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Cryptosporidium (part i of 2) page 5 of 13 <br /> �' i/ — � 1 <br /> considering virulence differences for different strains). While most indications suggest <br /> that the dose required to induce infection is between 1 to 100 oocysts, one study <br /> indicated that doses of 10,000 oocysts were not capable of inducing infections in adult <br /> monkeys. Recently, Dupont and his colleagues completed a human feeding study which <br /> determined that the dose at which 20 percent of the subjects were infected was 30 <br /> oocysts while a 40 percent infection rate of the subjects was 100 oocysts (Figure 2). <br /> From this information, Haas and Rose propose that the minimum infectious dose is 1 <br /> oocyst. <br /> � � <br /> �� <br /> � s <br /> � � i <br /> � � <br /> � � 4 � <br /> 0.2 2 <br /> � • .0 <br /> � 160 100Q i0r 16� 1� <br /> da� <br /> ao0no�r. unee ahow �!6 ootdld�no� lnumb to flfled does�rs�Pa�s Wrw. <br /> 6ers rMiw rr�rn6�r of aubJ�ds wed Aer daae. <br /> R�f�reror: 77 <br /> Flgur� � <br /> Doss Respanse D�ta oF DupoM <br /> Ft t� E�oner�tial �lodsl <br /> The immunocompromised apparently are the most susceptible population. There is <br /> some indication that deficiencies in the immune system during pregnancy make <br /> pregnant women more susceptible to a prolonged bout of cryptosporidiosis. It is <br /> reported that when CD4 counts (CD4 counts indicate the level of T-helper celis in the <br /> immune system) are less than 180, the host will usually be unable to clear the <br /> disease. The CD4 counts for normal individuals typically are between 800 and 900. <br /> Significant virulence differences exist between strains of Cryptosporidium. For example, <br /> comparing two different isolates showed a similar rate of infection but twice the <br /> incidence of diarrheal illness. <br /> Relative Public Health Significance <br /> Assessing the relative public health significance of Cryptosporidium is complex because <br /> of the different responses between immunocompetent and immuno-compromised <br /> individuals. Sexually transmitted diseases typically account for the highest incidence of <br /> infectious diseases followed by gastrointestinal illnesses. <br /> Major Facts <br /> ■ Several points frame the question of public health significance: <br /> http://sfwater.org/printContent.cfm/C ID/446/ 10/19/2004 <br />