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Medical Microbiology > Tissue Worms > Flashcards

Tissue Worms Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

What is a parasite?

A

An organism that requires another organism (host) for all or part of its life cycle

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2
Q

What is Parasitology?

A

The study of parasites

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3
Q

What is entomology?

A

The study of insects

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4
Q

What is medical entomology?

A

The study of insects of medical importance

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5
Q

What is a vector?

A

An organism that transmits parasite between hosts e.g. Sand fly or Mosquito

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6
Q

What is a host?

A

An organism that harbours a parasite (humans or other animals)

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7
Q

What is a primary (definitive) host?

A

The host where the parasite reaches maturity (sexual reproduction if it occurs)

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8
Q

What is a secondary (intermediate) host?

A

A host where the parasite spends part of its life cycle

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9
Q

What is a dead end host?

A

A host where parasite cannot complete its life cycle

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10
Q

What is host specificity?

A

The range of organisms an individual parasite can utilise as a host (fewer=more specific)

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11
Q

What is zoonoses?

A

Infection transmitted between vertebrate animals and humans

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12
Q

What causes Dracunculiasis?

A

The nematode Drancunclus medinensis

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13
Q

What are other names for Dracunculiasis?

A

Guinea worm

Medina worm

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14
Q

How many cases of Dracunculiasis have there been in recent years?

A
    1. 5 million cases (p.a.) in 1980’s

* 15,000 (p.a.) around 2000

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15
Q

What causes Dracunculiasis?

A

Drinking contaminated water

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16
Q

Describe the etiology of Dracunculiasis

A

• Humans become infected when they drink water containing the infected Cyclopshe
• Also known as a Water flea – Type of crustacean
• The Cyclops becomes infected with nematode larvae
The vector for this infection is a copepod (Cyclops spp.)

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17
Q

What are the symptoms of Dracunculiasis?

A

Blistering

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18
Q

What can result from the blisters caused by Dracunculiasis?

A

From the initial infection it usually take 1 year before the blister develops on the skin
• The male worm can be 10-40 cm in length
• The female can be up to 100 cm in length

The formation of the blister causes a burning sensation as the worm emerges through the skin
• Risk of secondary infection / tetanus

This encourages the desire to immerse the limb in water
This then contaminates the water for everyone else in the community

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19
Q

Describe the life cycle of the Drancunclus medinensis

A

Person drinks water containing infected copepod
Larvae are release and penetrate the duodenum

The larvae moult twice as they migrate around the body before males and females mate in the subcutaneous tissue
• Males die after mating

Fertilised females migrate to the subcutaneous tissue usually on the lower limbs
• A blister forms on the surface of the skin

When the limb enters water the blister bursts and the female release more larvae
Larvae are then swallowed by the copepods
• The cycle begins again

20
Q

What happens when a person infected with Dracunculiasis immerses their blisters in water?

A

In contact with water the female worm prolapses
her uterus

This release the larvae
The larvae infect copepods
This contaminates the water supply for other users

This is a particular problem if large numbers of people share a water supply

21
Q

How is Dracunculiasis treated?

A

There is no drug treatment
• The worm can be removed by slowing winding it around a stick/twig
• This is a long process and can take up to a month

22
Q

What are the complications that can occur in an individual with Dracunculiasis?

A
  • Ulceration
  • Secondary infection
  • Tetanus
23
Q

How can Dracunculiasis be prevented?

A
  • Infections can be prevented by filtering water through a nylon gauze
  • Drinking straw with a nylon filer
  • Unglazed pottery

Copepods are too big to pass through. This removes the copepods and prevents the infection

24
Q

Describe the eradication campaign for Dracunculiasis

A

Expected to be the next infection after small pox to be eradicated (Carter Centre)
• Eradicated in India, Pakistan, Yemen, Senegal, Cameroon, Chad, Kenya & Uganda
• Still prevalent in Ghana & Sudan – Major announcement last year

25
Q

What causes Lymphatic filiariasis?

A

Nematode worms:

  • Wuchereria bancrofti
  • Brugia malayi
26
Q

What vectors are involved in the transmission of Lymphatic filiariasis?

A

Mosquitos:

  • Anopheles, Culex, Aedes (Pacific islands)
  • Mansonia (Brugia malayi)
27
Q

Describe the epidemiology of Lymphatic filiariasis

A
  • 120 million people in 80 countries (40 million disfigured)
  • 1/3 in India, 1/3 in Africa
  • Remainder in South Asia, Pacific & Americas
28
Q

What are the symptoms of Lymphatic filiariasis?

A

Adult worms live in the lymphatic system (4-6
years) - Producing millions of microfilariae
• Cause local inflammation, fibrosis, obstruction (lymphatic filariasis)
• Eosinophilia; later, enlargement and thickening of the skin (elephantiasis)

29
Q

What is the lymphatic system?

