Gastrointestinal Parasites

Question 1

Giardia lamblia

Answer 1

The intestinal protozoan that causes giardiasis. Flagellate protozoan distributed all over the world. Giardiasis is a zoonosis, and it may be acquired by ingestion of water contaminated by feces from animal or human carriers.

Question 2

Source of G. lamblia infection

Answer 2

Acquired by ingestion of the cyst form of the parasite, often from water contaminated with feces. They may also be transferred via oral-anal route. Can be transmitted in cold and warm climates. They are resistant to chlorine.

Question 3

Giardiasis symptoms

Answer 3

Produces a mild but persistent diarrheal illness. Signs of malnutrition due to malabsorption may occur in chronic cases.

Malabsorption of fats can lead to greasy, foul-smelling stools, diarrhea associated with unabsorbed fatty acids in the lumen, deficiencies of fat-soluble vitamins (A, D, E, and K)

Question 4

G. lamblia-mediated damage

Answer 4

• Stomach acid stimulates the cysts to transform into the vegetative trophozoite form in the duodenum
• Giardia trophozoites attach to the epithelium using a sucking disk
• Not invasive and does not produce bloody diarrhea or metastatic infection.
• Host response characterized by submucosal infiltration of chronic inflammatory cells and effacement of normal villi, which results in malabsorption

Question 5

Diagnosing giardiasis

Answer 5

Direct identification of parasites in stool or in duodenal aspirates. It may take three or more stool examinations to find cysts in an infected patient or carrier

Antigen detection assays for giardiasis are available and more sensitive.

Question 6

Treatment of giardiasis

Answer 6

Metronidazole

Even still, relapses may occur

Question 7

Metronidazol

Answer 7

Metronidazole is of the nitroimidazole class.

It inhibits nucleic acid synthesis by disrupting the DNA of microbial cells. This function only occurs when metronidazole is partially reduced, and because this reduction usually happens only in anaerobic bacteria and protozoans, it has relatively little effect upon human cells or aerobic bacteria.

Question 8

Prevention of giardiasis

Answer 8

Boiling or filtering drinking water from lakes or streams, or treating it with adequate amounts of iodine can prevent giardiasis

Question 9

Helminths

Answer 9

Can be divided into intestinal helminths and tissue and blood helminths. The largest parasites which affect humans. Often cause chronic, well tolerated infections. May cause morbidity in large numbers in the form of malnutrition or substantial host immune response.

Reproduce sexually, and therefore a human must harbor both a male and female in order for eggs to be produced (Tapeworms are hermaphroditic, and are thereby an exception to this rule).

Question 10

Major types of helminth relevant to medicine

Answer 10

• Roundworms: Cylindrical rather than flattened. The body wall is composed of a cuticle that sometimes has ridges called alae. The alimentary canal is complete
• Flukes: Dorsoventrally flattened and symmetric body. They possess both an oral and ventral sucker. They lack a body cavity.
• Tapeworms: Flattened, elongated, and consist of segments called proglottids. They have a scolex (head) which bears organs of attachment, a neck (where segments proliferate), and a chain of proglottids called the strobila. There is no alimentary canal. Hermaphroditic.

Question 11

Helminth life cycle

Answer 11

Critical stages of development take place in exterior, arthropod, or animal reservoires. For this reason, human-to-human transmission is impossible.

Generally, established helminths within humans are not eliminated by the immune response. Most helminth infections resolve spontaneously when the adult parasites reach senescence (after a few months or years, they die of old age).

Question 12

Definitive host

Answer 12

Harbor the adult, sexual form of the parasites

Question 13

____ is often regarded as a characteristic host response to parasitic infection.

Answer 13

Eosinophilia is often regarded as a characteristic host response to parasitic infection.

Question 14

Ascaris lumbricoides

Answer 14

One of the largest of the human parasites (up to ~30cm). Frequently encountered worldwide.

Affects ~25% of humans worldwide, especially prevalent in the southern United States.

Question 15

Source of A. lumbricoides infection

Answer 15

Eggs present in stool of infected. Require several weeks in warm environment to mature to infective phase. So, restricted to warm climates where soil is contaminated with untreated human feces. The eggs must be ingested to complete the cycle.

Soiled hands in the mouth or by eating food contaminated with soil containing eggs. Fruits and vegetables growing near to the ground become contaminated by direct contact with fecally contaminated soil. Newly acquired worms pass through the lung before settling in the GI tract

Question 16

A. lumbricoides-mediated disease and damage

Answer 16

A few Ascaris are generally well tolerated, but a large worm load can cause serious illness. May result in transient pneumonia (as new worms pass through lungs). May be quite severe if the patient has been previously sensitized.

Intestinal obstruction may occur if GI load is high. May cause peritoneal inflammation or bile duct obstruction by moving about and penetrating the intestinal wall. May result in malnutrition.

Question 17

Diagnosing ascariasis

Answer 17

Diagnosed by stool examination. Each adult female worm releases approximately 200,000 eggs per day into the intestine. Adult worms can be passed in stool, and are readily identified

Question 18

Treatment of ascariasis

Answer 18

Several anti-parasitic drugs are available and effective in treating Ascaris infection of the GI tract. Medical treatment with the anti-helminthics typically relieves intestinal obstruction without surgical intervention.

