Parasitology Flashcards
What is a parasite?
An organism that lives on or in a host organism and gets its food from or at the expense of its host.
What are the three main classes of parasites that can cause disease in humans?
- Protozoa - unicellular organisms
- Helminths - worms
- Ectoparasites - fleas and scabes etc
What are protozoa?
Protozoa are microscopic, single celled organisms that can be free-living or parasitic in nature.
Example of a free-living protozoa
Amoeba - people can be infected by these from water
How do parasites in nature act?
Rely on a host for survival
Multiplication of protozoa
Able to multiple in humans unlike worm infections, allowing serious infections
Transmission of protozoa
- Protozoa living in the human intestine can be transmitted by the fecal-oral route e.g. diarrhoea can disseminate and be transmitted to other people.
- Protozoa living in blood or tissues are transmitted by an arthropod vector e.g. malaria taken up by an arthropod can then be transmitted.
How are protozoa classified usually?
Depending on the mode of movement
Amoeba
Single-cell organisms that moves by pushing out its pseudopodia e.g. Entamoeba
Flagellates
Propelled by flagellum e.g. Giardia, Leishmania
Types of protozoa
Amoeba
Flagellates
Ciliates
Sporozoa
Sporozoa
Organisms whose adult stage is not motile e.g. Plasmodium Cryptosporidium
Entamoeba histolytica
- Causes amoebic dysentery
- Section of intestine shows that there is a part of the epithelial surface with an ulcer formation; an undermined edge and loss of the epithelium completely.
Giardia lamblia
Causes giardiasis - diarrhoeal infection - where the giardia has flagella.
Trichomonas Vaginalis
Motile protozoa that causes vaginal discharge
Toxoplasma gondii
Common parasite worldwide
Infection from cat faeces; tend to infect children
Cryptosporidium
- Common cause of epidemic diarrhoea worldwide
- Major course of moderate diarrhoea and growth stunting in children
- Also opportunisitc infection of those who are immunosuppressed.
Leishmania spp.
Protozoa infection which causes severe systemic illness but also causes cutaneous disease and cutaenous ulcers.
Common in the middle east and parts of Africa.
Trypansoma cruzi
Causes chagas disease
Endemic in parts of Latin America
Trypansoma brucei (gambiense/rhodesiense)
Sleep disease
What are helminths?
Large, multicellular organisms (worms) that are generally visible to the naked eye in their adult stages. In their adult stage, they cannot multiple in humans.
Main groups of helminths
- Nematodes (roundworms)
- Trematodes (flukes)
- Cestodes (tapeworms)
Example of a nematode
Ascarids can be expelled in stool
Example of trematode
Schistosoma - the outer is a male and the female lives in a groove in the male. They have their sexual reproductive stage in the human host.
Soil transmitted helminth nematodes
- Ascaris lumbricoides (most common, can be 20-30cm long)
- Trichuris trichiura (3-4 cm long, attenuated and lives in the large intestinal mucosa)
- Hookworm spp (Cause anaemia, look like tadpoles, feed off the blood in the small intestinal mucosa)
- Enterobius vermicularis (itchy bum, very common).
Filarial parasites nematodes
- Wuchereria bancrofti (lives in blood and lymphatics, tends to cause inflammation in the lymphatics and blockage).
- Loa loa (the eye worm, crawls across the sclera).
- Onchocera volvulus (lava stage can infest the eye and cause blindness)
- Dracunculus medinensis (in the kidneys, a worm that lives in superficial tissues, causing itching and blisters, peeing causes the female to release all its lava
Other nematode examples
- Toxocara canis/cati: caught in the UK from infection with cats and dogs
- Trichinella spiralis: causes trichinosis
Medically important trematodes
- 3 species of schistosoma mansoni/haematobium/japonicum which are adult worms that live in the mesenteric vessels
- Clonorchis sinensis (fluke): lives in bile duct, chronic infections can cause cholangiocarcinoma, can occur by eating raw fish (common in Asia)
- Fasciola hepatica (fluke): liver fluke which used to be common in wales.
- Paragonimus spp.: Lung infection that causes a TB like illness.
Medically important cestodes
- Taenia saginata: Beef tapeworm that is a chronic infection of a large worm.
- Taenia solium: Pig tapeworm that if infects the humans at the wrong lifecycle stage, it can cause cysticercosis (nodules in the brain caused by exposure to the lava stage of parasite infection) - associated with epilepsy.
- Echinococcus granulosus: normally, infection of dogs, but release of eggs from dogs can cause large cysts in the organs such as the liver.
What are ectoparasites?
Blood sucking arthropods such as ticks, fleas, lice and mites that attach or burrow into skin and remain there for relatively long periods of time. For example, a mosquito isn’t a parasite as it doesn’t stay for long.
