Inter-laboratory comparison of the CD-1 neonatal mouse logistic dose-response model for Cryptosporidium parvum oocysts

Anti-Cryptosporidium parvum activity of Artemisia judaica L. and its fractions: in vitro and in vivo assays

Background

This study investigates the toxic activity of Artemisia judaica ethanolic extract (ArEx) as well as its phenolic fraction (ArPh), and terpenoid fraction (ArT) against Cryptosporidium parvum (C. parvum) oocysts.

Methods

Over a 4 months period, estimation of the total phenolic (TPC), total flavonoids (TFC), and total terpenoids contents (TTC) in ArEx; investigation of the in vitro antioxidant activity of ArEx, ArPh, and ArT; evaluation of ArEx, ArPh, and ArT toxic activity against C. parvum oocysts using MTT assay; parasitological analysis on ArPh-treated C. parvum oocysts and comet assay were performed both in vitro and in vivo (infectivity).

Results

The ArEx TPC, TFC, and TTC was 52.6 ± 3.1 mgGAE/g, 64.5 ± 3.1 mg QE/g, and 9.5 ± 1.1 mg Linol/g, respectively. Regarding the phytochemical in vitro antioxidant activity, the ArPh exhibited the highest antioxidant activity compared to the ArEx and ArT. The ArPh showed promising free radical scavenging activity of DPPH and ABTS^•+ with IC₅₀ values of 47.27 ± 1.86 μg/mL and 66.89 ± 1.94 μg/mL, respectively. Moreover, the FRAP of ArPh was 2.97 ± 0.65 mMol Fe⁺²/g while its TAC was 46.23 ± 3.15 mg GAE/g. The ArPh demonstrated toxic activity against C. parvum oocysts with a potent IC₅₀ value of 31.6 μg/mL compared to ArT (promising) and ArEx (non-effective). ArPh parasitological analysis demonstrated MIC₉₀ at 1000 μg/ml and effective oocysts destruction on count and morphology. ArPh fragmented oocysts nuclear DNA in comet assay. Beginning at 200 μg/mL, ArPh-treated oocysts did not infect mice.

Conclusion

To combat C. parvum infection, the phenolic fraction of A. judaica L. shows promise as an adjuvant therapy or as a source of potentially useful lead structures for drug discovery.

Efficacy of ultraviolet light-emitting diodes (UV-LED) at four different peak wavelengths against Cryptosporidium parvum oocysts by inactivation assay using immunodeficient mice

As an alternative to using ultraviolet (UV) lamps, which are made with mercury that is toxic to the environment and human health, UV light-emitting diodes (UV-LEDs) are expected to be effective for inactivating microorganisms in water. Although UV-LEDs have been reported to be effective against bacteria and viruses, the effectiveness of UV-LEDs against Cryptosporidium parasites has not been fully evaluated. As we report here, we have developed an in vivo quantitative inactivation assay for C. parvum oocysts using immunodeficient mice. Using the assay, we evaluated the effectiveness of treatment by UV lamp (254 nm) at approximately 1000 μJ/cm² (for 3 s at a distance of 95 mm) compared to inactivation by commercially available UV-LEDs (with peak wavelengths of 268, 275, 284, and 289 nm). The shed patterns of oocysts after treatment with 284- and 289-nm wavelength UV-LEDs were significantly delayed compared to that after treatment with a UV lamp. These findings provide the first suggestion that UV-LEDs are effective against these parasites, as assessed using commercially available 350-mA UV-LEDs under conditions of fixed exposure distance and time.

Inhibitory activity of chitosan nanoparticles against Cryptosporidium parvum oocysts

Cryptosporidium is a ubiquitous harsh protozoan parasite that resists many disinfectants. It remains viable and infective for a long time in water and food causing global outbreaks. Chitosan (the deacetylated chitin molecule) was used in its nanosuspension form to evaluate its effect against Cryptosporidium parvum. The experiments were performed in vitro in serial concentrations and confirmed in mice in vivo infectivity assay. Chitosan nanoparticles (Cs NPs) were toxic to Cryptosporidium oocysts. The effect appeared to decrease the number of Cryptosporidium oocysts and altered their content. The destruction rate of oocysts was dependent on the dose of chitosan and the time of exposure (P < 0.05). Higher doses of Cs NPs over a prolonged period exhibited a significantly higher destruction rate. Using staining and light microscopy, remarkable destructive changes were observed in the oocysts' morphology. The minimal lethal dose for > 90% of oocysts was 3000 μg/ml, no mice infections in vivo were observed. The results in this study elucidate Cs NPs as an effective anti-cryptosporidial agent.