Question 19

Intestinal tapeworms

Answer 19

Transmitted between humans (in which they usually reside in the GI tract) and an animal or fish host (in which they reside in the muscle and other tissues.)

Attaches to the intestinal wall by a head (scolex) that has sucking disks or grooves.

In their intermediate animal host, tapeworms penetrate deep tissues and develop into infective, cystic larval forms.

Question 20

Source of tapeworm infection

Answer 20

GI infection with the most common human tapeworms are acquired by eating uncooked or inadequately cooked beef, pork, or fish.

Cows and pigs become infected by ingesting human feces containing the parasite’s eggs. Thus these infections are transmitted when humans defecate in areas where these domestic animals graze or feed.

Cysticercosis is a special case, meriting its own discussion.

Question 21

Tapeworm-mediated disease and damage

Answer 21

Larvae hatch in human intestine and mature into adults in GI lumen. Can live within a human host and grow for decades.

Most patients are asymptomatic, but some have nausea, diarrhea, and weight loss. The infection is usually noted only because of the presence of motile tapeworm segments (proglottids) in the stool

Question 22

Diagnosing tapeworm infection

Answer 22

• Most tapeworm infections are diagnosed by stool examination.
• Proglottids are macroscopic and can be seen by the naked eye. The eggs are large enough (31 to 43 μm in diameter) to be seen using lowpower magnification
• Although the eggs of pork and beef tapeworms are identical, their proglottids can be distinguished by the experienced observer.

Question 23

Treatment of tapeworm infection

Answer 23

Most patients (>90%) with intestinal tapeworms are cured with a single dose of an anti-parasitic medication. Those who are not cured often have nausea or vomiting with their first treatment and typically respond to a second treatment with the drug.

Question 24

Prevention of tapeworm infection

Answer 24

Intestinal tapeworms can be prevented by cooking meat to kill the parasites, or by keeping human feces from contaminating livestock feed or grazing areas

Question 25

Cysticercosis

Answer 25

The larvae of a these pork-transmitted species of tapeworm infect deep tissues of humans and cause diseases that may have severe manifestations.

Don’t get confused: the pork tapeworm, T. solium, can cause either GI infection or deep tissue infection. Which of these it causes depends on whether it is acquired from the pig (in pork) or human (by the fecal-oral route).

Question 26

Source of Taenia solium infection

Answer 26

• Human cysticercosis is acquired by ingesting Taenia solium eggs from the feces of humans who harbor an adult tapeworm
• Cysticercosis can be spread from one person to another person, or it may develop endogenously in a person who carries a tapeworm (autoinfection)

Question 27

Cysticerosis-mediated disease and damage

Answer 27

• Eggs hatch after passing through stomach acid.
• Larvae lodge under skin or organs, develop a cyst wall and fibrous capsule (courtesy of host chronic inflammation).
• When cysts die, they cause potent acute inflammation. Also magnifies tissue displacement
• Symptoms may not become apparent for as long as ~50 years post infection
• Cysts outside CNS rarely produce symptoms.
• Cysts within CNS cause cerebral dysfunction, seizure, blindness, elevated intracranial pressure

Question 28

Diagnosing Cystericerosis

Answer 28

Because cysticerci can be present in patients without evidence of intestinal infection, T. solium cysticercosis is usually diagnosed by its deep tissue manifestations (including brain lesions visible by CT scan).

A positive serologic test for antibodies to T. solium is helpful, especially among persons who live in areas of low incidence. The test is typically negative in persons with intestinal T. solium infection only.

Question 29

Treatment of Cystericerosis

Answer 29

Several anti-parasitic drugs are effective in the treatment of cysticercosis. Drug treatment kills the organism and decreases the size of the lesions. During treatment, central nervous system symptoms may transiently worsen because of the inflammatory response to dying cysticerci.

The concomitant use of steroids typically alleviates the headaches and seizures that the treatment can cause

Question 30

Prevention of Cystericerosis

Answer 30

Steps which reduce intestinal T. solium infection will prevent cysticercosis as well. Steps to prevent fecal-oral spread will also prevent cysticercosis

Question 31

Ascaria lumbricoides lifecycle

Answer 31

Question 32

Cysticercosis is acquired by ____, while intestinal Taeniasis is caused by ____.

Answer 32

Cysticercosis is acquired by ingestion of eggs, while intestinal Taeniasis is caused by ingestion of larval cysts.

Question 33

Giardia uptakes ____ to prevent the activity of intestinal lipases.

Answer 33

Giardia uptakes bile salts to prevent the activity of intestinal lipases.

This is what causes the poor fat and fat soluble vitamin absorption in cases of chronic giardia.

Question 34

If you ingest T. solium from pork, ____.

If you ingest T. solium from human feces, ____.

Answer 34

If you ingest T. solium from pork, you will have a GI infection.

If you ingest T. solium from human feces, you will have a systemic infection.