TAENIASIS
Taeniasis is a tapeworm (cestode) infection
acquired by the ingestion of raw or undercooked meat of infected animals. Although many
species exist, two species, Taenia saginata and Taenia solium,
cause pathology in humans. T. saginata is associated with the
ingestion of the worm's larval form found in infected beef while T. solium
is associated with that of infected pork.
T. saginata is also commonly known as beef
tapeworm. T. solium is similarly referred to as pork tapeworm.
Both species are worldwide in distribution-- approximately 100 million cases of infection world-wide annually.
Approximately 50 million cases of these cases are
T. saginata while the other 50 million are T. solium related. Incidence of infection of T.solium is also based on other
factors such as religious inhibitions on eating pork, inspection of pork before
distribution for human consumption, and high degrees of sanitation which limits
exposure of the intermediate hosts (such as pigs and cattle) to human feces.
Classification
Kingdom: Animalia
Phylum: Platyhelminthes
Class: Cestodes
Order: Cyclophyllidea
Family: Taeniidae
Genus: Taenia
Species: saginata/ solium
Other members of
Taenia that exist but humans are dead end hosts include:
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T. taeniformis
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T. bremneri (Syn. T. confusa)
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T. africanus
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T. multiceps
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T. serialis
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T. glomerulatus
Researchers believe that about 2 million
years ago, African hominids (our early ancestors), who scavenged for
food or preyed on antelope and other bovids, were exposed to tapeworm
colonization. These worms were using hyena and large cats as
definitive hosts and bovids as intermediate hosts. This occurred before
the origin of modern humans and substantially earlier than the
domestication of swine and cattle and the development of agriculture.
The conclusion was inferred from an examination of host and parasites
evolutionary histories and from evidence for the rate of molecular
evolution between species of Taenia.
Rather than humans' acquiring
Taenia from cattle and pigs, researchers believe man gave tapeworms to these
domestic animals, since the association between Taenia
and hominids was established before the domestication of these food
animals. It was not until about 10,000 years ago, with the development
of agriculture, that cattle, swine, and companion carnivores became
intermediate hosts. DNA analysis of the worms also suggest that
prehumans acquired these tapeworms before cattle and swine.
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Microscopy TAKE EXTREME CARE IN PROCESSING THE SPECIMENS! INGESTION OF TAENIA SOLIUM EGGS CAN RESULT IN CYSTICERCOSIS! The eggs of Taenia spp. are indistinguishable from each other, as well as from other members of the Taeniidae. The eggs measure 30-35 micrometers in diameter and are radially-striated. The internal oncosphere contains six refractile hooks.
Scoleces of Taenia spp. The scolex of T. solium contains four large suckers and a rostellum containing two rows of large and small hooks. There are usually 13 hooks of each size. The scolex of T. saginata has four large suckers but lacks the rostellum and rostellar hooks. The scolex of T. asiatica has rudimentary hooklets in a wart-like formation.
D: Scolex of T. solium. Note the four large suckers and rostellum containing two rows of hooks. Proglottids of Taenia spp. Gravid proglottids are longer than wide and the two species, T. solium and T. saginata, differ in the number of primary lateral uterine branches: T. solium contains 7-13 lateral branches and T. saginata 12-30 lateral branches. Proglottids of T. asiatica are similar to T. saginata and usually have more than 12 primary uterine branches.
Cross-sections of proglottids of Taenia spp., stained with hematoxylin and eosin (H&E)
J: Higher magnification of Figure I showing a close-up of the eggs. Note the characteristic striations, typical for the taeniids. Not visible in these images are the hooks commonly seen in cestode eggs. Hooks do not stain with H&E but are refractile and visible with fine focusing of the microscope.
L: Close-up of a cross-section of a Taenia sp. proglottid stained with hematoxylin and eosin (H&E), showing numerous calcareous corpuscles. Image courtesy of Ameripath. Adults of Taenia spp. Adults can reach a length of 2-8 meters, but the scolex is only 1-2 millimeters in diameter.
