Lecture 25: people, parasites & pets

Where does the hookworm reside

The GI tract

What are parasites

Organisms living in or on another living organism, obtaining part or all of its organic nutrition, commonly exhibiting some degree of adaptive structural modification and potentially causing damage to its host.

Example of parasitic symbionts

Wasp & caterpillar

Prevalence

The proportion (%) of a population infected at a given point in time or the total number of infections in a country or across the world.

Incidence

Refers to the rate of infection: the frequency of a population acquiring infection in a unit of time (e.g. 3% per week)

Intensity

Measure of the number or density of parasites per host (e.g. Ectoparasites on skin or % of red blood cells infected with malaria) - measure of virulence

Mortality

The number of deaths in a given time or the percentage of deaths among all cases of infection.

Morbidity

The level of ill health or disability among all cases of infection or among the general population.

DALYs

Disability adjusted life years. A measure of overall disease burden: the number of healthy years lost to disease, disability or early death.

Prevalence, mortality & DALYs of vector borne disease

1. Ascaris roundworm: 800M 60K 10.5M

2. Schistosomiasis: 250M 280K 1.4M

3. Malaria: 219M 435K 55M

4. Sleeping sickness: 70K 48K 1.5M

Transmission of ascaris roundworm

Directly through faecal contamination of soil

How is schistosomiasis transmitted

Snail-borne

How is malaria transmitted

Mosquito borne

How is sleeping sickness transmitted

Tsetse fly-borne

Classifications of parasites

• Protozoa

• Helminths

• Arthropods

What type of parasites are Protozoa

Unicellular either intracellular or extracellular

What type of parasites are Helminth's

Multicellular, extracellular

Examples of helminths

• Metazoan roundworms (nematodes)

• Schistosomes

• Tapeworms

Examples of parasitic arthropods

ectoparasites: ticks, fleas & ice

What has the co-evolution of hosts & parasites led to

• Many pathogens have adapted to not harm the host in order to survive

• Exquisite tuning to specific host

• The infection is not always pathogenic

How are parasitic Protozoa classified

Simple classification by cellular organisation

Classifications of parasitic Protozoa & examples

• Ameboid: entamoeba histolytica (amoebic dysentery)

• Kinetoplastid: trypanosoma (sleeping sickness), Leishmania (Kala-Azar)

• Apicomplexa: plasmodium (malaria), toxoplasma gondii (toxoplasmosis)

What classification of Protozoa are flagellated

Kinetoplastid

Which classification of Protozoa have an apical complex

Apicomplexa

How are helminths classified

Based off of simple morphology: roundworm or flatworm

Classifications & examples of helminths

1. Nematoda: intestinal nematodes

2. Platyhelminthes: Trematodes(Schistosoma mansoni) and Cestodes(Taenia Tapeworm)

Where do intestinal nematodes live

Small or large intestine

Where do trematodes such as the schistosoma mansoni live

Hepatic vasculature

Where do cestodes such as the taenia tapeworm live

Intestinal tract

what do ectoparasites feed on

animals

Classifications and examples of ectoparasites

• Insects: Lice and fleas

• Arachnids: Ticks

Where are parasites ubiquitous

Wildlife

Polyparasitism

Polyparasitism is a condition where an organism is simultaneously infected with two or more different parasites which is common in areas with a high level of parasitic infection such as the tropics.

Polyparasitism in the Wood Mouse

• 12 Nematodes

• 7 cestodes

• 4 trematodes

• 10 protozoa

Canine Helminth

Toxocara Canis

Cat Protozoan

Toxoplasma Gondii

Sheep Helminth

Echinococcus granulosis

What is Toxocara Canis

Predominant intestinal roundowrm found in dogs and foxes who become infected by ingesting eggs from soil.

Where is Toxocara Canis larvae in accidental human hosts.

Migrates in soft tissue.

Which humans are at most risk of Toxocara Canis

Young people with PICA who may eat soil.

