Sporozoa Are a Unique Class of Intracellular Protozoa Distinguished by Their Alternating

CHAPTER 50 Sporozoa

Sporozoa are a unique class of intracellular protozoa distinguished by their alternating cycles of sexual and asexual reproduction. Two sporozoan infections, malaria and toxoplasmosis, are common diseases of humans; together, they affect more than one third of the world's population and kill or deform perhaps a million neonates and children each year. A third infection, cryptosporidiosis, has only recently been found to be an important cause of diarrhea, particularly in immunocompromised hosts. Plasmodium

I. Parasitology

1. The plasmodia are sporozoa in which the sexual and asexual cycles of reproduction are completed in different host species 2. Sexual phase occurs within the gut of mosquitoes which transmit the parasite while feeding on a vertebrate host 3. Within the RBCs of the vertebrate, the plasmodia reproduce 4. Four species, Plasmodium vivax, P. ovale, P. malariae, and P. falciparum, infect humans

A. MORPHOLOGY

1. Morphology of the parasite and the infected RBC vary by stage and species 2. The appearance of each of the four species of plasmodia that infect humans is sufficiently different to allow their differentiation in stained smears 3. Schizonts and merozoites are not present in the peripheral blood 4. Gametocytes are large and banana shaped

B. LIFE CYCLE OF MALARIAL PARASITES

1. Mosquito ingests gametocytes from blood of infected human 2. Sporozoites from oocyst reach mosquito salivary glands 3. Humans infected by mosquito bite 4. Rapid infection of hepatocytes starts asexual cycle in humans 5. Erythrocytic cycle begins with merozoite attachment to RBC receptor 6. Trophozoites multiply in RBC to form new merozoites 7. In 48–72 hours, RBCs rupture, releasing merozoites to infect new RBCs 8. Intrahepatic dormancy causes relapses with P. vivax and P. ovale

C. PHYSIOLOGY

1. Species of plasmodia vary in ability to attack subpopulations of erythrocytes 2. RBC Duffy antigen and glycoprotein A are RBC receptors 3. Sickle cell trait limits intensity of P. falciparum infection 4. Other hemoglobinopathies can also exert protection 5. Changes induced in erythrocyte membrane include alteration of its lipid concentration, modification of its osmotic properties 6. Binding to endothelium may cause microinfarcts 7. Malarial parasites metabolize anaerobically, synthesize their own folate

D. GROWTH IN THE LABORATORY

1. Successful in vitro completion of the entire sporogonic cycle, from ookinete to sporozoite, has been achieved 2. Impact has been on the introduction of methods for testing the sensitivity of P. falciparum to chemotherapeutic agents II. MALARIA

A. EPIDEMIOLOGY

1. Distribution in tropical areas worldwide 2. The intensity of malarial transmission depends on the density and feeding habits of suitable mosquito vectors 3. Clinical manifestations muted with hyperendemicity 4. Malaria kills 1–3 million annually; mostly children 5. Imported malaria may develop months after travel

B. PATHOGENESIS

a. Fever

1. Irregular and hectic fever associated with RBC rupture 2. Synchronization of sporulation causes cyclic fever

b. Anemia

1. Destruction of normal and parasitized RBCs causes anemia 2. In falciparum malaria massive intravascular hemolysis known as blackwater fever can occur

c. Circulatory Changes

1. Vasodilatation leads to a decrease in the effective circulating blood volume and hypotension 2. Intense parasitemias P. falciparum is capable of producing and the adhesion of infected RBCs to the endothelium of visceral capillaries 3. Blood flow decreased to vital organs

d. Cytokines

1. Elevated levels of IL-1 and TNF are consistently found in patients with malaria 2. Elevated cytokine levels might precipitate cerebral malaria by increasing the sequestration of P. falciparum–parasitized erythrocytes in the cerebral vascular endothelium

e. Other Pathogenic Phenomena

1. Thrombocytopenia is common in malaria and appears to be related to both splenic pooling and a shortened platelet lifespan 2. Glomerulonephritis in acute falciparum malaria and progressive renal disease in chronic P. malariae malaria probably result from the host immune response

