12. Nematodes (infective larvae) Flashcards
Which nematodes are infective in their egg stages?
Trichuris trichiura
Ascaris lumbricoides
Enterobius vermicularis
which nematodes are infective in their larval stage (L3)?
Necator americanus
Ancylostoma duodenale
Strongyloides stercoralis
Trichinella spiralis
which is the new world hookworm?
Necator americanus
what is the old world hookworm
Ancylostoma duodenale
what is the most prehistoric evidence for hookworm disease?
coprolites - fossilized human feces
Where have coprolites been identified?
NA, SA and europe
where are the oldest caprolites from and how old are they?
Brazil, over 7000 y.o.
which is the second most common nematode in humans?
hookworm
what is the “Order” of hookworms
Strongylidae - large order containing important pathogens of man and domestic animals
what are the Superfamilies of Order Strongylidae?
- The Superfamily Strongyloidea (the hookworms in man)
- The Superfamily Trichostrongyloidea, intestinal nematodes important in many domesticated animals (e.g. Haemonchus brasiliensis in rodents)
- The Superfamily Metastrongyloidea (the lungworms, in domesticated animals)
what are the two hookworms capable of causing infection in humans?
Ancylostoma duodenale
Necator americanus
what determines where you will find which worms?
physical landscape: geographic features
- soil
- climate
what is required by the first two larval stages of hookworms L1 and L2 in order to undergo the first two molts, M1 and M2, to yield the L3 stage?
A. duodenale and N. americanus, the larvae require soil containing an equal mixture of sand and silt and a smaller proportion of clay
how long are hookworms
0.5 to 1.5 mm long,
A. duodenale slightly longer than N. americanus
what is found at the posterior end of the male worm? (nematodes)
a characteristic copulatory bursa, used to catch and hold the female nematode during mating
what is found near the center of the female worm, slightly anterior in NECATOR, slightly posterior in ANCYLOSTOMA
a vulva
what is the habitat of adult hookworms
attach to the wall of the small intestine, the jejunum - usually attached to the mucous layer
what is the morphology of the eggs in hookworms
- bluntly rounded,
- thin shelled,
- and are almost - indistinguishable between the different species,
whos eggs are larger, Ancylostoma or Necator?
Ancylostoma
how do the L3 larvae of hookworms usually infect humans
the eggs hatch in the soil and they usually burrow through the feet
what is the path the L3 take through the DH body
- infect man by usually burrowing through the soles of the feet
- the larvae then migrate to infect the heart and lungs before passing into the trachaea, pharynx and then the small intestine
how does an accidental host occur in hookworms and what is the hookworm’s fate?
The larvae of several species of hookworms infecting domesticated animals may penetrate human skin, causing pathology even though they do not develop the adult parasites in man
describe the life cycle of hookworms
- Eggs are passed in the stool
- Under favourable conditions; moisture, warmth, shade, larvae hatch in 1-2 days
- The released Rabditiform larvae (L1) groe in feces and/or the soil
- After 5-10 days (and 2 molts) they become filariform larvae (L3) that are infective
- These infective larvae can survive 3 to 4 weeks in favourable environment conditions
- On contact with the human host, the larvae penetrate the skin and are carries through the veins to the heart and then to the lungs
- They penetrate into the pulmonary alveoli, ascend the bronchial three to the pharynx and are swallowed
- The larvae reach the small intestine, where they reside and mature into adults
- Adult worms live in the lumen of the small intestine, , where they attach to the intestinal wall with resultant blood loss by the host
- Most adult worms are eliminated in 1 to 2 years, but longevity records can reach several years
what happens sometimes to A. duodenale larvae?
- Some A. duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle)
what is a possible route of transmission for A. duodenale?
- In addition to penetrating the host skin, A. duodenale infection may also occur by the oral and trans-mammary route.
- N. americanus, however, requires a trans-pulmonary migration phase – (if it doesn’t do the transpulmonary… will not mature into the adult worm)
what does CLM stand for?
cuntaneous larval migrans
what is CLM also knows as?
creeping eruptions
what is CLM? give an example
is a zoonotic infection with hookworm species that do not use humans as a DH
(i.e. humans are the accidental host), the most common being A. braziliense and A. caninum.
what is the name for L3 larvae
filariform larvae
what are the two areas of pathology for hookworm infection?
