Cryptosporidium spp are well recognised as causes of diarrhoeal disease during waterborne epidemics and in immunocompromised hosts. Studies have also drawn attention to an underestimated global burden and suggest major gaps in optimum diagnosis, treatment, and immunisation. Cryptosporidiosis is increasingly identified as an important cause of morbidity and mortality worldwide. Studies in low-resource settings and high-income countries have confirmed the importance of cryptosporidium as a cause of diarrhoea and childhood malnutrition. Diagnostic tests for cryptosporidium infection are suboptimum, necessitating specialised tests that are often insensitive. Antigen-detection and PCR improve sensitivity, and multiplexed antigen detection and molecular assays are underused. Therapy has some effect in healthy hosts and no proven efficacy in patients with AIDS. Use of cryptosporidium genomes has helped to identify promising therapeutic targets, and drugs are in development, but methods to assess the efficacy in vitro and in animals are not well standardised. Partial immunity after exposure suggests the potential for successful vaccines, and several are in development; however, surrogates of protection are not well defined. Improved methods for propagation and genetic manipulation of the organism would be significant advances.

Purified Cryptosporidium parvum oocysts were exposed to ozone, chlorine dioxide, chlorine, and monochloramine. Excystation and mouse infectivity were comparatively evaluated to assess oocyst viability. Ozone and chlorine dioxide more effectively inactivated oocysts than chlorine and monochloramine did. Greater than 90% inactivation as measured by infectivity was achieved by treating oocysts with 1 ppm of ozone (1 mg/liter) for 5 min. Exposure to 1.3 ppm of chlorine dioxide yielded 90% inactivation after 1 h, while 80 ppm of chlorine and 80 ppm of monochloramine required approximately 90 min for 90% inactivation. The data indicate that C. parvum oocysts are 30 times more resistant to ozone and 14 times more resistant to chlorine dioxide than Giardia cysts exposed to these disinfectants under the same conditions. With the possible exception of ozone, the use of disinfectants alone should not be expected to inactivate C. parvum oocysts in drinking water.

A new species of Cryptosporidium is described from the feces of domestic cattle, Bos taurus. Oocysts are structurally similar to those of Cryptosporidium muris described from mice but are larger than those of Cryptosporidium parvum. Oocysts of the new species are ellipsoidal, lack sporocysts, and measure 7.4 x 5.5 microm (range, 6.0-8.1 by 5.0-6.5 microm). The length to width ratio is 1.35 (range, 1.07-1.50). The colorless oocyst wall is < 1 microm thick, lacks a micropyle, and possesses a longitudinal suture at one pole. A polar granule is absent, whereas an oocyst residuum is present. Oocysts were passed fully sporulated and are not infectious to outbred, inbred immunocompetent or immunodeficient mice, chickens or goats. Recent molecular analyses of the rDNA 18S and ITS1 regions and heat-shock protein 70 (HSP-70) genes demonstrate this species to be distinct from C. muris infecting rodents. Based on transmission studies and molecular data, we consider the large form of Cryptosporidium infecting the abomasum of cattle to be a new species and have proposed the name Cryptosporidium andersoni n. sp. for this parasite.

In vitro cell cultures were compared to neonatal mice for measuring the infectivity of five genotype 2 isolates of Cryptosporidium parvum. Oocyst doses were enumerated by flow cytometry and delivered to animals and cell monolayers by using standardized procedures. Each dose of oocysts was inoculated into up to nine replicates of 9 to 12 mice or 6 to 10 cell culture wells. Infections were detected by hematoxylin and eosin staining in CD-1 mice, by reverse transcriptase PCR in HCT-8 and Caco-2 cells, and by immunofluorescence microscopy in Madin-Darby canine kidney (MDCK) cells. Infectivity was expressed as a logistic transformation of the proportion of animals or cell culture wells that developed infection at each dose. In most instances, the slopes of the dose-response curves were not significantly different when we compared the infectivity models for each isolate. The 50% infective doses for the different isolates varied depending on the method of calculation but were in the range from 16 to 347 oocysts for CD-1 mice and in the ranges from 27 to 106, 31 to 629, and 13 to 18 oocysts for HCT-8, Caco-2, and MDCK cells, respectively. The average standard deviations for the percentages of infectivity for all replicates of all isolates were 13.9, 11.5, 13.2, and 10.7% for CD-1 mice, HCT-8 cells, Caco-2 cells, and MDCK cells, respectively, demonstrating that the levels of variability were similar in all assays. There was a good correlation between the average infectivity for HCT-8 cells and the results for CD-1 mice across all isolates for untreated oocysts (r ‫؍‬ 0.85, n ‫؍‬ 25) and for oocysts exposed to ozone and UV light (r ‫؍‬ 0.89, n ‫؍‬ 29). This study demonstrated that in vitro cell culture was equivalent to the "gold standard," mouse infectivity, for measuring the infectivity of C. parvum and should therefore be considered a practical and accurate alternative for assessing oocyst infectivity and inactivation. However, the high levels of variability displayed by all assays indicated that infectivity and disinfection experiments should be limited to discerning relatively large differences.

