Parasitic adaptation in platyhelminthes

                  Parasitic adaptation in platyhelminthes

                                    Adaptation

•      Fitness of an organism to its environment

•      It is the characteristic which results in suitable & convenient morphological & functional correlation between an organism & its environment

                                          Parasitic adaptation

•      Platyhelminthes have undergone profound adaptation to suit their parasitic modes of life

•      These adaptations- parasitic adaptations

•      Are of morphological & physiological nature

1.                              Morphological adaptations

       Body covering

      Organs of adhesion

      Organs of locomotion

      Organs of nutrition (Trophic organs)

      Neurosensory system

       Reproductive system

                                          Body covering

Thick tegument frequently provided with scales affords suitable protection to the parasite

This thick protoplasmic layer is continually renewed by mesenchymal cells forming it

                                    B. Organs of adhesion

•      For a firm grip on/in the host’s body, some special organs of adhesion are needed

•      Flatworms are variously armed with suckers, hooks & spines

•      Suckers may be with/without hooks/spines

•     

                                       Organs of locomotion

•      Locomotion is actually an effort of procuring food

•      But parasites habitually inhabit such places in host’s body, where sufficient food is available without effort

•      Thus, organs of locomotion such as cilia of turbellarians- absent in parasitic forms

•      Locomotory organs present in free living larvae of parasitic forms

•      Miracidium possess cilia & cercaria bears a tail for locomotion

•     

                     D. Organs of nutrition (Trophic organs)

•      Food of parasite comprises readily available & digested/ semi digested food of the host

•      Elaborate organs of nutrition not needed

•      Trematodes have an incomplete gut & in most cases a suctorial pharynx for sucking food

•      An eversible pharynx is present in free living turbellarians

•      In cestodes, parasite freely bathes in digested food of host which is absorbed directly

•      Thus, total absence of alimentation in tapeworms

                                     E. Neurosensory system

•      Need for quick & efficient “response to stimuli” is associated with free active life & not with a quiet parasitic life in a safe environment

•      In parasites therefore, there is preferred reduction of nervous system & a total absence of sense organs

•      But the free living miracidium possesses eye spots

                                    F. Reproductive system

•      Best developed system in helminth parasites, designed & preferred to meet the need for tremendous egg production

•      Parasitic flatworms with a few exceptions like Schistosoma, are monoecious (hermaphrodite)

•      Hermaphroditism is of distinct advantage to the parasite because:

1. It ensures copulation even when a few individuals are present

2. After copulation both individuals lay eggs, doubling the rate of production

3. In absence of companion parasite can reproduce offspring

•      In cestodes reproductive system is much more elaborate & each mature proglottid possesses 1 or2 complete sets of male & female genitalia

•      In gravid proglottid all other organs of the system degenerate to make room for the uterus which becomes highly enlarged & branched to accommodate large number of eggs

2.                                   Physiological adaptations

 a. Protective mechanism                                      b. Anaerobic respiration                                                                                                                                                                           c. Osmoregulation                                                 d. High fertility

                                      

a.                                    Protective mechanism

•      Inside the alimentary canal the parasites have to protect  themselves from the action of digestive juices of host

•      Tapeworms accomplish this:

1. By stimulating walls of gut to secrete mucus, which then forms a protective clothing around parasite

2. By secreting antienzymes to neutralize the digestive enzymes of host

3. By probably continually renewing their protective body covering i.e., tegument

b.                                Anaerobic respiration

•      Environment in gut & bile ducts is devoid of free oxygen

•      Flatworms inhabiting these places, therefore, respire anaerobically by breaking down glycogen

•     

c.                                   Osmoregulation

•      Osmotic pressure of endoparasite’s body fluids, especially in case of trematodes is almost the same as that of host

•      This renders osmoregulation unnecessary

•      But in intestinal tapeworms, osmotic pressure is little higher

•      This permits ready absorption of host’s digested food by tapeworms

•     

d.                                        High fertility

•      Eggs produced by a parasitic flatworm face a very uncertain future while passing through the complex life cycle, these potential offsprings face several hazards as a result of which a very small percentage of total eggs produced reaches adulthood

