Parasitology - Hookworms

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Necator americanus and Ancylostoma duodenale (Hookworm)

Infection

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Infection is acquired through the active penetration of filariform larvae into the skin. Note the long slender appearance, short esophagus

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Necator americanus and Ancylostoma duodenale (Hookworm)

Adult parasite

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The adult Necator americanus suck blood, but feed on villus tissue

Male and female worms: the female's tail is pointed, whereas the male worm has a hand-like posterior bursa

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Necator americanus and Ancylostoma duodenale (Hookworm)

Pathology

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Iron-deficiency anemia is the major consequence of infection, and arises from the adult worms' penchant for sucking blood

First bite of the worm into intestine causes pain. Adult suck blood to obtain O2 from blood and munch on villus tissue. Secrete anti-coagulant and once the worm moves on, the location still bleeds for 3 days, so the host continues to lose blood. Necator americanus sucks 10 times less blood than Ancylostoma duodenale.

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Necator americanus and Ancylostoma duodenale (Hookworm)

Diagnosis

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Hookworm eggs in feces: there are several nematode eggs that can be confused with those of hookworm. Their presence may lead not only to false diagnosis, but also to what appears to be failure of treatment

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Necator americanus and Ancylostoma duodenale (Hookworm)

Overview

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The worms inhabit the small intestine and feed on host intestinal mucosa and blood.

Leads to the development of the outhouse because the worms can only crawl 4 feet. And also why southern men are so sloth like.

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Necator americanus and Ancylostoma duodenale (Hookworm)

Life Cycle

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Infection begins when the filariform larvae actively penetrate the cutaneous tissues, usually through a hair follicle or an abraded area. Skin invasion may be facilitated by release of larval hydrolytic enzymes. Once in subcutaneous tissues, the larvae enter capillaries and are carried passively through the bloodstream to the capillaries of the lungs. The 3rd stage larvae break out of the alveolar capillaries and complete the migratory phase of the life cycle by crawling up the bronchi and trachea over the epiglottis and into the pharynx. There they are swallowed and then proceed into the stomach. Ancylostoma duodenale larvae are also infective orally. Following maturation and copulation, the female worms lay eggs. The eggs pass out in the feces and larvae hatch and develop in the soil and then wait for contact with a host.

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Necator americanus and Ancylostoma duodenale (Hookworm)

Life Cycle in endemic areas

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In endemic areas, where reinfections are continual, the larvae of A. duodenale (but not N. americanus) is interrupted. After entering the host, these larvae penetrate into bundles of skeletal muscles and become dormant. They can later resume their development and complete the life cycle. Larval arrest in human tissue occurs during times of the year when the external environment is unfavorable for worm development in the soil. Larval arrest also occurs during pregnancy, and continues after parturition.

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Necator americanus and Ancylostoma duodenale (Hookworm)

Clinical disease

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Iron deficiency anemia, failure to thrive syndrome (idiopathic endocrinopathy)

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Necator americanus and Ancylostoma duodenale (Hookworm)

Treatment

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Mebendazole

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Necator americanus and Ancylostoma duodenale (Hookworm)

Life CycleNecator americanus and Ancylostoma duodenale (Hookworm)

Prevention and Control

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Invention of outhouse, sanitary disposal of feces

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Ancylostoma caninum

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Ancylostoma of dog. Can't differentiate dog from human skin. Once inside the human, can tell. In human it crawls around trying to escape, and releasing acidic components that cause an immune reaction. Causes a "creeping eruption"

Tx: Thiabendazole - it has anti-inflammatory properties and is absorbed through the skin to kill the worm.

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Strongyloides stercoralis

Infection

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Infection is acquired by penetration of the skin by the filariform larva found in fecally contaminated soil. This stage may also be ingested from contaminated water.

