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Category: Immunology; Clinical Microbiology
Overview of Parasitic Pathogens, Page 1 of 2
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This chapter focuses mainly on parasites that cause serious diseases in humans. Among the parasitic protists, there are two major pathogenic groups, the apicomplexans, named for the anterior complex of organelles involved in host cell invasion, and the flagellated kinetoplastids, named for the large mass of DNA contained within their single, elaborate mitochondrion. Parasitic helminths belong to one of two major phyla, Nematoda or Platyhelminthes. Well-known, free-living members of these phyla are Caenorhabditis elegans and the planaria, respectively. A theme that has emerged from the study of parasitic diseases is that the decision by the host to make the wrong type of immune response can have disastrous consequences. Intestinal nematodes are the most prevalent helminth parasites. Species such as Ascaris lumbricoides, the hookworms (Necator americanus, Ancylostoma duodenales), whipworms (Trichuris trichiura), and Strongyloides stercoralis-together referred to as soil-transmitted helminths since they are acquired through contact with earth that has been contaminated with feces from infected humans infect billions of people worldwide. Most parasitic infections are chronic in nature; presumably chronicity provides the selection advantage of increasing the potential for transmission in many cases.
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Protozoan infections. A summary of the major protozoan infections of humans and available experimental models. The lists of pathogens are not exhaustive.
Helminth infections. A summary of the major helminth infections of humans and available experimental models. The lists of pathogens are not exhaustive.
Concomitant immunity. A primary immune response fails to clear the primary infection, (— · —, – – – –) but is able to prevent the establishment of a superinfection following secondary or subsequent exposures to the same pathogen. This contrasts with the more conventional notion of protective immunity in which the primary infection is cleared by the primary immune response and the emergent memory immune response confers strong resistance to secondary infection with the same pathogen (———).
Chronic infection. Many bacterial and viral infections are completely cleared within a relatively short time after acquisition (———). In contrast, many parasitic infections are chronic. Chronicity can take many forms, including the establishment of a population of organisms that is long-lived and stable, as is the case for schistosomes (which do not replicate within the definitive host), (– – – –), the control, but not clearance, of a population of replicating parasites, as is the case in certain types of leishmaniasis (—· —) and the cyclical expansion and contraction of replicating parasite populations that is consistent with the emergence of new antigenic variants and their sequential control by the immune response, as is the case for Trypanosoma brucei infection (......).