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  • 7/22/2019 Cacing usus


    Helminths: Pathogenesis andDefenses

    Derek Wakelin



    Helminth is a general term for a parasitic worm. The helminthsinclude the Platyhelminthes or flatworms (flukes and tapeworms)and the Nematoda or roundworms.


    All helminths are relatively large (> 1 mm long); some are verylarge (> 1 m long). All have well-developed organ systems and mostare active feeders. The body is either flattened and covered withplasma membrane (flatworms) or cylindrical and covered withcuticle (roundworms). Some helminths are hermaphrodites; othershave separate sexes.


    Helminths are worldwide in distribution; infection is most commonand most serious in poor countries. The distribution of thesediseases is determined by climate, hygiene, diet, and exposure tovectors.


    The mode of transmission varies with the type of worm; it mayinvolve ingestion of eggs or larvae, penetration by larvae, bite ofvectors, or ingestion of stages in the meat of intermediate hosts.

    Worms are often long-lived.


    Many infections are asymptomatic; pathologic manifestationsdepend on the size, activity, and metabolism of the worms. Immuneand inflammatory responses also cause pathology.

    Host Defenses


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    Nonspecific defense mechanisms limit susceptibility. Antibody- andcell-mediated responses are important, as is inflammation. Parasitessurvive defenses through many evasion strategies.


    Helminths - worms - are some of the world's commonest parasites(see Ch. 86). They belong to two major groups of animals, theflatworms or Platyhelminthes (flukes and tapeworms) and theroundworms or Nematoda. All are relatively large and some are verylarge, exceeding one meter in length.

    Their bodies have well-developed organ systems, especiallyreproductive organs, and most helminths are active feeders. Thebodies of flatworms are flattened and covered by a plasmamembrane, whereas roundworms are cylindrical and covered by atough cuticle. Flatworms are usually hermaphroditic whereasroundworms have separate sexes; both have an immensereproductive capacity.

    The most serious helminth infections are acquired in poor tropical

    and subtropical areas, but some also occur in the developed world;other, less serious, infections are worldwide in distribution. Exposureto infection is influenced by climate, hygiene, food preferences, andcontact with vectors. Many potential infections are eliminated byhost defenses; others become established and may persist forprolonged periods, even years. Although infections are oftenasymptomatic, severe pathology can occur. Because worms arelarge and often migrate through the body, they can damage thehost's tissues directly by their activity or metabolism. Damage alsooccurs indirectly as a result of host defense mechanisms. Almost allorgan systems can be affected.

    Host defense can act through nonspecific mechanisms of resistanceand through specific immune responses. Antibody-mediated,cellular, and inflammatory mechanisms all contribute to resistance.However, many worms successfully avoid host defenses in a varietyof ways, and can survive in the face of otherwise effective hostresponses.


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    Transmission of Infection

    Helminths are transmittedto humans in many different ways(Fig. 87-1). The simplest is by accidental ingestion of infectiveeggs(Ascaris, Echinococcus, Enterobius, Trichuris) or larvae (somehookworms). Other worms have larvae that actively penetratethe skin(hookworms, schistosomes, Strongyloides). In severalcases, infection requires an intermediate host vector. In somecases the intermediate vector transmits infective stages when itbites the host to take a blood meal (the arthropod vectors offilarial worms); in other cases, the larvae are contained in thetissues of the intermediate host and are taken in when a humaneats that host (Clonorchis in fish, tapeworms in meat and fish,

    Trichinella in meat). The levels of infection in humans thereforedepend on standards of hygiene (as eggs and larvae are oftenpassed in urine or feces), on the climate (which may favor survivalof infective stages), on the ways in which food is prepared, and onthe degree of exposure to insect vectors.

    FIGURE 87-1 Entry and localization of pathogenic helminths.


