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2 types of hosts with regards to life cycles
- Definitive: in which parasite reaches sexual maturity/ sexual reproduction occurs
- Intermediate: required for parasite development, but parasite does not reach sexual maturity
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3 major endoparasitic groups
- protozoa (reproduce in host)
- metazoa (no direct replication in definitive host)
- helminths
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4 types of protozoa and their diseases... where does reproduction happen?
- Leishmania: leishmaniasis
- Plasmodium: malaria
- Trypanosomes: sleeping sickness
- Babesia: tick borne diseases in cattle
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3 groups of helminths
- nematodes (ascaris, hookworms, filarial worms)
- trematodes (schistosomes)
- cestomes (tapeworms)
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3 groups of nematodes
- Filarial nematodes: Wuncheria bancrofti, Onchocerca volvulus
- Ascaris (lumbricoides)
- Hookworms (Necator americanis)
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why is investigating life cycles so important in parasitology?
Numerous developmental forms may have different pathologies, have different immune evasion mechanisms, be susceptible to different control measures
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2 types of life cycles of parasites
- Direct: single host species
- Indirect: multiple host species eg malaria with mosquito vector
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What is Ascaris lumbricoides? Size? Life span? Egg production?
Hookworm (nematode), 15-35cm, 1 year, ~200,000 eggs produced per day
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Describe life cycle of Ascaris
- Adults in small intestine
- Eggs laid and passed out in faeces
- Infective larvae within eggs
- Ingestion
- Larvae hatch and penetrate villi of small intestine into portal circulation
- Arrive at lungs, mature, penetrate alveoli, coughed up, swallowed
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What type of immune response to intestinal helminths provoke?
- Th2
- IL3, 4, 5: increased IgE, eosinophils, mastocytosis
- Increased mucus and smooth muscle contraction -> expulsion of worms
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How does Ascaris cause damage to host? (7)
- Intestinal blockage (fatal with several 100 worms)
- Lung damage (Eg pneumonia due to infection of pools of blood caused by burrowing)
- Abdominal pain, nausea, vomiting
- Protein and fat deficiency
- Impaired Vitamin A absorption
- Lactose intolerance
- Allergic reactions to worm metabolites: rashes, eye pain, asthma
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Describe resilience of ascaris
- As infective larvae within eggs, can survive in soil for over 10 years
- Sensitive to UV light (eg sunlight)
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Example of Hookworm, size
Necator americanis, 1cm
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Hookworm life cycle
- Adults in small intestine, eating blood
- Eggs passed in faeces
- Eggs HATCH to release free living larvae
- Infective larvae enter host by skin penetration
- Migrate to lungs, coughed up, swallowed
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What do hookworms feed on?
Blood
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What does Ascaris feed on?
Liquid food in small intestine
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How long can hookworms live? Egg production?
- Up to 15 years
- around 10,000 eggs produced per day
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How do hookworms cause damage to host?
- Laceration of small intestine: up to 200ml blood lost per day - anaemia
- Abdominal pain, loss of appetite
- Protein and iron drain: can cause catastrophic malnutrition in children with stunted growth and impaired cognitive development
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6 things hookworms do to enable survival in host
- Induce apoptosis in T cells
- Release factors inc Superoxide dismutase (prevent ROS destruction), AchE (anti-inflammatory), proteases
- Inhibit neutrophil migration
- Produce anti-coagulant molecules so that blood does not clot as they feed
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2 types of filarial nematodes and the diseases they cause, sizes
- Wuncheria bancrofti: elephantiasis, 4-10 cm
- Onchocera volvulus: river blindness, 20-50 cm
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Life cycles of filarial nematodes. Differences between the 2 types.
- INDIRECT (Wb = wuncheria bancrofti, Ov = onchocera volvulus)
- Wb in lymph nodes, Ov in subcutaneous nodules
- Microfilaria released into blood (Wb) or skin (Ov)
- Transmission via mosquito (Wb) or blackfly (Ov)
- Infective larvae enter wound during feeding
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What parasite causes elephantiasis? Describe pathology
- Wuncheria bancrofti
- Live in lymph nodes, cause inflammatory response
- Lymph return is obstructed
- Progressive infiltration of fibrous connective tissue eg in scrotum, legs, arms
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What parasite causes river blindness? Describe pathology
- Onchocera volvulus
- Microfilaria degenerate in skin: elephantiasis, dermatitis, blindness (fibrous tissue in eye causing sclerosing keratitis)
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Describe bacterial symbiosis with nematodes
Wolbacteria: use antibiotics to kill bacteria, nematodes cannot reproduce and also die
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Describe life cycle of Toxoplasma gondii
- T. gondii in gut epithelial cells of cat
- Oocysts shed in faeces
- DIRECT: ingestion by cat
- INDIRECT: ingestion by intermediate host (human, mouse, rat, cow, sheep, pig)
- Cysts develop due to immune response to parasites in tissues
- Can cross placenta and damage foetus
- Tissues eaten by cat
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Describe T. gondii cell invasion
- Active process
- T. gondii enclosed in parastophorous vacuole: host proteins excluded from membrane to prevent degradation
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2 ways in which T. gondii avoids immune system
- Parasitous vacuole with host proteins excluded
- Interference with NF-kB translocation to nucleus
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Pathogenesis in T. gondii
- Cysts in heart, brain, CNS, skeletal muscle - can be asymptomatic for years
- Immunosuppression: dissemination: ocular toxoplasmosis and fatal CNS disorders
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4 public health measures to control parasitic diseases
- proper sewage disposal
- food
- water
- education
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3 methods of vector control
- drainage schemes
- spraying
- bite avoidance
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