Parasitology

Question 1

What is a parasite?

Answer 1

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

-often parasites are adapted to their host so they don’t cause pathology. Some parasites are not well adapted to their host and cause a nasty inflammatory response or death.

Question 2

There are three main classes of parasites that can cause disease in humans - what are they?

Answer 2

1. Protozoa (unicellular organisms)
2. Helminths (worms)
3. Ectoparasites (fleas, scabes)

Question 3

What are protozoa?

Answer 3

microscopic, single-celled organisms that can be free- living (amoeba) or parasitic in nature

• tend to live in their host and depend on them for survival
• able to multiply in humans allowing serious infections to develop from a single organism

Question 4

difference betweens helminths and protozoa?

Answer 4

protozoa multiply, helminths don’t

Question 5

transmission of protozoa?

Answer 5

• 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 6

how are protozoa classified?

Answer 6

by mode of movement

Amoeba
Flagellates, e.g. Leishmania
Ciliates e.g. Balantidium
Sporozoa – organisms whose adult stage is not motile e.g. Plasmodium

Question 7

name some medically important protozoa infections

Answer 7

• Leishmania spp.
• Malaria (Plasmodium spp.)
• Toxoplasma gondii – common parasite worldwide, infection from cats faeces or poorly cooked food
• Cryptosporidium – major cause of growth delay in children, faecel contamination to a water source, also an opportunistic infection – can cause chronic diarrhoea in patients with HIV
• Entamoeba histolytica - causes amoeban dysentery, area of the epithelial surface in the uterus where there is an ulcer formation.
• Giardia lamblia – common cause of diarrhoea, have flagella that propel them through the intestinal contents, tend to be around the epithelium of the host and cause epithelial damage, such as blunting of the villi in the small intestine
• Trichomonas vaginalis – motile, causes vaginal discharge

Question 8

what are helminths?

Answer 8

• large multicellular organisms
• generally visible to the naked eye in their adult stages
• in their adult form, helminths cannot multiply in humans

-three main groups of helminths are human parasites:

Nematodes (roundworms)
Trematodes (flukes)
Cestodes (tapeworms)

Question 9

in helminths, what is the parasite burden dependent on?

Answer 9

your exposure to the infective stage in the environment

you could build up a high parasite burden over a period of time through repeated exposures to infections in the environment

Question 10

what type of helminths are medically important?

Answer 10

all 3 types

nematodes

• Soil-transmitted helminths
• Filarial parasites

trematodes
- Schistosoma mansoni/haematobium/japonicum – the adult worms live in the mesenteric vessels

cestodes
-Taenia saginata

Question 11

What are ectoparasites?

Answer 11

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 12

Medically important ectoparasites

Answer 12

1. Mites
   - scabies, an itchy rash between fingers, wrist and flexures, people scratch and get secondary infections (impetigo)

• trombiculid – when you walk around in the tropics in freshly mown grass, can cause cellulitis because people scratch so much

2. Ticks
   - Hard and Soft
3. Lice
   - Pediculus humanus
4. Flies
   - Botflies

Question 13

what do parasites often have?

Answer 13

complex life cycles

Question 14

types of parasitic hosts and vectors?

Answer 14

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 15

example of a parasite

Answer 15

schistomiasis

• complex life cycle
• walk in water contaminated with human faeces, the eggs develop and infect snail (intermediate organism)
• the snail becomes an amplifier, asexual reproduction in the snail produce circharae which infect human in the water, and develop into adults when in humans

Question 16

parasitic infections - location in the world?

Answer 16

the poorer the country, the more parasites there are.

also, the further north or south you go there seem to be fewer parasitic infections because the environment becomes increasingly inhospitable for both humans and parasites.

Question 17

DALY’s?

Answer 17

disease adjusted life years - a way to measure morbidity

many infections are not fatal but they are chronic, and DALY’s helps measure the effect of the levels of disability.

Question 18

Determinants of parasite infections?

Answer 18

Depends on mode of transmission and opportunities for transmission

• Faeco-oral
• Household sanitation
• Access to clean water
• Personal hygiene behaviours
• Food
• Regulations and government controls
• Complex life cycles
• Education
• Country-level and regional control programmes

Question 19

Protozoa - Chagas disease

Answer 19

Life cycle of the parasite Trypansoma cruzi

The bug feeds on you at night - doesn’t transmit the infection through its bite, but when it defecates

When you scratch the bite, you may scratch the faeces through your skin

It enters the body and paralyses the nerve and muscle cells.

These cells rupture and release lots of tryptomastogotes, taken up by a bug at a later blood meal. Endemic in Latin America.

Question 20

Phases of Chagas disease

Answer 20

acute and chronic

5-10% develop chronic Chagas immediately after acute disease

Question 21

acute chagas

Answer 21

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

Occurs within 3 weeks, symptoms last 8-10 wks 
Generally mild or asymptomatic 
Local swelling (Romaña) 
Nodule/chagoma
Fever
Anorexia 
Lymphadenopathy 

• Hepatopsplenomegaly
• Acute myocarditis
• Meningoencephalitis
• Fatality <5% of symptomatic

Question 22

chronic chagas

Answer 22

Chronic inflammatory response to persistent parasites in muscle and nerve cells

• Autoimmune mechanisms
• Predominance of Th1 cytokines and CD8+ T cells

HEART

• damage to the muscle of the heart wall, causing cardiomyopathy
• thrombus formation causing a stroke
• causes sudden death related to arrhythmias

GUT

• esophagus, rectum, and sigmoid colon affected
• damage to the nervous system in the gut, interferes with peristalsis and the gut is less able to move contents on and it starts to swell, producing mega organs. megacolon - constipation