Cryptosporidium parvum-Infected Neonatal Mice Show Gut Microbiota Remodelling Using High-Throughput Sequencing Analysis: Preliminary Results

BACKGROUND

During the last decade, the scientific community has begun to investigate the composition and role of gut microbiota in normal health and disease. These studies have provided crucial information on the relationship between gut microflora composition and intestinal parasitic infection, and have demonstrated that many enteric pathogen infections are associated with altered gut microflora composition. In this study, we investigated the effects of Cryptosporidium parvum infection (zoonotic protozoan affecting a large range of vertebrates) on both qualitative and quantitative composition of gut microbiota in a CD-1 neonatal mouse model.

METHODS

5-day-old neonate mice were experimentally infected with 10⁵Cryptosporidium parvum Iowa oocysts by oesophageal gavage. The intestinal microbiota of both infected (Cp+) and uninfected (Cp-) mice groups was examined by high-throughput sequencing of the bacterial 16S rDNA gene V3-V4 hypervariable region.

RESULTS

The most consistent change in the microbiota composition of Cp+ mice was the increased proportion of bacterial communities belonging to the Phylum Bacteroidetes. In contrast, the microbiota of Cp- mice was associated with increased proportions of several Firmicutes and Actinobacteria phyla members.

CONCLUSION

For the first time, our study provides evidence of an association between cryptosporidial infection and gut dysbiosis, thus contributing valuable knowledge to the as-yet little-explored field of Cryptosporidium-microbiota interactions in a neonatal mouse model.

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.

Cryptosporidiosis Drug Discovery: Opportunities and Challenges

The apicomplexan parasite Cryptosporidium is the second most important diarrheal pathogen causing life-threatening diarrhea in children, which is also associated with long-term growth faltering and cognitive deficiency. Cryptosporidiosis is a parasitic disease of public health concern caused by Cryptosporidium parvum and Cryptosporidium hominis. Currently, nitazoxanide is the only approved treatment for cryptosporidium infections. Unfortunately, it has limited efficacy in the most vulnerable patients, thus there is an urgent need for a safe and efficacious cryptosporidiosis drug. In this work, we present our current perspectives on the target product profile for novel cryptosporidiosis therapies and the perceived challenges and possible mitigation plans at different stages in the cryptosporidiosis drug discovery process.

Efficacy of amine-based disinfectant KENO™COX on the infectivity of Cryptosporidium parvum oocysts

Cryptosporidium parvum is a zoonotic protozoan parasite that may cause severe neonatal diarrhoea or even mortality in newborn ruminants: its oocysts are extremely resistant to normal environmental conditions and to most common disinfectants. KENO™COX, a patent pending amine-based formula, was tested for its ability to inactivate C. parvum oocysts. The Daugschies assay (2002), a standardized assay for chemical disinfection initially described for Eimeria spp., was adapted for C. parvum oocysts. KENO™COX diluted in water at 2% and 3% concentration and incubated with oocyst suspensions for 2h, allowed a significant reduction in viability, lysing 89% and 91% of oocysts respectively. Infectivity of the remaining C. parvum oocysts was assessed by inoculation to C57 Bl/6 neonatal mice. Each mouse received 2.5 μl of a suspension initially containing 500,000 oocysts before contact with KENO™COX. Six days post inoculation, the intestinal parasite load was significantly reduced by 97.5% with KENO™COX 2% compared to that of the mice inoculated with untreated parasites. KENO™COX 3% completely eliminated infectivity of oocysts. The number of oocysts remaining infectious in the inoculum treated with KENO™COX 2% was calculated from an inoculated dose-response curve: it was estimated at about 48.6 oocysts among the 500,000 oocysts initially treated corresponding to 99.99% of inhibition. These results demonstrate the high efficacy of KENO™COX against C. parvum oocysts. Combined with an appropriate method of cleaning, the application of KENO™COX may be a useful tool to reduce cryptosporidial infectious load on farm level.