Medically important ectoparasites
- Mites
- Ticks: Hard and Soft
- Lice
- Flies
Mites
- Scabies: itchy rash that can infect between the fingers, wrists and flextures. If scratched off, it can cause secondary infections
- Trombiculid: also called chiggers. Acquired in the tropics, on freshly grown grass. Itchy ankles, red mites that can cause reactions and cellulitis
Lice
Pediculus humanus capitis - head lice: Nits - eggs laid on hairs
Pediculus humanus humanus - body lice
Pthirus pubis - pubic lice
Flies
Botflies - cause myiasis (maggot lives under the skin)
What are NTDs?
Neglected tropical diseases (NTDs) include parasitic diseases such as lymphatic filariasis (elephantiasis), onchocerciasis and guinea worm disease.
Where do parasitic infections affect the most?
In the tropics and subtropics, as well as in more temperate climates. Affect more than 1 billion people in largely rural areas of low-income countries.
Type of hosts
Intermediate: host in which larval or asexual stages develop. For example, cysticercosis where humans are the intermediate host.
Definitive: host in which adults or sexual stage occurs.
Why are vectors important?
- Mechanical when no development of parasite in vector - just transmit the parasite.
- Biological where some stages of life cycle occur within the vector
Schistosomiasis life cycle
- Exposed to contaminated water with eggs and faeces.
- Eggs develop into a miracidia which affects the intermediate host (the snail).
- The snail acts as an amplifier and releases thousands of cercanae.
- These infect humans when they walk into contaminated water..
- The cercanae will develop into adults in the human host and undergo sexual reproduction.
- The female will release thousands of eggs into the environment.
- Then, the life cycle will continue again.
Geographic distributions of parasitic infections
- The poorer the country, the more parasites. However, in certain areas (extreme south or extreme north), ther is less infection because the environment becomes more inhospitable.
- Many parasite infections in Europe, but a lot of them are endemic in the animal populations but limited opportunities for transmission to humans. If it does occur, it is very sporadic.
DALYs
Disease/Disability adjusted life years:
- used as a way to measure the morbidity to parasitic infections.
- Most infections are in Asia, Sub-saharan Africa and Central America.
Different modes of transmission
Faeco-oral: spread of diarrhoea
- Household sanitation
- Access to clean water
- Personal hygiene behaviours
Food:
- Animal husbandry - how farms look after cattle
- Surveillance
- Regulations and government controls
Complex life cycles:
- Distributions of vectors and intermediate/definitive hosts
- Many restricted to tropical regions
- With global warming, the distribution of vectors is moving north away from the tropics.
- Increase in parasitic infections in high income countries.
Others:
- Government resources and level of human development/per capita income
- Education
- Country-level and regional control programmes (also how effective these are)
- Availability of cheap and efficacious treatments
- Construction and building regulations (e.g. Chagas)
- Urban vs rural residence
- Environment sanitation
Chagas disease
- Transmitted by the triatomine bugs (kissing bugs) which live in houses which are made of thatched roofs, or walls with hools etc.
- Chickens are a source of food for triatomine bugs also dogs.
- > Simple measures like keeping animals and chickens away from the house
- > Improve condition of house reduce opportunity of transmission.
Life cycle of chagas disease
- The bug feeds on the human at night, it does not transmit the infection via a bite but rather through defecation on you.
- When scratching the bite, it will move the faeces through the skin into the body through the mucosa.
- It multiples in the nerves and muscles.
- These cells rupture into thousands of trypomastigotes and then these can be taken up by the blood circulation.
Where is chagas disease endemic?
In latin America, but also present in Europe and the USA, because of migration.
What are the three phases of chagas disease?
- Acute
- Chronic “indeterminate”
- “Determinate” Chronic disease
Acute chagas disease
- Incubation of 1-2 weeks after bite
- Up to months after transfusion
- Trypanosomes in blood (circular things)
Chronic “indeterminate” chagas disease
- Lifelong infection - parasite is still there but in low levels
- Generally trypanosomes not detectable but often positive for parasite DNA (can do PCR which will pick up the parasite DNA).
- Seropositive (blood serum test for pathogen)
- 60 - 70%
- Normal ECG and X-rays
“Determinate” Chronic chagas disease
- Seropositive
- 30-40% of infected 10-30 years after infection
- 5-10% develop chronic chagas immediately after acute disease
- Chagas and CVD
Symptoms of acute chagas
- Occurs within 3 weeks
- Generally mild or asymptomatic: local swelling (romana); nodule or chagoma (on hand), fever, anorexia and lymphadenopathy
- 1-2% diagnosed
- Symptoms last 8-10 weeks
- Rarely (young and IS); Hepatosplenomegaly , Acute myocarditis, Meningoencephalitis and fatality < 5% of symptomatic.
