US 6,565,803 B1Grant

Method for the inactivation of cryptosporidium parvum using ultraviolet light

Issue Date:2003-05-20

•14 Claims

•2 Drawing Sheets

Abstract

A method for the inactivation of Cryptosporidium oocysts, Giardia cysts and similar organisms comprising irradiating water with ultraviolet light in doses of from about 1 mJ/cm²to about 175 mJ/cm².

Metadata

Assignee

Calgon Carbon Corporation

Inventors

James R. Bolton
R. D. Samuel Stevens
Bertrand Dussert

Application Information

Application Number:US 09/300,325

Filing Date:1999-04-27

Priority Date:1998-05-13

Art Unit:7

Classifications

IPC:

C02F 132

Field of Search:

422 23422 24210748

Patent Drawings (2 sheets)

Description

Cross-Reference

[0002] The present application is a continuation-in-part application of Ser. No. 09/078,116, filed May 13, 1998, entitled METHOD FOR PREVENTING REPLICATION IN CRYPTOSPORIDIUM PARVUM USING ULTRAVIOLET LIGHT, now U.S. Pat. No. 6,129,893.

Field of the Invention

[0003] The present invention relates to a method for inactivating Cryptosporidium parvumin water and in particular to a method for the prevention of Cryptosporidium parvumand other protozoans, such as Giardia muris,from establishing infection in human hosts, as measured by the ability to infect neo-natal mice, using low doses of ultraviolet light.

Background of the Invention

[0004] It has been generally well recognized that it is necessary to kill or inactivate protozoan oocysts so that they cannot infect susceptible hosts. This is especially important in drinking water. One such method is the use of ultraviolet (“UV”) light. The prior art teaches that a UV dose of at least 3000 mJ/cm²is required to inactivate Cryptosporidium parvum(Lorenzo-Lorenzo et al., J. Parasitol. 1993, 79, 67-70) and Giardia muris(E. L. Jarol, “Effect of Disinfectants on Giardia Cysts”, CRC Critical Reviews in Environmental Control, 1988, 18, 1-28). Snowball and coworkers (UK Patent Application #9416287.2, Nov. 8, 1984; Wat. Res., 1995, 29, 2583-2586) developed an apparatus that first filtered out Cryptosporidium oocysts and then exposed them to UV doses of 700-800 mJ/cm². The patent teaches the use of 2 μm screen filters to trap Cryptosporidium oocysts which are then irradiated with a bank of low-pressure Hg lamps for a UV dose of 350-400 mJ/cm². The filter is then backwashed onto a second filter and the irradiation is repeated for a total dose of 700-800 mJ/cm². The patent discloses that the treatment “kills” the organisms.

[0005] M. J. Lorenzo-Lorenzo, M. E. Area-Mazea, I. Villacorta-Martinez de Maturana and D. Duran-Oreiro [“Effect of Ultraviolet Disinfection of Drinking Water on the Viability of Cryptosporidium parvumOocysts”, J. Parasitol.1993, 79(1), 67-70] report the prevention of infection in mice after exposure to at least 150 min. of UV from a (presumably) low-pressure Hg lamp. Although the paper is not clear, it can be inferred that the UV dose applied was over 5000 mJ/cm²to obtain better than 2 logs reduction in infectivity. The authors claim that exposure to UV for 150 min. or more “eliminates” infectivity, but they give no mechanism other than to say “UV radiation disrupts DNA by causing formation of thy[ia]mine dimers, and high levels may lead to cell death”. At the UV doses they applied, the effects observed almost certainly arose from cell death.

[0006] In a paper by A. Bushnell, W. Clark, J. Dunn and K. Salisbury [“Pulsed Light Sterilization of Products Packaged by Blow-Fill-Seal Techniques”, Pharm. Engin.1997, September/October, 74-83], a pulsed UV technique for “sterilizing” surfaces containing bacteria, fungi, spores, viruses, protozoa and oocysts is described. The required UV doses were reported to be over 1000 mJ/cm². The effectiveness of the method was assayed using mouse infectivity. At the reported UV doses, the effects were believed to be due to cell death.

