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SEM 4: Medical Microbiology > Pathogenesis of Parasitic Infections > Flashcards

Pathogenesis of Parasitic Infections Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

What are the three main schistosomiasis species?

A
  • Schistosoma mansoni
  • Schistosoma haematobium
  • Schistosoma japonicum
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2
Q

Schistosoma mansoni

A

Affects the hepatic and intestinal system but only found in Latin America

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3
Q

Schistosoma haematobium

A

Affects the urinary tract found in sub-saharan Africa but also find S. Mansoni

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4
Q

Schistosoma japonicum

A

Affects the hepatic and intestinal system but only in Asia.

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5
Q

Schistosoma Lifecycle

A
  1. Eggs shed from infected humans.
  2. Eggs hatch and release miracidia.
  3. Miracidia penetrate snail tissue.
  4. Sporocysts develop in snail (successive generations).
  5. Free-swimming cercariae released from snail into water.
  6. Cercariae penetrate skin.
  7. Cercariae lose tails during penetration and become schistosomulae.,
  8. It enters circulation.
  9. Migration to protal blood in liver and maturation into adults.
  10. Paired adult worms migrate to faeces and urine.
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6
Q

What causes cercarial dermatitis?

A

Occurs when exposure to cercariae from animal or bird schistosomes. It requires pre-sensitisation. Allergic-type reaction to the presence of cercariae in water source. This causes cercarial dermatitis.

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7
Q

What is the key feature of the immune response of cercarial dermatitis?

A

Granuloma formation

  • This is T2 delayed type hypersensitivity
  • Eggs become organised in granulomas
  • Repeated insults and tissue repair leads to fibrosis and organ damage.
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8
Q

Hepato-intestinal schistosomiasis

A
  • Infections with s. mansoni and s. japonicum
  • Pathology caused by immune response to eggs
  • The adults are in the mesenteric vessels, the female releases thousands of eggs and these go to the intestines through the capillaries. They are pushed by the immune response through the intestinal wall, the mucosa, and then excreted.
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9
Q

How is liver damage caused by schistosoma?

A
  • Chronic exposure causes a loss of inflammation repair and fibrosis.
  • Also, pipestem occurs which is when fibrosis occurs - this is typical of advanced schistosoma liver disease.
  • Also get hepatomegaly and splenomegaly.
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10
Q

How is urinary schistosomiasis caused?

A

The adult parasites live in the vessels around the bladder and release eggs into the bladder. These are then pushed through the mucosa of the bladder and secreted into the urine.

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11
Q

When does haematuria occur in urinary schistosomia?

A

Peeing blood is common in adolescence. Almost seen as a sign of ageing in some countries.

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12
Q

Bladder pathology in urinary schistosomiasis

A

Due to inflammation in the bladder wall, due to the eggs, there is damage to the bladder wall. This can lead to the development of cancer. Bladder cancer is common in patients with urinary schistosomiasis.

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13
Q

What is onchocerciasis?

A
  • Also called river blindness which used to be a major cause of blindness particularly in West Africa.
  • Caused by filarial parasite (onchocerca volvulus) - can be up to 50 cm long but is coiled up.
  • Transmitteed by blackflies.
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14
Q

Geographical distribution of onchocerciasis

A

Equatorial regions of Africa and Central and South America

Largely controlled in South America

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15
Q

Life cycle of onchocerciasis helminths

A
  1. Blackfly bites you and transmits infectious larvae
  2. Larvae develops under the skin into adults.
  3. Female and male mates and the female releases thousands of larvae.
  4. These produce microfilariae that is found in skin and lymphatics of connective tissue.
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16
Q

What is the vector for onchocerciasis?

A

Simulium which feeds on skin.

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17
Q

What is the pathology of onchocerciasis?

A
  • Repeated episodes of inflammation to the presence of microfilariae leads to permanent damage and scarring in skin and eyes.
  • In the eyes, it can cause blindness
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18
Q

How does the parasite affect the host?

