Chapter 23 Eukaryotic Parasites of Medical Importance: Protozoa and Helminths* *Lecture notes are to be used as a study guide only and do not represent the comprehensive information you will need to know for the exams. Parasitology covers protozoa and helminths (worms) that live on or in a host body. Parasites are spread to humans by other humans, animal hosts, and vectors. These notes focus on representative species and genera and are by no means comprehensive. Chapter figures are especially helpful in following the complex life cycles of these organisms. Section 5.7 and 5.8 and some figures in the text chapter 5 may be helpful. 23.1 The Parasites of Humans Parasitology is the study of eukaryotic parasites. Pathogenic protozoa are a diverse group of unicellular, eukaryotic organisms which cause a variety of diseases See Systems Profile 23.1. 23.2 Major Protozoan Pathogens Some protozoans propagate only as trophozoites, the active feeding stage found in hosts, whereas others alternate between a trophozoite and a cyst stage (cysts are dormant, stress resistant forms (fig. 5.23). While many reproduce asexually, some have more complex life cycles with sexual and asexual phases that can be carried out in multiple hosts. Table 23.2 lists the more common drugs used for protozoan infections. Infective Amoebas Amoebas are motile by means of pseudopodia cytoplasmic extensions which allow it to crawl across surfaces. A. Entamoeba histolytica is the agent of amebiasis, or amebic dysentery. This type of infection is spread worldwide and affects some 500 million people in the tropics alone. The organism alternates between trophozoite and cyst stages (fig. 23.1). Cysts released in feces are carried and spread through unsanitary water and food. See Pathogen Profile #1. Ingested cysts transform into trophozoites that invade the large intestine causing ulceration and dysentery (fig. 23.2 and fig. 23.3). Treatment: metronidazole 1, fiodoquinol, and chloroquine are effective in eliminating the organism. 1 Metronidazole (Flagyl ) is the most common treatment for protozoal infections and, unusually, for Clostridium difficile infections also. 1
B. Naegleria fowleri is a common free-living amoeba (fig. 23.4) that only infects humans accidentally. Found in fresh or brackish (mildly salty) water, ponds, lakes, and occasionally swimming pools and hot tubs (especially if improperly chlorinated), the amoeba are forced into the nasal passages during swimming. It can burrow through the nasal mucosa and infect the CNS resulting in acute meningitis. The hemorrhage almost always proves fatal. Treatment: the infection proceeds so quickly that usually no treatment is effective. 23.3 The Flagellates (Mastigophorans) Trichomonads: Trichomonas Species The principal human parasite of this group is Trichomonas vaginalis, the cause of a common sexually transmitted disease known as trichomoniasis that infects the vagina, cervix, and urethra (in both sexes)(fig. 23.6). Painful, itchy urethritis with discharge. Approximately 50% carriers are asymptomatic. Treatment: metronidazole. Giardia intestinalis and Giardiasis Giardia lamblia is an intestinal parasite that causes giardiasis (fig. 23.7). G. lamblia s natural reservoir is animal intestines, and the source of human infection is cyst-containing fresh water and food. Symptoms of giardiasis: severe diarrhea (may be chronic in immunocompromised patients), abdominal pain, and flatulence. Giardiasis is preventable by disinfecting water (boiling or iodine), but chlorine at the concentrations used in swimming pools does not kill the cysts. Treatment: responds well to metronidazole or quinacrine. Hemoflagellates: Vector-Borne Blood Parasites Hemoflagellates, as the name suggests, are protozoans that occur in blood infections. Mostly tropical zoonoses spread by insect vectors. Most of these pathogens have complex life cycles with various stages maturing in insect and human (or other animal) hosts (Table 23.3). (i) Trypanosoma have tapering, flagellated cells. T. brucei agent of African sleeping sickness (fig. 23.8). Vector: tse-tse fly. The bite of the fly inoculates the skin; the trypanosome invades the circulatory system where it multiplies causing damage to the spleen, lymph nodes, and brain. Chronic symptoms include: sleepiness, tremors, paralysis, and coma. T. cruzi agent of Chagas disease (endemic to Central and South America) (fig. 23.9). Vector: reduvid (kissing) bug. Insect feces inoculate a cutaneous portal (cuts & scrapes or the bite site of the insect). Also know to be transmitted via insect feces in food. Mature trypanosomes invade circulatory system causing chronic inflammation in organs (esp. the heart and brain) (fig. 23.10). Treatment is problematic; disease is lifethreatening. Possible emerging disease in Texas. 2
