Parasitology

Question 1

Parasite

Answer 1

An organism living in or on another organism, the host, and benefitting at the expense of the host.
‘Parasitology’ is generally concerned with protozoan & metazoan eukaryotic parasites, although all viruses & many pathogenic bacteria are parasitic.
Majority of animals are parasitic

Question 2

Endoparasites

Answer 2

Invade host, including the gut

Question 3

Ectoparasites

Answer 3

Generally external to the host, e.g ticks

Question 4

Vector

Answer 4

An agent, often a biting insect, that is responsible for the transmission of a disease organism

Question 5

Host

Answer 5

Organism that gives food & shelter to another organism.

Question 6

Definitive host:

Answer 6

Host in which the parasite reaches sexual maturity or in which parasite sexual reproduction occurs

Question 7

Intermediate host

Answer 7

Host which is required for parasite development but one in which the parasite does not reach sexual maturity

Question 8

Protozoa

Answer 8

Endoparasitic
Reproduce directly in host
e.g Trypanosomes (sleeping sickness), Leishmania spps (leishmaniasis), Plasmodium spss (malaria), Babesia spps (tick borne disease of cattle)

Question 9

Metazoa

Answer 9

Endoparasitic
No direct replication in definitive host
Helminths: Nematodes (ascaris, hookworms, filarial worms), Treematodes (Schistosomes), Cestodes (tapeworms)

Question 10

Direct life cycle

Answer 10

Involves only a single host species

Question 11

Indirect life cycle

Answer 11

Involves 2 or more host species, e.g life cycles that involve a vector, or a predator-prey relationship

Question 12

Zoonosis

Answer 12

Disease found mainly in animals but may be passes to humans.
Animal host serves as a major resevoir

Question 13

Ascaris lumbricoides (suum)

Classification, size?
Life cycle?
Resilience?
Immune response?

Answer 13

Nematode 15-35cm long.
Direct life cycle.
Life span ~1 year. ~1.3bn infected.

Adults (m+f) in small intestine -> eggs laid (~200,000/day) & parred out with faeces -> embryogenesis & development to infective larva within eggshell in soil -> ingestion of infective larvae -> larvae hatch, penetrate villus of small intestine enter portal circulation -> arrive at lungs, penetrate into alveoli, migrate up respiratory tract into pharynx -> swallowed -> REPEAT.

Eggs embryonate in soil. Infective stage is v resilient (up to 10 years) but sensitive to sunlight.

Intestinal helminths generally elicit an TH2 response: IgE, mastocytosis, eosinophilia. TH2 cytokines & mast cell degranulation leads to increased mucus production & smooth muscle contraction.

Question 14

Ascaris lumbricoides:

Damage to host

Answer 14

Largely direct
Heavy infection can cause fatal intestinal blockage
Damage to lungs during migration.
Allergic response to worm metabolites, strong TH2 response.
Abdominal pain, nausea, vomiting
Nutritional;
- impaired Vit A absorption
- lactose intolerance
- decreased protein & fat utilisation.
Detrimental effects on physical & cognitive development0 some 1.5m children never recover.

Question 15

Hookworms e.g Necator americanis

Answer 15

Direct life cycle
Nematode, 1cm long
May live up to 15 years.

Adults in small intestine feed on blood, lay eggs (~10,000/day) -> eggs passed in faeces -> eggs hatch releasing free-living infective larvae -> infective larvae enter host by skin penetration -> migrate to lungs, coughed up, swallowed.

Infections flourish when soil becomes contaminated with human faeces carrying hookworm eggs.
Juveniles will not develop below 17 degrees.

Survival in host:

• anticoagulant molecules to prevent blood clotting as they feed.
• secrete molecules that provide protective shield against immune attack e.g AChE, superoxide dismutase, proteinases, pro-apoptotic factor which induces apoptosis in

Question 16

Hookworms damage to host

Answer 16

Largely direct.
Worms attach to mucosa of small intestine & feed on blood. In heavy infections patients may lose up to 200ml/day.
Anaemia. Can lead to stunted growth & impaired cognitive development.

Moderate infection- slight intermittent abdominal pain, loss of normal appetite.

Question 17

Wucheria bancrofti

Answer 17

Filarial nematode. 4-10cm
Indirect life cycle
Transmitted by blood feeding vector

Causes filariasis (elephantisis) 
Complex immune response of long duration is major contributor to pathology. 
- adult worms in lymph nodes -> inflammatory response -> lymph return to lymph nodes is obstructed -> lymph 'piles up' greatly dilating the duct -> progressive infiltration of fibrous connective tissue. 
Most commonly affected: scrotum, legs, arms.

Question 18

Onchocerca volvulus

Answer 18

Filarial nematode. 20-50cm
Indirect life cycle
Transmitted by blood feeding vector

Causes river blindness
Complex immune response of long duration is major contributor to pathology.
- Microfilaria cause pathology.
- Adult worms in subcutaneous nodules are least pathogenic, but may sometimes cause elephantisis & dermatisis from degenerating microfilaria in skin.
- Microfilaria in skin may invade eyes, cause inflammation followed by build up of fibrous tissues - blindness.

Question 19

Toxoplasma gondii

Answer 19

Intracelular parasitic protozoa.
Direct & indirect life cycles. (v complex diagram, involves cats)
In humans, major cause of birth defects. Death/ disability if fetus is infected.
Causes spontaneous abortion in sheep.
~13% world infected, higher where raw meat is eaten. Most infections are asymptomatic/ mild. May disseminate & cause serious problems in the immunosuppressed.

Question 20

Toxoplasma gondii invasion

Answer 20

Invades many cell types (unusual for a parasitic protozoa, most are specific about cell type)- both phagocytic & non-phagocytic

Enters by an active process mediated by the parasite’s cytoskeleton. Host cell does not play an active role in uptake. Ends up within the parasitophorous vacuole.

Parasitophorous vacuole is critical to survival within macrophages: as forms, excludes any host TM proteins, including host factors that govern acidification etc.
TG also interferes with NF-KB translocation to nucleus.