Schistosomiasis (1990) Part 3 of 3

at about 4 weeks after infection the first Sakari emerge from the snail and they're soon escaping daily in swarms they're about half a millimeter long and are actively swimming organisms that travel through the water with characteristic movements usually tail first their life is short about 48 hours at most for they have no functional gut and their rapidly expended energy Reserve cannot be replaced the Saria has an elongated body region and a long forked tail it also has an oral and a ventral sucker by which it adheres to any substrate the external surface of the secaria is covered with spines that are backwardly directed these help it to penetrate the skin of its next host since they ensure that the organism can only travel in a forward Direction each Saria has a secretory complex comprising three types of unicellular glands which open through the oral sucker this electron micrograph shows the folded gland openings the glands contain powerful proteolytic enzymes which help the secaria to digest its way through the skin of the host here Sakari are penetrating the tail skin of a mouse they attach with their suckers digest and wriggle their way through the skin once the body is inside the tail drops off but remains active the parasite now called the schistosomulum is safely within its new host the sequence is now repeated in diagram a Saria attaches and makes a hole in the epidermis it deepens the aperture and enters it by elongating its body shedding its tail it migrates through the dermis and fat layer where it turns seeking a blood vessel on finding one it enters and starts its Journey around the body of the host it's carried in the bloodstream first to the lungs by this time it's become longer and considerably more slender and lost its midbody spines although spines are retained at both ends the spines together with rhythmic activity enable it to migrate a along the tiny lung capillaries which are often smaller in diameter than the lava itself the flukes leave the lungs through the pulmonary veins and pass via the left side of the heart to the liver the earliest migrants arrive in the liver 6 to 8 days after infection they become short and squat begin to feed and their intestines fill with dark hematin pigment 2 to 3 week old juvenile flukes are now more recognizable as schistosomes but they have a convoluted system of folds and ridges on the surface by week four however these folds are less conspicuous and dome-shaped elevations appear on the dorsal surface of males they clearly represent early stages in the development of surface tubic male and female fluk pair at around week five and the male tubic take on their characteristic spiny appearance the paired flukes migrate through the portal vein to the mesenteric veins and egg laying begins that completes the life cycle of the schistosome we now have some observations on one of the most intriguing features of the life cycle the transition from free swimming Saria to parasitic schistosomulum the Saria has to adapt very rapidly to its move from fresh water to body fluid a temperature change of about 20° and a hostile environment the most dramatic changes take place at the outer surface of the secaria this is a diagrammatic section through a secaria the surface is spiny and totally covered by a typical three L membrane the outer layer is the glyco Kix this is an electron micrograph of the three- layered membrane under the membrane is the tegument a citium with no cell walls its nuclei are located in deeper cell bodies joined to the tement by narrow Twisted connections these are layers of muscle cells this is how the section appears under the electron microscope when the secaria enters the tissues of its new host large numbers of membranous secretion granules are synthesized in the subtegumental cells and pass up into the tegument the bodies join together and then connect with the surface of the lava to liberate their membranes to the exterior the original trilaminate membrane and the glycocalix are cast off in a mass of microvilli this action not only serves to distract the host's immune system it also sensitizes the host against subsequent schistosome infection the parasite rapidly covers itself with a new seven layered membrane derived from the membranous bodies the lava further confuses the host by disguising its surface with proteins derived from the host's red blood cells at an early stage of infection it's also thought that the host produces a class of antibodies which attach to the invading parasites their function is rather puzzling in that they're not aggressive towards the parasite furthermore they serve to block the killing of the Parasite by other toxic antibodies later there's a switch so that the toxic antibodies predominate this could explain the finding that young children playing in infected water have no resistance to schistosomiasis and may become heavily infected but as they get older they develop an effective immune response that kills newly invading parasites and thus limits the number of shist asss they carry because man eventually develops this partial immunity concerted efforts are being made in many Research Laboratories to analyze the mechanisms responsible for killing the flukes and to characterize the parasite antigens against which the host's responses are directed present research gives hope that it will be possible to design and produce a vaccine for human use it would be an acceptable desirable first step in Disease Control if the vaccine could reduce the debilitating pathology caused by schistosome eggs schistosomes like many other parasites have a variety of ways of avoiding the lethal effects of immunity shedding of microvilli using host protein as a disguise and the development of blocking antibodies these are but three of their many tricks a really successful vaccine will have to be several jumps ahead of these highly sophisticated evasive strategies until that time comes we must rely on improved standards of hygiene to break the life cycle of the parasite and drugs to reduce the infection but the problems of expense the need for repeated Drug Administration and the everpresent threat of drug resistance mean that researchers are working against the clock to develop an effective vaccination program [Music]