[0007] In a paper by R. LaFrenz [“High Intensity Pulsed UV for Drinking Water Treatment”, Proc. AWWA WQTC Conference,Denver, Colo., November, 1997], a similar pulsed system was described. While very few details were given, it appears that mouse infectivity assay was used and 6 logs of “inactivation” of Cryptosporidium was obtained at energy levels of approximately 200 mJ/cm²and greater. The paper claims that the pulsed UV overcomes the “DNA repair mechanism”; however, the UV doses applied are much larger than required with either a steady-state medium pressure or low pressure Hg lamp, as shown herein.

[0008] From the references cited above, we infer that the prior art teaches that very large UV doses (>200 mJ/cm²and up to 5000 mJ/cm²) are required to inactivate Cryptosporidium by “killing” the organisms. Accordingly, it is an object of the invention to provide a method using ultraviolet light to treat water in an effective way so that Cryptosporidium oocysts cannot infect susceptible hosts or, in other words, to “disinfect” the water in regard to Cryptosporidium oocysts that may be present. It is another object of the invention to provide a method using ultraviolet light from a medium-pressure mercury lamp to render the Cryptosporidium oocysts unable to infect. It is yet another object of the present invention to provide a method using ultraviolet light that is cost-effective in treating drinking water to eliminate the potential for infection by Cryptosporidium oocysts and Giardia cysts. The final object of the invention is to provide a method using ultraviolet light from a low-pressure mercury lamp to render Cryptosporidium oocysts and Giardia cysts unable to infect.

Summary of the Invention

[0009] Generally it has been discovered that it is not necessary to “kill” pathogens, such as Cryptosporidium parvumor Giardia muriswith ultraviolet light in order to prevent infection; one need only apply enough ultraviolet light to prevent the organism from “replicating”. The method of the present invention prevents replication (cell mitosis) by inactivating the DNA to prevent infection. The UV doses required to prevent replication are orders of magnitude lower than required to “kill” the oocysts. This means that the cost of UV treatment to prevent infection by Cryptosporidium oocysts will be markedly lower.

[0010] It has been found that when biological organisms are exposed to ultraviolet light (UV) in the range of 200-300 nm, the UV can be absorbed by DNA, RNA, and proteins. Absorption by proteins can lead to rupture of cell walls and death of the organism. Absorption by DNA or RNA (specifically by thymine bases) is known to cause inactivation of the DNA or RNA double helix strands through the formation of thymine dimers. If enough of these dimers are created in DNA, the DNA replication process is disrupted and hence, in mitosis, the cell cannot replicate. Cells that cannot replicate cannot infect. The present invention utilizes UV doses substantially lower (to achieve the state of hindered replication) by orders of magnitude than those required to cause oocyst death.

[0011] The present invention preferably utilizes a broad band (200-300 nm) medium-pressure mercury UV lamp to achieve the disinfection. In another embodiment of the invention, a low-pressure mercury (essentially monochromatic) UV lamp can be used. The dose required with a medium-pressure lamp was measured to be 11 mJ/cm²to achieve better than 5.9 log disinfection. From this it can be inferred that a dose of 7 mJ/cm²will achieve better than 4 log disinfection (99.99%) and 3.6 mJ/cm²will achieve better than 2 log disinfection (99%). For low pressure lamps a dose of 8 and 16 mJ/cm²was required to achieve 4.1 and 4.3 log disinfection, respectively. Thus, the dose levels of UV are significantly lower than those used before resulting in significantly lower power levels needed to achieve the results. It has been found that inactivation of Cryptosporidium and similar organisms such as Giardia occurs at dosages from about 1 mJ/cm². Accordingly, the method provides a substantial improvement in the cost effectiveness of UV for the disinfection of contaminated drinking water as regards to Cryptosporidium oocysts and Giardia cysts that may be present. Other advantages will become apparent from a perusal of the following detailed description of a presently preferred embodiment of the invention.

