Intestinal nematodes Flashcards
What are nematodes? What regions of the body can they infect?
Nematodes = are roundworms with unsegmented bodies
Locations
* Intestinal
* Blood
* Lymphatics
* Subcutaneous
What are the common intestinal nematodes (scientific and common names)
- Enterobius vermicularis - pinworm
- Trichuris trichiuria - whipworm
- Ascaris lumbricoides - roundworm
- Ancylostoma duodenale and Necator americanus - hookworm
- Strongyloides stercoralis
- Trichinella spiralis
Describe the eggs of E. vermicularis
- The eggs of Enterobius vermicularis measure 50—60 µm by 20—30 µm.
- They are transparent, elongate to oval in shape, and slightly flattened on one side.
- They are usually partially embryonated when shed.
What is the scientific name for pinworm?
Enterobius vermicularis
How is E. vermicularis transmitted?
Via ingestion of embryonated eggs from contaminated food/feces
Where do E. vermicularis mature?
Ileum and colon
What are the common symptoms of E. vermicularis
- Asymptomatic
- Pruritis ani
- Vulvovaginitis
- Sterile pyuria
How is E. vermicularis infection (enterobiasis) diagnosed?
Scotch tape method in early morning before bowel movement as adult female migrates nocturnally and lays eggs. Eggs can then be mounted and visualized
What is the treatment for E. vermicularis infection?
- Albendazole or Mebendazole x1 dose
- Repeat in 2 weeks
- Treat others if sharing room
- Wash bedsheets
Describe the lifecycle of E. vermicularis
- Gravid adult female Enterobius vermicularis deposit eggs on perianal folds.
- Infection occurs via self-inoculation (transferring eggs to the mouth with hands that have scratched the perianal area) or through exposure to eggs in the environment (e.g. contaminated surfaces, clothes, bed linens, etc.)
- Following ingestion of infective eggs, the larvae hatch in the small intestine
- The adults establish themselves in the colon, usually in the cecum
- The time interval from ingestion of infective eggs to oviposition by the adult females is about one month. At full maturity adult females measure 8 to 13 mm, and adult males 2 to 5 mm; the adult life span is about two months. Gravid females migrate nocturnally outside the anus and oviposit while crawling on the skin of the perianal area . The larvae contained inside the eggs develop (the eggs become infective) in 4 to 6 hours under optimal conditions
True or false: E. vermicularis infection causes eosinophilia
False: E. vermicularis does not cause eosinophilia
What is the scientific name of whipworm?
Trichuris trichiuria
What is the treatment for T. trichiruia infection?
Albendazole or Mebendazole for 3 days
Describe the eggs of T. trichiruria
- Trichuris trichiura eggs are 50-55 micrometers by 20-25 micrometers.
- They are barrel-shaped, thick-shelled and possess a pair of polar “plugs” at each end
- The eggs are unembryonated when passed in stool.
Describe the lifecycle of T. trichiuria
- The unembryonated eggs are passed with the stool
- In the soil, the eggs develop into a 2-cell stage
- Then an advanced cleavage stage
- Then they embryonate
- Eggs become infective in 15 to 30 days. After ingestion (soil-contaminated hands or food), the eggs hatch in the small intestine, and release larvae
- Larvae mature and establish themselves as adults in the colon
- The adult worms (approximately 4 cm in length) live in the cecum and ascending colon. The adult worms are fixed in that location, with the anterior portions threaded into the mucosa. The females begin to oviposit 60 to 70 days after infection. Female worms in the cecum shed between 3,000 and 20,000 eggs per day. The life span of the adults is about 1 year.
What are the common symptoms of T. trichiuria infection
- 90% asymptomatic
- Rectal prolapse due to peristalsis to expel
- Abdominal pain
- Diarrhea
- Anemia of chronic disease
- Colitis
- Growth retardation in children
What is the scientific name for roundworm?
Ascaris lumbricoides
How large are adult A. lumbricoides worms?
Up to 45 cm
Largest of the nematodes that infect humans
Describe the lifecycle of A. lumbricoides
- Adult worms live in the lumen of the small intestine.
- A female may produce approximately 200,000 eggs per day, which are passed with the feces . Unfertilized eggs may be ingested but are not infective.
- Larvae develop to infectivity within fertile eggs after 18 days to several weeks , depending on the environmental conditions (optimum: moist, warm, shaded soil).
- Infective eggs are swallowed
- Larvae hatch
- Larvae then invade the intestinal mucosa, and are carried via the portal, then systemic circulation to the lungs.
- The larvae mature further in the lungs (10 to 14 days), penetrate the alveolar walls, ascend the bronchial tree to the throat, and are swallowed.
- Upon reaching the small intestine, they develop into adult worms. Between 2 and 3 months are required from ingestion of the infective eggs to oviposition by the adult female. Adult worms can live 1 to 2 years.
True or false: A. lumbricoides do not cause eosinophilia
False: A. lumbricoides do cause eosinophilia due to the obligatory extra-intestinal migration through tissues
Describe the appearance of fertilized eggs of A. lumbricoides
- Fertilized eggs are are rounded and have a thick shell with an external mammillated layer that is often stained brown by bile.
- In some cases, the outer layer is absent (known as decorticated eggs).
- Fertile eggs range from 45 to 75 µm in length.
What are the common symptoms of A. lumbricoides infection
Larval phase
* Eosinophilia
* Pneumonitis
Adult phase
* Nutritional malabsorption
* Impaired growth
* Abdominal discomfort to small bowel obstruction and infarction
* Wandering ascariasis into hepatobiliary tree can cause hepatobiliary obstruction
Migrating ascariasis
* Loeffler’s syndrome (eosinophilic pneumonia that mimics asthma)
* Biliary-pancreatic symptoms
* Anastomotic perforation
What is the treatment for A. lumbricoides infection?
- Albendazole 400mg in a single dose
- Heavy infections may need repeat dosing
How is A. lumbricoides infection diagnosed?
- Stool microscopy
- Larvae in sputum
- Adults passed in stool or coughed up can be identified by features = 3 lips
What are the scientific names of the two most common human hookworms?
- Ancylostoma duodenale
- Necator americanus
Describe the lifecycle of human hookworms
- Eggs are passed in the stool , and under favorable conditions (moisture, warmth, shade),
- Rhabditiform larvae hatch in 1 to 2 days and become free-living in contaminated soil. These released rhabditiform larvae grow in the feces and/or the soil
- After 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective . These infective larvae can survive 3 to 4 weeks in favorable environmental conditions.
- On contact with the human host, typically bare feet, the larvae penetrate the skin and are carried through the blood vessels to the heart and then to the lungs. They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed.
- The larvae reach the jejunum of the small intestine, where they reside and mature into adults. Adult worms live in the lumen of the small intestine, typically the distal jejunum, where they attach to the intestinal wall with resultant blood loss by the host image . Most adult worms are eliminated in 1 to 2 years, but the longevity may reach several years.
Describe the egg of human hookworms
- Thin-shelled
- Colorless
- Measure 60-75 µm by 35-40 µm
Eggs of Ancylostoma and Necator cannot be differentiated microscopically
How is human hookworm infection diagnosed?
Microscopic identification of eggs in the stool
