60. Parasitology

Question 1

What is a parasite?

Answer 1

A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host.

There are three main classes of parasites that can cause disease in humans:

1) Protozoa
2) Helminths
3) Ectoparasites.

Question 2

What are protozoa?

Answer 2

• Protozoa are microscopic, single-celled organisms that can be free- living or parasitic in nature.
• They are able to multiply in humans allowing serious infections to develop from a single organism.

• Transmission:

• Protozoa living in the human intestine can be transmitted by the fecal-oral route
  - Protozoa living in blood or tissues are transmitted by an arthropod vector

Question 3

What are helminths?

Answer 3

• Helminths are large, multicellular organisms (worms) generally visible to the naked eye in their adult stages. In their adult form, helminths cannot multiply in humans.
• There are three main groups of helminths that are human parasites:

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

Question 4

What are ectoparasites?

Answer 4

Blood-sucking arthropods such as ticks, fleas, lice, and mites that attach or burrow into the skin and remain there for relatively long periods of time (e.g., weeks to months).

Question 5

Parasites- general comments

Answer 5

•Parasites often have complex life cycles
•Type of host
- Intermediate – host in which larval or asexual stages develop
- Definitive – host in which adult or sexual stage occurs
•Vectors
- Mechanical when no development of parasite in vector
- Biological when some stages of life cycle occur

Question 6

Determinants of parasite infections

Answer 6

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 CYCLE
- 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 (eg Chagas)
• Urban vs. rural residence
• Environmental sanitation

Question 7

Phases of disease

Answer 7

•Acute

• Incubation 1-2 wks after bite
• Up to months after transfusion
• Trypanosomes in blood

•Chronic ‘indeterminate’

• Lifelong infection
• Generally trypanosomes not detectable but often positive for parasite DNA
• Seropositive
• 60-70%
• Normal ECG and X rays

•‘Determinate’ Chronic disease

• Seropositive
• 30-40% of infected 10-30 years after infection
• 5-10% develop chronic Chagas immediately after acute disease

Question 8

Chronic Chagas

Answer 8

CARDIAC

• Damage to the conduction of the heart – arrhythmias
• Damage to the heart muscle ~ cardiomyopathy
• Apical aneurysms and thrombus formation

DIGESTIVE

• Damage to the NS of the gut, and it starts to swell
• megacolon ~ Constipation

Question 9

Chagas pathogenesis

Answer 9

ACUTE

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

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

CHRONIC

• 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