Parasitic Protozoa, Helminths, and Arthropod Vectors Chapter 23 Parasitic Protozoa, Helminths, and Arthropod Vectors

Parasitic Diseases Protozoan and helminthic parasites exist worldwide Occur among people in rural, undeveloped, or overcrowded places Emerging as serious threats in developed nations Parasitic infections often involve several hosts Definitive host Intermediate host Parasites can infect humans in one of three ways

Routes by which humans acquire parasitic infections Figure 23.1

Protozoan Parasites of Humans Protozoa that enter the body via ingestion have two morphological forms Trophozoite Feeding and reproducing stage that lives within the host Cyst Infective form that survives in the environment Undergo excystment when ingested developing into trophozoites Trophozoites undergo encystment before leaving the host in feces Parasites presented based primarily on their mode of locomotion Ciliates, amoebae, flagellates, and apicomplexans

Protozoan Parasites of Humans Ciliates Protozoa that use cilia in their trophozoite stage Balantidium coli Only ciliate known to cause disease in humans Commonly found in animal intestinal tracts Humans infected by food or water contaminated with feces containing cysts Trophozoites attach to mucosal epithelium lining the intestine Infections generally asymptomatic in healthy adults Balantidiasis occurs in those with poor health Persistent diarrhea, abdominal pain, and weight loss

Protozoan Parasites of Humans Amoebae Protozoa with no truly defined shape Move and acquire food through the use of pseudopodia Found in water sources throughout the world Few cause disease

Protozoan Parasites of Humans Amoebae Entamoeba Carried asymptomatically in the digestive tracts of humans No animal reservoir exists Infection occurs by drinking water contaminated with feces containing cysts Three types of amebiasis can result from infection Luminal amebiasis Invasive amebic dysentery Invasive extraintestinal amebiasis Maintaining clean water is important in prevention

Trophozoite of Entamoeba histolytica Figure 23.2

Protozoan Parasites of Humans Amoebae Acanthamoeba and Naegleria Cause rare and usually fatal brain infections Common inhabitants of natural and artificial water systems Individuals who wash their contact lenses with tap water can become infected

Protozoan Parasites of Humans Amoebae Acanthamoeba and Naegleria Acanthamoeba disease Occurs through cuts, scrapes, the conjunctiva, or inhalation Acanthamoeba keratitis due to conjunctival inoculation Amebic encephalitis is the more common disease Naegleria disease Infection occurs when swimmers inhale contaminated water Amoebic meningoencephalitis can result Prevention difficult since organisms are environmentally hardy

Protozoan Parasites of Humans Flagellates Protozoa that possess at least one flagellum Number and arrangement of flagella important to determining the species The flagellates include members of the genera Trypanosoma, Leishmania, Giardi, and Trichomonas

Protozoan Parasites of Humans Flagellates Trypanosoma cruzi Causes Chagas’ disease Endemic in Central and South America Opossums and armadillos are the primary reservoir Transmission occurs through bite of insects in genus Triatoma “Kissing bugs” feed preferentially from blood vessels in the lips

The life cycle of Trypanasoma cruzi Figure 23.3

Protozoan Parasites of Humans Flagellates Trypanasoma cruzi Chagas’ disease Progresses through four stages Acute stage characterized by chagomas Generalized stage An asymptomatic chronic stage Symptomatic stage characterized by congestive heart failure following formation of pseudocysts Parasite-induced heart disease is a leading cause of death in Latin America

Mature trypomastigotes of Trypanasoma cruzi Figure 23.4

Protozoan Parasites of Humans Flagellates Trypanosoma brucei Causes African sleeping sickness The insect vector is the tsetse fly Humans usually infected when bitten by infected tsetse flies

The life cycle of T. brucei Figure 23.5

Protozoan Parasites of Humans Flagellates Trypanasoma brucei African sleeping sickness Progresses through three stages if left untreated Site of the fly bite becomes a lesion with dead tissue Fever, lymph node swelling, and headaches Meningoencephalitis Infections characterized by cyclical waves of parasitemia Clearing tsetse fly habitats reduce the cases of disease

Protozoan Parasites of Humans Flagellates Leishmania Causes leishmaniasis Endemic in parts of the tropics and subtropics Wild and domestic dogs and small rodents are common hosts Leishmania have two developmental stages Amastigotes Multiply in host’s macrophages and monocytes Promastigotes Develop extracellularly within a vector’s gut

The life cycle of Leishmania Figure 23.6

Protozoan Parasites of Humans Flagellates Leishmania Three clinical forms of leishmaniasis often observed Cutaneous leishmaniasis Mucocutaneous leishmaniasis Visceral leishmaniasis Most cases of leishmaniasis heal without treatment Prevention limited to reducing exposure to reservoir and vector

