(4) Introduction to parasites Flashcards
What is a parasite?
An organism which lives in or on another organism (its host) and benefits by deriving nutrients at the other’s expense
Do parasites always cause disease?
Not necessarily
The parasite derives all benefits from the association and the host may either be harmed or may suffer the consequences of this association (a parasitic disease)
What is a host?
An organism which harbours the parasite, may be animals or human
What is symbiosis?
Living together; close, long-term interaction between two different species
Give 3 types of symbiosis
- mutualism
- parasitism
- commensalism
What is mutualism?
An association in which both species benefit from the interaction eg. in termites
What is parasitism?
An association in which the parasite derives benefit and the host gets nothing in return but always suffers some injury
What is commensalism?
An association in which the parasite only is deriving benefit without causing injury to the host (rarely occurs)
Give 3 different types of host
- definitive host
- intermediate host
- paratenic host
What is a definitive host?
Either harbours the adult stage of the parasite or where parasite utilises the sexual method of reproduction
What is the definitive host in the majority of human parasitic infections?
Man
What is the intermediate host?
Harbours the larval or asexual stages of the parasite
Some parasites require 2 intermediate hosts in which to complete their life cycle
What is the paratenic host?
Host where the parasite remains viable without further development
What are the 2 classifications of parasite
- protozoa
- helminths
What are protozoa?
Micro-parasites (small)
What are helminths?
Macro-parasites (large, worms)
Give an example of a flagellate protozoa
Giardia lamblia
Give an example of an amoeboid protozoa
Entamoeba sp
Give an example of a sporozoan protozoa
Plasmodium sp
Cryptosporidium sp
Give an example of a trypanosome protozoa
Leishmania sp
What are the 2 major types of helminths?
- platyhelminths (flatworms)
- nematodes (round worms)
What are the 2 types of flatworm?
- cestodes (tape worms)
- trematodes (flukes)
What are the 2 types of round worm?
- intestinal nematode
- tissue nematode
Give an example of a cestode (tape worm)
Taenia sp
Give an example of a trematode (fluke)
Schistosoma sp
Give an example of an intestinal nematode
Ascaris lumbricoides
Give an example of a tissue nematode
Wuchereria bancrofti
What are 3 types of parasitic life cycles
- direct
- simple indirect
- complex indirect
What is a direct life cycle characterised by?
One host
Give an example of a direct life cycle
- host = bird
- infective embryonated eggs are eaten by bird
- bird sheds parasite eggs into environment in faeces
- eggs mature in environment and become infective
- infective embryonic eggs are eaten by bird
What is an indirect life cycle characterised by?
Two hosts
Give an example of a simple indirect life cycle
- bird sheds parasite eggs into environment in faeces
- sowbug eats eggs of parasite (intermediate host)
- eggs hatch in sowbug and infective larvae develop within sowbug
- bird eats sowbug and becomes infected
How is a complex indirect life cycle different from a simple indirect life cycle?
There are more intermediate hosts
Give an example of a complex indirect life cycle
- bird sheds parasite eggs into the environment in faeces
- eggs eaten by amphipod (first intermediate host) where first and second stage larvae develop
- amphipod is eaten by amphibian (second intermediate host) where infective stages of larvae develop
- bird feeds on amphibian and becomes infected to complete life cycle
Give an example of when a paratenic host might be part of the life cycle
Fish (paratenic host) eats the amphibian and larvae encyst in body of fish - no further development of the parasite
(the fish gets eaten by bird and life cycle is completed)
What happens in the definitive host?
The sexual reproductive stage
Ascariasis, schistosomiasis and hydatid disease are all what?
Diseases caused by helminths
Malaria and cryptosporidiosis are both what?
Diseases caused by protozoa
What type of parasite causes ascariasis?
Macroparasite - intestinal nematode
Ascaris lumbricoides
Describe the prevalence of ascariasis
More than 1 billion people affected
Peak prevalence in 3-8 year olds
Common in areas of poor hygiene
South-east Asia mainly
How many eggs can one adult ascaris lumbricoides worm produce in one day?
200,000 eggs per day
How is ascaris lumbricoides acquired?
By ingestion of eggs
What type of life cycle does ascaris lumbricoides have?
Direct
Describe the life cycle of ascaris lumbricoides
Direct life cycle
- eggs shed in faeces
- develop in environment
- ingest eggs through faecally contaminated food
- passes into intestines
- migrates into portal circulation
- transported to lungs
- swallow it and passes back into out intestines
How many hosts are there in the life cycle of ascaris lumbricoides?
