Chapter 23

Parasitic Protozoa, Helminths, and Arthropod Vectors

Parasitic Diseases

• Protozoan and helminthic parasites exist worldwide

• Often occur among people living in rural, undeveloped, or overcrowded places

• Emerging as serious threats in developed nations

Parasitic Diseases

• Parasitic infections often involve several hosts

• Definitive host

• Mature forms of the parasite are present and usually reproducing

• Intermediate host

• Immature parasites undergo various stages of maturation in these hosts

• Parasites can infect humans in one of three ways

ingestion, mucus membranes, direct injection

Protozoan Parasites of Humans

• Most protozoa that enter the body via ingestion have two morphological forms

• Trophozoite

• Feeding and reproducing stage that lives within the host

• Cyst

• Dormant form that can survive in the environment and is infective to new hosts

• Cysts undergo excystment when ingested by a host and develop into trophozoites

Protozoan Parasites of Humans

• Trophozoites undergo encystment before leaving the host in the feces and becoming available to infect other hosts

• Parasites are presented here based primarily on their mode of locomotion

• Ciliates, amoebae, flagellates, and the typically nonmotile apicomplexans

Ciliates

• Protozoa that use cilia in their trophozoite stage

• Balantidium coli is the only ciliate known to cause disease in humans

• Commonly found in animal intestinal tracts

• Humans become infected by consuming food or water contaminated with feces containing cysts

• Trophozoites attach to the mucosal epithelium lining the intestine

• B.coli infections are generally asymptomatic in healthy adults

Ciliates

• Balantidiasis occurs in those with poor health

• Characterized by persistent diarrhea, abdominal pain, and weight loss

• Dysentery results in severe infections

• Presence of trophozoites is diagnostic for the disease

• Prevention relies on good personal hygiene and efficient water sanitation

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

Entamoeba histolytica

• Carried asymptomatically in the digestive tracts of humans

• No animal reservoir exists

• Infection usually occurs by drinking water contaminated with feces that contain cysts

• Trophozoites migrate to the large intestine where they multiply

Entamoeba histolytica

• 3 types of amoebiasis can result from infection

• Luminal amoebiasis

• Least severe form that is asymptomatic

• Invasive amoebic dysentery

• More common form of infection

• Characterized by bloody, mucus-containing stools and pain

• Invasive extraintestinal amoebiasis

• Trophozoites carried via the bloodstream throughout the body

• Maintaining clean water is important in prevention

Acanthamoeba and Naegleria

• Cause rare and usually fatal brain infections

• Common inhabitants of natural waterways as well as artificial water systems

• Contact lenses wearers who use tap water to wash their lenses can become infected

• Acanthamoeba diseases

• Infection occurs through cuts or scrapes, the conjunctiva, or through inhalation

• Keratitis results from conjunctival inoculation

• Amoebic encephalitis is the more common disease

Acanthamoeba and Naegleria

• Naegleria disease

• Infection occurs when swimmers inhale contaminated water

• Amoebic meningoencephalitis results when trophozoites migrate to the brain

• Prevention is difficult because these organisms are environmentally hardy

Flagellates

• Protozoa that possess at least one flagellum

• Number and arrangement of the flagella are important to determining the species

• The flagellates include members of the genera Trypanosoma, Leishmania, Giardi, Trichomonas

Trypanosoma brucei

• Causes African sleeping sickness

• The insect vector is the tsetse fly

• Humans are usually infected when bitten by tsetse flies infected while feeding on infected animals

African Sleeping Sickness

• Progresses through three stages if left untreated

• The wound created at the site of the fly bite becomes a chancre with necrotic tissue

• Presence of the parasites in the blood generates fever, lymph node swelling, and headaches

• Invasion of the central nervous system results in meningoencephalitis

African Sleeping Sickness

• Infections are characterized by cyclical waves of parasitemia

• T. brucei changes its surface glycoproteins with each wave of replication resulting in a ineffective immune response

• Clearing of tsetse fly habitats and use of insecticides help reduce the cases of disease

