The classification schemes for the Protozoa seem to be evolving even quicker than the organisms themselves. Within the context of this book a classification scheme used by classical parasitologists in textbooks has been adopted. It is based largely on light and electron microscopy and modes of locomotion, but considers current evolutionary thinking based on comparative genetics. Within the context of this scheme, the Protozoa are considered a subkingdom within the kingdom Protista.
The subkingdom Protozoa, includes the following phyla: Sarcomastigophora, including the flagellates and amebas; Apicomplexa, including malaria parasites, Cryptosporidium and Toxoplasma; Microsporidia, including the microsporidia; and Ciliophora, including the ciliates (Table 48–2).
TABLE 48–2Classification of Protozoan Parasites ||Download (.pdf) TABLE 48–2 Classification of Protozoan Parasites
Kingdom - Protista
Subkingdom - Protozoa
Phylum - Sarcomastigophora
Subphylum - Mastigophora
Genera - Leishmania, Trypanosoma, Giardia, Trichomonas and Dientamoeba
Subphylum - Sarcodina
Genera - Entamoeba, Acanthamoeba, and Naegleria
Phylum - Apicomplexa
Genera - Plasmodium, Toxoplasma, Cryptosporidium,
Cyclospora, Isospora, Sarcocystis, and Babesia.
Phylum - Microsporidia
Genera - Enterocytozoon and Encephalitozoon
Phylum - Ciliophora
Genus - Balantidium
The Sarcomastigophora are an extremely diverse group including true flagellates of the subphylum Mastigophora and parasites such as those belonging to the genera Leishmania, Trypanosoma, Giardia, Trichomonas, and Dientamoeba. Mastigophorans include those that are obligate intracellular parasites (Leishmania), parasites of the blood vascular system (Trypanosoma), intestinal track (Giardia), or genital–urinary track (Trichomonas). The subphylum Sarcodina can also be found within this phylum and include the important genera Entamoeba, Acanthamoeba, and the ameba–flagellate Naegleria.
The Apicomplexa also represent a diverse group of organisms that have been placed together phylogenetically because of the presence of complex apical organelles in life cycle stages responsible for cellular invasion. All are obligate intracellular parasites for most of their life cycles. Parasites in this taxonomic grouping include members of the genera Plasmodium, Toxoplasma, Cryptosporidium, Cyclospora, Isospora, Sarcocystis, and Babesia.
The Microsporidia include an opportunistically important group of parasites called the microsporidia. Many infections in this group are seen in immunocompromised patients with the more important genera being Enterocytozoon and Encephalitozoon. The Ciliophora include a single genus, Balantidium, which is only occasionally encountered in humans.
Protozoa range in size from slightly more than 1 to more than 100 μm. They are single-celled organism and have a true membrane-bound nucleus. The nucleus contains clumped or dispersed chromatin and a central nucleolus or karyosome. The shape, size, and distribution of the nucleus can be useful in distinguishing protozoan species from one another.
The cytoplasm is frequently divided into an inner endoplasm and a thin outer ectoplasm. The granular endoplasm is concerned with nutrition and often contains food reserves, contractile vacuoles, and undigested particulate matter. The ectoplasm may be organized into specialized organelles of locomotion. In some species, these organelles appear as blunt, dynamic extrusions known as pseudopods. In others, highly structured thread-like cilia or flagella arise from intracytoplasmic basal bodies. Flagella are longer and less numerous than cilia and possess a structure and a mode of action distinct from those seen in prokaryotic organisms.
Endoplasm contains nutrients
Ectoplasm has organelles of locomotion
Most parasitic Protozoa are heterotrophic and must assimilate organic nutrients. This assimilation is accomplished by engulfing soluble or particulate matter in digestive vacuoles, processes termed pinocytosis and phagocytosis, respectively. In some species, food is ingested at a definite site, the peristome or cytostome. Food may be retained in special intracellular reserves, or vacuoles. Undigested particles and wastes are extruded at the cell surface by mechanisms that are the reverse of those used in ingestion. The intracellular location of many of these parasites means that host cells may have to be modified to accommodate for transport and assimilation of nutrients. This is especially true among the apicomplexans and parasites like Leishmania. Many parasitic protozoans are facultative anaerobes in their definitive host (E histolytica and Giardia are excellent examples). The African trypanosomes must switch from an inefficient anaerobic to a more efficient mode of aerobic respiration when they take up residence in their vector. This is accomplished by profound changes that take place within the kinetoplast–mitochondrial complex of these organisms. The malaria parasite P falciparum has been found to complete its development within the microaerophilic environment of post capillary venules. This discovery now allows investigators to extensively cultivate this parasite in vitro. Some parasitic Protozoa are amitochondriate and utilize specialized organelles such as mitosomes (Giardia) or hydrogenosomes (T vaginalis) where terminal events of electron transfer in anaerobic respiration occur.
✺ Many Protozoa are facultative anaerobes
✺ Nutrients engulfed by phagocytosis or pinocytosis
Survival is ensured by fastidious reproductive and protective techniques. Reproduction in many parasitic protozoans is accomplished primarily by simple binary fission. In one phylum of Protozoa, the Apicomplexa, a cycle of multiple fission (schizogony) alternates with a period of sexual reproduction (sporogony). A similar mode of reproduction is seen in the microsporidia although somewhat modified. Many Protozoa, when exposed to an unfavorable milieu, become less active metabolically and secrete a cyst wall capable of protecting the organism from physical and chemical conditions that would otherwise be lethal. In this form, the parasite is better equipped to survive passage from host to host in the external environment. Giardia, Entamoeba, Naegleria, Cryptosporidium, Cyclospora, and others are all capable of forming environmentally protective cysts or oocysts. The microsporidia produce spores. Immunoevasive mechanisms described in later chapters also contribute to survival of these parasites within the host.
