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Define a parasite 

An organism that lives on or in a host organism and gets food from or at the expense of its host 


What are the three classes of parasites? 

  1. Protozoa - unicellular organisms 
  2. Helminths - Worms 
  3. Ectoparasites - fleas of scabes 


Describe protozoa

  • Microscopic, single celled organisms that can be free living (like amoeba) or parasitic in nature 
  • They are able to multiply in humans allowing serious infections to develop from a single organism 



How can protozoa be transmitted? 

  • Protozoa living in human intestine can be transmitted by the faecal-oral route 
  • Protozoa living in blood can be transmitted by an arthropod vector 


Provide examples of the different types of protozoa (FACS)

Protozoa are mainly classified by their mode of movement  

  • Flagellates, e.g. Giardia, Leishmania
    • propelled by flagella
  • Amoeba, e.g. Entamoeba
    • Moves by pushing out of pseudopodia
  • Ciliates e.g.Balantidium
    • use cilia around them
  • Sporozoa– e.g. Plasmodium, Cryptosporidium
    • organisms whose adult stage is not motile


  • Entamoeba histolytica → Causes amebic dysentery
  • Giardia lamblia → Causes diarrhoea, contains flagella which propel through intestinal contents causes blunting of villi in small intestine
  • Trichomonas vaginalis → Motile protozoa which causes vaginal discharge
  • Malaria (Plasmodium spp.)
  • Toxoplasma gondii → Common parasite worldwide, transmitted from cat faeces to children
  • Cryptosporidium → Cause of epidemic diarrhoea, can cause growth stunting in children, can also cause chronic diarrhoea in patients with HIV (opportunistic diarrhoea)
  • Leishmania spp. → Causes systemic illness and cutaneous ulcers
  • Trypanosoma cruzi → Causes Chagas disease, endemic parts of Latin America
  • Trypansoma brucei (gambiense/rhodesiense)→ Acute inflammatory illness 


Describe Helminths and their main groups 

  • Large, multicellular organisms (worms) visible to naken eye at later stages 
  • In their adult form, helminths cannot multiply in humans 

Three main groups of helminths 

  1. Nematodes (roundworms) 
  2. Trematodes (flukes)
  3. Cestodes (tapeworms) 


List some medically important helminths (nematodes) 

- Ascaris lumbricoides - the most common helminth parasite (can go up to 20-30 cm)
- Trichuris trichiura – 3-4 cm, lives inside the large intestinal mucosa
- Hookworm spp. – causes anaemia, they feed off of the blood supply of the small intestinal mucosa
- Enterobius vermicularis – ‘itchy bum worm’, common

- Wuchereria bancrofti – lives in the blood, enters lymphatics and causes inflammation, causing elephantitis
- Loa loa – eye worm
- Onchocerca volvulus - can cause blindness
- Dracunculus medinensis – guinea worm infection, lives in superficial tissues, female causes itching which causes a blister; victim goes to water to relive itching, which is when female releases her larvae

- Toxocara canis/cati - get from cats/ dogs

- Trichinella spiralis – comes through contaminated meat, causing trichonosis


List some medically important helminths (trematodes and cestodes) 

- Schistosoma mansoni/haematobium/japonicum – live in the mesenteric vessels
- Clonorchis sinensis – (fluke) lives in the bile duct, can cause cholangiocarcinoma, common in parts of asia, comes from eating poorly cooked fish
- Fasciola hepatica – (liver fluke) used to be common
- Paragonimus spp. – lung worm infection, can cause a TB-like illness

- Taenia saginata – beef tapeworm, causes a chronic infection
- Taenia solium – pig tapeworm infection, if get infected at a certain lifecycle stage, can cause cycsticercosis, it has a world-wide distribution and is associated with epilepsy
- Echinococcus granulosus – causes Hytadid disease which causes very large cysts in the organs such as liver, infection of dogs


Define Ectoparasites 

  • Bloos sucking athropods (e.g fleas, ticks, lice and mites) 
  • These attach or burrow into the skin and remain there for relatively long periods of time (e.g weeks to months) 


Define some medically important ectoparasites 

  • Mites
    • Scabies
    • Trombiculid
  • Ticks
    • Hard
    • Soft
  • Lice (head, body and pubic)
    • Pediculus humanus capitis
    • Pediculus humanus humanus
    • Pthirus pubis
  • Flies
    • Botflies – Cause myiasis (maggot lives under the skin)


What are the two types of hosts that parasites can infect? 

Two types of hosts are: 

  • Intermediate → Host in which larval or asexual stages develop 
  • Definitive → Host in which adult or sexual stage occurs 


What are the two types of vectors ?

  • Mechanical when there is no development of the parasite in the vector 
  • Biological when some stages of life cycle occur in the vector 


What is the geographical distribution of parasitic infections? 

  • The poorer the country = the more parasites present 
  • You wont get NTDs (neglected tropical diseases) in wealthier and colder parts of the world 


Describe some determinants of parasitic infections 

⇒ Depends on mode of transmission and opportunities for transmission

  • Faeco-oral
    • Household sanitation
    • Access to clean water
    • Personal hygiene behaviours
  • Food
    • Animal husbandry
    • Surveillance
    • Regulations and government controls
  • Complex life cycles
    • Distributions of vectors and intermediate/definitive hosts
  • Others
    • Government resources and level of human development/per capita income
    • Education
    • Country-level and regional control programmes
    • Availability of cheap and efficacious treatments
    • Construction and building regulations (egChagas)
    • Urban vs. rural residence
    • Environmental sanitation


What is chagas disease? 