Detection of UV-induced thymine dimers in individual Cryptosporidium parvum and Cryptosporidium hominis oocysts by immunofluorescence microscopy

To investigate the effect of UV light on Cryptosporidium parvum and Cryptosporidium hominis oocysts in vitro, we exposed intact oocysts to 4-, 10-, 20-, and 40-mJ x cm-2 doses of UV irradiation. Thymine dimers were detected by immunofluorescence microscopy using a monoclonal antibody against cyclobutyl thymine dimers (anti-TDmAb). Dimer-specific fluorescence within sporozoite nuclei was confirmed by colocalization with the nuclear fluorogen 4',6'-diamidino-2-phenylindole (DAPI). Oocyst walls were visualized using either commercial fluorescein isothiocyanate-labeled anti-Cryptosporidium oocyst antibodies (FITC-CmAb) or Texas Red-labeled anti-Cryptosporidium oocyst antibodies (TR-CmAb). The use of FITC-CmAb interfered with TD detection at doses below 40 mJ x cm-2. With the combination of anti-TDmAb, TR-CmAb, and DAPI, dimer-specific fluorescence was detected in sporozoite nuclei within oocysts exposed to 10 to 40 mJ x cm-2 of UV light. Similar results were obtained with C. hominis. C. parvum oocysts exposed to 10 to 40 mJ x cm-2 of UV light failed to infect neonatal mice, confirming that results of our anti-TD immunofluorescence assay paralleled the outcomes of our neonatal mouse infectivity assay. These results suggest that our immunofluorescence assay is suitable for detecting DNA damage in C. parvum and C. hominis oocysts induced following exposure to UV light.

Cryptosporidium species found in cattle: a proposal for a new species

Humans and animals are infected worldwide by apicomplexan parasites of the genus Cryptosporidium. Yet, parasitologists are continuously surprised by the expanding complexity of this genus. Over the past 20 years, cattle were identified as being a reservoir host for taxa transmitted from animals to humans. However, a remarkable assemblage of species affects cattle, including both cattle-specific, in addition to a zoonotic, species. To clarify species classification, Cryptosporidium pestis n. sp. is proposed for the species formerly recognized as the "bovine genotype" of C. parvum. The observed increasing complexity of Cryptosporidium species, along with recent advances in knowledge, should be reconsidered in the context of past records, and not vice versa. In this way, the gaps in our understanding of Cryptosporidium species can be identified and addressed in a scientific manner.

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.

A Bayesian analysis of mouse infectivity data to evaluate the effectiveness of using ultraviolet light as a drinking water disinfectant

Modelling disinfectant performance using Bayesian hierarchical methods can overcome problems with traditional methods and lead to improved estimates. Animal and cell-culture assays are used to estimate the degree of inactivation of a microorganism produced by a given disinfectant dose. Assay data traditionally are analyzed with logistic model or most probable number (MPN) method. These methods are limited particularly when assays show all (or no) animals or cells to be infected-estimates are reported as greater than (or less than) a measurement limit (i.e., censored data). The proposed Bayesian approach (1) properly models the propagation of uncertainty through the data analysis/modelling process, resulting in reduced model uncertainty, and (2) uses appropriate probability distribution models for the response variables, avoiding the censored data problem and more accurately describing statistical error when estimating dose-response behavior. This paper applies the Bayesian hierarchical models to logistic and MPN data from published papers for the ultraviolet (UV) inactivation of Cryptosporidium. Results are compared to those from three alternative models. The Bayesian model estimates a significantly lower UV dose for a given level of Cryptosporidium inactivation than the alternative models, due mainly to the reduced model uncertainty.

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.

UV inactivation of Cryptosporidium hominis as measured in cell culture

The Cryptosporidium spp. UV disinfection studies conducted to date have used Cryptosporidium parvum oocysts. However, Cryptosporidium hominis predominates in human cryptosporidiosis infections, so there is a critical need to assess the efficacy of UV disinfection of C. hominis. This study utilized cell culture-based methods to demonstrate that C. hominis oocysts displayed similar levels of infectivity and had the same sensitivity to UV light as C. parvum. Therefore, the water industry can be confident about extrapolating C. parvum UV disinfection data to C. hominis oocysts.

Irreversible UV inactivation of Cryptosporidium spp. despite the presence of UV repair genes

