Parasitology

Question 1

Identify the three main groups of parasites that infect humans

Answer 1

1) Protozoa:

• Unicellular, microscopic organisms that can cause diseases such as malaria, toxoplasmosis, giardiasis, and amoebic dysentery
• Some protozoa such as plasmodium species (malaria) have complex life cycles involving different stages and hosts

2) Helminths:

• Multicellular, often large, worms, divided into 3 groups:
• Nematodes (roundworms): Examples include pinworm, hookworm and the giant roundworm
• Trematodes (flukes): These include the liver fluke and the blood flukes
• Cestodes (tapeworms): beef tapeworm, pork tapeworm and the fish tapeworm

3) Ectoparasites:

• live on the surface of the host, blood-sucking arthopods
• Examples include insects like lice, fleas, ticks, and mites
• While they can cause localised skin conditions, many ectoparasites are vectors for disease-causing organisms, and can transmit diseases such as Lyme disease (ticks), plague (fleas), and Typhus (lice)

Question 2

Describe and explain the geographic distributions of parasitic infections and the determinants of that distribution

Answer 2

Depends on mode of transmission and opportunities for transmission

1) Faeco-oral:

• Household sanitation
• Access to clean water
• Personal hygiene behaviours

2) Food:

• Animal husbandry
• Surveillance
• Regulations and government controls

3) Complex life cycles:

• Distribution of vectors and intermediate/definitive hosts

4) 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 (e.g. Chagas)
• Urban vs rural residence
• Environmental sanitation

Question 3

Describe and explain the range of diseases caused by different parasites

Answer 3

Protozoa - Chagas disease:

A tropical parasitic disease caused by the protozoan parasite Trypanosoma Cruzi, mainly transmitted to humans by the feces of triatomine bugs, known as “kissing bugs,” when they bite and ingest blood

The disease has two phases: the acute phase and the chronic phase

Acute:

• Tissue damage caused by inflammatory response to parasite in nests of amastigotes in cardiac, skeletal, and smooth muscle
• symptoms are usually mild and non-specific, such as fever, fatigue, body aches, rash, and swelling at the site of the infection
• In some cases, severe acute symptoms such as severe inflammation of the heart muscle (myocarditis) or brain (meningoencephalitis) can occur
• Parasite killing by antibodies, activated innate immune response and Th1 pro-inflammatory cytokines

Indeterminate:

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

Chronic (20-30%):

• Chronic inflammatory response to persistent parasite in muscle and nerve cells
• cardiac complications (e.g., heart rhythm abnormalities, heart failure)
• and gastrointestinal complications (e.g., enlarged esophagus or colon, leading to difficulties in eating or defecating)

Protozoa - leishmania:

Leishmaniasis is caused by the protozoan parasite Leishmania, transmitted to humans by the bite of infected female phlebotomine sandflies

There are several different forms of leishmaniasis, including cutaneous leishmaniasis, mucocutaneous leishmaniasis, and visceral leishmaniasis

1) Cutaneous Leishmaniasis: causes skin sores at the site of the sandfly bite that may start out as papules or nodules and may eventually ulcerate

2) Mucocutaneous Leishmaniasis: In this form, the parasites spread to the mucous membranes of the nose, mouth, and throat. It can lead to partial or total destruction of these mucous membranes and surrounding tissues, causing significant disfigurement

Visceral Leishmaniasis: Also known as kala-azar, this is the most severe form of the disease and is potentially fatal if left untreated. The parasites affect internal organs such as the spleen, liver, and bone marrow. Symptoms include fever, weight loss, enlargement of the spleen and liver, and anaemia

Acute lesions:

• Tissue damage caused by inflammatory response to presence of parasite in macrophages
• Parasite killing by Th1 pro-inflammatory responses and macrophage killing

Latency:

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

Relapse (rare):

• Alteration in immune response (i.e. change in Th1 vs. immune regulation secondary to HIV, malnutrition) may trigger relapse