Quantitative flow cytometric evaluation of maximal Cryptosporidium parvum oocyst infectivity in a neonate mouse model

Survival and infectivity of Toxoplasma gondii and Cryptosporidium parvum oocysts bioaccumulated by Dreissena polymorpha

AIMS

The study was aimed to understand the depuration process of Cryptosporidium parvum and Toxoplasma gondii oocysts by zebra mussel (Dreissena polymorpha), to consider the use of the zebra mussel as a bioremediation tool.

MATERIALS AND METHODS

Two experiments were performed: (i) individual exposure of mussel to investigate oocyst transfers between bivalves and water and (ii) in vivo exposure to assess the ability of the zebra mussel to degrade oocysts.

RESULTS

(i) Our results highlighted a transfer of oocysts from the mussels to the water after 3 and 7 days of depuration; however, some oocysts were still bioaccumulated in mussel tissue. (ii) Between 7 days of exposure at 1000 or 10 000 oocysts/mussel/day and 7 days of depuration, the number of bioaccumulated oocysts did not vary but the number of infectious oocysts decreased.

CONCLUSION

Results show that D. polymorpha can release oocysts in water via (pseudo)faeces in depuration period. Oocysts remain bioaccumulated and infectious oocyst number decreases during the depuration period in zebra mussel tissues. Results suggest a degradation of bioaccumulated C. parvum and T. gondii oocysts.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlighted the potential use of D. polymorpha as a bioremediation tool to mitigate of protozoan contamination in water resources.

Mouse Models for Use in Cryptosporidium Infection Studies and Quantification of Parasite Burden Using Flow Cytometry, qPCR, and Histopathology

Cryptosporidiosis threatens life of young children in developing countries and newborn calves around the world. No vaccine or therapy can prevent or cure this diarrhea-inducing enteric disease caused by Cryptosporidium spp. protozoan parasites. There is an essential need to discover new therapeutic drugs efficient in reducing parasite burden in infected individuals. Research therefore relies on reliable small animal models of cryptosporidiosis. Here, we present excellent mouse models which can efficiently mimic pathogenesis of human and bovine cryptosporidiosis. We also describe methods to purify C. parvum oocysts from stool and intestine of infected mice to facilitate oocyst quantification. Moreover, we present protocols using flow cytometry, quantitative polymerase chain reaction, and histopathology to accurately quantify parasite burden in stool or intestine samples.

Delivery of SA35 and SA40 peptides in mice enhances humoral and cellular immune responses and confers protection against Cryptosporidium parvum infection

BACKGROUND

Cryptosporidium parvum is a major cause of diarrhea in children and ruminants at the earliest stages of life. Maternal antibodies represent the main shield of neonate mammals for most of the infections. Two recombinant antigens (SA35 and SA40), portions of two C. parvum proteins, were tested for their ability to induce immune responses in adult mice and for protection on neonate BALB/c mice born from females immunised by mucosal delivery of both peptides.

METHODS

Adult BALB/c mice were intraperitoneally immunised with SA35 and SA40, separately or mixed, and their immune response was characterised. Furthermore, BALB/c pregnant mice were immunised by mucosal delivery with an SA35/40 mix, before and during pregnancy. Soon after birth, their offspring were infected with two doses (1 × 105 and 5 × 103) of C. parvum oocysts and the parasitic burden was determined at 5 and 9 days post-infection.

RESULTS

Intraperitoneal immunisation with SA35 and SA40 induced specific IgG and IgG1 in serum, specific IgA in the intestinal mucosa, increase of CD3+/CD4+ and CD30+ cells in splenocytes, which produced IFN-γ. Neonates born from immunised mice and infected with 1 × 105 oocysts showed a significant reduction of oocysts and intestinal forms (23 and 42%, respectively). A reduction of all parasitic forms (96%; P < 0.05) was observed when neonates were infected with 5 × 103 oocysts.

CONCLUSIONS

SA35 and SA40 peptides induce specific humoral and cell-mediated immune responses to C. parvum in adult mice. Moreover, mucosal administration of the SA35/40 mix in pregnant mice reduces C. parvum burden in their litters.