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Strongyloides stercoralis

Adult parasite

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Only the female adult is parasitic for humans, whereas both male and female free-living adults can be found in fecally contaminated soil

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Strongyloides stercoralis

Pathology

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Worms invade epithelial cells, induce cell death

Diarrhea of three to six weeks duration.
Malabsorption syndrome
Bacterial sepsis is a frequent sequela in hyper-infective strongyloidiasis" Secondary bacteremia/septicemia as larvae migrate throughout body and defecate microbes that they ingested in the large intestine. Death may occur due to overwhelming bacterial septicemia.

An intense inflammatory process in the lamina propia may also be seen

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Strongyloides stercoralis

Diagnosis

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The rhabditiform larva is the stage of this parasite that is commonly seen in freshly passed feces. This larval form must be distinguished from the larva of hookworm. Microscopic examination. Hard to see, so 6 stool samples is needed.

"String" test: lower string into small intestine and let it stay overnight and take end and put it on a slide to see if a worm can be seen

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Strongyloides stercoralis

Overview

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Parasitic nematode with worldwide distribution, but is particularly prevalent throughout tropical and subtropical regions.

Dogs and non-human primates can serve as reservoir hosts, and they can undergo a faculatative, free-living phase in soil, allowing for many potential sources of human infection.

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Strongyloides stercoralis

Life cycle

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Strongyloides stercoralis exists both as a free-living and a parasitic animal. The nematode lives embedded within a row of columnar epithelial cells in the small intestine, usually in the lamina propria of the duodenum and proximal jejunum. Reproduction is by parthenogenesis. There is apparently no parasitic male. The embryonated eggs hatch rapidly into first-stage larvae, which emerge into the lumen of the small intestine and proceed into the colon. There, they molt once, becoming second-stage rhabditiform (stumpy) larvae. Ordinarily, the rhabditiform larvae are deposited in soil with feces. Alternatively, they may molt into third-stage filariform (long, slender) larvae while still within the lumen of the colon, burrow into the mucosa, and enter the circulation directly. This aspect of the parasitic phase of the life cycle is known as autoinfection. Free living phase rhabiditiform larvae require warm, moist, sandy or loamy soil for the next developmental phase of the cycle to take place. In the proper soil, and under optimal environmental conditions, they develop to free-living adult worms. Adult worms of both sexes are found during the free-living phase. They mate, and the female produces embryonated eggs that hatch and molt twice to 3rd-stage filariform larvae (infectious stage). Usually, this happens when conditions become unfavorable for the continuation of the free-living phase (e.g. lack of nutrition, low moisture). The third-stage filariform larvae can remain in soil for several days. When the infective larvae encounter a suitable host, they penetrate the skin (can be unbroken skin) and begin the parasitic phase of the infection. Filariform larvae can also "swim" in aquatic environments, giving them a greater range in which to find a host as compared to hookworm hilariform larvae, which cannot do so. If filariform larvae fail to locate a host within 3 days, they expend all their stored glycogen and die.

Parasitic phase (homogenic life cycle): Third stage filariform larvae enter the host through the skin, a process facilitated by the release of a protease by the parasite. Upon entry into the host, the larvae probably enter venules and/or lymphatics before they are carried through the afferent circulation to the right heart, pulmonary artery, and pulmonary capillaries. The larvae rupture into the alveolar space, actively crawl up the respiratory tree, pass through the trachea into the pharynx, cross the epiglottis, and are swallowed. S. stercoralis may not always migrate through the lungs to reach the intestinal tract. The larvae undergo a final molt in the small intestine and become parasitic females. They begin egg production within 25-30 days after the initial infection.
Autoinfection, hyperinfection, disseminated infection: In some groups of patients, rhabditiform larvae develop within the colon to the infective filariform stage. The infective larvae may reenter the circulation before they migrate through the lungs and are swallowed. THis process is referred to as autoinfection. By this means, the parasite can cycle through the human body for many years

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Strongyloides stercoralis

Treatment

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Ivermectin - veterinary drug for parasites and humans


MOA: blocks Cl- ion channels, inhibits gama-aminobutyric acid receptor complex. neurotransmitter inhibitor that does not cross BBB.

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Strongyloides stercoralis

Prevention and Control

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Sanitary disposal of human feces

Worm can be transmitted through dog feces, but cannot control spread of dog feces.