Nematodes Flashcards
Disease-causing parasites depend on their infected host for survival and can be divided into two groups:
- Protozoans – microscopic, single-celled eukaryotes
- Helminths – macroscopic, multicellular worms
Three classes of helminths can infect humans:
- Trematodes (flukes)
- Cestodes (tapeworms)
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Nematodes (roundworms)
- Gastrointestinal form – use only one host to complete their life cycle
- Blood and tissue form – use multiple hosts to complete their life cycle
How abundant are Nematodes?
Extremely abundant (25,000 described, 1,000,000 estimated)
- 90,000 worms in a single rotting apple
- ~9 billion in 1 acre of farmland
- small predators or saprophytes
Describe Nematode habitats
- Free-living marine, freshwater, soil species
- Plant and animal parasites
What are the general characteristics of Nematodes?
- Eukaryotic
- Non-segmented, cylindrical, tapering at both ends
- Diecious or hermaphroditic
- 1 mm to 3 feet (eg. pinworm / guinea worm)
Nematodes – General Characters: Eukaryotic / Multicellular
- Differentiated organs
- Primitive nervous systems
- Primitive excretory systems
- Highly developed reroductive systems
How many molts and larval stages do Nematodes undergo to reach adult stage?
Nematodes undergo 4 molts and 4 larval stages to reach adult stage
L3-L4 -parasitic
Nematodes – Modes of Infection: Ingestion
- Embryonated eggs contaminating food & drinks, e.g. (A. lumbricoides, E. vermicularis & T. trichiura)
- Growing embryos in an intermediate host (infected cyclops) e.g. D. medinensis
- Encysted embryos in infected pig’s flesh e.g. Trichinella spiralis
Nematodes – Modes of Infection: Inhalation of infected dust
Containing embryonated eggs e.g. A. lumbricoides, E. vermicularis
Nematodes – Modes of Infection: Penetration of skin
Filariform larvae bores through the skin e.g. A. duodenale, S. stercoralis, N. americanus
Nematodes – Modes of Infection: Via blood sucking insects
e.g. filarial worms
Anthelminthic
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Piperazine:
- GABA mimetic, acts on GABA –gated Cl- channels, causes body wall muscle paralysis
-
Benzimidazoles (thiabendazole, albendazole, mebendazole):
- compromise the cytoskeleton through a selective interaction with β-tubulin
-
Imidazothiazole (levamisole) and tetrahydropyrimidines (pyrantel and morantel)
- nicotinic acetylcholine receptor agonists, elicit spastic muscle paralysis
Classification – Tissue Nematodes: lymphatic
Wuchereria bancrofti, Brugia malayi, Brugia timori
Classification – Tissue Nematodes: Subcutaneous
- Loa loa (african eye worm)
- Onchocerca volvulus (blinding filaria)
- Dracunculus medinensis (thread worm)
Classification – Tissue Nematodes: Conjunctiva
Loa loa
Tissue nematodes can induce disease in:
- Tissues
- Blood
- Lymph system
How many types of tissue nematodes use humans as definitive hosts?
4 major types
They can live for years in subcutaneous tissues and lymph vessels
Tissue nematodes discharge live offspring called microfilariae
- Circulate through the blood or tissue
- Can be ingested by blood sucking insects
Loiasis (Loa loa)
- Day-biting flies introduces L3 filarial larvae onto the skin of the human host
- The larvae develop into adults (subcutaneous tissue)
- Adults produce microfilariae
- The fly ingests microfilariae during a blood meal
- The microfilariae migrate to the thoracic muscles
- There the microfilariae develop into L1 larvae → L3 infective larvae
- The L3 infective larvae migrate to the fly’s proboscis and can infect another human when the fly takes a blood meal
Explain the Pathogenesis Of Loiasis
- The adult worms live between layers of connective tissue
- The microfilaria move into the lymph vessels of the body and eventually they move into the lungs where they spend most of their time.
- Most people with loiasis do not have any symptoms. The most common manifestations:
- Calabar swellings = localized, non-tender, usually found on the arms and legs and near joints.
- Itching can occur around the area of swelling or can occur all over the body.
- Eye worm is the visible movement of the adult worm across the surface of the eye and can cause eye congestion, itching, pain, and light sensitivity.
Treatment Of Loiasis
- The treatment of choice is diethylcarbamazine (DEC), which kills the microfilariae and adult worms.
- Albendazole used in patients who are not cured with multiple DEC treatments.