Just one (the definitive host)
What are the 2 clinical phases of ascariasis?
- lung migration phase
- intestinal phase
What symptoms does the lung migration phase of ascariasis cause?
Loefflers syndrome
Dry cough, dyspnea, wheeze, haemoptysis, eosinophilic pneumonitis
What symptoms does the intestinal phase of ascariasis cause?
- malnutrition
- migration (into hepatobiliary tree and pancreas)
- intestinal obstruction
- worm burden
What is a common sign of all helminth infections?
Raised eosinophils
How is ascariasis diagnosed?
Can actually see the eggs and the worms
How is ascariasis treated?
Albendazole
- benzimidazole
- prevents glucose absorption by worm
- worm starves
- detaches
- passed PR
How is the spread of ascariasis being controlled?
WHO “action against worms”
- improve sanitation
- education
- community targeted deworming
What is schistomiasis caused by?
Macro-parasite (helminth - platyhelminth (flatworm) - trematode/fluke)
What is schistomiasis also known as?
Bilharzia disease
Katayama fever
Swimmer’s itch
How many people are affected by schistomiasis?
200 million worldwide
predominantly in Africa
What are the different species of Schistosoma?
S. haematobium S. mansoni S. intercallatum S. japonicum S. mekongi
What does schistosomiasis result in?
Causes chronic disease resulting in bladder cancer and liver cirrhosis
What is the intermediate host in schistosomiasis?
Snails
What is the 10-stage life cycle of Schistosoma?
- schistosoma in faeces or urine
- eggs hatch releasing miracidia
- miracidia penetrate snail tissue
- sporocysts in snail (successive generations)
- cercariae released by nail into water and free-swimming
- penetrate skin
- cercariae lose tails during penetration and become schistosomulae
- circulation
- migrate to portal blood in liver and mature into adults
- paired adult worms migrate to: mesenteric venues of bower/rectum
Where else can the eggs of schistosoma have effects in distant sites?
- spine
- lung
S. haematobium is which type of schistosoma?
Urinary
S. mansoni, S. intercallatum, S. japonicum and S. mekongi are which type of schistosoma?
Hepatic/intestinal
What clinical features can urinary schistosomiasis cause?
- haematuria
- bladder fibrosis and dysfunction
- squamous cell CA bladder
What clinical features can hepatic/intestinal schistosomiasis cause?
- portal hypertension
- liver cirrhosis
How is urinary schistosomiasis diagnosed?
- terminal stream microscopy
- serology
How is hepatic/intestinal schistosomiasis diagnosed?
- stool microscopy
- rectal snip microscopy
- serology
What drug is used to treat schistosomiasis?
- praziquantel (parazinoisoquinoline derivative)
As well as treatment of long-term complications (more complicated)
What dosage of praziquantel is used to treat schistosomiasis?
40-60mg/kg with food
3 doses
8- hourly
How does praziquantel work to treat schistosomiasis?
Mechanism unknown
- increased ionic permeability tetanic concentration, detachment, death
How is praziquantel metabolised?
Well absorbed
Extensive 1st pass metabolism
Inactive metabolites excreted in urine
How is schistosomiasis controlled?
- chemical treatment to kill snail intermediate hosts
- chemoprohylaxis
- avoidance of snail-infested waters
- community targeted treatment, education and improved sanitation
What type of parasite causes Hydatid disease?
Macro-parasite (platyhelminth (flatworm) - cestode (tapeworm))
Humans are accidental hosts of Hydatid disease. What are the usual hosts?
Dogs = definitive host
Sheep, goats etc = intermediate host
Where is Hydatid disease found?
All over the world, wherever sheep are farmed
What is hydatid disease caused by?
Echinococcus sp. (E. granulosus - cystic and E. multilocularis - alveolar)
Where are the cysts in Hydatid disease found?
70% liver
20% lungs
Give some clinical facts about Hydatid disease
- may remain asymptomatic for years
- mass effect
- secondary bacterial infection
- cysts rupture - hypersensitivity
How is hydatid disease diagnosed?
- imaging
- serology
How is hydatid disease control?
- regularly worm dogs to reduce egg production
- hand hygiene
- safe disposal of animal carcasses/products of conception
What type of parasite is malaria caused by?