Assessing viability and infectivity of foodborne and waterborne stages (cysts/oocysts) of Giardia duodenalis, Cryptosporidium spp., and Toxoplasma gondii: a review of methods

Giardia duodenalis, Cryptosporidium spp. and Toxoplasma gondii are protozoan parasites that have been highlighted as emerging foodborne pathogens by the Food and Agriculture Organization of the United Nations and the World Health Organization. According to the European Food Safety Authority, 4786 foodborne and waterborne outbreaks were reported in Europe in 2016, of which 0.4% were attributed to parasites including Cryptosporidium, Giardia and Trichinella. Until 2016, no standardized methods were available to detect Giardia, Cryptosporidium and Toxoplasma (oo)cysts in food. Therefore, no regulation exists regarding these biohazards. Nevertheless, considering their low infective dose, ingestion of foodstuffs contaminated by low quantities of these three parasites can lead to human infection. To evaluate the risk of protozoan parasites in food, efforts must be made towards exposure assessment to estimate the contamination along the food chain, from raw products to consumers. This requires determining: (i) the occurrence of infective protozoan (oo)cysts in foods, and (ii) the efficacy of control measures to eliminate this contamination. In order to conduct such assessments, methods for identification of viable (i.e. live) and infective parasites are required. This review describes the methods currently available to evaluate infectivity and viability of G. duodenalis cysts, Cryptosporidium spp. and T. gondii oocysts, and their potential for application in exposure assessment to determine the presence of the infective protozoa and/or to characterize the efficacy of control measures. Advantages and limits of each method are highlighted and an analytical strategy is proposed to assess exposure to these protozoa.

Pilot-Scale Pulsed UV Light Irradiation of Experimentally Infected Raspberries Suppresses Cryptosporidium parvum Infectivity in Immunocompetent Suckling Mice

Cryptosporidium spp., a significant cause of foodborne infection, have been shown to be resistant to most chemical food disinfectant agents and infective for weeks in irrigation waters and stored fresh vegetal produce. Pulsed UV light (PL) has the potential to inactivate Cryptosporidium spp. on surfaces of raw or minimally processed foods or both. The present study aimed to evaluate the efficacy of PL on viability and in vivo infectivity of Cryptosporidium parvum oocysts present on raspberries, a known source of transmission to humans of oocyst-forming apicomplexan pathogens. The skin of each of 20 raspberries was experimentally inoculated with five 10-μl spots of an oocyst suspension containing 6 × 10(7) oocysts per ml (Nouzilly isolate). Raspberries were irradiated by PL flashes (4 J/cm(2) of total fluence). This dose did not affect colorimetric or organoleptic characteristics of fruits. After immunomagnetic separation from raspberries, oocysts were bleached and administered orally to neonatal suckling mice. Seven days after infection, mice were euthanized, and the number of oocysts in the entire small intestine was individually assessed by immunofluorescence flow cytometry. Three of 12 and 12 of 12 inoculated mice that received 10 and 100 oocysts isolated from nonirradiated raspberries, respectively, were found infected. Four of 12 and 2 of 12 inoculated mice that received 10(3) and 10(4) oocysts from irradiated raspberries, respectively, were found infected. Oocyst counts were lower in animals inoculated with 10(3) and 10(4) oocysts from irradiated raspberries (92 ± 144 and 38 ± 82, respectively) than in animals infected with 100 oocysts from nonirradiated raspberries (35,785 ± 66,221, P = 0.008). PL irradiation achieved oocyst reductions of 2 and 3 log for an inoculum of 10(3) and 10(4) oocysts, respectively. The present pilot-scale evaluation suggests that PL is an effective mode of decontamination for raspberries and prompts further applicability studies in industrial contexts.

Individual subject meta-analysis of parameters for Cryptosporidium parvum shedding and diarrhoea in animal experimental models

Cryptosporidium is a zoonotic protozoan parasite with public health importance worldwide. The objectives of this study were to (1) conduct a meta-analysis of published literature for oocyst shedding and diarrhoea outcomes, and (2) develop recommendations for standardization of experimental dose-response studies. Results showed that for the outcome of oocyst shedding in faeces, the covariates 'experimental species', 'immunosuppression', 'oocyst dose' and 'oocyst dose' × 'age' were all significant (P≤0.05). This study suggests that exposing mice, piglets, or ruminants, and using immunosuppressed experimental hosts, is more likely to result in oocyst shedding. For the outcome of diarrhoea in experimentally infected animal species, the key covariates 'experimental species', 'age' and 'immunosuppression' were significant (P≤0.2). Therefore, based on the results of this meta-analysis, these variables should be carefully reported and considered when designing experimental dose-response studies. Additionally, detection of possible publication bias highlights the need to publish additional studies that convey statistically non-significant as well as significant results in the future.