A
  • Larvae in the dermis of the skin; the parasite is actively downregulating the host immune response.
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19
Q

How is the downregulation of the parasite switched off?

A

By giving treatment such as diethylcarbamazine. There is a strong inflammatory response, with lots of eosinophils. There are eosinophil abscesses which are killing the microfilariae.

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20
Q

Why does the microfilariae in onchocerciasis cause a rash?

A

Rash due to a strong response to the killing of the microfilariae.

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21
Q

What are onchocercal nodules? What happens when you remove the nodules?

A

Adult females in fibrosis nodules under the skin. When you remove the nodules and dissect will show females under the skin. The males migrate between the nodules, fertilising the females.

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22
Q

Acute papular onchodermatitis

A

Looks like a typical rash

23
Q

Chronic onchodermatitis

A

Repeated inflammation over periods of time leads to damage to the elastin and collagen in the skin. This causes presspadermia (aged skin).

24
Q

Skin disease caused by onchocerciasis

A
  • Acute papular onchodermatitis
  • Chronic onchodermatitis
  • Sowda
25
Q

Anterior segment eye disease caused by microfilariae from onchocerciasis

A
  • Punctate keratitis - invasion of the cornea; microfilariae is being killed by inflammatory response.
  • Acute indocyclitis
  • Sclerosing keratitis - pacification of the cornea, occlusion of the cornea with blindness
26
Q

Posterior segment eye disease caused by microfilariae from onchocerciasis

A
  • Optic neuritis/atrophy - microfilariae in the optic nerve, sheathing of the retinal vessels due to inflammation. This is post-inflammatory optic atrophy which is an important cause of blindness.
  • Chorioretinopathy - affects the retina, small white spots in the eye where microfilariae is being killed by inflammatory response. There is damage to the retina. Underneath the retina, there is retinal pigment epithelium, if this dies, then the overlying neural cells die also. Choroid is underneath the retina will be seen.
27
Q

The immunopathogenesis of onchocerciasis in acute disease

A
  • Causes rapid allergy reactions to microfilariae dying in the skin.
  • Activation of mast cells, and other immune cells such as eosinophils to get eosinophilic abscesses.
  • Strong Th2 response with production of IL-4 to make IgE and IL-5 to recruit and activate eosinophils.
28
Q

The immunopathogenesis of onchocerciasis in chronic disease

A
  • The immune response starts to be shut down.
  • Modified Th2 response with the production of IL-10.
  • Production of Treg cells that are important in immune regulation.
  • Antibodies such as IgG4 which also have a role in immune regulation such that someone with chronic infection has no apparent response to the presence of microfilariae in their skin.
29
Q

Action of tics

A

Stick mouth parts in the skin which releases ‘cement’ to stick onto the skin. It feeds off blodd. These are important vectors of a large number of different infections.

30
Q

Disease transmitted by hard ticks

A

When filled with blood, the abdomen expands.

  • Tick typhus
  • Viral encephalitis
  • Viral fevers
  • Viral haemorrhagic fevers
  • Tularemia
  • Tick paralysis
  • Human babesiosis
31
Q

Diseases transmitted by soft ticks

A

Q fever

Relapsing fever

32
Q

How does ticks cause tick paralysis?

A

Progressive flaccidity due to a failure of acetylcholine liberation in the neuromuscular junction. Tick’s toxin produces a block in the motor nerve fibres.

33
Q

What type of diseases of hard-bodied ticks (ixodidae)?

A

Carriers of rickettsial, spirochaetal, viral, bacterial, and protozoan diseases.

34
Q

What type of soft-bodied ticks (ornithodorus)?

A

Vectors of endemic relapsing fever (caused by borrelia duttoni) and Q-fever.

35
Q

Lyme disease

A

A bacterial infection caused by ticks.

