1. What is Cryptosporidia? Describe its life cycle.

• Cryptosporidia are sporozoan protozoan parasites that infect the intestinal tract of a wide range of mammals including humans. 

Life Cycle

• The fully mature, infective oocysts are excreted in the stool of the parasitized animal. 
• Following ingestion by another animal,sporozoites are released from the oocyst and attach to the microvilli of the small bowel epithelial cells where they are transformed into trophozoites. 
• They divide asexually by multiple fission (schizogony) to form schizonts containing eight daughter cells known as type 1 merozoites. 
• Upon release from the schizont, each daughter cell attaches itself to another epithelial cell, where it repeats the schizogony cycle, producing another generation of type I merozoites.
•   Eventually, schizonts containing four type 2 merozoites are seen. Incapable of continued asexual reproduction, these develop into male (microgamete) and female (macrogamete) sexual forms. 
• Following fertilization, the resulting zygote develops into an oocyst that is shed into the lumen of the bowel. 
• The majority possess a thick protective cell wall that ensures their intact passage in the feces and survival in the external environment.
•   Approximately 20% fail to develop the thick wall. 
• The cell membrane ruptures releasing infective sporozoites into the intestinal lumen and initiating a new "autoinfective" cycle within the original host. 
• In the normal host, acquired immunity dampens both the cyclic production of type 1 merozoites and the formation of thin walled oocysts, halting further parasite multiplication and terminating the acute infection.
•   In the immunocompromised (AIDS) both processes continue resulting in chronic infection.

2. How is cryptosporidia acquired? 

• Domestic animals are the major reservoir in this country.
•  Transmission occurs by person to person, animal to person, and from the environment, particularly water. 
• The principle route of transmission is by direct fecal-oral spread. Waterborne outbreaks, including one in Milwaukee affecting 403,000 persons, have occurred.

3. How does cryptosporidia cause disease? 

• Organisms apear as spherical structures arranged in rows along the microvilli of epithelial cells.
•  They are covered by a double membrane derived from reflection, fusion and attenuation of the microvilli and are thus considered intracellular.
•  There may be villous atrophy and blunting, crypt hyperplasia and lengthening, and infiltration of the lamina propria with inflammatory cells. Mechanism of diarrhea is not known. 
• Studies in AIDS patients have shown secretory mechanisms unaffected by fasting and malabsorptive mechanisms with positive D-xylose and decreased absorption of vitamin B12 as well as steatorrhea.

4. How is it treated? 

• There is no palliative or curative treatment for cryptosporidiosis. 
• The disease is self-limited in the immunocompetent. 
• Currently paromomycin (a luminal antimicrobic) or azithromycin to treat infection or octreotide (a somatostatin analogue) to control the diarrhea are used in persons with AIDS. Bovine transfer factor and hyperimmune bovine colostrum have been experimentally shown to ameliorate the symptoms. 
• Recent publications have documented clinical resolution of cryptosporidiosis in patients who responded to highly active antiretroviral therapy.

5. What is Mycobacterium avium complex (MAC)?

• Mycobacterium avium complex is a group of related acid-fast organisms that grow only slightly faster than M. tuberculosis and can be divided into a number of serotypes. 
• Some cause disease in birds; others cause disease in mammals but not birds. 
• They are found in soil and water and in infected animals.

6. Describe the pathogenesis of infection with MAC in persons infected with HIV.

• Asymptomatic colonization after ingestion or inhalation precedes infection. 
• Symptomatic localized infection can occur either in the lung or the G1 tract. 
• The GI tract is probably the most common portal of entry. 
• Focal pneumonias are uncommon in AIDS and dissemination usually ensues. 
• Localized GI infection can occur from esophagus to rectum but the duodenum is most common.
•  Dissemination usually involves many organs-- most commonly the blood, bone marrow, liver, spleen. and lymph nodes; but the organism has been recovered from the eye, brain, meninges, CSF. skin, tongue. heart, lung, stomach, thyroid, breast, parathyroid, adrenals, kidney, pancreas, prostate, testis and urine. 
• Microscopically, tissues are filled with large numbers of distended histiocytes that on Ziehl-Neelson staining are packed with acid-fast bacilli.
•  The histologic picture in disseminated MAC disease is similar to that seen in lepromatous leprosy and reflects an inability of the host to mount an efftective cell-mediated immune response which would be manifest as a granulomatous reaction. Phagocytosis by macrophages in patients with AIDS appears to be intact but intracellular killing does not occur.

7. What are the clinical manifestations of infection with MAC in persons with AIDS? 

• Fever and weight loss are characteristic with chronic diarrhea, abdominal pain and signs of extrahepatic biliary obstruction occurring less commonly.
•  Severe anemia is common. Rarely reported are localized pneumonia, endobronchial lesions, arthritis, skin lesions and endophthalmitis.

8. How is infection with MAC diagnosed? What specimens are useful for culture?

• Diagnosis is established by recovery of the organism from a normally sterile site but blood, bone marrow, lymph node, and liver are most common.
•  Blood cultures are set up in specialized liquid medium with radiometric detection systems developed specifically for mycobacteria. 
• DNA probes are used to rapidly identify any growth of acid- fast- positive organisms.
• Tissue samples show infiltration with swollen macrophages containing large numbers of mycobacteria on acid-fast staining.

9. Is infection with MAC treatable? What medications are used in its treatment? For prevention?

• Yes, initial antimicrobial treatment should be with clarithromycin (high dose) or azithromycin plus ethambutol. 
• Additional drugs may be used but recent results of clinical trials have shown that addition of clofazimine to a two drug regimen containing one of the macrolides plus ethambutol provided no benefit.
• Besides clofazimine, amikacin , rifampin, rifabutin ciprofloxacin can all be used. AT LEAST TWO DRUGS SHOULD BE USED TO PREVENT EMERGENCE OF RESISTANT MUTANTS.
• Clinical response may require 2 - 8 weeks and therapy should be continued for life. PREVENTIVE THERAPY in patients at high risk (T-cell counts <50 -100) with rifabutin or clarithromycin or azithromycin being recommended for lifelong prophylaxis.