Nematodes

Question 1

Disease-causing parasites depend on their infected host for survival and can be divided into two groups:

Answer 1

• Protozoans – microscopic, single-celled eukaryotes
• Helminths – macroscopic, multicellular worms

Question 2

Three classes of helminths can infect humans:

Answer 2

• Trematodes (flukes)
• Cestodes (tapeworms)
• 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

Question 3

How abundant are Nematodes?

Answer 3

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

Question 4

Describe Nematode habitats

Answer 4

• Free-living marine, freshwater, soil species
• Plant and animal parasites

Question 5

What are the general characteristics of Nematodes?

Answer 5

• Eukaryotic
• Non-segmented, cylindrical, tapering at both ends
• Diecious or hermaphroditic
• 1 mm to 3 feet (eg. pinworm / guinea worm)

Question 6

Nematodes – General Characters: Eukaryotic / Multicellular

Answer 6

• Differentiated organs
• Primitive nervous systems
• Primitive excretory systems
• Highly developed reroductive systems

Question 7

How many molts and larval stages do Nematodes undergo to reach adult stage?

Answer 7

Nematodes undergo 4 molts and 4 larval stages to reach adult stage

L3-L4 -parasitic

Question 8

Nematodes – Modes of Infection: Ingestion

Answer 8

• 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

Question 9

Nematodes – Modes of Infection: Inhalation of infected dust

Answer 9

Containing embryonated eggs e.g. A. lumbricoides, E. vermicularis

Question 10

Nematodes – Modes of Infection: Penetration of skin

Answer 10

Filariform larvae bores through the skin e.g. A. duodenale, S. stercoralis, N. americanus

Question 11

Nematodes – Modes of Infection: Via blood sucking insects

Answer 11

e.g. filarial worms

Question 12

Anthelminthic

Answer 12

• 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

Question 13

Classification – Tissue Nematodes: lymphatic

Answer 13

Wuchereria bancrofti, Brugia malayi, Brugia timori

Question 14

Classification – Tissue Nematodes: Subcutaneous

Answer 14

• Loa loa (african eye worm)
• Onchocerca volvulus (blinding filaria)
• Dracunculus medinensis (thread worm)

Question 15

Classification – Tissue Nematodes: Conjunctiva

Answer 15

Loa loa

Question 16

Tissue nematodes can induce disease in:

Answer 16

• Tissues
• Blood
• Lymph system

Question 17

How many types of tissue nematodes use humans as definitive hosts?

Answer 17

4 major types

They can live for years in subcutaneous tissues and lymph vessels

Question 18

Tissue nematodes discharge live offspring called microfilariae

Answer 18

• Circulate through the blood or tissue
• Can be ingested by blood sucking insects

Question 19

Loiasis (Loa loa)

Answer 19

• 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

Question 20

Explain the Pathogenesis Of Loiasis

Answer 20

• 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.

Question 21

Treatment Of Loiasis

Answer 21

• 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.

Question 22

Classification – Intestinal Nematodes: Small Intestine

Answer 22

• Ascaris lumbricoides (round worm)
• Necator americanus (american hook worm)
• Ancylostoma duodenale (hook worm)
• Strongyloides stercoralis
• Trichinella spiralis (trichina worm)
• Capillaria philippinensis

Question 23

Classification – Intestinal Nematodes: Caecum and appendix

Answer 23

• Enterobius vermicularis (pin worm)
• Trichuris trichiura (whip worm

Question 24

Intestinal nematodes have the following characteristics:

Answer 24

• Tough outer cuticle
• Thousands of offspring are produced
• Eggs must incubate outside the host to become infective
• There is a larval form

Question 25

Intestinal nematode infection can produce:

Answer 25

* Malnutrition * Discomfort * Anemia * Occasionally death

Question 26

Severity of disease:

Answer 26

* Small worm load – asymptomatic * Large worm load – more serious disease Host immune defenses are slow to develop

Question 27

Enterobiasis (Enterobius vermicularis)

Answer 27

Definitive host: **Humans** Intermediate host: **None** Geographic distribution: **Cosmopolitan** Location: **caecum** AKA: **pinworm**

Question 28

Enterobiasis (Enterobius vermicularis) is the most common nematode infection in WESTERN WORLD

Answer 28

* 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

Question 29

Describe the Infection cycle of Enterobius vermicularis

Answer 29

1. Pinworms attach to the mucosa of the cecum 2. Females migrate down to the perianal tissue to lay eggs 3. Eggs rapidly mature and are infectious within hours 4. Eggs can be inhaled or swallowed 5. Eggs hatch in the upper intestine 6. Larvae migrate down to the caecum 7. Larvae mature into adults in 2-6 weeks

Question 30

Describe the pathogenesis of Enterobiasis

Answer 30

* 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

Question 31

Treatment Of Enterobiasis: **Mebendazole/albendazole (Vermox, Ovex)**

Answer 31

* 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

Question 32

Treatment Of Enterobiasis: **Pyrantel pamoate**

Answer 32

* Causes paralysis * The worm "loses its grip" on the intestinal wall and gets passed out of the system by feces

Question 33

Ascariasis (Ascaris lumbricoides)

Answer 33

Definitive host: **Humans** Intermediate host: **None** Geographic distribution: **Cosmopolitan** Location: **small intestines** AKA: **Giant roundworm**

Question 34

Ascariasis (Ascaris lumbricoides): Most common nematode infection WORLDWIDE

Answer 34

* 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

Question 35

Explain the Infection cycle of Ascaris lumbricoides

Answer 35

1. Adult worms live in the lumen of the small intestine. 2. A female lays eggs which are passed with the feces. 3. Infective eggs are swallowed. 4. The larvae hatch, penetrate the intestinal mucosa, invade the liver, and are carried to the lungs. 5. The larvae mature in the lungs, penetrate the alveolar walls, can be coughed up and swallowed. 6. Upon reaching the small intestine, they develop into adult worms.

Question 36

Describe the pathogenesis of Ascariasis

Answer 36

* 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

Question 37

Prolonged infection with heavy worm loads can cause:

Answer 37

* Malnutrition * Abdominal pain * Obstruction of the bile and pancreatic ducts

Question 38

Treatment Of Ascariasis

Answer 38

Treatment can be: * Albendazole/Mebendazole * Pyrantel pamoate * Piperazine citrate * Levamisole

Question 39

Trichinosis (Trichinella spiralis)

Answer 39

* 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**

Question 40

Explain the Infection cycle of Trichinella spiralis

Answer 40

* 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.

Question 41

Explain the pathogenesis of Trichinosis

Answer 41

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 * Complications – during migration: * Heart involvement can cause congestive heart failure * CNS involvement can cause encephalopathy or meningitis

Question 42

Treatment Of Trichinosis: **Albendazole / Mebendazole**

Answer 42

* 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

Question 43

Treatment Of Trichinosis: **Corticosteroids**

Answer 43

In case of complications