Unit 4: Phylum Nemathelminthes

Unit-4 Phylum Nemathelminthes General Characteristics of Nematoda: Nematodes present in the soil and feed on bacteria, fungi, and other small nematodes. They play an important role in nutrient recycling. Their body is bilaterally symmetrical. They are cylindrical in shape. They exhibit tissue level of organization. Their body has a cavity or pseudo coelom. Their alimentary canal is distinct with mouth and the anus. They are devoid of circulatory and respiratory systems properly. They are free-living or parasitic. They cause disease in the host. Fertilization is internal. Reproduction is sexual. Classification of Nematode: Nematodes are classified into two classes, such as: (a) Phasmidia (b) Aphasmidia (a) Phasmidia/Phasmidea: They are mostly parasitic. They are unicellular. Sense organs are present. Their excretory system has paired lateral canals. For example: Ascaris & Enterobius. (b) Aphasmidia/Aphasmidea: They are free-living. They are mostly marine and some are terrestrial. Sense organs are well developed. The body cuticle is smooth. Ex: Capillaria & Anticoma. Life Cycle, development & pathogenicity Ascaris: Systematic position: Phylum - Nematoda Class - Phasmidia Order - Ascaroidea Genus - Ascaris Species - Lumbricoidis Introduction: Ascaris lumbricoidis is a gastrointestinal parasite of man. Besides, it is also found in the intestine of apes, pigs, cattle, sheep, etc. It remains in the intestine & feeds on the partly digested food of the host. Ascaris is dioecious showing sexual dimorphism. Gonads are typically long, coiled & tubular. Life History & Development: Copulation & Fertilization Copulation takes place in the small intestine of the host. The male orients its body at a right angle to that of the female in such a way that its cloacal appears the vulva of the female. Fertilization takes place in the proximal end of the uterus or distal end of the oviduct. 2. Zygote: Immediately after fertilization the fertilized egg produces a thick clear & chitinous inner shell. As the egg passes down the uterine wall secretes an outer shell of albuminous protein. These eggs are also called as mammillated egg. Eggs are laid in the small intestine of the host & they pass out with the feces. They can remain alive in the soil for years. Under suitable condition of temperature, moisture & oxygen, eggs undergo cleavage & further development. 3. Cleavage & Early Development: Cleavage in Ascaris is of spiral & determinate type. The first division results in a dorsal (AB) and a ventral cell (P1), further the dorsal cell divides into an anterior (A) & a posterior cell (B). The ventral cell divides into an upper cell & a lower cell. The four-celled embryo thus formed is first T-shaped which soon becomes rhomboidal. In the next cleavage, AB divides into right & left cells (A1, A2 & B1, B2). P1 divides into MST & E1, P2 divides into P3 & C. Again, E divides into E1, E2 & P3 divides into P4 & D. P4 further divides into G1 & G2. The fate of various cells is fixed at this stage. A & B give rise to the ectoderm & similarly MST to mesoderm & E1, E2 to endoderm. G1 & G2 give rise to germ cell primordium & C & D to ectoderm & mesoderm. The gastrula grows in size to become an active juvenile within 10-14 days from the start of cleavage. The juvenile is also termed rhabditiform larva of the first stage & is not infective. In another week it moulds & becomes the second stage rhabditoid which is capable of infecting the host. 4. Infection to New Host: Man acquires infection by directly ingesting Ascaris eggs containing the infective second stage rhabditoid larva with contaminated food & water. In the small intestine, the egg shells dissolve by the action of digestive juice & the juvenile hatches out. 5. Later Development & Migration: After hatching, the juvenile performs active movements & bores through the epithelium of the host intestine & starts migration. The larva enters the hepatic portal infection which carries it to the liver. From the liver, it finally reaches the heart through the post caval vein. From the heart, it again is transported into the lungs via pulmonary artery. Remaining within the lungs for a few days, the larva increases in size. After about 6 days the 2nd stage larva moulds to become the 3rd stage larva. Another mould occurs after 10 days, resulting in the 4th stage larva. The 4th stage larva leaves the lungs & reaches the pharynx through the trachea from where it is coughed off & then swallowed for the 2nd