Parasites

Question 1

Define parasite

Answer 1

An organism that grows either in or on another organism and DERIVES A BENEFIT from that relationship, while its HOST DERIVES SOME HARM

Question 2

Explain the importance of parasitic diseases for human health from a global perspective.

Answer 2

(1) VERY prevalent
(2) Major impact on morbidity, mortality, DALY, esp in 3rd world; malaria kills 1-2 million, mainly children, per year.
(3) Parasitic disease are uncommon in the US, thus outcomes are often worse than elsewhere due to unfamiliarity with the parasites – a good patient history, including travel, is very important in Dx.

Question 3

Explain why the life cycles of parasites are important for the geographic distribution, diagnosis, and control of parasitic diseases.

Answer 3

An understanding of life cycles is essential for understanding how parasites are transmitted to man and how such transmission can be minimized or prevented.

Many different parasites’ life cycles are spread throughout a variety of species or a variety of phases, all of which can provide points at which it is possible to prevent their contraction.

For example, since malarial parasites have to go back and forth between humans and a certain species of mosquitos, by eradicating mosquitos it is possible to eliminate malaria.

Unlike protozoans, most helminths go through a developmental cycle in the human host but do not reproduce there. This means the number of worms found in an infected human body - which is usually related to the severity of the disease manifestation - has less to do with time since infection and more to do with the extent of the original exposure.

Question 4

Describe the taxonomic groups of helminths

Answer 4

1. Roundworms/Nematodes (Ascaris, pinworms, whipworms, hookworms, Strongyloides, Echinococcus, Trichinella, filarial worms, etc.)
2. Flatworms/Trematodes/Flukes (Schistosomes, lung fluke, etc.)
3. Tapeworms/Cestodes (beef tapeworm, pork tapeworm, fish tapeworm, etc.)

Question 5

Describe the taxonomic groups of protozoa

Answer 5

1. Amebas (Entamoeba histolytica, etc.)
2. Flagellates (Giardia, Trichomonas, Trypanosoma, Leishmania, etc.)
3. Ciliates (Balantidium coli)
4. Sporozoa (Cryptosporidium, Cyclospora, Plasmodium, Toxoplasma, etc.).
5. Microsporidia (Enterocytozoon, Septata, etc.)

Question 6

Explain the pathogenesis of schistosomiasis

Answer 6

Sub-Saharan Africa.

1. Live in fresh water
2. Hatch eggs, infect snails
3. Mature in snail, released back into water
4. Penetrate human skin on contact/ingestion
5. Lose tail, circulate to mesenteric veins, live there for years
6. Lay eggs that circulate to the liver and are shed in stool (one species goes the bladder and causes probs there)

Eggs laid in the liver cause hepatic scarring (portal hypertension, esophageal varices, etc) and severe ascites.
After irreversible liver damage has occurred, eradication of the worms does not resolve the problems.

and the liver, where they trigger the inflammatory response responsible for much of the damage caused in the disease.

Eggs can’t mature in the human body, so the number of adult worms in the body depends on the extent of exposure, not time since infection.

The histopathology of the inflamed tissue shows enormous numbers of eosinophils (which, as we know, is a marker for parasite infection).

Schistosomiasis control strategies are largely to avoid contact with infected water, mass treatment of population with anti-schistosomal drugs, use of snail-killing agents (which seems unfortunate for the hapless snails).

Question 7

Describe Malaria infection

Answer 7

Mosquito infects human with mature bug (Sporozoite)
Sporo migrates to hepatocytes to replicate and lyse the cell, releasing merozoite (immature form)
Meros get into bloodstream where this process repeats over and over in RBCs (secondary replication)
Some progeny form gametes in RBCs, which are later picked up by another mosquito
Gametes mate and mature inside mosquito’s stomach
Mature bugs make their way to the mosquito salivary glands

Complications include anemia, hepatosplenomegaly (esp spleen), microcirculation issues.

You can develop immunity, but need constant exposure to keep it going. Leave the endemic area and it’s gone.

Question 8

Describe some of the major current challenges to treatment and prevention of parasitic diseases.

Answer 8

Many parasites evade or subvert the protective immune responses of their hosts, and damage to host tissues is often the result of host immune responses.

Diseases caused by many parasites become clinically apparent when the number of parasites (the parasite burden) is high or when infection persists for long periods of time.

Development of effective vaccines against the major parasitic diseases of humans has been difficult and remains an important goal for world health. control measures deal with environmental (exposure) control or treatment

Because parasitic diseases are caused by eukaryotic pathogens, the biological bases for selective toxicity of anti-parasite drugs are quite different from those described previously for anti-bacterial and anti-viral drugs.