Can cross the placenta and cause congenital disease. (ii) Leishmania have a single, anterior flagellum Transmitted by blood sucking biting flies, vector: sand flies. Found in equatorial regions. Upon biting the host, leishmania will reside in macrophages (fig. 23.11). The infection can be local or systemic. 23.4 Apicomplexan Parasites Very small, intracellular parasites that have complex life cycles and lack motility in their mature stage. Infectious forms include sporozoites, fecal cysts called oocysts, and tissue cysts. Most diseases are zoonotic and vector-borne. Plasmodium: The Agent of Malaria Plasmodium causes malaria a life threatening disease. Four species within the genus Plasmodium are of concern. Humans are a primary host of the asexual phase of the parasite; the female Anopheles mosquito is the vector/host for the sexual phase. Distributed in a geographic belt around the equator (fig. 23.12), an estimated 300 to 500 million cases/year of malaria are diagnosed (with at least 2 million deaths of young people and infants every year). The U.S. sees approximately 1,000-2,000 cases a year, most of which occur in immigrants. See Pathogen Profile #2. Sporozoites enter the human circulatory system with mosquito saliva, penetrate liver cells, multiply, and form hundreds of merozoites. The merozoites infect, multiply in, and lyse red blood cells (fig. 23.13). Figure 23.13 illustrates the life cycle of Plasmodium. Symptoms: episodes of chills-fever-sweating, anemia, and organ enlargement. Treatment: chloroquine, quinine, or primaquine Coccidian Parasites T. gondii lives naturally in cats that harbor oocysts in their GI tract (fig. 23.15). Toxoplasma gondii causes toxoplasmosis in humans. Acquired by ingesting raw or rare meats containing tissue cysts or by accidentally ingesting oocysts from substances contaminated by cat feces. In humans, infection is usually mild and flu-like EXCEPT in immunodepressed patients (esp. AIDS) where the infection spreads to the CNS, causing brain damage and death. Can cross the placenta and cause fetal toxoplasmosis wherein the fetus will suffer brain and heart damage; for this reason: Pregnant women should NEVER handle cat litter boxes. 3
Treatment: pyrimethamine and sulfadiazine (alone or in combination; this is another combination that blocks folate synthesis). 23.5 A Survey of Helminth Parasites Parasitic helminths, or worms, are multicellular animals with a wide variety of special adaptations for parasitizing human hosts including: special mouth parts (hooks), degenerative organ structures (they don t waste energy with complex metabolisms when the host will digest food for them), and complex life cycles (fig. 23.17). Adult worms reproduce sexually producing eggs which hatch into larvae. Larvae then mature into adults in a multi-stage process. In some worms the sexes are separate, and in others they are hermaphroditic (both sexes in one organism). See Systems Profile 23.2. See Table 23.4, for major helminth infections of humans and their modes of transmission. (Don t memorize the whole thing it s just a good resource) Prevention: Many of the species which are acquired through the consumption of improperly cooked meats can be prevented by adequate cooking. Water sources can also be purified of contaminating species by boiling or other treatment (e.g. iodine, chlorination). It is almost always a good recommendation to cook meats well done. Treatment: Mebendazole is a broad-spectrum antihelminth agent used to treat intestinal roundworm, hookworm, and trichinosis. Niclosamide is specifically used for tapeworm infections Pyrantel is for pinworm (Enterobius