Rapid detection and enumeration of Giardia lamblia cysts in water samples by immunomagnetic separation and flow cytometric analysis

Giardia lamblia is an important waterborne pathogen and is among the most common intestinal parasites of humans worldwide. Its fecal-oral transmission leads to the presence of cysts of this pathogen in the environment, and so far, quantitative rapid screening methods are not available for various matrices, such as surface waters, wastewater, or food. Thus, it is necessary to establish methods that enable reliable rapid detection of a single cyst in 10 to 100 liters of drinking water. Conventional detection relies on cyst concentration, isolation, and confirmation by immunofluorescence microscopy (IFM), resulting in low recoveries and high detection limits. Many different immunomagnetic separation (IMS) procedures have been developed for separation and cyst purification, so far with variable but high losses of cysts. A method was developed that requires less than 100 min and consists of filtration, resuspension, IMS, and flow cytometric (FCM) detection. MACS MicroBeads were used for IMS, and a reliable flow cytometric detection approach was established employing 3 different parameters for discrimination from background signals, i.e., green and red fluorescence (resulting from the distinct pattern emitted by the fluorescein dye) and sideward scatter for size discrimination. With spiked samples, recoveries exceeding 90% were obtained, and false-positive results were never encountered for negative samples. Additionally, the method was applicable to naturally occurring cysts in wastewater and has the potential to be automated.

Evaluation of water treatment plant UV reactor efficiency against Cryptosporidium parvum oocyst infectivity in immunocompetent suckling mice

AIM

To assess the efficiency of a medium-pressure UV reactor under full-scale water treatment plant (WTP) conditions on the infectivity of Cryptosporidium parvum oocysts in an Naval Medical Research Institute (NMRI) suckling mice infectivity model.

METHODS AND RESULTS

Six/seven-day-old mice were administered orally 2-10x10(4)Cryptosporidium parvum oocysts. Compared with nonirradiated oocysts, 40 mJ cm(-2) UV irradiation of ingested oocysts resulted 7 days later in a 3.4-4.0 log10 reduction in the counts of small intestine oocysts, using a fluorescent flow cytometry assay.

CONCLUSION

Present data extend to industrial conditions previous observations of the efficiency of UV irradiation against Cryptosporidium parvum oocyst in vivo development.

SIGNIFICANCE AND IMPACT OF THE STUDY

Present results suggest that in WTP conditions, a medium-pressure UV reactor is efficient in reducing the infectivity of Cryptosporidium parvum oocysts, one of the most resistant micro-organisms present in environmental waters.

Cytometric approach for detection of Encephalitozoon intestinalis, an emergent agent

Encephalitozoon intestinalis is responsible for intestinal disease in patients with AIDS and immunocompetent patients. The infectious form is a small spore that is resistant to water treatment procedures. Its detection is very important, but detection is very cumbersome and time-consuming. Our main objective was to develop and optimize a specific flow cytometric (FC) protocol for the detection of E. intestinalis in hospital tap water and human feces. To determine the optimal specific antibody (Microspor-FA) concentration, a known concentration of E. intestinalis spores (Waterborne, Inc.) was suspended in hospital tap water and stool specimens with different concentrations of Microspor-FA, and the tap water and stool specimens were incubated under different conditions. The sensitivity limit and specificity were also evaluated. To study spore infectivity, double staining with propidium iodide (PI) and Microspor-FA was undertaken. Distinct approaches for filtration and centrifugation of the stool specimens were used. E. intestinalis spores stained with 10 microg/ml of Microspor-FA at 25 degrees C overnight provided the best results. The detection limit was 5 x 10(4) spores/ml, and good specificity was demonstrated. Simultaneous staining with Microspor-FA and PI ensured that the E. intestinalis spores were dead and therefore noninfectious. With the stool specimens, better spore recovery was observed with a saturated solution of NaCl and centrifugation at 1,500 x g for 15 min. A new approach for the detection of E. intestinalis from tap water or human feces that ensures that the spores are not viable is now available and represents an important step for the prevention of this threat to public health.

