free-living, others are parasitic and infect both plants and animals
• Their size can vary greatly, from just a few millimeters to over a meter in length
• Nematodes have a simple body structure that lacks features such as cilia and a distinct head
• Their bodies are elongated, cylindrical, unsegmented, and covered with a tough cuticle

Examples of Parasitic Nematodes Affecting Humans

1. Geohelminths (Soil-transmitted worms)

• Ancylostoma duodenale (Old World hookworm)
• Found in humans, as well as dogs and cats
• Adult males measure about 8 to 11 millimeters; females are slightly larger at 10 to 13 millimeters
• Worms attach to the intestinal wall and feed on blood, which can lead to iron deficiency anemia
• Larvae of animal hookworms may migrate under human skin and cause a condition known as cutaneous larva migrans, a red, itchy, serpentine rash
• Strongyloides stercoralis
• Has both a free-living and a parasitic life cycle
• Autoinfection can occur when larvae inside the body mature and reinfect the host without leaving
• The infective form is the filariform larva, which penetrates the skin to initiate infection
• The diagnostic form is the rhabditiform larva, found in stool samples

2. Tissue-dwelling Nematodes

• These nematodes inhabit tissues rather than body cavities
• They do not release eggs or larvae into the bloodstream and are not transmitted by insects
• Trichinella spiralis
• Causes trichinosis, a disease that results from eating undercooked meat containing encysted larvae
• The larvae are often found encased in muscle tissue
• Dracunculus species (e.g., Dracunculus medinensis)
• The female worm migrates to the skin, usually the leg, to release larvae into water
• Historically known as the "Guinea worm" and was once widespread but is now near eradication

3. Filarial Nematodes (Filariae)

• These worms produce larvae called microfilariae
• Microfilariae are transmitted by blood-sucking insects, such as mosquitoes or blackflies
• Wuchereria bancrofti
• Causes lymphatic filariasis, also known as elephantiasis
• The worms block lymphatic vessels, leading to severe swelling of the limbs or genitals
• Onchocerca species
• Onchocerca volvulus causes river blindness, transmitted by blackflies
• Adult worms form nodules under the skin, and microfilariae can invade the eyes, leading to blindness
• Treatment with Ivermectin, developed through Nobel Prize–winning research, is effective
• Dirofilaria species
• Commonly known as dog heartworm, but some species infect humans
• Transmitted by mosquitoes