Ultraviolet light is being considered as a disinfectant by the water industry because it appears to be very effective for inactivating pathogens, including Cryptosporidium parvum. However, many organisms have mechanisms for repairing ultraviolet light-induced DNA damage, which may limit the utility of this disinfection technology. Inactivation of C. parvum was assessed by measuring infectivity in cells of the human ileocecal adenocarcinoma HCT-8 cell line, with an assay targeting a heat shock protein gene and using a reverse transcriptase polymerase chain reaction to detect infections. Oocysts of five different isolates displayed similar sensitivity to ultraviolet light. An average dosage of 7.6 mJ/cm2 resulted in 99.9% inactivation, providing the first evidence that multiple isolates of C. parvum are equally sensitive to ultraviolet disinfection. Irradiated oocysts were unable to regain pre-irradiation levels of infectivity, following exposure to a broad array of potential repair conditions, such as prolonged incubation, pre-infection excystation triggers, and post-ultraviolet holding periods. A combination of data-mining and sequencing was used to identify genes for all of the major components of a nucleotide excision repair complex in C. parvum and Cryptosporidium hominis. The average similarity between the two organisms for the various genes was 96.4% (range, 92-98%). Thus, while Cryptosporidum spp. may have the potential to repair ultraviolet light-induced damage, oocyst reactivation will not occur under the standard conditions used for storage and distribution of treated drinking water.

Susceptibility of five strains of Cryptosporidium parvum oocysts to UV light

Previous evaluations of the effect of ultraviolet (UV) light on Cryptosporidium parvum oocysts have been limited to a single strain—the Iowa strain. This study investigated the response of five strains of C. parvum to UV. A collimated beam apparatus was used to apply controlled doses of monochromatic (254 nm) UV to oocysts of the Iowa, Moredun, Texas A&M, Maine, and Glasgow strains. Irradiation was measured using a calibrated radiometer and sensor. Inactivation was quantified through animal infectivity by inoculation of cohorts of CD‐1 neonatal mice with UV‐treated and untreated control oocysts of each strain followed by examination of intestinal sections for infection using hemotoxylin and eosin staining. A UV light dose of 10 mJ/cm² achieved at least 4‐log₁₀ inactivation of all strains evaluated. All five strains of C. parvum were shown to be highly susceptible to low levels of UV light.

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.

Chlorine inactivation of spores of Encephalitozoon spp

This report is an extension of a preliminary investigation on the use of chlorine to inactivate spores of Encephalitozoon intestinalis and to investigate the effect of chlorine on two other species, E cuniculi and E. hellem, associated with human infection. The 50% tissue culture infective doses of these three species were also determined. On the basis of the results obtained, it appears that chlorination of water is an effective means of controlling spores of these organisms in the aquatic environment.

Modifications to United States Environmental Protection Agency methods 1622 and 1623 for detection of Cryptosporidium oocysts and Giardia cysts in water

Collaborative and in-house laboratory trials were conducted to evaluate Cryptosporidium oocyst and Giardia cyst recoveries from source and finished-water samples by utilizing the Filta-Max system and U.S. Environmental Protection Agency (EPA) methods 1622 and 1623. Collaborative trials with the Filta-Max system were conducted in accordance with manufacturer protocols for sample collection and processing. The mean oocyst recovery from seeded, filtered tap water was 48.4% +/- 11.8%, while the mean cyst recovery was 57.1% +/- 10.9%. Recovery percentages from raw source water samples ranged from 19.5 to 54.5% for oocysts and from 46.7 to 70.0% for cysts. When modifications were made in the elution and concentration steps to streamline the Filta-Max procedure, the mean percentages of recovery from filtered tap water were 40.2% +/- 16.3% for oocysts and 49.4% +/- 12.3% for cysts by the modified procedures, while matrix spike oocyst recovery percentages ranged from 2.1 to 36.5% and cyst recovery percentages ranged from 22.7 to 68.3%. Blinded matrix spike samples were analyzed quarterly as part of voluntary participation in the U.S. EPA protozoan performance evaluation program. A total of 15 blind samples were analyzed by using the Filta-Max system. The mean oocyst recovery percentages was 50.2% +/- 13.8%, while the mean cyst recovery percentages was 41.2% +/- 9.9%. As part of the quality assurance objectives of methods 1622 and 1623, reagent water samples were seeded with a predetermined number of Cryptosporidium oocysts and Giardia cysts. Mean recovery percentages of 45.4% +/- 11.1% and 61.3% +/- 3.8% were obtained for Cryptosporidium oocysts and Giardia cysts, respectively. These studies demonstrated that the Filta-Max system meets the acceptance criteria described in U.S. EPA methods 1622 and 1623.

Comparison of UV inactivation of spores of three encephalitozoon species with that of spores of two DNA repair-deficient Bacillus subtilis biodosimetry strains

When exposed to 254-nm UV, spores of Encephalitozoon intestinalis, Encephalitozoon cuniculi, and Encephalitozoon hellem exhibited 3.2-log reductions in viability at UV fluences of 60, 140, and 190 J/m(2), respectively, and demonstrated UV inactivation kinetics similar to those observed for endospores of DNA repair-defective mutant Bacillus subtilis strains used as biodosimetry surrogates. The results indicate that spores of Encephalitozoon spp. are readily inactivated at low UV fluences and that spores of UV-sensitive B. subtilis strains can be useful surrogates in evaluating UV reactor performance.