Immunofluorescence flow cytometry technique for enumeration of the brown-tide alga, Aureococcus anophagefferens

A new immunologically based flow cytometry (IFCM) technique was developed to enumerate Aureococcus anophagefferens, a small pelagophyte alga that is the cause of "brown tides" in bays and estuaries of the mid-Atlantic states along the U.S. coast. The method utilizes a monoclonal antibody conjugated to fluorescein isothiocyanate (FITC-MAb) to label the surface of A. anophagefferens cells which are then detected and enumerated by using a flow cytometer. Optimal conditions for FITC-MAb staining, including solution composition, incubation times, and FITC-MAb concentrations, were determined. The FITC-MAb method was tested for cross-reactivity with nontarget, similarly sized, photoautotrophic protists, and the method was compared to an enzyme-linked immunosorbent assay (ELISA) using the same MAb. Comparisons of the IFCM technique to traditional microscopy enumeration of cultures and spiked environmental samples showed consistent agreement over several orders of magnitude (r(2) > 0.99). Comparisons of the IFCM and ELISA techniques for enumerating cells from a predation experiment showed a substantial overestimation (up to 10 times higher) of the ELISA in the presence of consumers of A. anophagefferens, presumably due to egested cell fragments that retained antigenicity, using the ELISA method, but were not characterized as whole algal cells by the IFCM method. Application of the IFCM method to environmental "brown-tide" samples taken from the coastal bays of Maryland demonstrated its efficacy in resolving A. anophagefferens abundance levels throughout the course of a bloom and over a large range of abundance values. IFCM counts of the brown-tide alga from natural samples were consistently lower than those obtained using the ELISA method and were equivalent to those of the polyclonal immunofluorescence microscopy technique, since both methods discriminate intact cells. Overall, the IFCM approach was an accurate and relatively simple technique for the rapid enumeration of A. anophagefferens in natural samples over a wide range of abundance values (10(3) to 10(6) cells ml(-1)).

A flow cytometric protocol for detection of Cryptosporidium spp

Cryptosporidium parvum is transmitted through water and can cause severe diarrhea. The diagnosis is usually based upon observer-dependent microscopic detection of oocysts, with rather low sensitivity and specificity. Our objective was to optimize a flow cytometric (FC) protocol for the detection of C. parvum. A specific monoclonal antibody conjugated with R-phycoerythrin was incubated with dead oocysts to determine the optimal antibody concentration. Serial concentrations of oocysts were stained with the optimized concentration and analyzed by FC. The lower detection limit was determined, and the possibility of cross-reaction was investigated using prokaryotic and eukaryotic microorganisms. A FC protocol was optimized to detect oocysts in spiked human stools. The optimal antibody concentration was found to be 3.0 mug/ml. The lowest number detectable was 2 x 10(3) oocysts/ml. Staining procedure was specific, as no cross-reactions were observed. This reliable and easy FC protocol allow the specific detection of Cryptosporidium oocysts, even at very low concentrations, which is important for public health and further studies of treatment efficacy.

Assessment of faecal contamination and the relationship between pathogens and faecal bacterial indicators in an estuarine environment (Seine, France)

The Seine estuary, one of the largest estuaries of the European northwest continental shelf, is subjected to numerous anthropogenic influences. Here we present an assessment of the microbial faecal contamination of the estuary water. The most vulnerable areas were defined on the basis of the fluxes of indicator organisms and the occurrence of Salmonella and Cryptosporidium sp. and Giardia sp. (oo)cysts. The microbial quality of the water changes from upstream to downstream: in the upstream area, contamination by faecal-indicator bacteria and Salmonella occurs during periods of high flow; in the urbanized area, mid-way between the uppermost areas of the estuary and its mouth, discharge from a wastewater treatment plant and a tributary degrade water quality; at the estuary mouth, the accumulation of microorganisms attached to particles in the maximum turbidity zone, particularly Clostridium perfringens spores and oocysts of Cryptosporidium, is accompanied by inputs of ThC and Escherichia coli from tributaries. In some areas, significant strong relations are observed between Salmonella, (oo)cysts of protozoan, and levels of faecal indicators.