Trypanosoma cruzi

• Causes Chagas’ disease

• Endemic in Central and South America

• Opossums and armadillos are the primary reservoir

• Transmission occurs through the bite of insects in the genus Triatoma

• The “kissing bugs” feed preferentially from the blood vessels in the lips

Chagas’ Disease

• Progresses through four stages

• Acute stage characterized by chagomas, which are swellings at the sites of each of the bites

• A generalized stage characterized by fever, swollen lymph nodes, myocarditis, and enlargement of the spleen, esophagus, and colon

• A chronic stage, which is asymptomatic and can last for years

• A symptomatic stage characterized primarily by congestive heart failure following the formation of pseudocysts, which are clusters of amastigotes in heart muscle tissue

Chagas’ Disease

• Parasite-induced heart disease is one of the leading causes of death in Latin America

Leishmania

• Causes leishmaniasis

• Endemic in parts of the tropics and subtropics

• Zoonotic protozoa commonly hosted by wild and domestic dogs and small rodents

• Transmission of the parasites occurs through the bite of infected female sand flies

• Leishmania have two developmental stages

• Amastigotes-multiply within the mammalian host’s macrophages and monocytes

• Promastigotes-develop extracellularly within the vector’s gut

Leishmaniasis

• 3 clinical forms of leishmaniasis are often observed

• Cutaneous leishmaniasis

• Produces large painless skin ulcers that form around the bite wound

• Mucocutaneous leishmaniasis

• Skin lesions enlarge to encompass the mucous membranes of the mouth, nose, and soft palate

• Visceral leishmaniasis

• Macrophages spread the parasite to the liver, spleen, bone marrow, and lymph nodes

Leishmaniasis

• Most cases of leishmaniasis heal without treatment and confer immunity

• Prevention is limited to reducing exposure to the reservoir and vector

Giardia intestinalis

• Causative agent of giardiasis

• Giardiasis is a common gastrointestinal disease in the United States

• Giardia are found in the intestinal tracts of animals and humans worldwide but also in the environment

• Infections usually result from ingestion of cysts in contaminated drinking water

• Giardia have a life cycle similar to that of Entamoeba

• The trophozoites are released into the small intestines where they multiply

• Trophozoites can either remain free in the lumen of the small intestine or attach to the intestinal mucosa

• Giardiasis can range from an asymptomatic infection to significant gastrointestinal disease

• Prevention of infections in endemic areas requires the use of filtered water

• Individuals recovering from infection must use good hygiene to prevent transmission

Trichomonas vaginalis

• Most common protozoan disease of humans in industrialized nations

• Parasite lives on the vulvas and in the vagina of women, and in the urethra and prostrates of men

• Transmitted almost exclusively via sex

• Occurs most often in people with a preexisting STD or in people with multiple sexual partners

• Infection of women results in vaginitis

• Infection of men is typically asymptomatic

• Prevention involves abstinence and safe sex

Apicomplexans

• Protozoa whose infective forms are characterized by an ornate complex of organelles at their apical end

• Parasites of animals

• Life cycles involve at least two types of hosts

• Schizogony is a major feature of apicomplexan life cycles

• Asexual reproduction producing multinucleate schizonts before the cells divide

• 4 important apicomplexan parasites

• Plasmodium, Toxoplasma, Cryptosporidium, Cyclospora

Plasmodium

• Causative agent of malaria

• 4 species cause malaria

• P. falciparum, P. vivax, P. ovale, and P. malariae

• Malaria is endemic throughout the tropics and subtropics

• Mosquitoes act as the vector for Plasmodium

• The Plasmodium life cycle has three prominent stages

Resistance to Malaria

• Various genetic traits increase the resistance to malaria in endemic populations

• Sickle-cell trait

• Sickle-shaped cells somehow resist penetration by Plasmodium

• Hemoglobin C

• Humans with two genes for hemoglobin C are invulnerable to 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 bind and infect erythrocytes

Malaria

• General symptoms of malaria are associated with synchronous cycles of erythrocyte lysis

• Fever correlates with erythrocyte lysis and most likely due to the immune response

• P. falciparum can cause a malaria form called blackwater fever

• Characterized by extreme fever, large-scale erythrocyte lysis, renal failure,and dark urine