Mucocutaneous leishmaniasis Figure 23.7

Protozoan Parasites of Humans Flagellates Giardia intestinalis Found in intestinal tracts of animals and in the environment Causative agent of giardiasis Common gastrointestinal disease in the United States Ingest cysts in contaminated water or when swimming Range from asymptomatic infection to gastrointestinal disease Giardia have a life cycle similar to that of Entamoeba Trophozoites multiply in the small intestines Prevent infections in endemic areas by use of filtered water

Trophozoite of Giardia intestinalis Figure 23.8

Protozoan Parasites of Humans Flagellates Trichomonas vaginalis Most common protozoan causing human disease in industrialized nations Lives in the genitourinary system of men and women Transmitted almost exclusively via sex Occurs in people with preexisting STD or multiple sex partners Infection of women results in vaginitis Infection of men is typically asymptomatic Prevention involves abstinence and safe sex

Trophozoite of Trichomonas vaginalis Figure 23.9

Protozoan Parasites of Humans Apicomplexans Alveolate protozoa Infective form characterized by ornate complex of organelles at their apical end Parasites of animals Life cycles involve at least two types of hosts Schizogony is major feature of apicomplexan life cycles Multinucleate schizonts form before the cells divide Four important apicomplexan parasites Plasmodium, Toxoplasma, Cryptosporidium, and Cyclospora

Protozoan Parasites of Humans Apicomplexans Plasmodium Causative agent of malaria Four species cause malaria P. falciparum, P. vivax, P. ovale, and P. malariae Malaria is endemic throughout the tropics and subtropics Mosquitoes are vector for Plasmodium The Plasmodium life cycle has three prominent stages

Geographical distribution and incidence of malaria Figure 23.10

The life cycle of Plasmodium Figure 23.11

Trophozoites of Plasmodium falciparum in erythrocytes Figure 23.12

Protozoan Parasites of Humans Apicomplexans Plasmodium Genetic traits increase malaria resistance in endemic areas Sickle-cell trait Sickle-shaped cells resist penetration by Plasmodium Hemoglobin C Two genes for hemoglobin C protect against malaria Glucose-6-phosphate-dehydrogenase deficiency Needed by the trophozoites to synthesize DNA Lack of Duffy antigens on erythrocytes P. vivax requires Duffy antigen to infect erythrocytes

Protozoan Parasites of Humans Apicomplexans Plasmodium Symptoms of malaria associated with cycles of erythrocyte lysis P. falciparum can cause blackwater fever High fever, erythrocyte lysis, renal failure, and dark urine Cerebral malaria results in tissue death in the brain Immunity develops if victim survives acute stage Periodic episodes become less severe over time Prevent malaria by limiting contact with mosquitoes

Protozoan Parasites of Humans Apicomplexans Toxoplasma T. gondii is causative agent of toxoplasmosis One of the world’s most widely distributed parasites Wild and domestic mammals and birds are major reservoir Cats are the definitive host Infection due to consumption of undercooked meat containing the parasite Ingestion or inhalation of contaminated soil can also occur The protozoan can cross the placenta and infect the fetus

The life cycle of Toxoplasma gondii Figure 23.13

Protozoan Parasites of Humans Apicomplexans Toxoplasma Most infections are asymptomatic Toxoplasmosis develops in a small number of people Fever-producing illness combined with other symptoms Usually is a self-limited infection Toxoplasmosis is more severe in two populations AIDS patients Fetuses Prevent by cooking meats and avoiding contaminated soil

Pseudocysts of Toxoplasma gondii Figure 23.14

Protozoan Parasites of Humans Apicomplexans Cryptosporidium parvum Causative agent of cryptosporidiosis (Cryptosporidum enteritis) Once thought to only infect livestock and poultry Humans can carry the parasite asymptomatically Infection usually results from drinking contaminated water Fecal-oral transmission can occur Causes severe diarrhea that can last up to two weeks Chronic Cryptosporidum enteritis indicator a person has AIDS Infection of AIDS patients can be life threatening

Oocysts of Cryptosporidium parvum Figure 23.15

Protozoan Parasites of Humans Apicomlexans Cyclospora cayetanensis Water borne apicomplexan Linked to raspberries imported from Central and South America Infection occurs from ingestion of oocysts in contaminated food or drink Environmental reservoir is unknown Symptoms include cramps, watery diarrhea, myalgia, and fever AIDS patients can experience more severe disease Thoroughly washing fruits and vegetables can prevent infection

Helminthic Parasites of Humans Helminths are macroscopic, multicellular, eukaryotic worms Life cycles are complex Intermediate hosts are often needed to support larval stages Adult worms are either dioecious or monoecious Three groups of helminthes Cestodes (tapeworm) Trematodes (fluke) Nematodes (roundworm)

Helminthic Parasites of Humans Cestodes Flat, segmented intestinal parasites All tapeworms lack digestive systems All possess the same general body plan