Quantification of the infectivity of Cryptosporidium parvum by monitoring the oocyst discharge from SCID mice

Doses of 1-10(5) oocysts of Cryptosporidium parvum HNJ-1 were inoculated into severe combined immunodeficient (SCID) mice, and the discharge of oocysts was monitored for 30 days post inoculation. None of the mice discharged any oocysts after oral inoculation of one oocyst. Only one of five SCID mice discharged oocysts after oral inoculation of 10 oocysts, and the prepatent period was 17 days. The other four mice did not discharge any oocysts. All the SCID mice discharged oocysts after oral inoculation of 10(2)-10(5) oocysts. The prepatent periods were 13-17, 8-10, 8, and 10 days in SCID mice inoculated with 10(2), 10(3), 10(4), and 10(5) oocysts, respectively. A proportional correlation was observed between inoculation doses of oocysts ranging from 10 to 10(4) oocysts and the corresponding prepatent periods. The prepatent period can be used to evaluate the infectivity of C. parvum oocysts.

Effect of ferric oxyhydroxide grain coatings on the transport of bacteriophage PRD1 and Cryptosporidium parvum oocysts in saturated porous media

To test the effect of geochemical heterogeneity on microorganism transport in saturated porous media, we measured the removal of two microorganisms, the bacteriophage PRD1 and oocysts of the protozoan parasite Cryptosporidium parvum, in flow-through columns of quartz sand coated by different amounts of a ferric oxyhydroxide. The experiments were conducted over ranges of ferric oxyhydroxide coating fraction of lambda = 0-0.12 for PRD1 and from lambda = 0-0.32 for the oocysts at pH 5.6-5.8 and 10(-4) M ionic strength. To determine the effect of pH on the transport of the oocysts, experiments were also conducted over a pH range of 5.7-10.0 at a coating fraction of lambda = 0.04. Collision (attachment) efficiencies increased as the fraction of ferric oxyhydroxide coated quartz sand increased, from alpha = 0.0071 to 0.13 over lambda = 0-0.12 for PRD1 and from alpha = 0.059 to 0.75 over lambda = 0-0.32 for the oocysts. Increasing the pH from 5.7 to 10.0 resulted in a decrease in the oocyst collision efficiency as the pH exceeded the expected point of zero charge of the ferric oxyhydroxide coatings. The collision efficiencies correlated very well with the fraction of quartz sand coated by the ferric oxyhydroxide for PRD1 but not as well for the oocysts.

Infectivity of Cryptosporidium hominis and Cryptosporidium parvum genotype 2 isolates in immunosuppressed Mongolian gerbils

One-month-old dexamethasone-immunosuppressed Mongolian gerbils were challenged with 1 oocyst to 2 x 10(5) oocysts from two isolates genotyped as Cryptosporidium hominis and C. parvum (genotype 2), respectively. A similar dose-dependent gut infection was obtained, and the initial genotype maintained for 21 to 22 days. The data suggest that immunosuppressed gerbils provide a reliable rodent model of persistent C. hominis infection.

Detection and enumeration of Cryptosporidium sp. oocysts in sewage sludge samples from the city of Florianópolis (Brazil) by using immunomagnetic separation combined with indirect immunofluorescence assay