• Cerebral malaria results when tissue death occurs in the brain

• Immunity gradually develops if the victim survives the acute stage of malaria

• Periodic episodes become less severe over time

• Prevention requires limiting contact with the mosquitoes carrying Plasmodium

Toxoplasma gondii

• Causative agent of toxoplasmosis

• One of the world’s most widely distributed parasites of humans

• Wild and domestic mammals and birds are the major reservoir

• Cats are the definitive host

• Infection results from consumption of undercooked meat containing the parasite

• Ingestion or inhalation of contaminated soil can also occur

• The protozoa can cross the placenta and infect the fetus

Toxoplasmosis

• Most infections are asymptomatic

• Toxoplasmosis develops in a small number of people

• Fever-producing illness with various other symptoms

• Usually is a self-limited infection

• Toxoplasmosis is more severe in two populations

• AIDS patients

• Fetuses

• Prevention is aided by thoroughly cooking meats and avoid contact with contaminated soil

Cryptosporidium parvum

• Causative agent of the zoonotic disease, cryptosporidiosis

• 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, especially in day care facilities

• Cryptosporidiosis results in severe diarrhea that can last up to 2 weeks

• Chronic cryptosporidiosis is an indicator disease for the clinical stages of AIDS

• Infection of AIDS patients can be life threatening

• Oral rehydration is used to treat the disease as drugs are ineffective against the parasite

Cyclospora cayetanensis

• Water-borne apicomplexan

• Linked in particular to raspberries imported from Central and South America

• Infection occurs from the ingestion of oocysts on contaminated berries, fruits, vegetable, or drinking water

• Environmental reservoir is unknown

• Symptoms include cramps, watery diarrhea, myalgia, and fever

• AIDS patients can experience more severe disease

• Thoroughly washing fruits and vegetables prior to eating can prevent infection

Key Features of Protozoan Parasites of Humans – see table 23.1 in textbook- know everything about at least one organism from each group!!!

Helminthic Parasites of Humans

• Helminths are macroscopic, multicellular, eukaryotic worms

• Reproductive systems and life cycles are complex

• Intermediate hosts are often needed to support larval stages

• Adult worms are either dioecious or monoecious

• Dioecious worms have male and female sex organs in separate worms

• Monoecious worms have both sex organs

• 3 groups of helminths

• Cestodes-tapeworms

• Trematodes-flukes

• Nematodes-roundworms

Cestodes

• All tapeworms lack digestive systems

• All possess the same general body plan

Taenia

• Taenia saginata is the beef tapeworm

• Taenia solium is the pork tapeworm

• Cattle and swine serve as the intermediate hosts

• Humans living in close proximity to livestock have the highest incidence of infection

• Cattle and swine become infected by eating contaminated vegetation

• Humans ingest cysticerci in raw or undercooked meat

• Adults attach to the intestinal epithelium

• Most individuals shed proglottids without experiencing symptoms

• Blockage of the intestine can occur if the tapeworm is large

• Thoroughly cooking or freezing meat is the easiest method of prevention

Echinococcus granulosus

• Tapeworm of canines

• Canines are infected by eating cysticerci in various herbivore hosts

• Humans are accidental intermediate hosts

• Infections occurs by consuming food or water contaminated from dog feces

Echinococcus granulosus

• Causes hydatid disease

• Larvae invade the circulatory system and travel throughout the body

• Form hydatid cysts

• Symptoms of hydatid disease follow enlargement of cysts in infected tissue and result from tissue dysfunction

• Large numbers of hydatid cysts can be fatal

• Infections are prevented with good hygiene practices and avoiding fecal-oral transmission from infected pets

Trematodes

• Flukes are flat, leaf-shaped worms

• Lack complete digestive systems

• Oral and ventral suckers enable attachment to host tissues to obtain nutrients

• Geographical distribution is limited because the intermediate host is limited

• Grouped according to the site in the body they parasitize

Blood Flukes: Schistosoma

• Dioecious blood flukes

• Causative agent of schistosomiasis

• Humans are the principal definitive host

• 3 geographically limited species infect humans

• S. mansoni-found in the Carribean, Venezuela, Brazil, Arabia, and Africa

• S. haemotobium-found only in Africa and India

• S. japonicum-found in China, Taiwan, the Phillipines, and rarely in Japan

Blood Flukes: Schistosoma

• Cercariae burrow through the skin of humans who contact contaminated water

• Larvae mature and mate in the circulatory system

• Eggs move to the lumen of the intestines or of the urinary bladder and ureters

• Dermatitis may occur at the site where the cercariae entered

• Infections can become chronic and can be fatal

• Prevention depends on improved sanitation and avoiding contact with contaminated water

