Medical Microbiology: Pathogenesis of Parasitic Infections Flashcards
What is Leishmaniasis?
- A parasitic disease caused by Leishmania parasites
What are the different types of Leishmaniasis?
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Visceral leishmaniasis
- Asia: Leishmania donovani
- Middle East/Africa/Asia: L. infantum variants
- Latin America: L. chagasi
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Cutaneous Leishmaniasis
- Old world disease: Mediterranean/Middle East - L. Infantum/L. major/L. tropica
- New world disease: Central and South America - L. braziliensis/amazonensis/mexicana
Describe the life cycle of Leishmania
- Sand fly bites you and transfers promastigote into the body
- Promastigotes invades macrophages
- Inside macrophage promastigotes form nests of amastigotes
- This causes macrophage to burst resulting in release of amastigotes which go on to infect other cells
What is the vector for leishmaniasis?
- Lutzomyia/Phlebotomus
What causes diffuse cutaneous leishmaniasis?
- Occurs as a result of someone not being able to produce an adequate immune response to the parasite
Describe the pathogenesis of cutaneous leishmaniasis
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Acute lesions
- Due to tissue damage caused by inflammatory response to presence of parasites in macrophages
- Parasite killed by Th1 pro-inflammatory responses and macrophages
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Latency
- Regulatory immune response which causes parasites to remain dormant long-term
- Characterized by balance of Th1 and anti-inflammatory responses
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Relapse (rare)
- Alteration in immune response e.g. change in Th1 vs immune regulation due to HIV infection
- Mucocutaneous disease associated with strong but inadequate inflammatory response to parasites that have metastasized to mucosa
- Diffuse cutaneous leishmaniasis associated with uncontrolled parasite replication
What are the 3 main species of Helminths that can cause Schistosomiasis?
- Schistosoma mansoni
- Schistosoma haematobium
- Schistosoma japonicum
Decribe the life cycle of Schistosoma
- People exposed to infective stage in contaminated water
- Get infected with cercariae which migrate through the body
- Cercariae fbecome adults in mesenteric system
- Female and male mate and female releases eggs into mucosal epithelium
- Eggs get excreted either through faeces or urine
What is cercarial dermatitis?
- Allergic type reaction caused by exposure to cercariae from animal or bird schistosomes
- Requires pre-sensitization
What is a key feature of the immune response to Schistosomiasis?
- Granuloma formation - Eggs from female become organized in granulomas
- Repeated insults and tissue repair leads to fibrosis and organ damage
What is Hepato-intestinal schistosomiasis?
- Schistosomiasis infection caused by S.mansoni and S. japonicum
- Pathology caused by immune response to eggs - eggs are pushed through intestinal wall and mucosa and are then excreted
- This causes hepatosplenomegaly
What diseases/conditions can urinary schistosomiasis cause?
- Haematuria - blood in the urine
- Can also lead to bladder cancer due to inflammation of bladder wall caused by eggs
What is onchocerciasis?
- Major blinding disease caused by filarial parasite (Onchocerca volvulus)
- Transmitted by blackflies
Describe the life cycle of Onchocerca volvulus
- Blackfly bites you and transmits infectious larvae
- Larvae migrates under the skin and develops into adults
- Male and female adult mate and female releases 1000s of larvae called microfilariae
- Microfilariae get taken up by blackfly and develop in blackfly until they can be trasnmitted themselves
What is the vector of Onchocerca volvulus?
- Simulium
Describe the pathology of Onchocerciasis
- Repeated episodes of inflammation due to presence of microfilariae leads to permanent damage and scarring in skin and eyes
What diseases/conditions are caused by Onchocerciasis?
- Onchocercal nodules - only problematic if around joints
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Skin disease
- Acute papular onchodermatitis
- Chronic onchodermatitis
- Sowda
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Eye disease
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Anterior segment (iris)
- Punctate keratitis
- Acute iridocyclitis
- Sclerosing keratitis
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Posterior segment (retina)
- Optic neuritis/atrophy
- Chorioretinopathy
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Anterior segment (iris)
What is punctate keratitis?
- Condition caused by death of small groups of cells in the cornea as a result of Microfilariae being killed by immune response
What is sclerosing keratitis?
- Condition caused by repeated inflammation of the cornea
- This results in opacification of the cornea causing blindness
Describe the immune response that occurs due to onchocerciasis
- Rapid allergic reactions that kill microfilariae in skin
- Activation of mast cells which recruit other immune system cells e.g. eosinophils
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Acute stage
- Strong TH2 response which produces IL-4 and IL-5
- IL-4 leads to IgE production and IL-5 leads to recruitment and activation of eosinophils
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Chronic stage
- Host immune response starts to be regulated and shut down
- Modified TH2 response - production of IL-10, TReg cells and IgG4 antibodies
How do ticks infect their hosts?
- Tick sticks its mouth into skin and releases “cement” which keeps it in the skin
- Once stuck in skin it feeds off blood
- Tick also releases toxins into body which produces a block in motor nerve fibres
What are some of the diseases transmitted by hard ticks?
- Tick typhus
- Viral encephalitis
- Viral fevers
- Viral haemorrhagic fevers
- Tick paralysis
What are some of the diseases transmitted by soft ticks?
- Q fever
- Relapsing fever
What are some characteristics of head lice?
- Suck blood from scalp and lay eggs on hair
- Easily spread by close contact, sharing of combs and brushes etc.
What are some characteristics of body lice?
- Suck blood from body and lay eggs on clothing
- Spread via bodily contact, sharing of clothing or bedding
- Can cause vector diseases such as:
- Epidemic typhus
- Trench fever
- Relapsing fever
What are some characteristics of pubic lice?
Describe the life cycle of the botfly (Dermatobia hominis)
- Mid-flight it grabs a mosquito and lays its eggs on the mosquito
- Mosquito goes on to bite animal, the change in temperature causes eggs to hatch and larvae to invade the skin
What is Myiasis?
- Parasitic infestation of the body of a live animal by Botfly larvae (maggots) which grow inside the host while feeding on its tissue
What drugs can be used to treat infections caused by different types of parasites?
- Tinidazole has shorter dose regimens compared to other drugs used to treat protozoa infections - 1g once a day for 3 days
- Metronidazole has more adverse reaction and has longer dose course - 1 or 2 week course
- Benznidazole used to treat chagas disease but causes very bad adverse reactions so people can’t complete course
- Heavy metals used to treat Leishmaniasis
How can parasitic infections be controlled?
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Behaviours
- Education
- Introducing Hand washing and hygiene behaviours
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Environmental interventions
- Spraying of residual insecticides for household vectors
- Mosquito nets for malaria
- Improved housing
- Sewage disposal and potable water
- Drainage of swamps
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Poverty reduction
- Micro-financing
Why must treatment of parasitic infections in endemic settings be given periodically over long periods of time?
- Because re-infections are rapid and because the treatment kills larval rather than adult stages
Give some examples of treatments of parasitic infections given periodically over a long period of time
- A single dose of albendazole is given to high risk groups such as school children up to every 4 months to control STH infections
- A single dose of ivermectin is given to endemic communities (mass drug administration) every 6 or 12 months to control onchocerciasis
- A single dose of praziquantel is given to endemic communities (mass drug administration) every 6 or 12 months to control schistosomiasis