Brief Description of the Drawings

[0012] FIG. 1 is a chart that shows the correlation between bench-scale and demonstration-scale tests and the difference between “in vitro” and “in vivo” methods.

[0013] FIG. 2 is a chart that shows the correlation between tests using low-pressure versus medium-pressure mercury are UV lamps.

Presently Preferred Embodiment

[0014] Experiments were conducted on two different sets of apparatus: a bench-scale collimated beam setup and a demonstration-scale UV reactor.

[0015] A well-known bench-scale collimated beam apparatus was used in the test. An upper lamp housing can contain either a 15 W low-pressure Hg lamp (monochromatic at 254 nm) or a 1 kW Rayox medium-pressure Hg lamp (emitting over a broad range from 200-300 nm). Each lamp has its own power supply in the lower housing. A black plastic collimator (48 cm long and 6.4 cm diameter) extends vertically down from the lamp housing with a pneumatically-driven shutter in between. The cell suspension (in finished water from the Mannheim Water Treatment Plant, Kitchener, Ontario, Canada) to be irradiated is placed in a Petri dish (with a stir bar) on top of a stirring motor below the collimator and exposed for a fixed length of time to achieve the desired UV dose. The UV irradiance is measured with an Interational Light Model 1400 Radiometer with a Model SED240 detector. Proper account is taken of the variation of the detector sensitivity with wavelength, the attenuation of irradiance in the water and of the irradiance distribution over the top of the Petri dish. The UV dose (mJ/cm²) is the average UV irradiance (mW/cm²) in the water times the exposure time (s).

[0016] Demonstration-scale challenges of Cryptosporidium parvumand Giardia muriswere carried out on filtered water at the Mannheim Water Treatment Plant in Kitchener, Ontario, Canada with a 111 L (29.4 gal) UV reactor containing 6×1 kW Rayox Sentinel™ medium-pressure UV lamps mounted horizontally across a tower type UV reactor. The organisms were introduced upstream of a static mixer ahead of the reactor and collected on 1 micron filters after the reactor. The overall flow rate during each test was about 215 gpm (814 L/min). The filters were shipped to St. Albans, Vt. where the organisms were extracted from the filters and concentrated. All organisms were subjected to in vitro assays (fluorogenic vital dyes and excystation); four of the Cryptosporidium samples were shipped to the Department/Laboratory of the University of Arizona for mouse infectivity assays.

[0017] The UV-treated oocysts were enumerated with a hemocytometer, using bright-field microscopy to determine the concentration of oocysts present in each tube. These preliminary counts were used to calculate the dilutions that were necessary for preparation of oocyst inocula for neonatal mice.

[0018] Upon arrival of the oocysts at the University of Arizona, the infectivity of the oocysts was determined by their inoculation into 4 to 6-day-old CD-1 outbred mice. The mice were challenged with oocyst inocula prepared in sterile water adjusted to pH 7. All inocula were prepared by serial dilution from a pre-enumerated oocyst suspension. A calibrated pipette was used for all dilutions and following dilution, oocysts inocula were re-enumerated before they were fed to the mice. The counts were performed and cross-checked by two technicians. The oocysts were administered orally in 10 μL inocula with a dedicated, calibrated pipette equipped with a standard tip. The inocula were administered slowly with the mouse held gently in the palm of the technician's hand until the entire inoculum was swallowed. The animals were sacrificed seven days post-inoculation and approximately 2 cm of terminal ileum was excised. The tissue samples were fixed in formalin, embedded in paraffin, sectioned, mounted on microscope slides, stained, and examined for the presence of endogenous stages of C. parvumin the brush-borders of the intestinal villi. Specimens with parasites were scored positive and those without parasites were scored negative.

[0019] The UV dose (mJ/cm²) applied in the reactor was calculated from the average irradiance (mW/cm²) (determined from a sophisticated point source summation model of the reactor) times the residence time in the reactor (about 8.3 s). The UV dose was varied by turning one or two lamps on at “low” or “full” power.

