Norwalk virus in water was found to be more resistant to chlorine inactivation than poliovirus type 1 (LSc2Ab), human rotavirus (Wa), simian rotavirus (SAll), or f2 bacteriophage. A 3.75 mg/liter dose of chlorine was found to be effective against other viruses but failed to inactivate Norwalk virus. The Norwalk virus inoculum remained infectious for five of eight volunteers, despite the initial presence of free residual chlorine. Infectivity in volunteers was demonstrated by seroconversion to Norwalk virus. Fourteen of 16 subjects receiving untreated inoculum seroconverted to Norwalk virus. Illness was produced in four of the eight volunteers and in 11 of 16 control subjects. A similar Norwalk virus inoculum treated with a 10 mg/liter dose of chlorine produced illness in only one and failed to induce seroconversion in any of eight volunteers. Free chlorine (5 to 6 mg/liter) was measured in the reaction vessel after a 30-minute contact period. Norwalk virus appears to be very resistant to chlorine which may explain its importance in outbreaks of waterborne disease.

The kinetics of inactivation of simian rotavirus SAl1 by chlorine, chlorine dioxide, and monochloramine were studied at 5°C with a purified preparation of single virions and a preparation of cell-associated virions. Inactivation of the virus preparations with chlorine and chlorine dioxide was studied at pH 6 and 10. The monochloramine studies were done at pH 8. With 0.5 mg of chlorine per liter at pH 6, more than 4 logs (99.99%) of the single virions were inactivated in less than 15 s. Both virus preparations were inactivated more rapidly at pli 6 than at pH 10. With chlorine dioxide, however, the opposite was true. Both virus preparations were inactivated more rapidly at pH 10 than at pH 6. With 0.5 mg of chlorine dioxide per liter at pH 10, more than 4 logs of the single-virus preparation were inactivated in less than 15 s. The cell-associated virus was more resistant to inactivation by the three disinfectants than was the preparation of single virions. Chlorine and chlorine dioxide, each at a concentration of 0.5 mg/liter and at pH 6 and 10, respectively, inactivated 99% of both virus preparations within 4 min. Monochloramine at a concentration of 10 mg/liter and at pH 8 required more than 6 h for the same amount of inactivation.

Drinking water disinfection provides the final barrier to transmission of a wide variety of potentially waterborne infectious agents including pathogenic bacteria, viruses, and protozoa. These agents differ greatly in their innate resistance to inactivation by disinfectants, ranging from extremely sensitive bacteria to highly resistant protozoan cysts. The close similarity between microorganism inactivation rates and the kinetics of chemical reactions has long been recognized. Ideally, under carefully controlled conditions, microorganism inactivation rates simulate first-order chemical reaction rates, making it possible to predict the effectiveness of disinfection under specific conditions. In practice, changes in relative resistance and deviations from first-order kinetics are caused by a number of factors, including microbial growth conditions, aggregation, and association with particulate materials. The net effect of all these factors is a reduction in the effectiveness and predictability of disinfection processes. To ensure effective pathogen control, disinfectant concentrations and contact times greater than experimentally determined values may be required. Of the factors causing enhanced disinfection resistance, protection by association with particulate matter is the most significant. Therefore, removal of particulate matter is an important step in increasing the effectiveness of disinfection processes.

Sieves and nylon screens were used to separate primary sewage effluent solids into particle fractions of <7or >7-p.m size. The efficiency of separation was determined by using a particle counter. Indigenous coliforms associated with the particle fractions were tested for their resistance to chlorine and monochloramine. Coliforms associated with the <7-p.m fraction were inactivated more rapidly by 0.5 mg of chlorine per liter at 5°C and pH 7 than coliforms associated with the >7-p.m fraction. Homogenization of the >7-p.m fraction not only resulted in an increase in the number of <7-jim particles, but also increased the rate of inactivation to a rate similar to that of the <7-p.m fraction. With 1 mg of monochloramine per liter at 5°C and pH 7, particle size had no appreciable effect on the rate of inactivation. At pH 8, however, the <7-jim fraction was inactivated more rapidly than the >7-p.m fraction. The time required for 99% inactivation of the particle fractioins with monochloramine at pH 7 or 8 was 20to 50-fold greater than the time required for the same imount of inactivation with chlorine at pH 7. The results indicate that coliforms associated with sewage effluent particles are inactivated more rapidly with 0.5 mg of chlorine per liter than with 1.0 mg of monochloramine per liter. However, >7-p.m particles can have a protective effect against the disinfecting action of chlorine.

Of all the current potential alternatives to free residual chlorine for drinking water disinfection (ozone, chlorine dioxide, and chloramines), ozone is the most potent biocide. Chlorine dioxide is about on a par with hypochlorous acid, but in contrast to free residual chlorine, its efficiency increases substantially as pH increases in the range at which disinfection is usually applied. Chloramines are weaker biocides than hypochlorite ion, the least efficient form of free residual chlorine. Precise quantitative ranking of biocidal efficiencies is not possible because efficiencies differ with different microorganisms and experimental conditions. Laboratory studies are not always directly applicable to disinfectant use in the field.

Campylobacterjejuni and closely related organisms are important bacterial causes of acute diarrheal illness in the United States. Both endemic and epidemic infections have been associated with consuming untreated or improperly tteated surface water. We compared susceptibility of three C. jejuni strains and Escherichia coli ATCC 11229 with standard procedures used to disinfect water. Inactivation of bacterial preparations with 0.1 mg of chlorine and 1.0 mg of monochloramine per liter was determined at pH 6 and 8 and at 4 and 25°C. Under virtually every condition tested, each of the three C. jejuni strains was more susceptible than the E. coli control strain, with greater than 99% inactivation after 15 min of contact with 1.0 mg of monochloramine per liter or 5 min of contact with 0.1 mg of free chlorine per liter. Results of experiments in which an antibiotic-containing medium was used suggest that a high proportion of the remaining cells were injured. An animal-passaged C. jejuni strain was as susceptible to chlorine disinfection as were laboratory-passaged strains. These results suggest that disinfection procedures commonly used for treatment of drinking water to remove coliform bacteria are adequate to eliminate C. jejuni and further correlate with the absence of outbreaks associated with properly treated water.

Cysts of Giardia lamblia from symptomatic and asymptomatic carriers had similar resistances to chlorination.

Giardia muris cysts, isolated from mouse faeces, excysted routinely at levels greater than 90%, when induced in 1X Hanks' supplemented with 17 mM glutathione, 29 mM L-cysteine-HCl, and 50 mM NaHCO3 for 30 minutes at 35 degrees C, followed by washing and suspension in trypsin-Tyrode's solution at pH 8.0. Although trypsin was not required in this final step, it enhanced the escape of the trophozoites from their cysts. G. muris excystation was dependent upon the length of the induction period, pH, oxidation-reduction potential and temperature. Optimal induction conditions for excystation were: an induction period of 5 to 30 min; pH of 2; 120 mV oxidation-reduction potential; and a temperature around 35 degrees C. A gradual decline in excystation occurred as pH and oxidation-reduction potential were changed to 7 and 57 mV, respectively. There was a pronounced increase in excystation percentages with increasing temperatures between 0 and 37 degrees C. At 40 degrees C and above, the G. muris cysts showed signs of inactivation. The thermal death point of G. muris cysts was determined to be about 54 degrees C. G. muris cysts showed no polarity; however, the tail or posterior trophozoite portion always emerged through one end of the cyst first. Cytokinensis began within the first hour after excystation. This method always produced extremely active, normal-looking G. muris trophozoites.