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(2020-21 BY2MI1) Microbiology II > Nematodes > Flashcards

Nematodes Flashcards

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1
Q

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

A
  • Protozoans – microscopic, single-celled eukaryotes
  • Helminths – macroscopic, multicellular worms
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2
Q

Three classes of helminths can infect humans:

A
  • 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
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3
Q

How abundant are Nematodes?

A

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
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4
Q

Describe Nematode habitats

A
  • Free-living marine, freshwater, soil species
  • Plant and animal parasites
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5
Q

What are the general characteristics of Nematodes?

A
  • Eukaryotic
  • Non-segmented, cylindrical, tapering at both ends
  • Diecious or hermaphroditic
  • 1 mm to 3 feet (eg. pinworm / guinea worm)
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6
Q

Nematodes – General Characters: Eukaryotic / Multicellular

A
  • Differentiated organs
  • Primitive nervous systems
  • Primitive excretory systems
  • Highly developed reroductive systems
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7
Q

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

A

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

L3-L4 -parasitic

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8
Q

Nematodes – Modes of Infection: Ingestion

A
  • 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
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9
Q

Nematodes – Modes of Infection: Inhalation of infected dust

A

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

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10
Q

Nematodes – Modes of Infection: Penetration of skin

A

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

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11
Q

Nematodes – Modes of Infection: Via blood sucking insects

A

e.g. filarial worms

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12
Q

Anthelminthic

A
  • 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
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13
Q

Classification – Tissue Nematodes: lymphatic

A

Wuchereria bancrofti, Brugia malayi, Brugia timori

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14
Q

Classification – Tissue Nematodes: Subcutaneous

A
  • Loa loa (african eye worm)
  • Onchocerca volvulus (blinding filaria)
  • Dracunculus medinensis (thread worm)
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15
Q

Classification – Tissue Nematodes: Conjunctiva

A

Loa loa

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16
Q

Tissue nematodes can induce disease in:

A
  • Tissues
  • Blood
  • Lymph system
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17
Q

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

A

4 major types

They can live for years in subcutaneous tissues and lymph vessels

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18
Q

Tissue nematodes discharge live offspring called microfilariae

A
  • Circulate through the blood or tissue
  • Can be ingested by blood sucking insects
19
Q

Loiasis (Loa loa)

A
  • 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
20
Q

Explain the Pathogenesis Of Loiasis

A
  • 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.
21
Q

Treatment Of Loiasis

A
  • 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.
22
Q

Classification – Intestinal Nematodes: Small Intestine

A
  • Ascaris lumbricoides (round worm)
  • Necator americanus (american hook worm)
  • Ancylostoma duodenale (hook worm)
  • Strongyloides stercoralis
  • Trichinella spiralis (trichina worm)
  • Capillaria philippinensis
23
Q

Classification – Intestinal Nematodes: Caecum and appendix

A
  • Enterobius vermicularis (pin worm)
  • Trichuris trichiura (whip worm
24
Q

Intestinal nematodes have the following characteristics:

A
  • Tough outer cuticle
  • Thousands of offspring are produced
  • Eggs must incubate outside the host to become infective
  • There is a larval form
25
Q

Intestinal nematode infection can produce:

A
  • Malnutrition
  • Discomfort
  • Anemia
  • Occasionally death
26
Q

Severity of disease:

A
  • Small worm load – asymptomatic
  • Large worm load – more serious disease

Host immune defenses are slow to develop

27
Q

Enterobiasis (Enterobius vermicularis)

A

Definitive host: Humans

Intermediate host: None

Geographic distribution: Cosmopolitan

Location: caecum

AKA: pinworm

28
Q

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

A
  • 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
29
Q

Describe the Infection cycle of Enterobius vermicularis

A
  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
30
Q

Describe the pathogenesis of Enterobiasis

A
  • 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
31
Q

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

A
  • 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
32
Q

Treatment Of Enterobiasis: Pyrantel pamoate

A
  • Causes paralysis
  • The worm “loses its grip” on the intestinal wall and gets passed out of the system by feces
33
Q

Ascariasis (Ascaris lumbricoides)

A

Definitive host: Humans

Intermediate host: None

Geographic distribution: Cosmopolitan

Location: small intestines

AKA: Giant roundworm

34
Q

Ascariasis (Ascaris lumbricoides): Most common nematode infection WORLDWIDE

A
  • 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
35
Q

Explain the Infection cycle of Ascaris lumbricoides

A
  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.
36
Q

Describe the pathogenesis of Ascariasis

A
  • 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
37
Q

Prolonged infection with heavy worm loads can cause:

A
  • Malnutrition
  • Abdominal pain
  • Obstruction of the bile and pancreatic ducts
38
Q

Treatment Of Ascariasis

A

Treatment can be:

  • Albendazole/Mebendazole
  • Pyrantel pamoate
  • Piperazine citrate
  • Levamisole
39
Q

Trichinosis (Trichinella spiralis)

A
  • 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
40
Q

Explain the Infection cycle of Trichinella spiralis

A
  • 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.
41
Q

Explain the pathogenesis of Trichinosis

A

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
42
Q

Treatment Of Trichinosis: Albendazole / Mebendazole

A
  • 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
43
Q

Treatment Of Trichinosis: Corticosteroids

A

In case of complications

(2020-21 BY2MI1) Microbiology II (16 decks)

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