First: the pathology associated with the presence of the adult parasite in the intestine
Second: the pathology associated with the penetration and migration of the larval worms within the skin
what is used by the hookworm to attach to the intestinal wall ?
their buccal capsules
how do the hookworms obtain nutrients once they are the adult worm in the intestine?
the hookworm mouthparts penetrate blood vessels, and the parasites obtain nutrition by sucking blood
how much blood does Necator americanus take: per day, per worm?
30µl
how much blood does Ancylostoma duodenale take: per day, per worm?
up to 260 µl
what does a heavy infection lead to?
chronic iron deficiency anemia
what is ground itch? in what infection is it common?
an allergic reaction at the site of parasitic penetration and entry
is common in patients infected with N. americanus
what can result if the larvae begin to break into alveoli and travel up the trachea?
cough and pneumonitis
how is hookworm infection diagnosed?
- Characteristic worm eggs on microscopic examination of the stools, although this is not possible in early infection
are the eggs of N. americanus and A. duodenale distinguishable?
no
how can you identify the genus of a hookworm?
culture to release the larvae
how do BZAs work?
- kill adult worms by binding to the nematode’s β-tubulin and subsequently inhibiting microtubule polymerization within the parasite
- This blocks the uptake of glucose and other nutrients, resulting in the gradual immobilization and eventual death of the worm
at what stage is albendazole effective (in this case we are referring to hookworms, although it is used in many infections)?
intestinal (adult) stage and during the stage that the parasite is still migrating under the skin (L3)
what is the common name for Strongyloides stercoralis?
Threadworm
where is the habitat of Strongyloides stercoralis?
The adult parasite lives in tunnels in the mucosa od the small intestine, or is free-living
what is the morphology of the Strongyloides stercoralis eggs?
- ellipsoid
- thin wall
- containing larvae
where do the Strongyloides stercoralis eggs hatch?
in the intestine of the DH, the feces will reveal the L1 rhabditiform larvae
where can L1 larvae be found? what is it called?
in freshly isolated feces
rhabditiform larvae
what makes the life cycle of Strongyloides stercoralis more complicated than other nematodes?
it alternates between free-living and parasitic cycles, and its potential for autoinfection and multiplication within the host
describe the free-living life cycle of Strongyloides stercoralis
- The rhabitiform larvae (L1) passed in the stool (1) – [parasitic cycle] – can either molt twice and become infective larvae, L3 (direct development) (6) or molt four times and become free-living adult males and females (2) that mate and produce eggs (3) from which rhabditiform larvae hatch (4)
- The rhabditiform larvae in turn can either develop (5) into a new generation of free-living adults – [as represented in (2)] – or into infective filariform larvae, L3 (6)
- The filariform larvae (L3) penetrate the human host skin to initiate the parasitic cycle (6)
describe the parasitic cycle of Strongyloides stercoralis
- Filariform larvae (L3) in contaminated soil penetrate the human skin (6), and are transported to the lungs where they penetrate the alveolar spaces; they are carried through the bronchial tree to the pharynx, are swallowed and then reach the small intestine (7)
- In the small intestine, they molt twice and become adult female worms (8)
- The females live threaded in the epithelium of the small intestine and by parthenogenesis produce eggs (9), which yield rhabditiform larvae (L1)
- The rhabditiform larvae (L1) can either be passed in the stool (1) – [free-living cycle] – or can cause autoinfection
describe the parasitic cycle of Strongyloides stercoralis in autoinfection
In autoinfection, the rhabditiform larvae (L1) become infective filariform larvae (L3), which can penetrate either the intestinal mucosa (internal autoinfection) or the skin of the perianal area (external autoinfection); in either case, the filariform larvae may follow the previously described route, being carried successively to the lungs, the bronchial tree, the pharynx, and the small intestine where they mature into adults; or they may disseminate widely in the body.
how does ivermectin work on Strongyloides stercoralis
- Ivermectin kills by interfering with nervous system and muscle function, in particular by enhancing inhibitory neurotransmission
- The drug binds and activates glutamate-gated chloride channels (GluCls)
