Chapter 27 - Nematodes: Oxyurids (Pinworms)

Order Oxyurida

• Commonly called the pinworms because females typically have slender, sharp-pointed tails• Have a large esophogeal bulb and cuticle is swollen in anterior• They are the only endoparasites with haplodiploidy - males are haploid and develop parthenogenetically; females are diploid, developing from fertilized eggs

• Pinworms are common among birds, mammals, reptiles and amphibians, but are rare among fishes

• Two species are known to infect humans: Enterobius vermicularis and E. gregorii

Enterobius vermicularis

• Common parasite of younger children• Males with coiled tail; females with pointed tail and distension of the body due to the large number of eggs in the uteri

Male pinworm

Life Cycle

• Sexually mature worms usually inhabit the colon of the human intestinal tract but they can spread to adjacent regions of the small intestine• Worms feed on bacteria and epithelial cells• Males die following copulation while egg-bearing females migrate to the perianal and perineal regions• Stimulated by lower temperature and low oxygen levels, they deposit their eggs and then die• More eggs are released when the female body ruptures• The eggs upon deposition contain immature larvae• The infective 3rd stage larva develops within the egg after leaving the body of the female worm

Pinworm Life Cycle

Life Cycle cont.

• Infection and reinfection occurs when eggs containing the infective larvae are ingested by the host• Such eggs are usually picked up from bed cloths or fingernails contaminated when the host scratches the perianal zone to relieve the itching caused by the nocturnal migration of the female worms• However, the light weight eggs are sometimes airborne and so can also be inhaled

Life Cycle cont.

• Retroinfections occur when 3rd stage larvae hatch from perianally located eggs and migrate back up the host’s intestinal tract• Ingested eggs usually hatch shortly after reaching the duodenum• The escaping larvae undergo molts and development as they migrate posteriorly, becoming sexually mature when they arrive at the colon

Pathology and Diagnosis

• Pinworms are usually not very pathogenic• Clinical symptoms are often include anal itching• Heavy infections in children may result in sleeplessness, loss of weight, abdominal pain, etc.• Diagnosis is verified when adult worms and/or eggs are detected• The female worms emerge at night and are usually visible in the perianal and perianal regions• Adult worms are also observed in the feces as well• Eggs are observed in the feces only about 5% of the time• The most reliable procedure for finding eggs is to press a strip of scotch tape on the perianal skin, remove it, and place it on a clean microscope slide for examination