Cardiac Chronic Chagas
- After 10-30 years, this is gradually developed.
- Change in the immune response, where the host starts to recognise the parasites again causing inflammation.
- Damage to the condution of the heart
- Damage to the muscle (cardiomyopathy - enlarged heart).
- Apical aneurysms
- Causes sudden death - due to ventricular failure
Digestive chronic chagas
- Damage to the nervous system in the gut and interferes with the peristalsis
- The gut is unable to effectively move contents and starts to swell.
- Develops in 10-15% of patients with chronic infections
- Affects the oesophagus, rectum and sigmoid colon mostly.
- Presents as constipation
- Complications: Faecaloma, Obstruction, Sigmoid Volvulus, Ulceration, Perforation and Death
The natural course of chagas
- Acute illness - mild or asymptomatic.
- Mild appearance of parasites in the blood.
- Immune response controls the parasite numbers - “Indeterminant phase” follows.
- Over a 10-30 years, the chronic chagas disease associated with cardiac and digestive problems.
Acute chagas pathogenesis
- Tissue damage caused by inflammatory response to parasites in nests of amastigotes in cardiac, skeletal, and smooth muscle.
- Parasite killing by antibodies, activated innate immune response and Th1 pro-inflammatory cytokines. Th1 is effective in killing chagas disease.
Indeterminate chagas pathogenesis
- Regulatory immune response characterised by IL-10 and Il-17.
- Balance between killing parasites and protecting the host.
Chronic chagas pathogenesis
- Chronic inflammatory response to persistent parasites in muscle and nerve cells which leads to loss of toleration.
- Autoimmune mechanisms
- May vary by parasite strain and tissue tropism
- Associated with ressursion of parasite responses: predominance of Th1 cytokines and CD8+ T cells.
Where is visceral leishmaniasis prominent?
- Causes a large systemic, illness, mostly in India.
- In Asia: Leishmania donovani
- In Middle East/Africa/Asia: L. infantum variants
- Latin America: L. chagasi
Where is cutaneous leishmaniasis most prominent?
Old world: Mediterranean/Middle East - L. infantum/L. major/ L. tropica
New world: Central and South America - L. braziliensis/amazonensis/mexicn
Leishmaniasis Life Cycle
- Caused by a sandfly - when the sandfly bites you, it transmits a promastigote.
- The promastigote invades immune cells such as macrophages.
- Inside, it forms nests of amastigotes, this eventually bursts and releases lots of amastigotes which infects other cells.
What is the vector (carrier) for leishmania?
In latin america, its lutzomyia.
In Asia, its phlebotomus.
Sandflies are common in tropical regions and affect poorer built houses. They live in the cracks. They love to feed on chickens as a source of blood. They are able to get through mosquito nets.
What is the reservoir for leishmania?
Domestic and sylvatic animals - goats and sloths are the normal reservoirs. However, humans can become infected and also dogs. These are amplifier hosts.
What is cutaneous leishmaniasis?
- Normally after a sandfly bite, a papel forms. This gradually spreads and forms a lesion. The centre becomes necrotic and leads to an ulcer formation.
Other forms of leishmania tropica
- In the mediterranean, leishmania tropica which leaves large patches on the skin.
- Lesions can be reactivated.
- Multiple ulcers
- Leishmania can travel along the lymphatic vessels
Diffuse cutaneous leishmaniasis
Inadequate immune response so have a large population of parasites
Mucocutaneous Leishmaniasis
- Small portion of individuals after infection, years later will develop a stuffy nose.
- Then, there will be lesions around the nasal area, breakdown of the nasal septum.
- This is reactivatoin of the parasite in the mucosal area.
Pathogenesis of Cutaneous Leishmaniasis
- Acute lesions:
- Tissue damage caused by inflammatory response to presence of parasites in macrophages.
- Parasite killing by Th1 pro-inflammatory responses and macrophage killing.- > Release cytokines such as interferon gamma that activate macrophages to kill parasites that are inside the cells.
- Latency:
- Parasite present in lesions but not activated.
- Parasites remain present long-term. Regulatory immune response characterised by balance of Th1 and anti-inflammatory responses. - Relapse (rare)
- Alteration in the immune response (i.e. change in Th1 vs. immune regulation secondary to HIV, malnutrition) may trigger release.
- > Mucocutaneous disease associated with strong but inadequate inflammatory response to parasites that have metastasized to mucosa.- > Diffuse cutaneous leishmaniasis associated with uncontrolled parasite replication.
- > Recividans - reccurence of lesions at an old ulcer site.
What do histological sections show in leishmania?
Show immune cells, such as macrophages, packed with amastigotes.