Representative Liver Flukes: Fasciola

• Fasciola hepatica and F. gigantica

• Infect sheep and cattle worldwide

• Humans can act as accidental definitive hosts

• Infection occurs when metacercariae are ingested from watercress or other vegetation

• Migration of the parasite from the intestine to the liver is accompanied by acute disease

• Chronic infections occur when flukes reside in the bile ducts

• Symptoms coincide with episodes of bile duct obstruction and inflammation

Nematodes

• Roundworms are long, cylindrical worms that taper at each end

• Possess complete digestive tracts and have a protective cuticle

• All nematodes are dioecious

• Females are usually larger than males

Feature of the Life Cycle of Roundworms

• Parasites of almost all vertebrate animals

• Have a number of reproduction strategies

• Most intestinal nematodes shed their eggs into the lumen of the intestine

• Eggs are eliminated in feces

• Eggs are consumed in contaminated food or water

• Some intestinal nematodes release their eggs into the soil

• Larvae actively penetrate the skin of a host

• Inside the body, they travel to the intestine

• Other nematodes encyst in muscle tissue and are consumed in raw or undercooked meat

• Mosquitoes transmit a few species of nematodes

• Adult sexually mature stages are found only in definitive hosts

Two major types of roundworms

• Intestinal roundworms

• Tissue roundworms

Ascaris lumbricoides

• Causative agent of ascariasis

• Most common nematode infection worldwide

• About 1 Billion people infected

• Eggs are shed in feces; very resistant shells

• Swallowed eggs hatch; larvae invade the intestinal wall

• Migrate through bloodstream to lungs; are coughed up and swallowed; adults live in the intestine

• Preventing fecal-oral spread from infected individuals can help limit the disease

Ascaris lumbricoides-egg picture-recognize it!!

Ascaris lumbricoides

Hookworm

• Ancylostoma and Necator

• Second most common nematode infection

• 800 million infected people wordwide

• Eggs are produced by females living in the intestine, passed in the stool

• Larvae hatch and live in soil.

• When in contact with skin, they burrow into the skin and migrate to heart and lungs.

• Preventing fecal-oral spread from infected individuals can help limit the disease

Hookworm life cycle-know what’s special about it!!

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 can often be asymptomatic

• Intense perianal itching is the main symptom when they do occur

• Preventing fecal-oral spread from infected individuals can help limit the disease

Wuchereria bancrofti

• Causative agent of filariasis

• Filarial nematodes

• Infects the lymphatic system or subcutaneous tissue

• Transmitted by various genera of female mosquitoes

• Mosquitoes ingest the immature forms, or microfilariae, when taking blood meals from infected human hosts

• Mosquitoes then transmit the parasite back to humans at their next meal

Wuchereria bancrofti

• Lymphatic filariasis is asymptomatic for years

• Acute symptoms, when they develop, are due to lymphatic dysfunction

• Elephantiasis is the end result

• Cutaneous and subcutaneous tissue enlarge and harden in areas where lymph has accumulated

• Usually occurs in the lower extremities

• Prevention relies on avoiding infected mosquitoes

Key Features of Helminthic Parasites of Humans

Arthropod Vectors

• Vectors are animals that carry microbial pathogens

• Arthropods are a common vector

• Some arthropods also serve as hosts for the pathogens they transmit

• Disease vectors belong to two classes of arthropods

• Arachnida

• Insecta

Arachnids

• Adult arachnids have four pairs of legs

• Ticks and mites resemble each other morphologically

• Ticks are the most important arachnid vectors

• Serve as vectors for bacterial, viral, and protozoan diseases

• Second only to mosquitoes in the number of diseases they transmit

• Hard ticks are the most prominent disease vector

• Transmit Lyme disease, Rocky Mountain spotted fever, tularemia, relapsing fever, and tick-borne encephalitis