36
Q

Geographical distribution of ticks

A

Ticks are found worldwide

37
Q

Action of head lice

A
  • Suck blood from scalp and lay eggs on hair

- Common and easily spread by close contact, sharing of combs, brushes, hats etc.

38
Q

Action of body lice

A
  • Suck blood from body and lay eggs on clothing
  • Uncommon and spread bodily contact, sharing of clothing or bedding
  • Vector diseases (epidemic typhus, trench fever, relapsing fever)
  • For example, in Ethiopia - where people will share bedding etc
39
Q

What increases the chance of sucking lice diseases transmission?

A

Lousiness related to sanitation:

  • Crowded conditions
  • Long periods without bathing or changing clothes
40
Q

Pthiridae

A

Crab lice, pubic lice

  • Broad, flat lice that appear crab-like
  • Mid and hind legs are stout with very large claws
  • Abdominal segments have distinct lateral lobes
41
Q

Pthirus pubus

A

Single species confied to human pubic region

  • Bites cause irritation and typical rash
  • Spread by close body contact (usually sex)
  • No diseases
42
Q

Lifecyle of the dermatobia hominis (botfly)

A
  1. Lays an egg on the mosquito
  2. Mosquito plants egg onto a host animal
  3. Larvae is laid
43
Q

Geographical distribution of the dermatobia hominis

A
  • Mostly found in central and Latin America.
  • More common in the tropical part of the country but also found in the highlands
  • Myiasis is what occurs in people with the larvae
44
Q

Medical action of the botfly

A
  • The botfly sticks the body inside the lesion, then the head sticks out of the top.
  • It tends to be removed.
  • It sticks its head out as it needs air.
  • It needs to be cut out or will die.
  • It has spines which will stick to the tissue.
  • It will usually fall out though.
45
Q

Drugs that control protozoa infections

A
  • Tinidazole: shorter dose regiments
  • Metronidazole: more adverse reactions; 1-2 week course
  • Nitazoxanide
  • Benznidazole - chagas disease used but causes very adverse reactions
  • Heavy metals (meglumine antimoniate)
46
Q

Drugs that control helminths infections

A
  • Albendazole/Mebendazole: single dose, very effective.
  • Praziquantel: effective treatment for trematodes, flukes
  • Ivermectin
  • Diethylcarbamazine
  • Pyrantel
47
Q

Drugs that control ectoparasite infections

A
  • Ivermectin: effective for most ectoparasites; kill lice, botfly etc; single dose
  • Benzyl/malathion lotions: put on shampoos to treat head lice for example, however not as effective.
48
Q

How can behaviours control parasite infections?

A
  • Education to help avoid infections

- Hand washing and hygiene behaviours

49
Q

What environmental interventions can be used to control infections?

A
  • Spraying of residual insecticides for household vectors - for example prevention of chagas and malaria
  • Mosquito nets for malaria
  • Improved housing - problem in some parts of latin america, the bugs will live in the trees and come down to the house at night so spraying the house might not be as effective.
  • Sewage disposal and potable water
  • Drainage of swamps: to reduce chance of malaria
50
Q

What are poverty reduction can be used to control infections?

A

Micro-financing etc

51
Q

Why does treatment need to be periodical?

A

Needs to be given over long periods of time because re-infections are rapid or because the treatment kills larval rather than adult stages.

52
Q

Control of STH infections

A

A single dose of albendazole is given to high risk groups such as school children up to every 4 months to control STH infections.

53
Q

Control of Onchocerciasis in endemic communities

A

A single dose of ivermectin (only kills the larvae not the adults) is given to endemic communities (mass drug administration) every 6 or 12 months to control onchocerciasis for 15 years or so.

54
Q

Control of schistosomiasis

A

A single dose of praziquantel is given to endemic communities (mass drug administration) every 6 or 12 months to control schistosomiasis.

SEM 4: Medical Microbiology (15 decks)

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