The parasites covered in this chapter are categorized into two major groups: parasitic protozoa and parasitic helminths.

Protozoa are unicellular eukaryotes that form an entire kingdom. Classifying protozoan parasites into taxonomic groups is an ongoing process, and their status is often in a state of flux. For this reason, this chapter separates the parasitic protozoa into four traditional groups based on their means of locomotion and mode of reproduction: flagellates, amebae, sporozoa, and ciliates. Table 1 lists several medically important protozoan parasites by the organ system they infect, the mode of infection, diagnosis, treatment, and geographic location.

Table1. Synopsis of Protozoan Infections by Organ System

Table1. Synopsis of Protozoan Infections by Organ System (Continued)

(1) Flagellates have one or more whip-like flagella and, in some cases, an undulating membrane (eg, trypanosomes). These include intestinal and genitourinary flagellates (Giardia and Trichomonas, respectively) and blood and tissue flagellates (Trypanosoma and Leishmania). (2) Amebae are typically ameboid and use pseudopodia or protoplasmic flow to move. They are represented in humans by species of Entamoeba, Naegleria, and Acanthamoeba. (3) Sporozoa undergo a complex life cycle with alternating sexual and asexual reproductive phases. The human parasites, such as Cryptosporidium, Cyclospora, and Toxoplasma, and the malarial parasites (Plasmodium species) are all intracellular parasites. (4) Ciliates are complex protozoa bearing cilia distributed in rows or patches, with two kinds of nuclei in each individual. Balantidium coli, a giant intestinal ciliate of humans and pigs, is the only human parasite representative of this group, and because the disease is considered rare, it is not covered in this chapter.

Formerly listed with the sporozoa, because they possess polar filaments within a spore, microsporidia include more than 1000 species of intracellular parasites that infect invertebrates (mostly insects) and vertebrate hosts. In humans, microsporidians are opportunistic parasites of immunocompromised patients, including those undergoing chemotherapy and organ transplants.

Pneumocystis jiroveci was long considered a protozoan parasite but has been shown to be a member of the fungi rather than the protozoa. It causes interstitial plasma cell pneumonitis in immunosuppressed individuals and is considered an opportunistic pathogen.

The majority of parasitic helminths that infect humans belong to two phyla: Nematoda (roundworms) and Platyhelminthes (flatworms).

(1) Nematodes are among the most speciose and diverse animals. They are elongated and tapered at both ends, round in cross-section, and unsegmented. They have only a set of longitudinal muscles, which allows them to move in a whip like, penetrating fashion; a complete digestive system that is well adapted for ingestion of the host’s gut contents, cells, blood, or cellular breakdown products; and a highly developed separate-sexed reproductive system. They shed their tough cuticles (molt) as they undergo development from larvae to adults, and the eggs and larval stages are well suited for survival in the external environment. Most human infections are acquired by ingestion of the egg or larval stage, but nematode infections can also be acquired from insect vectors and skin penetration. (2) Platyhelminths are flatworms that are dorsoventrally flattened in cross-section and are hermaphroditic, with a few exceptions. All medically important species belong to two classes: Trematoda (flukes) and Cestoda (tapeworms).

Trematodes are typically flattened and leaf shaped with two muscular suckers. They have a bifurcated gut and possess both circular and longitudinal muscles; they lack the cuticle characteristic of nematodes and instead have a syncytial epithelium. Trematodes are hermaphroditic, with the exception of the schistosomes (blood flukes), which have male and female worms that exist coupled together within small blood vessels of their hosts.

The life cycle of human trematodes is typically initiated when eggs are passed into fresh water via feces or urine. Eggs develop, hatch, and release a ciliated miracidium, which infects a snail host that is usually highly specific to the fluke species. Within the snail, the miracidium develops into a sporocyst, which contains germinal cells that ultimately develop into the final larval stage—the cercariae. These swim out of the snail and encyst as metacercariae in a second intermediate host or on vegetation, depending on the species. Most fluke infections are acquired by ingestion of the metacercariae. The cercariae of schistosomes, however, directly penetrate the skin of their hosts and do not encyst as metacercariae.

Cestodes, or tapeworms, are flat and have a ribbon-like chain of segments (proglottids) containing male and female reproductive structures. Adult tapeworms can reach lengths of 10 m and have hundreds of segments, with each segment releasing thousands of eggs. At the anterior end of an adult tapeworm is the scolex, which is often elaborated with muscular suckers, hooks, or structures that aid in its ability to attach to the intestinal wall. Adult tapeworms have no mouth or gut and absorb their nutrients directly from their host through their integument.

The life cycle of cestodes, like that of the trematodes, is usually indirect (involving one or more intermediate hosts and a final host). Eggs are excreted with the feces and ingested by an intermediate host (invertebrate, such as a flea, or vertebrate, such as a mammal); the larvae develop into certain forms that are peculiar to the specific species within the intermediate host (eg, cysticercus in the case of Taenia solium or hydatid cyst with Echinococcus granulosus). Cestode larvae are generally eaten, and the larva develops into an adult worm in the intestine of the final host.

اخر الاخبار

اخبار العتبة العباسية المقدسة

جناح العتبة العباسية المقدسة في معرض طهران الدولي يشهد فعاليات متنوِّعة

شعبة مدارس الكفيل تطلق حملتها السنوية لدعم العوائل المتعففة في شهر رمضان المبارك

بأجواء رمضانية مميزة.. مجموعة مشاتل الكفيل تستقبل العوائل العراقية يوميًا

المجمع العلمي: الختمة القرآنية الرمضانية المركزية المرتلة تشهد تنوعًا في القراءات والمشاركات

المزيد

الآخبار الصحية

دراسة تكشف "السبب الخفي" وراء ترك التمارين الرياضية

كشف التوقيت "المثالي" لتغيير فرشاة الأسنان

بلاستيك في أنسجة بشرية.. دراسة تكشف مفاجأة طبية

دراسة تحذر من الإفراط في ممارسة الرياضة.. كيف يضر دماءك؟

المزيد

الآخبار التكنلوجية

"ديب سيك" تحجب أحدث نماذجها عن شركات الرقائق الأميركية

"سامسونغ" تطلق سلسلة هواتف "غالاكسي إس 26"

حملات سرية ورسائل تشويه.. كيف يتم استغلال "تشات جي بي تي"؟

أثر نفسي غير متوقع للرياضة.. مرتبط بالغضب

المزيد

مواضيع ذات صلة

Entamoeba histolytica (Intestinal and Tissue Ameba)

Giardia lamblia (Intestinal Flagellate)

Toxoplasmosis

The Lung Flukes

The Liver Flukes

Echinococcus granulosus

Taenia saginata

Taenia solium

Hymenolepis diminuta

Hymenolepis nana

Dipylidium caninum

Diphyllobothrium latum