[0020] A subsequent set of bench-scale experiments was conducted to assess differences between low-and medium-pressure Hg lamps. The conditions for these experiments were essentially the same as that for the bench-scale experiments described above except that one set of experiments was conducted using the low-pressure Hg lamp and one set with the medium-pressure Hg lamp.

Summary of the Results

[0021] Assays

[0022] Two in vitro assays (fluorogenic vital dyes and maximized excystation) and one in vivo assay (neonatal mouse infectivity) were used to assess the viability and infectivity of the Cryptosporidium parvumoocysts for both the bench- and demonstration-scale experiments. In addition, one in vitro assay (maximized excystation) was performed in the demonstration scale experiments for Giardia muriscysts. Also in vivo testing for Giardia was performed.

[0023] Bench-Scale Study

[0024] Viability and Infectivity of Untreated and Process Control Oocysts of Cryptosporidium

[0025] The initial viability assessment in the trip control (untreated oocyst suspensions) indicated high viability with the fluorogenic vital dyes (91%±2%) and maximized in vitro excystation (76%±4%. These results were corroborated with mouse infectivity, which indicated that 75 oocysts were necessary for 56% infection in CD-1 neonatal mice. In the process control (oocysts subjected to all experimental procedures without exposure to UV light), the fluorogenic vital dyes indicated a viability of 85±3% and maximized in vitro excystation a viability of 86%. For the process controls, an inoculum of 50 oocysts caused approximately 80% infection in neonatal mice. These data were used to “normalize” the experimental viabilities for the in vitro assays by multiplying by (1/0.85=1.18) in the case of fluorogenic vital dyes and by (1/0.86=1.16) in the case of maximized excystation.

[0026] Viability and Infectivity of UV-exposed Oocysts

[0027] Four UV doses were examined to assess their effect on oocyst viability and infectivity. The normalized in vitro viability data for the UV-exposed oocysts are given in Table 1a. The log viability factor is the logarithm of the ratio of the viability percentage relative to that of the process control.

[0028] The in vivo neonatal mouse infectivity data are given in Table 1b and were ascertained from microscopic examination of tissue sections derived from the ileum of individual mice, seven days past infection. The percentage infectivity for a given dose was determined from the ratio of infected mice to the total number of mice receiving that dose. In order to establish a log viability for infection, the percentage infectivity values were extrapolated from a log it dose response model, which was generated from previous infectivity studies and has the following equation.

[0029] response log it=−7.536+3.867 log₁₀Dose=ln[P/(1−P)]

[0030] where P is the proportion of mice infected. The response log it is very similar to that described by Finch et al. [Finch, G. R., Daniels, C. W., Black, E. K., Schaefer III, F. W. and Belosovic, M. 1993. “Dose Response of Cryptosporidium parvumin Outbred Neonatal CD-1 Mice”. Applied and Environmental Microbiology. 59(11):3661-3665.] and is used to determine the number of infectious oocysts in a given oocyst inoculum.

[0031] For example, oocyst exposure to 123 mJ/cm²of medium-pressure UV caused infection in 1 of 25 mice at an inoculum of 1.0×10⁵oocysts. Substitution of these data into the log it response equation indicates

[0032] Response log it=ln[0.04/0.96]=−3.178

[0033] Substituting:

[0034] −3.178=−7.536+3.867 log₁₀Dose

[0035] log₁₀(no. of infectious oocysts)=(7.536−3.178)/3.867=1.127

[0036] No. of infectious oocysts=13

[0037] This calculation indicates that following treatment of oocysts with UV and administration of an inoculum of 1.0×10⁵oocysts, approximately 13 oocysts were capable of causing infection in mice.

[0038] The log viability for infection was calculated by the following equation:

[0039] log viability (for infection)=log₁₀[(no. of infectious oocysts)/(original inoculum)]

[0040] In the example,

[0041] log viability (for infection)=log₁₀[13/100,000]=−3.9

[0042] These infectivity data in Table 1b indicate that the in vitro assays greatly underestimate oocyst inactivation when compared to in vivo mouse infectivity.