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People infected with
adult taenia often are asymptomatic. Infected people may become aware of
infection by noticing proglottid segments of the tapeworm in their feces.
Symptoms of infection, if any, are general: nausea, intestinal upset, vague
abdominal symptoms such as hunger pains, diarrhea and/or constipation, or chronic
indigestion. Increased eosinophils may be a sign of infection.
A more severe form of taeniasis, cystercercosis, can occur upon
ingestion of T. solium eggs found in the feces of infected humans. These eggs hatch in
the small intestine and migrate to various tissues of the body and form cysts.
T. saginata rarely causes cystercercosis.
The pathology associated with
cystercercosis depends on which organs are infected and the number of
cysticerci. An infection consisting of a few small cysticerci in the liver or
muscles would likely result in no overt pathology and go unnoticed. Those that
form in voluntary muscle tend to be asymptomatic, but may cause myositis,
with accompanying fever and eosinophilia. On the other hand, a few
cysticerci, if located in a particularly "sensitive" area of the body, might
result in irreparable damage. For instance, a cysticercus in the eye might lead
to blindness, a cysticercus in the spinal cord could lead to paralysis, or a
cysticercus in the brain (neurocysticercosis) could lead to traumatic
neurological damage or epileptic seizures. For this reason, cysticerci gather
more attention when they occur in the central nervous system or the eye rather
than when they develop in voluntary muscles.
In humans, it is the ingestion
of under-cooked beef (T. saginata) or pork (T. solium) containing
the larval cysts.
Intermediate hosts, such as
cows and pigs, are infected with the tapeworm when they come into contact with
the worm's eggs located in the feces of infected humans.
In addition to humans, T. Saginata also persists and
has a developmental stages in the cow. T. solium persists and has
developmental stages in pigs.
Vector
None
Incubation
Period
It takes about 5 to 12 weeks for the worm to mature
into adulthood in the human intestine. Usually only a single worm is present at
at time. However, multiple worms have been known to inhabit the human body.
T. solium may survive up to 25 years or more.
T. saginata
Larvae - Cysticerci are approximately 7.5 - 10mm
wide by 4 - 6 mm in length and are are found in the cow after ingestion of the
worm eggs.
Adults - The adult tapeworms have an average length
of ~ 5 meters, consisting of approximately 1000 proglottids, but may grow up to
25 meters in length. Thus the adult form of T. saginata may be larger
than that of T. solium.
The scolex has four suckers but no hooks, which is
found on Taenia solium. The scolex in this tapeworm is slightly
larger than that of T. solium, at approximately 2mm in diameter. Gravid proglottids
are shed either in the feces or leave the anus on their own. When the
proglottids reach the external environment desiccation occurs and the eggs are
released when the proglottid bursts.
Eggs - similar to T. solium eggs. (picture on
right)
T. solium
Egg: spherical, 6-hooked tapeworm larvae with
egg shell.
Larvae - These small cysticerci are approximately 6- 18mm wide by 4 - 6mm
in length when found in the muscles or subcutaneous tissues of their
intermediate host (generally, pigs). The cysticerci may
however be found in other tissues such as those of the central nervous system
where they may grow much larger, up to several cm in diameter.
Adults - The adult tapeworms have an average length
of ~3 meters, but can grow up to 8 meters in length. The worm contains
between 800 and 1000 proglottids. The gravid proglottids (right) is
approximately 12mm long. T. saginata has 15 to 20 branches on each
side, while T. solium has 7 to 13.
The
scolex (left) in this tapeworm may be differentiated from T. saginata as
it is equipped with a double crown at approx. 30 hooks.
The
scolex (left) in this tapeworm may be differentiated from T. saginata as
it is equipped with a double crown at approx. 30 hooks.
Images obtained from:
The life cycles of T. saginata and T.
solium are very similar. Life cycle differences include possible
autoinfection route in T. solium, and the different intermediate hosts
for each parasite.
T. saginata
Definitive host (sexual
reproduction): human.
Intermediate host: cattle
1.)Feces of humans contain
the eggs of the tapeworm.