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.

Quantitation of Cryptosporidium parvum infection in cell culture using a colorimetric in situ hybridization assay

A quantitative colorimetric in situ hybridization assay was developed for detecting Cryptosporidium parvum infection in cell cultures using a digoxigenin-labeled probe targeting 18S rRNA. Intra-cellular developmental stages of C. parvum such as trophozoites and meronts were clearly discerned by light microscopy as localized areas of dark purple/black precipitate against a colorless background. Infections developed focally and the term infectious focus was applied to each cluster of developmental stages. There were no significant differences in the number of infectious foci following 24 h or 48 h incubation. However, 24 h and 48 h dose response curves were significantly different when infectivity was measured as the number of developmental stages per monolayer, with an average of 5.3-fold more stages following 48 h incubation. When infectivity was expressed as the number of infectious foci per inoculum oocyst converted to a percentage, it was demonstrated that the rate of infection decreased with increasing oocyst age. Oocysts of the Iowa isolate that were 7-10 days old demonstrated 7.8+/-2.4% infectivity (mean +/- standard deviation) compared to 4.2+/-0.8% for 21-28 day-old oocysts and 1.4+/-1.3% for 42-70 day-old oocysts. The assay also detected infection with other genotype 2 oocysts and a genoptye 1 isolate. This assay provides a direct quantitative approach for measuring C. parvum infectivity in cell culture.

Low-pressure UV inactivation and DNA repair potential of Cryptosporidium parvum oocysts

Because Cryptosporidium parvum oocysts are very resistant to conventional water treatment processes, including chemical disinfection, we determined the kinetics and extent of their inactivation by monochromatic, low-pressure (LP), mercury vapor lamp UV radiation and their subsequent potential for DNA repair of UV damage. A UV collimated-beam apparatus was used to expose suspensions of purified C. parvum oocysts in phosphate-buffered saline, pH 7.3, at 25 degrees C to various doses of monochromatic LP UV. C. parvum infectivity reductions were rapid, approximately first order, and at a dose of 3 mJ/cm(2) (=30 J/m(2)), the reduction reached the cell culture assay detection limit of approximately 3 log(10). At UV doses of 1.2 and 3 mJ/cm(2), the log(10) reductions of C. parvum oocyst infectivity were not significantly different for control oocysts and those exposed to dark or light repair conditions for UV-induced DNA damage. These results indicate that C. parvum oocysts are very sensitive to inactivation by low doses of monochromatic LP UV radiation and that there is no phenotypic evidence of either light or dark repair of UV-induced DNA damage.

Comparison of Cryptosporidium parvum viability and infectivity assays following ozone treatment of oocysts

Several in vitro surrogates have been developed as convenient, user-friendly alternatives to mouse infectivity assays for determining the viability of Cryptosporidium parvum oocysts. Such viability assays have been used increasingly to determine oocyst inactivation following treatment with chemical, physical, or environmental stresses. Defining the relationship between in vitro viability assays and oocyst infectivity in susceptible hosts is critical for determining the significance of existing oocyst inactivation data for these in vitro assays and their suitability in future studies. In this study, four viability assays were compared with mouse infectivity assays, using neonatal CD-1 mice. Studies were conducted in the United States and United Kingdom using fresh (<1 month) or environmentally aged (3 months at 4 degrees C) oocysts, which were partially inactivated by ozonation before viability and/or infectivity analyses. High levels of variability were noted within and between the viability and infectivity assays in the U.S. and United Kingdom studies despite rigorous control over oocyst conditions and disinfection experiments. Based on the viability analysis of oocyst subsamples from each ozonation experiment, SYTO-59 assays demonstrated minimal change in oocyst viability, whereas 4',6'-diamidino-2-phenylindole-propidium iodide assays, in vitro excystation, and SYTO-9 assays showed a marginal reduction in oocyst viability. In contrast, the neonatal mouse infectivity assay demonstrated significantly higher levels of oocyst inactivation in the U.S. and United Kingdom experiments. These comparisons illustrate that four in vitro viability assays cannot be used to reliably predict oocyst inactivation following treatment with low levels of ozone. Neonatal mouse infectivity assays should continue to be regarded as a "gold standard" until suitable alternative viability surrogates are identified for disinfection studies.