C. IMMUNITY

1. Initial immune response limits parasite multiplication but does not eliminate infection (premunition) 2. Antibody-mediated immunity important 3. Antigenic variation could play a role in persistence 4. Suppressor T lymphocytes in spleen may protect parasites III. MALARIA: CLINICAL ASPECTS

A. MANIFESTATIONS

1. Incubation period prolonged by suppressant use 2. Malarial paroxysm: cold, hot, wet stages 3. Typical paroxysms after 2–3 weeks when sporulation is synchronized 4. Cerebral falciparum malaria often lethal

B. DIAGNOSIS

1. Thick and thin blood smears detect parasites 2. Acridine orange stains of centrifuged parasites in quantitative buffy coat is rapid 3. Simple card antigen detection procedures detect a protein (HRP2) excreted by P. falciparum within minutes.

C. TREATMENT

1. Treatment of malaria requires the destruction of three parasitic forms: the erythrocytic schizont, the hepatic schizont, and the erythrocytic gametocyte 2. Unfortunately, no single drug accomplishes all three goals

a. Termination of Acute Attack

1. Chloroquine inhibits hemoglobin degradation by parasite 2. Artemisinins prevent gametocyte development 3. Resistance of chloroquine and other drugs now common with P. falciparum 4. Combinations of one of the artemisinins in combination with quinine/quinidine, antifolate–sulfonamide compounds, mefloquine, or halofantrine may be necessary

b. Radical Cure

1. In P. vivax and P. ovale infections, hepatic schizonts persist and must be destroyed to prevent reseeding of circulating erythrocytes with consequent relapse 2. Primaquine, an 8-aminoquinaline, is used for this purpose

D. PREVENTION a. Personal Protection

1. Mosquito protection with screens and repellents 2. Chemoprophylaxis choice must consider resistance in area b. General

1. Reduce human reservoir and eradicate mosquitoes 2. Attempts at eradication have failed c. Vaccines

1. Recombinant DNA procedures enabled scientists to clone and sequence the genes encoding such antigens suitable for vaccine 2. Subunit vaccines using a protein fragment from the outer surface of P. falciparum fused with a hepatitis B protein have shown promise

TOXOPLASMA GONDII I. PARASITOLOGY

1. Asexual and sexual cycles in felines 2. Three forms of human disease A. MORPHOLOGY

a. Oocyst

1. The oocyst is ovoid, measures 10 to 12 μm in diameter, and possesses a thick wall 2. With maturation two sporocysts appear, and later four sporozoites may be discerned within each sporocyst

b. Tachyzoite (Trophozoite)

1. The term "trophozoite" refers to the asexual proliferative forms responsible for cell invasion and clinical disease 2. Crescent or arc shaped, measures 3 by 7 μm, and can invade all nucleated cell types

C. Tissue Cysts

1. Cysts measure 10 to 200 μm in diameter 2. Contained organisms, referred to as bradyzoites, are similar to tachyzoites, but are smaller and divide more slowly

B. LIFE CYCLE

a. Definitive Host

1. Sexual reproduction of T. gondii occurs only in the intestinal tract of felines, most importantly in the domestic cat 2. Infection in cat ileal cells 3. Fusion of gametes leads to oocyst formation; shed in feces 4. Sporulate in external environment

b. Intermediate Hosts

1. Mature oocysts infect human hosts orally 2. Released sporozoites invade macrophages 3. Cysts develop and can persist for life of host II. TOXOPLASMOSIS

A. EPIDEMIOLOGY

a. Prevalence and Distribution

1. Worldwide distribution among mammals and birds 2. Prevalence of positive serologic evidence for the disease increases with age

b. Transmission

i. Ingestion of Oocysts

1. 1% of domestic cats excrete oocysts at any given time 2. Increased hazard to children by close contact with contaminated areas like a cat's litter box

ii. Ingestion of Tissue Cysts

1. Cysts present in meat 2. Handling and/or ingestion of raw or undercooked meat carries risk of disease

iii. Congenital

1. Approximately 1 of every 500 pregnant women acquires acute toxoplasmosis 2. Transplacental transmission highest in third trimester

iv. Miscellaneous

1. Transmitted by transfusions and organ transplants 2. Possible for infection to occur after contact with body fluids containing trophozoites