Features of tapeworm morphology Figure 23.16

Generalized life cycle of some tapeworms of humans Figure 23.17

Helminthic Parasites of Humans Cestodes Taenia Taenia saginata is the beef tapeworm Taenia solium is the pork tapeworm Cattle and swine serve as the intermediate hosts Humans living close to livestock have highest incidence of infection Adults attach to the intestinal epithelium Most individuals shed strobila without having symptoms Intestinal blockage can occur if the tapeworm is large

Helminthic Parasites of Humans Cestodes Echinococcus Echinococcus granulosus is the canine tapeworm Canines infected by eating cysticerci in various herbivore hosts Humans are accidental intermediate hosts Consume food or water contaminated from dog feces Causes hydatid disease Larvae travel throughout body and form hydatid cysts Symptoms follow enlargement of cysts in infected tissue Prevent with good hygiene practices

X ray of hydatid cyst Figure 23.18

Helminthic Parasites of Humans Trematodes Flukes are flat, leaf-shaped worms Lack complete digestive tract Attach to host tissues via a ventral sucker to obtain nutrients Geographical distribution limited because the intermediate host is limited Grouped according to the site in the body they parasitize

Some features of fluke morphology Figure 23.19

Representations of fluke life cycles Figure 23.20

Representative Liver Fluke: Fasciola Trematodes Representative liver fluke: Fasciola Fasciola hepatica and F. gigantica Infect sheep and cattle worldwide Humans are accidental definitive hosts Ingest metacercariae from aquatic vegetation Parasite migrates from the intestine to the liver Chronic infections occur when flukes reside in the bile ducts Symptoms coincide with episodes of bile duct obstruction and inflammation

Helminthic Parasites of Humans Trematodes Blood flukes: Schistosoma Dioecious blood flukes Causative agent of schistosomiasis Humans are the principal definitive host Three geographically limited species infect humans S. mansoni S. haemotobium S. japonicum

Helminthic Parasites of Humans Trematodes Blood flukes: Schistosoma Cercariae burrow through skin of humans who contact contaminated water Larvae mature and mate in the circulatory system Eggs move to the intestines or the urinary bladder and ureters Dermatitis may occur at the site where cercariae entered Infections can become chronic and can be fatal Prevention relies on improved sanitation and avoiding contaminated water

An egg of Schistosoma mansoni Figure 23.21

Helminthic Parasites of Humans Nematodes Roundworms are long, cylindrical worms that taper at each end Possess complete digestive tracts Have a protective outer cuticle Parasites of almost all vertebrates

Helminthic Parasites of Humans Nematodes Features of the life cycle of roundworms Number of reproduction strategies Intestinal nematodes shed eggs into lumen of the intestine Eggs are consumed in contaminated food or water Larvae of some intestinal nematodes hatch in the soil Larvae actively penetrate the skin of a host Some nematodes encyst in muscle tissue Consumed in raw or undercooked meat Mosquitoes transmit a few species of filarial nematodes Adult sexually mature stages are found only in definitive hosts

Ascaris lumbricoides Figure 23.22

Helminthic Parasites of Humans Nematodes Ascaris Ascaris lumbricoides Endemic in the rural southeastern United States Causative agent of ascariasis Most common nematode infection of humans worldwide Eggs are ingested in water or on vegetables Adult worms grow and reproduce in the small intestine Most infections are asymptomatic Proper sanitation and hygiene are important for prevention

Helminthic Parasites of Humans Nematodes Ancylostoma and Necator Two hookworms infect humans Ancylostoma duodenale Necator americanus Larvae burrow through skin Travel to the heart, lungs, and eventually the small intestine Adult worms suck the blood of their hosts Causes chronic anemia, iron and protein deficiencies Ground itch occurs at the site of larvae penetration Wearing shoes in endemic areas helps prevent exposure

Mouth of Ancylostoma duodenale Figure 23.23

Helminthic Parasites of Humans Nematodes Enterobius vermicularis Commonly known as the pinworm Most common parasitic worm found in the United States Humans are the only host for Enterobius Female pinworms deposit their eggs in the anus Infections are often asymptomatic Intense perianal itching occurs if symptomatic Preventing fecal-oral spread from infected individuals limits disease

Eggs of Enterobius vermicularis Figure 23.24

Helminthic Parasites of Humans Nematodes Wuchereria bancrofti Filarial nematode Infects the lymphatic system Causative agent of filariasis Transmitted by various genera of female mosquitoes Mosquitoes ingest microfilariae when feeding on human Mosquitoes then transmit the parasite to new individual Remains asymptomatic for years Lymphatic damage can result in elephantiasis Prevention relies on avoiding infected mosquitoes

A microfilaria of Wuchereria bancrofti in blood Figure 23.25

Elephantiasis in a leg Figure 23.26

Arthropod Vectors Vectors are animals that carry microbial pathogens Arthropods are common vectors Some arthropods are biological vectors Serve as hosts for the pathogens they transmit Disease vectors belong to two classes of arthropods Arachnida Insecta