Pathogenesis of Parasite Infections Flashcards
What are the three main species of Schistosomiasis and where are each found?
These are helminths
Three main species
- Schistosoma mansoni- found generally in parts of Latin America and sub-Saharan Africa
- S. haematobium- found in Africa and parts of the Middle East
- S. japonicum- found in far East Asia
What is the life cycle of schistosomiasis like?
You become infected by cercariae in contaminated water where the infected intermediate host, the snails, release thousand of cercariae
The cercariae will borrow through the skin and migrate through the systemic circulation and eventually either to the portal venous system around the liver and intestine for mansoni and japonicum or around the bladder, the vesicle plexus for haematobium
The adults will live in this region then release eggs and these will embolise in the capillaries and eventually will push through the mucosa through into the lumen of the bladder or the intestine where they are excreted in the urine or faeces
How does schistosomiasis cause cercarial dermatitis?
Exposure to cercariae from animal or bird schistosomes
Requires pre-sensitization
Allergic-type reaction
- The cercariae burrow into the skin and is met through a strong host immune response leadign to these rash-like lesions
What is the key feature of the immune response against schistosomiasis?
Key features of immune response is granuloma formation
Caused when there is backflow and the eggs go back to the liver instead of the intestine and that causes granuloma formation
Eggs become organized in granulomas particularly TH-2 granulomas
- As the egg is captured by TH2 cytokines such as IL13 and 4 and eosinophils
Repeated insults and tissue repair leads to fibrosis and organ damage
What happens in urinary schistosomiasis?
The first clinical manifestation of a urinary schistosomiasis would be haematuria; urinating blood
Within the bladder, urinary schistosomiasis can cause long term damage with repeated episodes of inflammation which can result in neoplastic changes in the bladder and its mucosa
You can get schistosomal papillomas and tubercules and nodular carcinomas
What is onchocerciasis and where is it found?
Major blinding disease
Caused by filarial parasite (Onchocerca volvulus )
Transmitted by blackflies
It used to be highly endemic in parts of Americas and equatorial regions of Africa
What is the life cycle of onchocerciasis like?
Transmitted by a blackfly which takes a blood meal and transmits the L3 larvae into the skin which will migrate through the skin and develop into adults
The females are very long and wound up in fibrous nodules into the skin and the males will go from female to female fertilising them
The females will release thousand of microfilariae into the kin which will infest the skin and also get into the eyes
These are taken up by the black flies in the blood meal
Pathology:
Repeated episodes of inflammation to presence of microfilariae leads to fibrosis and permanent damage and scarring in skin and eyes
What is the vector of onchocerciasis?
The vector; Simulium
Digs a hole in the skin and leaves a pool of blood and sucks form the pool of blood when it feeds
What are the skin and eye onchocerciasis disease features?
Skin disease - Acute papular onchodermatitis- where microfiliariae have been killed by the immune response - Chronic onchodermatitis-with repeated inflammation you get damage to elastin of skin leading to an ageing look of the skin - sowda Eye disease - Anterior segment: · Punctate keratitis · Acute iridocyclitis · Sclerosing keratitis - Posterior segment: · Optic neuritis/atrophy · Chorioretinopathy
How does punctate keratitis occur in those with onchocerciasis?
Eye becomes infested with microfilariae, you can see small white spots in the eye
Those pale spots represent where the microfilariae have invaded the corneal stroma and become immobilised by inflammatory infiltrates around it
With repeated episodes you end up with pacification of the cornea
How does chorioretinopathy occur in those with onchocerciasis?
Damage to the pigment epithelium, which is crucial for maintaining the metabolic requirements of the overlaying neural retina
So if the pigment epithelium dies the neural retina dies
Describe the immunopathogenesis of onchocerciasis.
In early disease, with punctate keratitis, initially there is this TH2 immune response with activated eosinophils
It activates B cells to produce IgE
And there is a lot of inflammation
With chronic infections, this system shuts down and you get alternatively activated macrophages, you get modified TH2 cells and adaptive TReg cells all of which serve to suppress this inflammatory response
Instead of producing IgE you produce IgG4 and you get suppression of eosinophils by blocking IL-5
What are the two variants of ticks and what does each transmit?
Hard ticks which transmit: - Tick typhus - Viral encephalitis - Viral fevers - Viral haemorrhagic fevers - Tularaemia - Tick paralysis Soft ticks which can transmit: - Q fever - Relapsing fever
Why are ticks medically important?
They can cause:
Mechanical injury by the bite
Tick paralysis- progressive flaccidity due to failure of acetylcholine liberation in the neuromuscular junction
Tick’s toxin produces a block in the motor nerve fibres
Hard-bodied ticks are carriers of rickettsial (such as Lyme disease), spirochaetal, viral, bacterial and protozoan diseases
Soft bodied ticks are vectors of endemic relapsing fever and Q fever
What are the different types of lice?
Headlice:
- Suck blood from scalp and lay eggs on hair
- Common and easily spread by close contact, sharing of combs, brushed, hats etc.
Body lice:
Suck blood from body and lay eggs on clothing
Uncommon and spread by bodily contact, sharing of clothing or bedding
Vector disease (epidemic typhus, trench fever, relapsing fever)
