Diphyllobothrium latum

                          Diphyllobothrium latum

                                         Systematic position

                                    Phylum- Platyhelminthes

                                         Class- Cestoda

                                    Subclass- Eucestoda

                                   Order- Pseudophyllidea

                               Genus- Diphyllobothrium

                                       Species- latum

           History

•      Linnaues, 1758

•      Lühe, 1910

•      Commonly known as fish tapeworm/ the broad tapeworm

                      Geographical distribution

•      Central Europe, America, Japan & Central Africa

•      Not yet reported from India

                                     Habitat

•      Adult worms live in small intestine (ileum) of man, also in dog, cat, fox & other fish eating mammals

           Morphology

•      Adult worm yellowish grey in colour with dark central markings caused by egg-filled uterus

•      Measures 3-10 m in length

•      Individual may live for a period up to 5-13 years

•      Scolex (head) elongated, spoon –shaped, measures 2-3mm by 1 mm

•      Bears 2 slit-like grooves (bothria) situated on dorsal & ventral surfaces

•      No rostellum & hooklets

•      Neck thin, unsegmented & much longer than head

•      Proglottids/ segments 3,000- 4,000

•      Segments greater in breadth than length

•      Mature segment measures 2-4 mm by 10-20 mm, practically filled with male & female reproductive organs

•      Terminal segments are apt to be shrunken & empty owing to constant discharge of eggs through uterine pore

•      Later dried-up segments break-off from body, not singly but in chains & passed in host’s faeces

•      3- genital pores comprising the openings of vas deferens, vagina & uterus lying close to one another

•      Ovary bilobed

•      Uterus large & remains coiled in centre of each segment in form of a rosette

•      Eggs are passed out in host’s faeces in large numbers

•      Oval, brown, 70 µm by 45 µm, contains abundant yolk granules & unsegmented ovum

•      Inconspicuous operculum present at one end with a small knob at other end

•      Does not float in saturated solution of common salt

•      Eggs not infective to man

                                       Larval Stages

•      Passed first in water & then in respective intermediate hosts

•      3-satges of larval development

•      1^st stage larva is coracidium- develops from egg in water

•      2^nd stage larva is procercoid- prsent inside Cyclops (1^st Intermediate host)

•      3^rd stage larva is plerocercoid- found in freshwater fish (2^nd Intermediate Host)

•      A single egg gives rise to a single larva

                                                   Life cycle

•      Worm passes its life cycle in one definitive host & 2 intermediate hosts

                                         Definitive hosts

•                                            Man, dog, cat

•                                   Man is optimum host

•                        Adult present in the small intestine

                                      Intermediate hosts

•      1^st intermediate host is a fresh water crustacean, a cyclops/ a diaptomus

•      2^nd intermediate host is a freshwater fish, pike, trout, salmon, perch & other fish

               Development of egg in water & liberation of coracidium

•      Operculated eggs are liberated through faeces of definitive hosts in water

•      A spherical ciliated embryo with 3-pairs of hooklets-coracidium develops within each egg- shell in course of 1-2 weeks

•      Mature coracidium (40-55 µm) escapes into water, ingested by a Cyclops

                       Larval development inside Cyclops

•      Inside intestine of Cyclops coracidium loses cilia & supporting cubical cells

•      Penetrates through the intestinal wall, comes to rest inside body cavity & in about 3 weeks , transformed into an elongated solid body with a caudal spherical appendage with 6  (useless) hooks- Procercoid larva

•      Cyclops with the procercoid larva is in turn devoured by second intermediate host- fresh water fish

•      Cyclops cannot house more than 2 procercoids

                            Larval development inside fish

•      In intestine of fish, procercoid (55 µm) after freeing itself, passes through gut-wall & rests into liver, muscles/ voluminous fat in mesentery & proceeds to develop further

•      In 1-3 weeks it develops into a plerocercoid/ sparganum larva

•      It has now lost its spherical caudal appendage & a depression at anterior end representing the withdrawn & inverted head of future adult worm

•      Larval body is white, somewhat flattened, marked by irregular unsegmented wrinkles

•      Smaller bodies lie straight in flesh but larger ones  remain bent & twisted

                 Infection of man & development of adult worm

•      Plerocercoid larva is infective to man

•      Not destroyed by ordinary salting, pickling/ smoking & therefore with the eating of these insufficiently cooked fish/ raw roe man is infected

•      Inside intestine of man plerocercoid larva develops into an adult worm & after having attained sexual maturity in about 5-6 weeks, starts discharging eggs which are passed along with faeces

•      Cycle is thus, repeated

                                             Pathogenicity

•      Man is infected by ingestion of imperfectly cooked infected fish/ roe containing plerocercoid larvae

•      Infection of D. latum in man- Diphyllobothriasis

•      Symptoms are gastro-intestinal disturbances & anemia

•      In persons having a tendency/ racial tendency d. latum infection precipitates Addisonian’s anaemia (macrocytic)

•      There is an early eosinophilia

                                                  Diagnosis

•      Established by microscopical examination of faeces for characteristic operculated eggs

•      Segments passed with faeces may be recognized by character of uterus & position of genital pores

•                                          Treatment

•      Antihelminthic drugs like mepacrine, dichlorophen, niclosmide

•     

•                                           Prophylaxis

•      Prevention of pollution of water by efficient disposal of water

•      Personal prophylaxis in endemic area may be taken by properly cooking fish before eating

•      In endemic areas infection is maintained by dogs & cats fed on the offals of fish, this practice should be stopped.