TABLE 1a

Normalized Viability Factors (percent)
for in vitro Bench-Scale Tests

TABLE 1b

Percentage Neo-natal Mouse Infectivity for in vitro Bench-Scale Tests

[0043] Demonstration Scale Study

[0044] Viability and Infectivity of Untreated and Process Control Cryptosporidium Oocysts and Giardia Cysts

[0045] The initial viability assessment in the trip control (untreated Cryptosporidium oocyst suspensions) indicated high viability with the fluorogenic vital dyes (82%±4%) and maximized in vitro excystation (81%±8%). These results were corroborated with mouse infectivity, which indicated that 75 oocysts were necessary for 35% infection in CD-1 neonatal mice. In two process controls (oocysts subjected to all experimental procedures without exposure to UV light), the fluorogenic vital dyes indicated an average viability of 77±5%, while maximized in vitro excystation indicated an average viability of 38%±8% for Cryptosporidium and 53%±23% for Giardia. For the process controls, an inoculum of 50 oocysts caused approximately 44% infection in neonatal mice. These data were used to “normalize” the experimental viabilities for the in vitro assays by multiplying by (1/0.72=1.39) in the case of fluorogenic vital dyes and by (1/0.38=2.63) in the case of maximized excystation for Cryptosporidium oocysts and (1/0.53=1.89) for Giardia.

[0046] Viability and Infectivity of UV-exposed Oocysts

[0047] In the demonstration-scale disinfection experiments, five UV doses were examined to assess their effects on Cryptosporidium parvumviability with three doses used to asses infectivity. Only in vitro excystation was used to assess the viability of Giardia muriscysts.

[0048] The normalized in vitro viability data for the UV-exposed oocysts and cysts are given in Table 2a and the in vivo neonatal mouse infectivity data are given in Table 2b. These data again indicate that the in vitro assays greatly underestimate oocyst inactivation when compared to in vivo mouse infectivity.

TABLE 2a

Normalized Viability Factors (percent) for
in vitro Demonstration-Scale Tests

TABLE 2b

Percentage Neo-natal Mouse Infectivity for in vitro
Demonstration-Scale Tests for Cryptosporidium

[0049] Comparison of Bench-and Demonstration-Scale Disinfection Studies to Assess Oocyst Inactivation

[0050] The oocyst inactivation data are illustrated in FIG. 1 as log(viability ratio versus UV). The viability ratio is defined as the ratio of the viability of the UV-treated oocysts to that of the process control versus UV dose. The dramatic difference between the in vitro (fluorogenic vital dyes and excystation) and in vivo (neonatal mouse infectivity) assays may be explained in the context that the in vitro assays measure integrity/permeability of the oocyst wall, not the ability of the oocyst to infect its host; whereas the in vivo assay measures the ability of the oocysts to infect a susceptible host.

[0051] Validation of the UV Dose for the Demonstration-scale Study

[0052] The UV dose for the demonstration-scale study depends on the average irradiance calculated from a complex mathematical model. It is thus important to have an independent assessment of the accuracy of the calculation. An examination of FIG. 1 shows an excellent agreement between the bench-scale and demonstration-scale studies especially considering the uncertainties associated with these assays. Thus the UV dose calculated in the demonstration-scale studies can be considered validated by the excellent agreement with the experimentally obtained data from the collimated beam tests.

[0053] Bench-Scale Study Comparing Effects of Low- and Medium-Pressure Hg Lamps

[0054] The effects of three low-pressure UV doses (8, 16 and 33 mJ/cm²) and two medium-pressure UV doses (11 and 20 mJ/cm²) on the viability and infectivity of Cryptosporidium parvumoocysts (suspended in Mannheim finished water) were examined.

[0055] Viability and Infectivity of Untreated and Process Control Oocysts of Cryptosporidium

[0056] The initial viability assessment in untreated oocyst suspensions indicated a viability of 80%±4% with the fluorogenic vital dyes and 71%±6% by maximized in vitro excystation. In the process control (oocysts subjected to all experimental procedures without exposure to UV light), the fluorogenic vital dyes indicated a viability of 68±4% and maximized in vitro excystation a viability of 67%. For the process controls, an inoculum of 50 oocysts caused approximately 53% infection in neonatal mice. These data were used to “normalize” the experimental viabilities for the in vitro assays by multiplying by (1/0.68=1.47) in the case of fluorogenic vital dyes and by (1/0.67=1.49) in the case of maximized excystation.