vermicularis) and hookworm infections. (OTC = Pin-Rid ). See Table 23.5, for a more comprehensive list of antihelminthic therapeutic agents. Intestinal Nematodes Cycle A Nematodes, or round worms, are filamentous with protective cuticles, circular muscles, complete digestive tracts, and separate sexes with well-developed reproductive systems. A. Ascaris lumbricoides (fig. 23.18) These intestinal nematodes are a very prevalent species indigenous to humans. Eggs ingested with food hatch into larvae and burrow through the intestine into the circulation. From there they travel to the lungs and pharynx and are swallowed. Back in the intestine, the adult worms complete the reproductive cycle. B. Enterobius vermicularis Also known as pinworm, this is a common childhood infection confined to the intestine. Symptoms include anal itching as worms exit the body (defecation is not necessary, i.e. they crawl out). 4
Intestinal Nematodes Cycle B C. Hookworms Hookworms have characteristic curved ends and hooked mouths (fig. 23.19). Two major species are: Necator americans (Western hemisphere) and Anclostoma duodenale (Eastern hemisphere). Both species share a similar life cycle: Humans shed eggs in feces, which hatch into larvae. Larvae burrow into the skin of the feet and the lower legs and travel from blood to lungs/pharynx and are swallowed. Adult worms reproduce in the intestine and complete the cycle. D. Trichinella spiralis T. spiralis causes trichinosis, a zoonosis in which humans are dead-end hosts. T. spiralis is acquired from eating undercooked or raw pork (or bear) containing encysted larvae. Larvae migrate from the intestine to blood vessels, muscle, heart, and CNS where they enter dormancy (fig. 23.20). Initial symptoms are flu-like: diarrhea, nausea, abdominal pain, and fever. Life-threatening if the brain and heart are infected. See Pathogen Profile #3. Treatment: poor prognosis once the larvae have encysted in muscles. 23.7 Flatworms: The Trematodes and Cestodes Trematodes, or flukes, are flatworms (platyhelminthes) with leaf-like bodies bearing suckers & hooks on the mouth and degenerated digestive systems. Most are hermaphroditic. Blood Flukes: Schistosomes Cycle D Blood flukes like Schistosoma species cause schistosomiasis (fig. 23.22). Prevalent in the tropics with an estimated 100 million active cases at any given time. Adult flukes live in humans and release eggs (via feces or urine) into water sources. The early larvae develop in the freshwater snail into a second larvae which can penetrate human skin (people report a tingling sensation when swimming in water heavily contaminated with this organism). They then migrate to the liver where they mature. Adults migrate to intestine or bladder (where they can cause bladder obstruction, resulting in blood in the urine) and shed eggs causing chronic organ enlargement. Cestodes (Tapeworm) Infections Cycle C These flatworms have long, very thin, ribbon-like bodies composed of units called proglottids and a scolex that grips host intestine. The scolex isn t a true mouth, rather, a series of hooks that aid in attachment. Each proglottid is an independent unit capable of absorbing food (which the human host has conveniently digested for it) and making and releasing eggs (hermaphrodites). 5
A. Taenia saginata T. saginata is the beef tapeworm for which humans are the definitive host (fig. 23.24 b). Animals are infected by grazing on land contaminated with human feces. Human infection occurs from eating raw beef in which the larvae have encysted. Larvae attach to the human intestine and mature into adults. An otherwise healthy adult human can harbor these worms almost asymptomatically for a long time (even years). B. Taenia solium T. solium is the pork tapeworm. Eggs consumed in improperly cooked pork will give rise to larvae which go on to encyst in many organs, including the brain (fig.23.24 and fig. 23.25). 23.8 The Arthropod Vectors of Infectious Disease Arthropods are invertebrates that include insects and arachnids. These can be ectoparasites that feed on the blood of the host, and at the same time transmit an infectious agent. See Table 23.6 for a summary of the arthropod vectors that harbor infectious agents. 6