2.)Egg
is ingested by cattle.
3.)Eggs
hatch to release hexacynth (six-hooked) larvae in small intestine.
4.)Larvae
migrate through the intestinal and enters the blood, lymph system.
5.)
Larvae is carried to
tissues such as heart and other muscles to develop cysticercus.
6.)
Man is infected by
ingesting uncooked meat containing cysticercus.
7.)
Once ingested, the scolex
of parasite attaches to the intestinal wall and grow into a mature tapeworm
which sheds eggs in the feces of the infected human.
T. solium
Definitive
host: human.
Intermediate
host: pig (accidental host: human)
1.)
Man ingests the larvae
by eating uncooked contaminated pork containing the larvae in cysticerci.
2.) Larvae develops into adult form
(only in humans)-- the tapeworm.
3.) Tapeworm
attaches to the intestinal lining of human and sheds its eggs in human feces.
4.) Pigs come
into contact with human feces and ingest the tapeworm eggs that contain the
eggs.
5.) Eggs
penetrate the small intestine of the pig,
enter the hepatic portal
vein, enter the general circulation
6.) Eggs
migrate to the skeletal or cardiac muscle where they form cysticerci.
Auto-infection route: Sometimes an infected human may ingest the eggs found
in its feces. If this is the case, then cysticerci may develop throughout the
tissues of the body, but particularly the brain as there is an affinity
for the brain.
Neurocysticercosis may result.
Summary of both routes (autoinfection not shown):
Taenia eggs and proglottids can be identified though microscopic identification. However, this technique is not possible during
the first 3 months following infection, prior to the development of adult tapeworms.
Microscopic examination of eggs does not distinguish between the two species.
Species can be distinguished, however, by examining proglottids. Also,
immunological tests can detect the presence of cysticerci and improved imaging
techniques such as CAT and MRI can be very useful in detecting cysticerci in
various organs.
The best way to prevent
taeniasis is to make sure meat is cooked thoroughly. Freezing to
-5˚C for 4 days, -15˚C for 3
days, or -24˚C for 1 day kills the larvae as well.
As with most cestodes,
treatment involves the use of Praziquantel. Niclosamide is also effective.
For T. solium, extra care
and hygiene (such as frequent hand washing) must be used to keep from ingesting
eggs and developing cysticerci in the brain. Surgery, praziquantel, and albendazole
can be used to treat cysticerci.
The area most affected by taeniasis currently is Irian Jaya,
Indonesia, the western half of New Guinea Island. In field surveys conducted in
2000 and 2001, researchers found that 5 (8.6%) of 58 local people and 7 (11%) of
64 local dogs living approximately 1 km from the local capital city, Wamena, in
Jayawijaya District, harbored adult tapeworms and cysticerci of T. solium.
Due to the prevalence of this tapeworm worldwide and increasing immigration and
foreign travel, T. solium will likely continue to emerge as an important
pathogen in the United States.
T. saginata
infection is common in areas of the world where beef is commonly eaten and human
sanitation is poor. It is commonly found throughout South America and in Africa
but is found in North America as well. T. saginata has worldwide
distribution, but unlike T. solium infection with T. saginata is
frequently encountered in the United States.
T.
solium is more
prevalent in poorer communities where humans live in close contact with pigs and
eat undercooked pork, and is very rare in Muslim countries. Infection with T.
solium is rarely encountered in the United States except areas of high
immigration from Mexico, Latin America, the Iberian peninsula, the Slavic
countries, Africa, India, Southeast Asia, and China.
In the U.S., laws have been passed
that requires meat inspection for cysticerci prior to meat being put on the
market of human consumption. Routine
serologic surveillance of cystercercosis and preventing cattle grazing in
contaminated areas will help prevent worm spread. Adequate cooking of meat
destroys the tapeworm larvae and will prevent infection by tapeworm. Good
hygiene and hand washing after using the toilet will prevent self-infection in a
person already infected with tapeworms in addition to contamination of
foodstuffs by human feces. Proper disposal of feces, to avoid
contamination of food, soil, and water, is important as well.
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