B. PATHOGENESIS AND IMMUNITY

1. Proliferation of trophozoites results in the death of involved host cells, stimulation of a mononuclear inflammatory reaction, and a parasite-specific secretory IgA response 2. Immunity is primarily cell-mediated 3. The cysts, which are found most frequently in the brain, retina, heart and skeletal muscle, normally produce little or no tissue reaction 4. Suppression of cell-mediated immunity may lead to the rupture of a cyst, release of trophozoites, and acute exacerbation of the disease 5. Dissemination in immunosuppressed subjects III. TOXOPLASMOSIS: CLINICAL ASPECTS

A. MANIFESTATIONS

a. Congenital Toxoplasmosis 1. Abortion and stillbirth are serious consequences 2. Infection in utero can produce malformations, chorioretinitis, and stillbirth

b. Normal Host

1. Fever and lymphadenopathy can mimic infectious mononucleosis 2. Visceral involvement may be manifested as meningoencephalitis, pneumonitis, myocarditis, or hepatitis

c. Immunocompromised Host

1. Primary infection or reactivation of latent infections can produce severe, widespread disease 2. AIDS patients develop encephalitis

B. DIAGNOSIS

1. Demonstration of parasite in histopathologic specimens Wright or Giemsa stain 2. Serodiagnosis detecting of IgG antibodies by indirect hemagglutination , indirect fluorescent antibody, or EIA tests is the primary approach 3. Rising titers of IgG or detection of IgM suggest acute infection or reactivation

C. TREATMENT AND PREVENTION

1. Usually, patients do not require therapy unless symptoms are particularly severe and persistent or unless vital organs, such as the eye, are involved 2. Spiramycin used to prevent congenital infection 3. Atovaquone is active against tachyzoites and cysts 4. Pregnant women and immunologically compromised hosts should avoid cat feces, particularly the changing of litter boxes.

CRYPTOSPORIDIA I. PARASITOLOGY

A. MORPHOLOGY

1. All strains regarded as a single species, the one that infects humans and cattle is often referred to as C. Parvum 2. Small spherical particles associated with microvilli 3. Oocysts are acid-fast

B. LIFE CYCLE

1. Mature, infective oocysts excreted in stools 2. Sporozoites are released from the oocyst and attach to the microvilli of the small bowel epithelial cells, where they are transformed into trophozoites 3. Divide asexually by multiple fission to form schizonts 4. Eventually, schizonts containing four merozoites are seen 5. Incapable of continued asexual reproduction, these develop into male (microgamete) and female (macrogamete) sexual forms 6. Following fertilization, the resulting zygote develops into an oocyst that is shed into the lumen of the bowel 7. Protective cell wall ensures survival of oocysts 8. Some thin-walled oocysts can autoinfect II. CRYPTOSPORIDIOSIS

A. EPIDEMIOLOGY

1. Animal reservoirs and person-to-person transmission both important 2. Infection rates highest in young children 3. Can be transmitted via contaminated water

B. PATHOGENESIS

1. The pathophysiology of the diarrhea is unknown, but its nature and intensity suggest that a cholera-like enterotoxin may be involved 2. Minimal intestinal pathology 3. Prolonged disease in AIDS patients III. CRYPTOSPORIDIOSIS: CLINICAL ASPECTS

A. MANIFESTATIONS

1. Explosive, profuse, watery diarrhea persists for 5 to 11 days and then rapidly abates 2. Self-limiting diarrhea in normal hosts 3. In patients with a broad range of immunodeficiencies the diarrhea is more severe 4. Unless the immunologic defect is reversed, the disease usually persists for the duration of the patient's life

B. DIAGNOSIS

1. The diagnosis of cryptosporidiosis is established by the recovery and identification of Cryptosporidium oocysts in a recently passed or preserved diarrheal stool 2. Detection of oocysts is by acid-fast or immunofluorescent stains

C. TREATMENT AND PREVENTION 1. No uniformly effective anticryptosporidial agent available at this time 2. When appropriate, withdrawal of cancer chemotherapy agents or immunosuppressive drugs may result in a cure 3. Strict stool precautions should be used for symptomatic patients