Parasitology and pathogenesis of parasite infections

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.

e.g may live in skin or blood

Question 2

What are the three main classes of parasites?

Answer 2

Protozoa

Helminths

ectoparasites

Question 3

What are protozoa?

How can they be transmitted?

Answer 3

Protozoa are microscopic, single-celled organisms that can be free-living or parasitic in nature

Able to multiply in humans allowing serious infections to develop from a single organism

Transmission:

• Protozoa living in human intestine can be transmitted by fecal-oral route
• Protozoa living in tissue

Question 4

Protozoa are classified by mode of movement

What are the types of protozoa?

Answer 4

Amoeba e.g Entamoeba

Flagellates e.g Giardia, Leishmania

Ciliates e.g Balantidium

Sporoza – organisms whose adult stage is not motile e.g Plasmodium, Cryptosporidium

Question 5

What are helminths?

What are the three main groups of helminths?

Answer 5

Helminths are large, multicellular organisms (Worms) generally visible to naked eye in adult stages.

In their adult form, helminths cannot multiply in humans

There are three main groups of helminths that are human parasites:

• Nermatodes
• Trematodes
• Cestodes

Question 6

What are the medically important helminths - nermatodes?

Answer 6

Soil-transmitted helminths

• Ascaris lumbricoides
• Trichuris trichiura
• Hookwork Spp
• Enterobius vermicularis – itchy anal margin in young children

Filarial parasites

• Wuchereria bancrofti
• Loa loa
• Onchocerca volvulus
• Dracunculus medinensis

Others

• Toxocara canis/cati
• Trichinella spiralis

Question 7

What are some examples of termatodes?

Answer 7

Schistosoma mansoni/haematobium/japonicum

Clonorchis sinensis

Fasciola hepatica

Paragonimus spp. - lung worm, transmitted through eating poorly cooked shrimps and crabs

Question 8

What are some examples of cestodes?

Answer 8

Taenia saginata – cow tapeworm

Taenia solium – pig tape worm, not as long

Echinococcus granulosus - production of lesions in viscera composed of multiple cysts

Question 9

What are ectoparasites?

Answer 9

Blood sucking arthropods suck as ticks, fleas, lice and mites that attach or burrow into skin and remain there for long periods of times e.g weeks to months

Question 10

What are some medically important ectoparasites?

Answer 10

Mites

• Scabies
• Trombiculid

Ticks

• Hard
• Soft

Lice

• Pediculus humanus capitis
• Pediculus humanus humanus
• Pthirus pubis – pubic lice

Flies

• botflies

Question 11

What are the two types of hosts of parasites?

Answer 11

Intermediate – host in which larval or asexual stages develop

Definitive – host in which adult of sexual stage occurs

Question 12

What are the determinants of parasitic infections?

Depends on mode of transmission and opportunities for transmission

Answer 12

Faceo-oral route

Prevented by

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

Food

Prevented by

• Animal husbandry
• Surveillance
• Regulations and government controls e.g of slaughter houses

Complex life cycles

• Distributions of vectors and intermediate/definitive hosts

Other factors determining opportunity for parasitic infections

• Education
• Government resources and level of human development
• 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 e.g proper sewage disposal

Question 13

What is the acute phase od chagase disease?

Answer 13

Incubation period 1-2 weeks after bite

Up to months after transfusion

Can see trypanosomes in blood

Question 14

What is the chronic phase of chagas disease?

Answer 14

Lifelong injection

Generally trypanosomes not detectable but often positive for parasite DNA

Seropositive

60-70% develop indeterminate chagas disease

Normal ECG and x rays

Question 15

What is determinate chronic chagas disease?

Answer 15

Seropositive

30-40% of infected , occurs 10-30 years after infection

5-10% develop chronic chagas immediately after acute disease – sevre or end organ damage

Question 16

What organs are affected by chornic chagas?

Answer 16

Heart, intestintal tract

• infect purkinje fibres causing fibrosis
• can cause arrthymias and damage to heart muscle, becomes thinner - heart doesn’t contract efficiently

digestive

• esophagus, rectum and sigmoid colon most affected
• persitalsis affected, causes enlagred colon due to impact faeces and constipation

Megacolon

• Presentation with constipation

Complications include

• Faecaloma
• Obstruction
• Sigmoid volvulus
• Ulceration
• perforation

Question 17

What are the clinical forms of cutaneous leishmaniasis?

Answer 17

Ulcers on face

Plaque that expands before it breaks down then forms ulcer

In ulcer if you take sample you can see parasite under microscope

Scars, can give immunity

Or may get re-activation, new lesions forming around old scars

Diffuse cutaneous leishmaniasis, nodules are packed with amastigotes

Mucocutaneous leishmaniasis, occurs in people that have already been infected, 10-15 years they get nasal stuffiness and damage to tissue, may cause loss of nasal septum. Difficult to treat

Question 18

What is the pathogenesis of cutaneous leishmaniasis?

Answer 18

Tissue damage caused by inflammatory response to presence of parasites in macrophages

Parasite killing by Th1 pro-inflammatory responses and macrophage killing

Question 19

When does relapse of cutaneous leishmaniasis occur?

Answer 19

Occurs when there is alternation in immune response e.g change in Th1 vs immune regulation secondary to HIV, malnutrition may trigger relapse

• Mucocutaneous disease associated with strong but inadequate inflamatory response to parasites that have metastasised to mucosa
• Diffuse cutaneous leishmaniasis associated with uncontrolled parasite replication
• Recividans – recurrence of lesions at old ulcer site