Classification – Intestinal Nematodes: Small Intestine
- Ascaris lumbricoides (round worm)
- Necator americanus (american hook worm)
- Ancylostoma duodenale (hook worm)
- Strongyloides stercoralis
- Trichinella spiralis (trichina worm)
- Capillaria philippinensis
Classification – Intestinal Nematodes: Caecum and appendix
- Enterobius vermicularis (pin worm)
- Trichuris trichiura (whip worm
Intestinal nematodes have the following characteristics:
- Tough outer cuticle
- Thousands of offspring are produced
- Eggs must incubate outside the host to become infective
- There is a larval form
Intestinal nematode infection can produce:
- Malnutrition
- Discomfort
- Anemia
- Occasionally death
Severity of disease:
- Small worm load – asymptomatic
- Large worm load – more serious disease
Host immune defenses are slow to develop
Enterobiasis (Enterobius vermicularis)
Definitive host: Humans
Intermediate host: None
Geographic distribution: Cosmopolitan
Location: caecum
AKA: pinworm
Enterobiasis (Enterobius vermicularis) is the most common nematode infection in WESTERN WORLD
- Mostly found in temperate climates of Europe and North America
- More than 200 million people are infected each year, most of these are children
- Readily transmitted where large numbers of children gather - nurseries, childcare facilities, and orphanages
Describe the Infection cycle of Enterobius vermicularis
- Pinworms attach to the mucosa of the cecum
- Females migrate down to the perianal tissue to lay eggs
- Eggs rapidly mature and are infectious within hours
- Eggs can be inhaled or swallowed
- Eggs hatch in the upper intestine
- Larvae migrate down to the caecum
- Larvae mature into adults in 2-6 weeks
Describe the pathogenesis of Enterobiasis
- The infection cycle takes about four weeks
- It can cause symptoms of insomnia, irritability from itching (pruritus ani)
- Skin abrasions can occur from scratching
- Female worms sometimes move to the genitourinary tract of females
- Secondary bacterial skin infection may develop from vigorous scratching to relieve pruritus
Treatment Of Enterobiasis: Mebendazole/albendazole (Vermox, Ovex)
- Active against both larval and adult stages of nematodes, and in the cases of roundworm and whipworm, kill the eggs, too
- Paralysis and death of the parasites occurs slowly, and elimination in the feces may require several days
- Mebendazole is usually taken as a single dose, but it can be taken for around 3 days for some infections
Treatment Of Enterobiasis: Pyrantel pamoate
- Causes paralysis
- The worm “loses its grip” on the intestinal wall and gets passed out of the system by feces
Ascariasis (Ascaris lumbricoides)
Definitive host: Humans
Intermediate host: None
Geographic distribution: Cosmopolitan
Location: small intestines
AKA: Giant roundworm
Ascariasis (Ascaris lumbricoides): Most common nematode infection WORLDWIDE
- Over 1000 million people worldwide are estimated to be infected with Ascaris lumbricoides and, of these, at least 20,000 die annually
- Female parasites can lay 250-500 thousand eggs per day, which are resistant to environmental pressure, and can be viable for up to 6 years
Explain the Infection cycle of Ascaris lumbricoides
- Adult worms live in the lumen of the small intestine.
- A female lays eggs which are passed with the feces.
- Infective eggs are swallowed.
- The larvae hatch, penetrate the intestinal mucosa, invade the liver, and are carried to the lungs.
- The larvae mature in the lungs, penetrate the alveolar walls, can be coughed up and swallowed.
- Upon reaching the small intestine, they develop into adult worms.
Describe the pathogenesis of Ascariasis
- Ascariasis results while larvae are in the lung or in the intestines
- If the worm load is small, infections can be asymptomatic
- If the worm load is large, symptoms can include:
- Fever, Coughing, Wheezing, Shortness of breath, pneumonia
Prolonged infection with heavy worm loads can cause:
- Malnutrition
- Abdominal pain
- Obstruction of the bile and pancreatic ducts
Treatment Of Ascariasis
Treatment can be:
- Albendazole/Mebendazole
- Pyrantel pamoate
- Piperazine citrate
- Levamisole
Trichinosis (Trichinella spiralis)
- Definitive host: Humans
- An infected animal serves as a definitive host and potential intermediate host.
- Geographic distribution: Cosmopolitan
- Most common in parts of Europe and the United States.
- Location (adults): small intestine
- Commonly involved muscles: diaphragm, intercostals, deltoid, pectoralis major, biceps
- AKA: Trichina worm
Explain the Infection cycle of Trichinella spiralis
- Ingestion of meat containing encysted larvae
- The larvae are released from the cysts (gastric acid and pepsin), and invade the small bowel mucosa where they develop into adult worms
- After 1 week, the females release larvae that migrate to the striated muscles where they encyst
- Encystment is completed in 4-5 weeks and the cysts may remain viable for several years.
Explain the pathogenesis of Trichinosis
ØTrichinella enters through the host vascular system and is distributed widely
- Only parasites that penetrate the skeletal muscle survive
- Clinical features depends on the stage:
-
Stage of intestinal invasion: 5-7 days, pain in abdomen, nausea, vomiting, diarrhoea
- Stage of larval migration: fever, urticarial rash, splinter haemorrhages, periorbital & facial oedema
- Stage of encystation: asymptomatic in light infections; myalgia, weakness in heavy infections
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Stage of intestinal invasion: 5-7 days, pain in abdomen, nausea, vomiting, diarrhoea
- Complications – during migration:
- Heart involvement can cause congestive heart failure
- CNS involvement can cause encephalopathy or meningitis
Treatment Of Trichinosis: Albendazole / Mebendazole
- Eliminates the worms from the intestine but has little effect on the cysts in muscle
- Decreases the likelihood of larval encystation, particularly if given within three days of infection
Treatment Of Trichinosis: Corticosteroids
In case of complications
