Pulse - October 2024 - 19

INDUSTRY INSIGHTS
Definitely Definitive or Definitively
Intermediate or Definitively Confused?
Christopher A. Lee, DVM, MPH, DACVPM
I
n August, People Magazine reported a case
of veterinary importance: Doctor Shares
Horrifying Scan of Patient's Legs Filled with
Parasites After They Ate Undercooked Pork.
By overly simplifying the pork tapeworm's life
cycle, the article risks misleading the reader:
Cysticercosis is a tapeworm infection that affects the brain, muscles,
and other tissues. It is caused by ingesting larval cysts of the
pork tapeworm (Taenia solium).
" Humans become infected with T. solium by ingesting cysts that
can be found in undercooked pork, " [Dr. Sam Ghali] explained.
The larvae " penetrate the intestinal wall and invade into the bloodstream
and from there can spread to literally anywhere in the entire
body. " - Etienne, V. (2024, August 28)
Did the individual obtain T. solium cysticerci (plural for cysticercus)
within his legs from dining upon undercooked pork?
" Yes. "
" No. "
" Well, it's complicated as some parasites deem an individual
as their intermediate host in one context and their definitive in
another. In this case, the cestode did both! "
The definitive host supports the parasite's maturation and sexual
reproduction. The intermediate host serves as a vessel for maturation
and/or asexual reproduction and transfers the parasite to the
definitive host.
A romantic cestode longs for a warm, long-lasting home where
they can sexually produce and raise their children. T. solium naturally
wishes the best for its definitive home or host. It anchors to
the warm, pink mucosa of the host's small intestine, with its armed
rostellum extending from the scolex or head. From the scolex, the
proglottid segments will grow, mature, and sexually reproduce.
The tapeworm absorbs the food within the lumen of the small
intestines without sucking blood or invading tissues. Typically, the
definitive host demonstrates no clinical signs.
The relationship perspective of the parasite toward the intermediate
host is often more sinister. For the cestode to establish
itself in its dream home, the intermediate host must become the
definitive host's dinner. Thus, T. solium hopes the pig becomes an
undercooked pork loin meal for a human definitive host.
Contrary to many helminth-intermediate host relationships, T.
solium causes few, if any, clinical signs in the pig. Does the tapeworm
know that the definitive host, H. sapiens, is highly particular
about the look of its food and prefers its prey in near perfect health?
Once a human eats pork with a viable (uncooked) cysticercus, the
PULSE
OCTOBER 2024
cestode nestles into the small intestine for what they hope is a long,
uneventful human life.
But what if the human stepped into the pig's cycle? Instead of a
definitive human eating an intermediate pork loin, an intermediate
human eats the infective egg from fecal-oral contamination.
After the tapeworm matures, the person's feces contain a steady
supply of eggs, which may be conveyed to the mouth by a lapse
in personal hygiene. When the eggs reach the stomach, the oncospheres
hatch out, enter the gut wall, and wander far and wide in
the body, slowly developing into cysticerci. Apparently, the milieu
intérieur of humans resembles that of swine closely enough to satisfy
the developmental requirements of the cysticercus. - Bowman,
D. Georgis' Parasitology for Veterinarians (p. 656).
The man in People was twice the victim: once as a definitive host
for eating the meat and, secondly, as an intermediate host from a
gap in hygiene. The latter event being far worse.
Many factors affect a parasite's decision with an individual host:
parasitic life stage, source of interaction, the animal's health, and
even an animal's reproductive status. The non-zoonotic fluke,
Alaria marcianae, uses cats as their definitive host unless their
infection occurs within a pregnant queen, where it changes its
course and invades the definitive kittens through trans-mammary
transmission. The mother just provides paratenic support to the
cestode.
Toxoplasma gondii likes to hedge its bets and uses cats as both
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Pulse - October 2024

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