  • Tropical parasitic infection caused by the protist 
    • Trypanosoma cruzi (protozoa)
  • It is transmitted through the faeces via the Reduviid kissing bug 
  • Endemic in Latin America but also present in USA and Europe (mainly spain) due to migration 


Describe the life cycle of trypanosoma cruzi (Chagas Disease)

  1. Parasite will first develop in the midgut of the reduviid kissing bug 
  2. Epimastigote→ Trypomastigote 
  3. The kissing bug will feed on the face and defecate, if defecation contains trypomastigotes it can infect the skin through scratching the infected area 
  4. The parasite will enter and multiply in nerve and muscle cells 
  5. This will cause the release of thousands of blood trypomastigotes 
  6. The life cycle is continued if another reduviid bug bites the individual taking up the infected blood 


How long is the incubation period in chagas? 


  • Incubation 1-2 weeks after bite 
    • (incubation means the time between when pathogen is first exposed and when symptoms show) 
  • Up to months after transfusion 
  • There will be presence of trypanosomes in blood 




What is the difference between chronic determinate and chronic indeterminate in chagas disease? 

CHRONIC 'INDETERMINATE' - will have posotive serology but will be asymptomatic (no clinical manifestations)  

  • Lifelong infection 
  • Generally trypanosomes not detectable but often posotive for parasite DNA (through PCR)
  • Seroposotive (+ in blood)
  •  60-70% reach this stage 
  • Normal ECG + X-rays 


  • Seroposotive 
  • 30-40% of infected 10-30 years after acute infection 
  • 5-10% develop chronic chagas immediately after acute disease
  • Involves heart and GI tract 


Describe acute chagas

  • This occurs within 3 weeks of exposure 
  • Mild and non-specific symptoms 
  • Fever, lymphadenopathy, anorexia 
  • Swelling at bite site (nodule/chagoma) or eyelid swelling (romana) 
  • 1-2 diagnosed, symptoms last 8-20 weeks  

RARELY (when young and immunosuppressed) 

  • Hepatosplenomegaly 
  • Acute myocarditis 
  • Meningoencaphalitis 


Describe how can acute chagas develop into chronic chagas? 


  • Acute chagas can develop into chronic chagas 10-30 years later 
  • This is where the host immune response will recognise the amastigotes and try to kill them causing more inflammation 
  • The T.cruzi antibodies will still be high 
  • Chronic chagas can be indeterminate (asymptomatic) or determinate (develop progressive symptoms) 


Describe what the cardiac effects that happen in chronic chagas 

  • Inflammation can cause damage to the heart such as 
    • Conduction system = cardiac arrhythmias 
    • Heart mucle and wall = cardiomyopathy 
    • Apical anuerysms = thrombus formaiton (can cause stroke) 
      • Cardiac damage can lead to sudden death 


What digestive changes can occur due to chagas? 

  • Develops in 10-15% of patients with chronic infections 
  • Rectum, sigmoid colon and oesophagus are most affected 
  • Can also cause damage to NS of gut = gut is unable to move contents and starts to swell. Thus, you get mega organs 
  • A megacolon presents with constipation and can lead to faecaloma, obstruction, sigmoid volvulus, ulceration and perforation 


Describe the pathogenesis of Chagas 


  • Tissue damage caused by inflammatory response to parasite in nests of amastigotes in cardiac, skeletal and smooth muscle
  • Parasite killing by antibodies, activated immune response and Th1 pro-inflammatory cytokines


  • Regulatory immune response characterized by IL-10 and IL-17


  • Chronic inflammatory response to persistent parasites in muscle and nerve cells
  • Autoimmune mechanisms
  • May vary by parasite strain and tissue tropism
  • Predominance of Th1 cytokines and CD8+ T cells


What is Leishmaniasis? 

  • Caused by a protozoan parasite called leishmania 
    • Different forms of leishmaniasis 
      • Visceral and Cutaneous (also different forms of cutaneous) 
        • Visceral affects the visceral organs 
  • Transmitted to humans via a sandfly 


Describe the leishmania life cycle 

  1. Leishmaniasis is caused by sand fly which bites you 
  2. This will transmit a promastigote 
  3. This will infect immune cells succh as macrophages 
  4. Here it will form nests of amastigotes 
  5. The cells then burst releasing the pathogen to infect other cells 
  6. Or a sand fly will bite you again taking up the infected blood causing continuation of the life cycle 


What is the vector in Leishmaniasis? 

The vector is a sand fly known as Lutzomyia/Phlebotomus 


How does leishmaniasis result in formation of ulcers on the skin? 

  • An individual will get bitten by a sand fly = formation of a papule that will spread 
  • The centre will become necrotic and result in ulcer formation 
  • Once an individual is infected they will usually develop immunity and not be infected again 


Diffuse cutaneous leishmaniasis is when there is not an adequate immune response, the skin will become packed with parasites 


Describe the pathogenesis of cutaneous leishmaniasis 

(acute lesions and latency) 


  • Tissue damage caused by inflammatory response to presence of parasites in macrophages
  • Parasite killing by Th1 pro-inflammatory responses and macrophage killing
    • Th1 will release beta and gamma interferon which will activate macrophages to kill parasites inside the cells


  • Parasites will remain present long-term
  • Regulatory immune response characterized by balance of Th1 and anti-inflammatory responses


Describe the pathogenesis of cutaneous leishmaniasis 


  • Alteration in immune response (i.e change in Th1 vs immune regulation secondary to HIV, malnutrition) may trigger relapse
  • Mucocutaneous disease is associated with strong but inadequate inflammatory response to parasites which have metastasized to mucosa
  • Diffuse cutaneous leishmaniasis is associated with uncontrolled parasite replication
  • Recividans = recurrence of lesions at old ulcer site

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