In the last few decades, agricultural reclamation of sewage sludge has increased in many countries. However, this practice must be adopted under recommended conditions to limit the risks associated with the presence of potential pathogens, including protozoan parasites such as Cryptosporidium. In the present work, we evaluated immunomagnetic separation (IMS) combined with an indirect immunofluorescence assay (IFA) in order to detect Cryptosporidium oocysts in sewage sludge samples obtained from the Municipal Wastewater Treatment Plant (MWTP) operating in the city of Florianópolis, State of Santa Catarina, Brazil. Integrated application of spontaneous sludge sedimentation and centrifugation of the supernatant, purification and concentration of oocysts through immunomagnetic separation as well as detection and quantification by means of indirect immunofluorescence using highly specific monoclonal antibodies, proved to be an effective multiple-analysis tool for monitoring the presence of Cryptosporidium oocysts in urban sewage sludge samples. Among 27 samples collected from MWTP, 20 (74.07%) were positive for Cryptosporidium sp. The efficiency of recovery by using the internal control ColorSeed (Biotechnology Frontiers Inc., Sydney, NSW, Australia) was medially 39.96 +/- 11.20%. Actually, oocysts were recovered from 100% of the positive samples tested and the number of Cryptosporidium oocysts found in the samples ranged from one to four. These results demonstrated a high recovery rate of Cryptosporidium oocysts in sewage sludge samples by using IMS-IFA analysis. In conclusion, such an approach can be applied to monitor the presence of the parasite and to determine the potential contamination of sludge destined for soil application.

Kinetics of spleen and Peyer's patch lymphocyte populations during gut parasite clearing in Cryptosporidium parvum infected suckling mice

Data from experimental and human cryptosporidiosis have established a major role of specific immunity in the control of Cryptosporidium parvum infection. In this work, alterations in spleen and Peyer's patch (Pp) lymphocytes were investigated in the course of a spontaneously resolutive gut cryptosporidiosis in four-day-old suckling NMRI mice infected with either 4 x 10(5) or 30 viable oocysts. Oocysts from entire small intestines, and spleen and Pp lymphocytes were examined using flow cytometry from day 7 to day 27 post-infection. Compared to uninfected animals, a 3-5 fold increase in the numbers of spleen TCR alphabeta+, CD4+, CD8+, TCR gammadelta+ and CD45R/B220+ lymphocytes was observed on day 17 post-infection in heavily infected animals. In Pp, more than ten-fold increases were observed, except for TCR gammadelta+ lymphocytes. At termination of infection, i.e. on days 21-23 after ingestion of 4 x 105 oocysts, T and B lymphocytes decreased rapidly in both organs, and remained lower than in uninfected animals on days 19-23 post-infection. In mice infected with 30 oocysts, similar alterations were observed in Pp, but not in spleen. Data suggest that in normally developing mice, clearance of gut C. parvum infection is associated with an initial increase in systemic and local lymphocyte numbers, followed by their decrease to below control levels during the recovery phase.

Indinavir reduces Cryptosporidium parvum infection in both in vitro and in vivo models

The use of highly active antiretroviral therapy in persons with acquired immunodeficiency syndrome has reduced the prevalence of infection with Cryptosporidium parvum and the length and severity of its clinical course. This effect has in most cases been attributed to the recovery of the host immunity; however, some works suggest that human immunodeficiency virus protease inhibitors, indinavir in particular, which is one of the human immunodeficiency virus protease inhibitors used in highly active antiretroviral therapy, may be capable of controlling Microsporidia and Cryptosporidium infections, which are refractory to other treatments. The objective of the present study was to investigate the effect of human immunodeficiency virus protease inhibitors on C. parvum infections. Since preliminary experiments using ritonavir, saquinavir, and indinavir showed a drastic reduction of C. parvum infection both in vivo (neonatal Balb/c mice) and in vitro (human ileocecal adenocarcinoma tumour cell line) models, indinavir alone was tested in successive experiments. In vitro, the treatment of the sporulated oocysts with different concentrations of indinavir reduced the percentage of human ileocecal adenocarcinoma tumour cell line infected cells in a dose-dependent manner. For established infection, the treatment with 50 microM of indinavir decreased the percentage of infected cells in a time-dependent manner. In vivo, mice treated with indinavir at the same time they were infected with the oocysts showed a 93% reduction in the number of oocysts present in the entire intestinal contents and a 91% reduction in the number of intracellular parasites in the ileum. For established infection, indinavir treatment reduced the number of oocysts in the entire intestinal content by about 50% and the number of intracellular parasites in the ileum by about 70%. These data show that indinavir directly interferes with the cycle of C. parvum, resulting in a marked reduction in oocyst shedding and in the number of intracellular parasites. Protease inhibitors could be considered as good candidates for the treatment of cyptosporidiosis in immunosuppressed persons.

Comparison of in vitro cell culture and a mouse assay for measuring infectivity of Cryptosporidium parvum

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.