Effects of Toxocara Canis

• Visceral Larva Migrans: Invades liver and causes fevers

• Ocular Larva Migrans: Invades the eye and impairs sight

What does the resilient coat of the Toxocara Canis do

Allows the eggs to survive in soil for moths and resist chemical treatments.

Life cycle of the Toxocara Canis

1. Eggs hatch in the stomach and release larvae

2. The larvae migrate to the lungs, liver or brain.

3. In dogs, the larvae migrate to the small intestine where they mature into adult worms

4. The adult females release eggs within faeces

What is different about the Toxocara Canis larvae in female dogs

The larvae remain arrested within the tissues and are reawakened when the female dog becomes pregnant as they respond to the pregnancy hormones. This means the larvae cross the placenta and infect puppies before birth or migrate into colostrum in order to infect milk.

How is Toxocara Canis controlled in dogs

Regular anthelmintic

Other name for Echinococcus granulosus

Hydatid Disease

what is Echinococcus granulosus

A rare sheep tapeworm disease

Life cycle of Echinococcus granulosus

1. Adult tapeworm in the small intestine of the definitive host releases eggs in the faeces.

2. The oncosphere of the egg hatches and penestrates the intestinal wall

3. Hydatid cysts form in various organs (usually liver and lung) of the intermediate host (sheep)

4. These cysts form adult tapeworms

Treatment of Echinococcus granulosus

• Surgery for hydatid cysts

• Tablets for dogs

• THERE IS NO CURE ONLY PREVENTION

Features of toxoplasma gondii

• Protozoan parasite

• Zoonotic

• Cosmopolitan

• Definitive host: cats

Definitive host meaning

Maintains the complete life cycle

For how long is an end stage host infected

For life

Life cycle of Toxoplasma Gondii

• Shed by definitive cat host less than 2 weeks after infection

• Oocysts shed in faeces for 24 ours and become infectious for humans

• Mice are also intermediate hosts which can reinfect cats

End stage hosts for Toxoplasma Gondii

Humans, pigs, sheep, cows

Tachyzoite meaning

Fast growing/multiplying

What do oocysts of toxoplasma gondii develop into

Tachyzoites to disseminate infection in the host

Stages of tachyzoïte dissemination

1. Attachment & invasion - broad cell specificity

2. Parasitophorous vacuole - avoids fusion with lysosomes

3. Rapid multiplication

Life cycle of toxoplasma gondii

• Oral ingestion of oocytes → bradyzoites → can reactivate to tachyzoites

What are bradyzoites

Dormant tissue cysts which cause chronic infection (eg. In brain)

What infection can tachyzoits cause in mammals

Congenital toxoplasmosis: Hydrocephaly, visual & mental impairment

Consequences of toxoplasma infection on pregnant women

The tachyzoites invade the foetus of a naïve mother and infect the brain in the 3rd trimester

Consequences of toxoplasma infection in mice

The cysts in their brain change their behaviour such as losing their fear of cats

How many people are infected with toxoplasma gondii

30% of humans

How do humans contract toxoplasma gondii

Cat litter, unwashed food, raw meat

When are humans more at risk of contracting toxoplasma gondii

Later in life, during pregnancy

Which ectoparasites are insects

Lice and fleas

Which ectoparasites are arachnids

Ticks

Head lice life cycle

• Egg (7-10 days)

• Nymph (10 days)

• Adult lays 50-150 eggs

What flea species are important to humans

1. Human flea (pulex irritans)

2. Cat flea (ctenocephalides felis)

3. Dog flea (ctenocephalides canis)

4. Oriental / tropical rat flea: xenopsylla cheopis

What ectoparasites transmit Lyme disease

Ticks (Ixodes ricinus)

What bacterium causes Lyme disease

Spirochete: borrelia burgdorfi

Symptoms of Lyme disease

• Erythema : bull's eye inflammation

• Subsequent myalgia, arthralgia, malaise

• Chronic arthritis, neuropathy, fatigue