Introduction to Diagnostic Medical Parasitology
Diphyllobothriasis is caused by the fish tapeworm Diphyllobothrium latum. The adult worm lives in the small intestine and reaches a length of up to 12 m and may consist of up to 3000 proglottids.
The life cycle involves three hosts. In the definitive host, millions of eggs are passed with the faeces. Within the egg, deposited in the water, a motile embryo (coracidium) develops, which after hatching infects copepods (water fleas). In this first intermediate host, procercoid larvae develop. If the infected copepod is eaten by a fish (second intermediate host), further development occurs: the procercoid becomes a plerocercoid larval stage. Infection of humans results from eating raw freshwater fish containing plerocercoid larvae of Diphyllobothrium latum. With new food habits in many parts of the world ("sushi" restaurants), the fish tapeworm is once more - on the attack - .
- The fish tapeworm occurs worldwide (Europe, northern Siberia, North America, western Africa, South-East Asia, Japan, Australia)
- Humans become infected by eating raw or undercooked fish (e.g. pike, perch, salmon)
- Besides humans, as an important final host, wild and domestic carnivore animals serve as reservoir hosts
- Pathology – provoked in most cases by a single worm – is light
- Hosts might be depleted of vitamin B12, resulting in rare cases in a macrocytic anaemia (especially in Scandinavia)
- Symptoms might include weakness, fatigue, diarrhoea, extreme hunger
- Anaemia is usually moderate
Patients often observe short segments of the tapeworm (chain of proglottids) in their stools.
It is not difficult to confirm diagnosis by detecting the numerous eggs by microscopy. D. latum eggs are morphologically similar to operculated trematode eggs. Measurements allow differentiating D. latum (length under 80 µm) from Paragonimus westermani, which has larger eggs.
No test developed
No test available
Serology inadequate for diagnosis
Besides parasitological methods, no special diagnostic strategy is needed!
Prevention and control
- Cestode larvae in fish are killed by heat or after freezing
- The control of transmission is difficult due to many reservoir hosts