Adult cestodes (intestinal tapeworms)
Adult cestodes (intestinal tapeworms)
Editor: Alessandro Bartoloni
Adult cestodes are also called tapeworms. Tapeworms have in common that: (i) they all live in the lumen of the intestine, (ii) most cause little pathology in humans. Tapeworms are very likely to appear on your tropical medicine tests!
Figure 1. Adult tapeworm
Section Contents
Beef tapeworm (Taenia saginata)
Background & Epidemiology
T. saginata is among the longest parasites (3-5 meters long!) to infect humans, and it is acquired by eating raw or undercooked beef that carries the larval (cyst) form. Although it’s called “beef” tapeworm, the only definitive hosts are humans. Taeniasis saginata is more common among places of the world where raw beef is eaten frequently (beware steak tartare lovers!!!) and where human feces are not disposed properly.
Life Cycle
Figure 2. Life cycle of Taenia saginata
Pathogenesis & Clinical Presentation
Most infections are asymptomatic. Occasionally, individuals can notice proglottids in stool, bedding or clothing, as they migrate out of the infected person overnight. Less commonly, individuals may experience epigastric fullness, postprandial nausea, or vomiting.
Diagnosis
1) Stool microscopy - can detect T. saginata eggs (eggs can stain acid fast!), 2) Inspection of proglottids (under the microscope, T. saginata has 12 or more branches on either side of the uterus). This may come up on your exam!!!
Treatment/Prevention
The drug of choice to treat T. saginata is praziquantel or niclosamide. Prevention of taeniasis includes: (i) proper disposal of human feces, (ii) cooking or freezing beef (avoid raw or undercooked beef).
Pork tapeworm (Taenia solium)
Background & Epidemiology
T. solium is acquired by eating raw or undercooked pork that carries the larval (cyst) form. Although it’s called “pork” tapeworm, the only definitive hosts are humans. Taeniasis solium is common among places of the world where human feces contaminates the environment of pigs and raw pork is eaten frequently. Thus, it is associated with poor sanitary conditions. Unlike T. saginata, ingesting eggs of T. solium can induce cysticercosis in humans.
Figure 3. Adult scolex of T solium
Life cycle
Figure 4. Life cycle of Taenia solium
Pathogenesis & Clinical Presentation
Most infections are asymptomatic. While it is true for T. saginata whose proglottids are motile, T. solium proglottids are not and are rarely observed in the perineum or clothing. Not to be confused with cysticercosis.
Diagnosis
1) Stool microscopy - can detect T. solium eggs (unlike T. saginata, eggs cannot stain acid fast!), 2) Inspection of proglottids (under the microscope, T. solium has 12 or less branches on either side of the uterus). This may come up on your exam!
Figure 5. Taenia spp eggs
Figure 6. Taenia solium proglottids
Treatment/Prevention
The drug of choice to treat T. solium is praziquantel or niclosamide. One should be very careful when using praziquantel in a tapeworm carrier as individuals can have neurocysticercosis! Prevention of taeniasis includes: (i) proper disposal of human feces, (ii) cooking or freezing beef (avoid raw or undercooked beef)
Fish tapeworm (Diphyllobothrium latum)
Background & Epidemiology
D. latum is the longest parasite (up to 25 meters long!) to infect humans, and it is acquired by eating raw or undercooked fish that carries the larval (plerocercoid) form. D. latum adults have a unique affinity for absorbing vitamin B12 and infections may be associated with vitamin B12 deficiency. Unlike taeniasis, D. latum has multiple animal reservoirs, including dogs, bears, cats, foxes, and other wild animals. Infection is seen more commonly in Northern Climates.
Life cycle
Figure 7. Life cycle of Diphyllotobthrium latum
Pathogenesis & Clinical Presentation
Most infections are asymptomatic. About half of the individuals may have vitamin B12 deficiency, but actual megaloblastic anemia occurs in <2% of patients. Eosinophilia has been reported.
Diagnosis
1) Stool microscopy - can detect D. latum eggs, 2) Inspection of proglottids.
Figure 8. D. latum proglottids
Figure 9. D. latum scolex
Figure 10. D. latum eggs
Treatment/Prevention
The drug of choice to treat D. latum is praziquantel or niclosamide. Prevention includes: (i) cooking or freezing fish.
Other tapeworms of minor medical importance
Hymenolepis nana (dwarf tapeworm) → measures 3-4 cm, has a worldwide distribution and is more common in children, and it is the only human tapeworm in which an intermediate host is not necessary. Humans become infected by ingesting the eggs present in feces from a tapeworm carrier. Adults only last 4-6 weeks, but proglottids can cross-fertilize and eggs can hatch inside of the intestine, allowing for an autoinfection cycle to occur. Most infections are asymptomatic, but heavy infection can have non-specific GI complaints. Diagnosis is made with stool microscopy. Treatment is with praziquantel or niclosamide.
Hymenolepis diminuta → it is commonly found in rats and mice but infrequently found in humans. Life cycle is complex involving rodents and beetles.
Figure 11. Hymenolepis spp. eggs
Dipylidium caninum (dog tapeworm) → unlike any other tapeworm we have learnt, D. caninum lives in the intestine of dogs. Humans become accidentally infected when ingesting adult fleas containing the larval stage. Does not usually cause any disease, but can be associated with eosinophilic gastroenteritis. Diagnosis is made with stool microscopy. Treatment is with praziquantel or niclosamide.
Figure 12. Life cycle of Dipylidium caninum
Figure 13. D. caninum eggs
Answer
- Strongyloides stercoralis
- Hymenolepis nana
- Capillaria philippinensis