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    Host Factors Influencing Susceptibility

    Human behavior is a major factor influencing susceptibility toinfection. If the infective stages of helminths are present in theenvironment, then certain ways of behaving, particularly with regard

    to hygiene and food, will result in greater exposure. Becausehelminths, with few exceptions (Strongyloides, Trichinella, sometapeworm larvae), do not increase their numbers by replicationwithin the same host, the level of infection is directly related to thenumber of infective stages encountered. Obviously, not everyexposure results in the development of a mature infection. Manyinfective organisms are killed by the host's nonspecific defensemechanisms. Of those that do become established, many aredestroyed or eliminated by specific defenses. The number of wormspresent at any one time therefore represents a dynamic balancebetween the rate of infection and the efficiency of defense. Thisbalance (which reflects the host's overall susceptibility) is altered bychanges in the host's behavior and ability to express forms ofdefense. Children are more susceptible to many helminths than areadults, and frequently are the most heavily infected members of acommunity. The waning of immune competence with age may alsoresult in increased levels of infection. Individuals differ genetically intheir ability to resist infection, and it is well known that in infectedpopulations, some individuals are predisposed to heavier infectionsthan others. Changes in diet may affect susceptibility, as do thehormonal-immune changes accompanying pregnancy and lactation.

    An important cause of increased susceptibility is the immunesuppression that accompanies concurrent infections with someother pathogens and the development of certain tumors. Similarly,immunosuppressive therapies (irradiation, immunosuppressantdrugs) may enhance susceptibility to helminth infection. A particularhazard in immunocompromised patients is the development ofdisseminated strongyloidiasis, in which large numbers of larvaedevelop in the body by autoinfection from relatively small numbersof adult Strongyloides stercoralis. It is interesting that the humanimmunodeficiency virus does not result in an overall increase insusceptibility to helminth infection.

    Parasite Factors Influencing Susceptibility

    The ability of hosts to control infection is offset by the ability ofparasites to avoid the host's defenses and increase their survival. Inaddition to their ability to evade specific immune defenses (seebelow), many worms are unaffected by the host's attempts to limittheir activities or to destroy them simply because they are large andmobile. Many important species measure several centimeters inlength or diameter (Ascaris, hookworms, hydatid cysts, Trichuris)

    and others may exceed one meter in length (tapeworms). Size alonerenders many defense mechanisms inoperative, as does the tough


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    cuticle of adult roundworms. The ability of worms to move activelythrough tissues enables them to escape inflammatory foci.

    Many of the pathogenic consequences of worm infections arerelated to the size, movement and longevity of the parasites, as the

    host is exposed to long-term damage and immune stimulation, aswell as to the sheer physical consequences of being inhabited bylarge foreign bodies.


    Direct Damage from Worm Activity

    The most obvious forms of direct damage are those resulting fromthe blockage of internal organs or from the effects of pressureexerted by growing parasites (Fig. 87-2). LargeAscaris ortapeworms can physically block the intestine, and this mayoccur after some forms of chemotherapy; migratingAscaris mayalso block the bile duct. Granulomas that form around schistosomeeggs may block the flow ofblood through the liver, and this maylead to pathological changes in that organ and elsewhere.Blockage of lymph flow, leading to elephantiasis, is associatedwith the presence of adult Wuchereria in lymphatics. Pressureatrophy is characteristic of larval tapeworm infections (hydatidcyst, the larva ofEchinococcus granulosus) where the parasitegrows as a large fluid-filled cyst in the liver, brain, lungs, or body

    cavity. The multilocular hydatid cysts caused by Echinococcusmultilocularis have a different growth form, metastasizing withinorgans and causing necrosis. The larvae ofTaenia solium, thepork tapeworm, frequently develop in the central nervous system(CNS) and eyes. Some of the neurological symptoms of the resultingcondition, called cysticercosis, are caused by the pressure exertedby the cysts.


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    FIGURE 87-2Pathogenesis: direct damage caused by largehelminths.

    Intestinal worms cause a variety of pathologic changes inthe mucosa, some reflecting physical and chemical damage to thetissues, others resulting from immunopathologic responses.Hookworms (Ancylostoma and Necator) actively suck blood frommucosal capillaries. The anticoagulants secreted by the wormscause the wounds to bleed for prolonged periods, resulting inconsiderable blood loss. Heavy infections in malnourished hosts areassociated with anemia and protein loss. Protein-losingenteropathies may also result from the inflammatory changes