Severe combined immune deficient (SCID) and athymic National Institutes of Health (NIH)-III (bg/nu/xid) mice were evaluated for susceptibility to Cryptosporidium parvum infections as neonates and as adults after challenge with inocula of 5 x 10(4) and 1 x 10(6) oocysts/mouse, respectively. SCID and NIH-III nude mice developed chronic infections that persisted over 12 weeks. Cryptosporidial organisms were observed in the small and large intestines of all mice. Colonization of the gallbladder and hepatobiliary duct epithelium occurred in a majority of the NIH-III nude mice and a smaller number of the SCID mice. Severe infections in both mouse strains were associated with clinical deterioration followed by death. Signs in these mice included dehydration, icterus, and emaciation. Deaths apparently resulted from hepatic dysfunction. Control mice experienced transient infections with no clinical signs. Immunodeficient mice should prove useful in studies of disease pathogenesis and in evaluating potential anticryptosporidial agents.

The humoral response of humans, calves, and horses to Cryptosporidium sporozoite antigens was evaluated using a western blot technique. Sera from calves, humans, and horses were obtained at various times following the detection of infection. Sera were reacted with detergent-solubilized, sodium dodecyl sulfate polyacrylamide gel-electrophoresed (SDS-PAGE) sporozoite antigens. The number of antigens recognized by immune sera from humans and animals increased with time postinfection. A 20-kDa antigen appears to be a major sporozoite surface determinant labeled via membrane protein biotinylation and recognized by mouse monoclonal antibodies using indirect immunofluorescence and western blotting. Detectable recognition of the 20-kDa band occurred in 3-wk postinfection (PI) sera from all species tested. Reactivity to the 20-kDa band diminished significantly in sera 5 mo PI or longer from infected humans with no known recurrence of cryptosporidial diarrhea. In contrast, 12-mo PI sera from an individual constantly exposed to oocysts under working conditions was as strongly reactive as the 3-wk convalescent sera. Serum reactivity to the 20-kDa antigen appears to be a good indicator of exposure to Cryptosporidium.

Cryptosporidium spp is a ubiquitous parasite that has long been recognized as a frequent cause of protozoal diarrhea in humans. While infections in immunocompetent hosts are usually self-limiting, immunocompromised individuals can develop severe, chronic, and life-threatening illness. Vaccine development or immunotherapy that prevents disease or reduces the severity of infection is a relevant option since efficacious drug treatments are lacking. In particular, children in developing countries might benefit the most from a vaccine since cryptosporidiosis in early childhood has been reported to be associated with subsequent impairment in growth, physical fitness, and intellectual capacity. In this review, immunotherapies that have been used clinically are described as well as experimental vaccines and their evaluation in vivo.

Differences in susceptibility to cryptosporidial infections were investigated between 2 strains of gamma interferon knockout (GKO) mice. Male C57BL/6J-Ifg and BALB/c-Ifg (GKO) mice, ages 8-10 wk, were inoculated with infectious oocysts at various doses. C57BL/6J-Ifg mice developed overwhelming infections and died 9-12 days after infection. Low inoculum doses (1 x 10(3)) did not increase the survival time significantly. The infection intensity in C57BL/6J-Ifg mice inoculated with 1 x 10(5) oocysts/mouse increased markedly on day 4 postinfection (PI) and continued to increase significantly over the next 6-7 days. Most of the C57BL/6J-Ifg mice exceeded 15% weight loss and died by day 10 PI. In contrast, BALB/c-Ifg mice developed moderate infections from which they recovered. The average parasite load in the BALB/c-Ifg mice was 100 times lower than in C57BL/6J-Ifg mice. Mice survived until termination of the experiment (39 days) even when 1 x 10(6) oocysts per mouse were used for inoculation. BALB/c-Ifg mice did not exhibit significant weight loss (or difference in stool consistency). These 2 mouse strains make excellent models for studying differences in recovering and nonrecovering immune mechanisms.