Micro-parasite
protozoa - sporozoan - plasmodium
What are the 4 species of Plasmodium (causing malaria)?
- P. falciparum
- P. vivax
- P. ovale
- P. malariae
What is the epidemiology of malaria?
- 300-500 million people infected every year
- 1-3 million deaths per year, mainly in young children
- 2000 cases per year in the UK
There are around 2000 cases of malaria per year in the UK. Which species are 70% of these cases caused by?
P. falciparum
What acts as a vector for malaria?
Anopheles (a genus of mosquito)
What do the malaria parasites do?
Rupture red cells
Block capillaries
Cause inflammatory reaction
What are the clinical signs and symptoms of malaria?
- fever and rigors
- cerebral malaria (confusion, headache, coma)
- renal failure (black water failure)
- hypoglycaemia
- pulmonary oedema
- circulatory collapse
- anaemia, bleeding and DIC
Fever and rigors are a symptom of malaria. How is this different in different types of malaria?
Fever and rigor = alternative days with falciparum malaria, every 48 or 72 hours with benign malaria
What is black water fever?
Complication of malaria - red blood cells burst in the bloodstream (haemolysis), releasing hemoglobin directly into the blood vessels and into the urine - leading to kidney failure
What is the key take-home message concerning malaria?
Returning traveller + fever = malaria (until proven otherwise)
How is malaria diagnosed?
- thick and thin microscopy
- serology - detection of antigen in blood
- PCR - detection of malarial DNA
How is malaria controlled?
- insecticide spraying in homes
- larvicidal spraying on breeding pools
- filling in of breeding pools
- larvivorous species introduced into mosquito breeding areas
- use of insecticide impregnated bed nets
- chemoprohylaxis
- vaccine????
Give some drugs used as falciparum antimalarials
- quinine
- artemether with lumefantrine (Riamet)
- atovaquone with proguanil (Malarone)
What type of parasite causes cryptosporidiosis?
Micro-parasite - sporozoan
Cryptosporidium parvum and hominis
What does cryptosporidiosis cause?
Diarrhoeal disease
How is cryptosporidiosis spread?
Human to human spread with animal reservoir (cattle, sheep, goats)
Faecal-oral spread
Where is cryptosporidiosis found?
World-wide distribution (especially temperature and tropical climates)
What is the life cycle of cryptosporidium?
- thick-walled oocyst (sporulated) exits host
- contamination of water and food with oocysts
- thick-walled oocyst ingested by host
How long is the incubation period in cryptosporidiosis?
2-10 days (usually 7 days)
How symptoms does cryptosporidiosis cause?
- watery diarrhoea with mucus (no blood)
- bloating
- cramps
- fever
- nausea
- vomiting
How long does cryptosporidiosis last for?
Usually self-limitng (up to 2 weeks)
In which population groups can cryptosporidiosis be severe in?
- very young
- very old
- immuno-compromised (60% of HIV patients infected go on to chronic infection - can lose up to 25 litres of fluid per day)
Who is at risk of cryptosporidiosis by human-human spread?
- regular users of swimming pools (can be resistant to chlorine)
- child care workers and parents
- nursing home residents/carers
- healthcare workers
- travellers
Who is at risk of cryptosporidiosis by animal-human spread?
- backpackers, campers, hikers
- farm workers
- visitors to farms/petting zoos
- consumers of infected dairy products
How is cryptosporidiosis diagnosed?
Faeces sample
- acid fast staining
- antigen detection by EIA
How is cryptosporidiosis treated (symptomatic)?
- rehydration etc
- nitazoxanide
How is cryptosporidiosis treated in the immunocompromised?
- paromomycin (to kill parasite)
- nitazoxanide (effectiveness is unclear)
- octreotide (reduce cramps and frequency)
- HIV patients, HAART should be quickly initiated
How is human-human spread of cryptosporidiosis controlled?
- hand hygiene
- filter or boil drinking water (cf. chlorination)
- isolate symptomatic patients in healthcare setting
- ensure symptomatic children are kept away fro school
How is animal-human spread of cryptosporidiosis controlled?
- pasteurise milk and dairy products
- boil or filter drinking water if camping
What are the most commonly used ANTI-PROTOZOAL drugs?
- metronidazole
- pentamidine
- nitazoxanide
- pyrimethamine
- antimalarias (treatment and prophylaxis)
What are the most commonly used ANTI-HELMINTHIC drugs?
- albendazole
- mebendazole
- ivermectin
- praziquantel
