37-38: Parasites Flashcards
definitive hosts
hosts in which the adult form matures
typically where sexual reproduction takes place
intermediate hosts
hosts which have larval or cyst form
parasitic worm infection statistics
1.5B people infected with soil-transmitted helminths worldwide
10-20% of the population infected with worms at any given time
human-human transmission vs zoonoses
some worm infections have high human-human transmission and are found primarily in humans
- humans as definitive or intermediate hosts
other helminths are zoonoses with distinct animal reservoirs and limited transmission to and between humans
soil-transmitted nematodes
ascaris, trichuris and hookworm
what are helminths divided into?
nematodes
- round worms
cestodes
- flat worms
trematodes
- flukes
how are soil-transmitted nematodes transmitted?
fecal-oral
lack of good sanitation
where do parasitic worms live?
mostly intestine
- ability to release eggs into the feces
- also a constant source of food
worms that have evolved to live outside the intestine
schistosoma worms
- blood flukes that reside in blood vessels
guinea worms
- peripheral tissues
intestinal worms
- traffic through lungs or other tissues as part of the life cycle
pinworm (nematode)
1 worm in the US
most common worm infection in the US
- prevalence in children can be as high as 50%
doesn’t cause harmful disease/cyst
no animal reservoir
- fecal-oral even with good sanitation
pinworm transmission
not a zoonosis
people infected by ingesting egg on fingers, clothing and other surfaces
childcare centres as common sites of outbreaks
how do we diagnose pinworms?
itching and examining for eggs by tape test
ascaris lumbricoides (nematode)
most common worm infection in the world
- more than 1B infections at any given time
human-only worm found everywhere
large round worms living in the small intestine
ascaris lumbricoides life cycle
larvae mature in liver, travel out capillaries into lungs where they are coughed up and swallowed to enter intestine (9 days after initial infection)
mature over 8-12 weeks into adults
eggs come out in the feces
can be prevented with good sanitation
ascaris lumbricoides statistics
807M-1.2B people infected
- most don’t get disease but just infection
<5% have disease
<0.5% have death or mortality
- still a lot with 1B infections
- more people die from ascaris infection then ebola
diagnosis of ascaris lumbricoides
finding eggs in stool or worms expelled
hookworms (nematodes)
parasites are transmitted through contaminated feces or from contact with larval form in soil
- can penetrate unbroken skin and doesn’t have to be swallowed
hook onto intestinal wall with teeth
ingest RBCs for nutrition
ancylostoma hookworms
can digest its way through human skin which is uncommon
hookworms eating
penetrate the intestine, find a blood vessel then suck in blood from the blood vessel
leads to a higher symptomatic rate because people have symptoms from anemia
trichinella (nematodes)
classic larval cyst
larvae make cysts in muscle that can be transmitted to humans when they eat undercooked pork
trichinella life cycle
rodents which are carnivorous
pigs interact with feces or eat the cyst, so the pig has cysts and then the human eats the pig
mostly, humans don’t go back and infect rodents
zoonosis from a pig/rodent or rodent/rodent cycle
filarial worms (nematode)
mosquito-born worm transmitted by mosquitos
humans are definitive hosts
filarial worms can cause lymphatic filariasis also known as elephantiasis
microfilariae secreted by adults into blood for uptake by mosquitos
- small larval forms which serve the purpose of eggs
microfilariae can build up in lymphatic vessels and block fluid uptake along with other infections
- creates hardened tissue
filarial worms as high on the list of neglected tropical diseases
100M infected, most problematic in SE Asia and Indonesia
worm infections can last for months or even years
guinea worm (nematode)
caused by ingestion of drinking water with copepods
primarily a zoonosis but humans can pass it to copepods who pass it to other humans
guinea worm infection
female worm extends anterior end through skin and forms a painful lesion up to 1 year after infection
burning sensation so you try to relieve it with water
water is a trigger for the guinea for to extend and release larvae into water
guinea worm removal
worm twisting
removal has to be gradual to prevent breaking off worm under skin
guinea worm is close to eradication
millions of infections in the 1980s down to 9 in 2022
still 490 infections in animals
- still has potential to infect humans
cestodes
flat segmented worms
primarily intestinal dwelling tapeworms
true parasites in that they lack a mouth, alimentary (intestinal/digestive) tract, circulatory system, body cavity and major metabolic pathways
absorb nutrient molecules through their body surface (tegument)
tapeworm anatomy
scolex is the anterior-most portion of the worm
- head
scolex armed with large hooks that alternate with small ones
neck followed by a chain of flat, ribbon-like segments
- proglottids
tænia life cycle
human agricultural disease
not a zoonosis
two species
- one passed through cows, other through pigs
- most common through pigs
cysts form in animal meat so if not cooked well enough, larval form and cyst are activated to become adult form
tapeworm infections cause taeniasis and worms are found worldwide
cysts take over muscle cells and form an inter-larval form that is like sleeping larvae
taeniasis occurs when raw/undercooked beef or pork is eaten
humans are definitive hosts but can also be intermediate hosts if infected with cysts
taenia sodium and cysticercosis
cysticercus in the brain can lead to neurological damage
cysts can go into a muscle cell and stay there forever
neurocysticercosis and epilepsy
when people have worms or are exposed to eggs, and there are cysts in the brain, this can be the greatest cause of acquired epilepsy
cysts in the brain can cause neurocysticercosis
neurocysticercosis and epilepsy
when people have worms or are exposed to eggs, and there are cysts in the brain, this can be the greatest cause of acquired epilepsy
cysts in the brain can cause neurocysticercosis
neurocysticercosis statistics
2.5-8.3M
trematodes (schistosomes)
blood flukes
life cycle includes a replication stage in water snails
- snails as intermediate hosts or animal vectors
schistosoma species
2 major forms of schistomiasis (intestinal and urogenital) caused by 5 main species of blood fluke
most common is the schistosome mansoni found in most of the world
schistosome life cycle
worm that can penetrate skin, lose their tail, go through circulation, go to the blood and live in the blood vessels of the liver or intestine
female lays egg, male fertilises the eggs which then have to get from the blood stream, penetrate through intestines and cell walls to the feces or urine
schistosoma disease
adult worms do not cause disease
- eggs released into tissues cause disease
eggs cause damage to the tissue and leak fluid from the blood into the peritoneum so the abdomen is filled with fluid
schistosoma granulomas
trap everything and form a granuloma around the egg
or the egg is successful, digests through the tissue and its released
inflammation causes tissue damage, but even if we try to get the worm out, it doesn’t do anything for adult worms which we have no immune response to