✺ Reproduction usually by binary fission
✺ Many Protozoa form resistant cysts as survival form
As for the Protozoa, the classification of helminth parasites is ever changing. The scheme used in this book is a more classical one and readily accepted and understood by most parasitologists. Accordingly, the various helminth groups discussed are placed into distinct phyla within the subkingdom Metazoa of the kingdom Animalia, which includes all multicellular organisms. These phyla include the Platyhelminthes with the important classes Trematoda (flat worms) and Cestoidea (tapeworm) the Nemathelminthes, or roundworms, and the Acanthocephala, or thorny-headed worms (Table 48–3).
TABLE 48–3Classification of Helminth parasites ||Download (.pdf) TABLE 48–3 Classification of Helminth parasites
Kingdom - Animalia
Subkingdom - Metazoa
Phylum - Platyhelminthes
Class - Trematoda
Genera - Schistosoma, Fasciola, Fasciolopsis, Clonorchis, and Paragonimus
Class - Cestoidea
Genera - Diphyllobothrium, Taenia, Echinococcus, and Hymenolepis
Phylum - Nemathelminthes
Genera - Trichuris, Trichinella, Capillaria, Strongyloides,
Necator, Ancylostoma, Ascaris, Toxocara, Wuchereria, Brugia, and Onchocerca
Phylum - Acanthocephala
Genus - Macrocanthorhynchus
The class Trematoda includes important parasites belonging to the genera Schistosoma, Fasciola, Fasciolopsis, Clonorchis, and Paragonimus. Cestoidea includes the tapeworm parasitic genera Diphyllobothrium, Taenia, Echinococcus, and Hymenolepis.
The Nemathelminthes include important parasites belonging to the genera Trichuris, Trichinella, Capillaria, Strongyloides, Necator, Ancylostoma, Ascaris, Toxocara, Wuchereria, Brugia, and Onchocerca.
The Acanthocephala contains only one genus, Macracanthorhynchus, considered to be of occasional importance to humans.
All helminths are multicellular organisms. The Trematoda vary in size from a few millimeters to several inches and are usually flat in shape. They all are invested in a tegument that is organized as a multicellular syncytium. Absorption of nutrients and excretion of wastes occur across the tegument. They are acoelomate with a body filled with parenchymal tissue. Embedded within this tissue are an incomplete digestive tract composed of ceca and the reproductive organs of both sexes. The schistosomes are an exception to this, have separate sexes, and are tubular in shape. Trematodes usually possess two suckers which help them to locomote and anchor to host tissue. Most trematodes have complex life cycles involving snails as a required first intermediate host in which asexual multiplication of larval stages takes place. Larval stages called cercaria emerge from snail and may either directly penetrate the final definitive host (the schistosomes), or encyst openly in the environment or within a second intermediate host (all other parasitic trematodes).
The Cestoidea, or tapeworms, have a flattened, ribbon-like body, or strobila, composed of segments called proglottids. Like the trematodes, they are invested in a tegument and are acoelomate with proglottids filled with parenchymal tissue. Each proglottid serves as an individual reproductive unit harboring both sets of male and female reproductive organs and are classified as being immature, mature, or gravid (egg filled). At the anterior end of the worm is a scolex (head region) that may or may not be armed with hooks. This body region is anchored to host tissue. Tapeworms continuously produce proglottids, which may be shed individually (apolysis) or as a chain once the eggs are released (anapolysis). They also possess a primitive nervous system that links the scolex to the proglottids. Tapeworms have complex life cycles that vary tremendously and will be discussed in chapters that follow. Infections by larval tapeworms usually result in greater pathogenesis to the host than infection by adult tapeworms. This is true of infections caused by Diphyllobothrium latum, T solium, E granulosus, and Echinococcus multilocularis.
The Nemathelminthes, or nematodes, have a cylindrical fusiform body and a tubular alimentary tract that extends from the mouth at the anterior end to the anus at the posterior end. They are invested in a tough cuticle that must be shed as they go through molts to become adults. They are considered as pseudocoelomate organisms with a body cavity filled with fluid. These worms possess only longitudinal muscles that allow them to flex and put pressure on internal organs so they can function. The sexes are separate, and the male worm is typically smaller than the female. An unusual feature in males is that sperms are ameboid and not flagellated. A variety of reproductive modes are used by these worms including oviparity (egg laying), ovoviviparity (egg followed by larval birth in utero), and parthenogenesis. First larval stages are considered as L1 upon hatching and molt four times to become adults. Life cycles vary tremendously within this group from being direct (Trichuris), indirect, and complex (Strongyloides), to those requiring an intermediate host (all filarial parasites). These life cycles will be discussed in chapters to follow. Helminth parasites are nourished by ingestion (nematodes) or absorption (trematodes and cestodes) of the body fluids, lysed tissue, or intestinal contents of their hosts. Carbohydrates are rapidly metabolized, and the glycogen concentration of the worms is high. Respiration is primarily anaerobic, although larval offspring frequently require oxygen. A large part of the energy requirement is devoted to reproductive needs. The daily output of offspring can be as high as 200 000 for some worms.
Protection from the host’s digestive and body fluids is afforded by the tegument or cuticle and the secretion of enzymes. Some worms, such as the schistosomes, can protect themselves from immunologic attack by the incorporation of host antigens into their tegument. The life span of the adult helminth is often measured in weeks or months, but some, such as the hookworms, filariae, and flukes, can survive within their hosts for decades, producing chronic infections with attendant morbidity or mortality.