time into the gut. In the intestine, it moulds for the last time to become an adult. Adults attain sexual maturity within 8-10 weeks & the average life span of Ascaris in the host is 9-12 months. Sometime the larva of Ascaris does not follow the usual migration route but reaches the brain or spinal cord or any such organ where it fails to survive. Pathogenicity & Prophylaxis: The disease caused by Ascaris is commonly called Ascariasis. Man gets infection by consuming contaminated food & water with infective eggs. Incidence of ascariasis is more in children than adults. The larvae often prove itself more dangerous by causing hemorrhages. In severe infection, they may cause acute pneumonia with fatal consequences. Frequently the infection is immediately followed by temperature, anemia & eosinophilia. It can also migrate into appendix, gall bladder & common bile duct and may cause inflammation of these structures. Therapy: The diagnosis is usually incidental when the host passes a worm in the stool or vomit. The eggs can be seen in smear of fresh fecal matter examined on a glass slide under a microscope. Ascariasis drugs like tetrachloroethylene, tetrachloroethylene, tetracycline, piperazine etc. can be administered. Prevention includes use of toilet facilities, safe excreta disposal, protection from dirt & soil, washing hands regularly, cutting of nails, avoiding contaminated food etc. Introduction: Wuchereria bancrofti, the filarial worm, is an endoparasite of human blood & lymph. It is a digenetic parasite completing its life cycle in two hosts. The final host is man while the secondary or intermediate host is the blood-sucking insect, i.e., the female mosquito of genus Aedes & Anopheles. The secondary host for Wuchereria bancrofti in India & China is Culex pipiens. Adult worms live coiled up in the lymph gland & lymph passages of man. Life Cycle: Copulation: Copulation takes place when matured individuals of both the sexes are present in the lymph gland. Larval Development in Man: The female worm is viviparous or oviparous, releasing numerous juveniles called microfilariae. Microfilariae are born in a very immature state. However, microfilariae discharge into lymph vessels and show active movement in the bloodstream. They migrate in the deeper blood vessels of the thorax. In the blood of man, they show nocturnal periodicity, i.e., they appear at night only (generally between 10 PM to 4 AM) but disappear during the rest of the day. When it comes to nighttime, it takes its meal, i.e., the blood meal, from the human body. Development in Secondary Host: Microfilariae, after reaching the body of the secondary host, undergo further development. Immediately after their entry into the stomach of the mosquito, they lose their sheath and penetrate the stomach wall within an hour or two. Then they migrate to the thoracic muscles, where they change first into larval stage L1 within 2 days. Within the next 3–7 days, they grow rapidly in muscles to become a more elongated form. This stage of larvae is called the second stage larvae. Finally, by the 10th–14th day, they become fully grown to a long juvenile, which is known as the infective stage of larvae. These larvae are infective and come to lie in the proboscis of the mosquito. Infection to New Human Host: When an infected mosquito bites a healthy human to suck blood, the infective juveniles come out from the proboscis and enter the human body. Finally, they pass into the lymph gland and lymph passages, where they are coiled up and develop into adult worms. Adult worms sexually mature within a period of 5-18 months and start the reproduction to repeat the life history again. The life span of adult worms is 5-10 years. Pathogenicity: The infection of Wuchereria bancrofti causes a disease known as Wuchereriasis. Generally, this disease is also known as Filariasis or Elephantiasis. The infection causes fever, headache, mental depression, etc. But during heavy infection, it causes the blockage of lymphatic vessels and glands, resulting in various pathological conditions. The most spectacular is the swelling of the affected body parts, generally the lower limb, scrotum, legs, and mammary glands. Treatment: No proper or satisfactory treatment is yet known. Infection may be reduced by the eradication of microfilariae from the circulation by administering hetrazan and compounds of antimony and arsenic. It is important for prevention from mosquito bite through mosquito nets; mosquito repellent is to be used.