[0057] Viability and Infectivity of UV-exposed Oocysts

[0058] The normalized in vitro viability data for the UV-exposed oocysts are given in Table 3a and the in vivo neonatal mouse infectivity data are given in Table 3b. These data again indicate that the in vitro assays greatly underestimate oocyst inactivation when compared to in vivo mouse infectivity. Also, there is a definite difference between the data for the low-pressure Hg lamp and that for the medium-pressure Hg lamp. None of the mice became infected in any of the medium-pressure experiments, whereas there were definite indications of infectivity for at least the two lowest low-pressure UV doses. To achieve 5.9 logs inactivation, it is preferable to administer at least between 11 and 22 mJ/cm²with a low-pressure Hg lamp. Typically, 11 mJ/cm²suffices for the medium-pressure Hg lamp. However, it has been found that there is little difference in UV sensitivity between the medium-pressure and low-pressure Hg lamps.

TABLE 3a

Normalized Viability Factors (percent) for in vitro
Bench-Scale Tests Comparing Low-Pressure (LP)
versus Medium-Pressure (MP) Hg Lamps

TABLE 3b

Percentage Neo-natal Mouse Infectivity for in vitro Bench-Scale Tests
Comparing Low-Pressure (LP) versus Medium-Pressure (MP) Hg Lamps

TABLE 4

Preliminary Results of the Dosage Relationship
of Cryptosporidium and Gardia.

[0059] While presently referenced embodiments of the invention have been described, the invention may be otherwise embodied within the scope of the appended claims.

Claims

What is claimed:

1. A method for the prevention of infection from cryptosporidium oocysts found in drinking water comprising irradiating said water with continuous ultraviolet light having predominant wavelength bands that falls within about 200 nanometers to about 300 nanometers with a dose of from about 1 mJ/cm²to about 175 mJ/cm².

2. A method as set forth in claim 1 wherein said dose is from about 3.5 mJ/cm²to about 175 mJ/cm².

3. A method as set forth in claim 1 or 2 wherein said ultraviolet light substantially comprises a wavelength of about 254 nanometers.

4. A method as set forth in claim 1 or 2 wherein said ultraviolet light is produced by one of a low pressure mercury lamp or a medium pressure mercury lamp.

5. A method as set forth in claim 4 wherein said ultraviolet light is produced by a low pressure mercury lamp.

6. A method as set forth in claim 4 wherein said ultraviolet light is produced by a medium pressure mercury lamp.

7. A method for the prevention of replication of cryptosporidium oocysts or giardia cysts in drinking water treatment comprising irradiating said water with a continuous source of light having predominant wavelength bands that falls within about 200 nm to about 300 nm and a dose of from about 1 mJ/cm²to about 175 mJ/cm².

8. A method as set forth in claim 7 wherein said dose is from about 3.5 mJ/cm²to about 175 mJ/cm².

9. A method as set forth in claim 7 or 8 wherein said light substantially comprises a wavelength of about 254 nanometers.

10. A method as set forth in claim 7 or 8 wherein said light is produced by one of a low pressure lamp or a medium pressure lamp.

11. A method as set forth in claim 10 wherein said light is produced by a low pressure lamp.

12. A method as set forth in claim 10 wherein said light is produced by a medium pressure lamp.

13. A method of treating drinking water containing contaminants including cryptosporidium comprising exposing said water to a continuous broad band of ultraviolet radiation with a dose of about 1.0 mJ/cm²to about 175 mJ/cm², wherein said exposing step is the sole process for rendering cryptosporidium in said drinking water noninfectious.

14. A method as set forth in claim 13 wherein said dose is from about 3.5 mJ/cm²to about 175 mJ/cm².

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