•      This threat to the very existence of species is suitably met by parasite which in its life time may produce eggs in millions

•      Reproductive organs of flatworms are accordingly developed

 

Giardia intestinalis

                                   Giardia intestinalis

                                                Systematic position

                                                Phylum- Protozoa

                                   Subphylum- Sarcomastigophora

                                         Class- Mastigophora

                                        Order- Polymastigina

                                             Genus- Giardia

                                       Species- intestinalis

                                                     History

—  First seen by Leeuwenhoek in 1681

—  Discovered in his own stool

                                  Geographical distribution

—                                           World-wide

                                               Habitat

—  Duodenum & upper part of jejunum of  man

                                     Morphology

—  Exists in two phases             :1.Trophozoite                                2.Cyst

                                                  Trophozoite

—  Looks like a tennis racket in flat view & like a split pear in longitudinal view

—  Dorsal surface convex, ventral surface concave with a sucking disc

—  14 µm long & 7 µm broad

—  Anterior end broad & rounded, posterior end tapers to a sharp point

—  Bilaterally symmetrical

—  All body organs paired

—  Two axostyles, two nuclei & four pairs of flagella

                                                        Cyst

—  Oval, 12 µm long, 7µm broad

—  Axostyles lie more or less diagonally, forming a sort of dividing line within cyst wall

—  4 nuclei, clustered at one end/ lie in pairs at opposite poles

—  Remains of flagella & margins of sucking disc may be seen inside cytoplasm

—  Acid environment causes parasite to encyst

                                                      Life cycle

—  In trophozoite stage parasite multiplies in man’s intestine by binary fission

—  Under unfavourable conditions in duodenum parasite encysts, usually in large intestine

—  In the cyst a thick resistant wall secreted by parasite, divides into two within cyst

—  Man becomes infected by ingestion of cysts

—  Just after 30 minutes of ingestion cyst hatches into 2 trophozoites & multiply in enormous numbers, colonize in duodenum

—  To avoid high acidity of duodenum Giardia localises in biliary tract

                                                  Pathogenicity

—  Sucking disc helps the parasite to attach from the convex surface to epithelial cells of intestine

—  Causes disturbance in intestinal function, leading to malabsorption of fat

—  Patient may complain looseness of bowels, mild steatorrhoea (passage of yellowish & greasy stools due to excess of fat)

—  Parasite is also capable of causing harm by toxic effects (allergy), traumatic, irritative & spoilative action

                                              Laboratory Diagnosis

—  Microscopical examination of freshly passed stool for demonstration of trophozoites & cysts

—  Trophozoites may be recovered both in bile A (aspirated from duodenum) & B (removed from bile duct) drawn by duodenal intubation

                                                       Treatment

—  Atebrin & acranil- effective for giardiasis

—  Schneider (1961) reported good results with a derivative of imidazole

—  Chloroquine in doses of 300 mg base once daily for 5 days is also effective

Phylum- porifera

                                       Phylum- porifera

                                                Characters.

—  Commonly known as “Sponges”

—  Multicellular, cellular level of organization

—  Solitary/ colonial, sessile

—  Shape vase-like cylindrical, tubular, many branched

—  Symmetry radial or none

—  Body wall- outer pinacoderm, middle mesenchyme, inner choanoderm

—  Cells loosely arranged, no definite layers

—  Body with many pores, canals & chambers- serving for water flow

—  Oscula present

—  Choanocytes or collar cells line special chambers

—  Skeleton of calcareous/ siliceous or spongin fibers

—  Digestion intracellular, respiratory, excretory organs absent, contractile vacuoles in some

—  Nervous system primitive

—  Hermaphrodite  but cross fertilization is rule

—  Asexual reproduction by budding/ gemmules

—  Sexual reproduction by ova & sperms

—  Regenerative power

—  Cleavage holoblastic

—  Development indirect by free swimming ciliated larva- Amphiblastula/ Parenchymula

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.