A

A network of vessels that conveys extracellular tissue fluid. Allows transportation of nutrients to every cell as lymph via the thoracic duct, back to the bloodstream

It consists of fine, blind-ended lymphatic capillaries which unite to form lymphatic vessels
Vessels have valves to prevent backflow of lymph

At various points along these vessels are lymph nodes. Lymph is similar in composition to plasma, but contains less protein, some cells (mainly lymphocytes)
• Milky in appearance

When lymphatic vessels become inflamed they can sometimes be seen as red streaks in the skin

30
Q

Describe the life cycle of Wuchereria bancrofti

A

Mosquito takes blood meal (L3 larvae enter skin)
Develop into adults in lymphatic system
Adults produce sheathed microfilariae that migrate
into lymph and blood channels
Mosquito takes blood meal (ingests Microfilariae)
Microfilariae shed sheaths, penetrate mosquito’s mid-gut and migrate to thoracic muscles
Develop into L1 larvae
Develop into L3 larvae
Migrate to head and mosquito’s proboscis

31
Q

How large are adult Wuchereria bancrofti?

A
  • Male – 4 cm long

* Female – 8-10 cm long

32
Q

What is the reason for the damage caused by Wuchereria bancrofti?

A

Adult worms live in lymphatic system. They produce microfilaria that collectively damage the lymphatic system

33
Q

What is nocturnal periodicity (with reference to Wuchereria bancrofti)?

A

Microfilariae travel around the bloodstream at night and hide in the lung capillaries by day

34
Q

When a patient has Lymphatic filiariasis, how often does Hydrocoele occur?

A

Hydrocoele (enlargement of the scrotum) occurs in 50% of men infected

35
Q

How is Lymphatic filiariasis diagnosed?

A

The standard method for diagnosing active infection is the identification of microfilariae in a blood smear by microscopic examination. The microfilariae that cause lymphatic filariasis circulate in the blood at night (called nocturnal periodicity). Blood collection should be done at night to coincide with the appearance of the microfilariae, and a thick smear should be made and stained with Giemsa or hematoxylin and eosin. For increased sensitivity, concentration techniques can be used.

Serologic techniques provide an alternative to microscopic detection of microfilariae for the diagnosis of lymphatic filariasis. Patients with active filarial infection typically have elevated levels of antifilarial IgG4 in the blood and these can be detected using routine assays. Binax® Now® card test can be used to give rapid result, and can be performed by untrained personnel

36
Q

How is Lymphatic filiariasis treated?

A

Albendazole (400mg single dose)
• Causes polymerisation of worm β-tubulin and microtubule formation

Other drugs can be used:
Ivermectin / diethylcarbamazine (DEC)
• 150 μg/kg / 6 mg/kg (Single doses)
• DEC/Ivermectin used in eradication campaigns
• Given for free by manufacturer
(They make their money as it is also used to treat
animal infections [lice, fleas])

• Doxycycline / Tetracycline can also be used

37
Q

How do DEC and Ivermectin treat Lymphatic filiariasis?

A

Both target the microfilariae (MF)
- Not adults (Usually combines with albendazole)

Ivermectin:

  • Hyperpolarises Glutamate sensitive chloride channels
  • Blocks nerve transmission & muscle function in the worm
  • Paralyses it

DEC – Mechanism unknown

38
Q

What is another name for Onchocerciasis?

A

River Blindness

39
Q

Describe the epidemiology for Onchocerciasis

A

Was the world’s second leading infectious cause of blindness, particular in:
• Africa (30 countries)
• Arabian peninsula
• Americas

Often associated with big river system

120 million people world-wide at risk:
• 96% in Africa
• 18 million infected
• 270 000 blind

40
Q

What organisms cause Onchocerciasis?

A
  • Onchocerca volvulus
  • Parasitic nematode
  • Produce microfilariae
41
Q

What are the symptoms of Onchocerciasis?

A

Serious visual impairment, including blindness
• Microfilariae penetrate the eye
• Sets up an immune response damaging the eye

 Elephantiasis:
• Lymphadenitis
• Resulting in elephantiasis of genitals - Hanging groin
• Damage to inguinal lymph vessel / node
• Bubo (buboes)
Skin problems
• Rashes, lesions, & intense itching
• Caused by microfiliariae
• Leopard skin (di-pigmentation)
• Lizard skin (wrinkled)
• Elephant skin (thickened)
• Nodules
42
Q

What is the vector involved in the transmission of Onchocerciasis?

A
  • Simulium spp - Black fly (Buffalo gnat)
  • Lay eggs in flowing water
  • Larvae attach to rocks
43
Q

Describe the life cycle of Onchocerca volvulus

A

Blackfly (genus Simulium) takes a blood meal (L3 larvae) enter bite wound
Larvae enter subcutaneous tissues
Adult worms develop in subcutaneous nodules
Adults produce microfilariae: found in skin and lymphatics
Blackfly takes a blood meal
Microfilariae penetrate blackfly’s mid-gut and migrate to thoracic muscles
L1 larvae develop
L3 larvae develop
L3 larvae migrate to head and blackfly’s proboscis

44
Q

How is Onchocerciasis treated?

A

Nodulectomy
• Removes adults therefore reducing number microfilariae released
African Programme for Onchocerciasis Control (APOC)
• Involves spraying insecticide into breeding sites
Donation of Ivermectin (Mectizan) by Merck & Co., Inc
• 2 doses 6 months apart repeated every 3 years
• Paralyses the microfilariae
• Stops progression towards blindness

45
Q

Is eradication of Onchocerciasis likely?

A 
Eradication has looked likely, however:
• Recent study from Ghana published in the Lancet
2007 suggested resistance is emerging
• Confirmed in 2011 in PloS
• Female worms are becoming resistant

Medical Microbiology (13 decks)

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