John C. Hoff

Effect of disinfectants on Giardia cysts

Due to a misstatement in the early literature, Giardia cysts have been considered for many years to be extremely resistant to disinfection. Recent studies using in vitro excystation as the criterion of viability have demonstrated that this concept is inaccurate. Several studies have shown that as little as 2 mg of chlorine per liter can kill greater than 99.8% of the Giardia cysts, while another study has suggested that only a slightly higher concentration of iodine than chlorine was equally cysticidal. Much lower concentrations were cysticidal when ozone (0.03 to 0.48 mg/l) was used as the disinfectant. Ultraviolet irradiation in excess of what can be feasibly generated commercially was ineffective against Giardia cysts. Several small‐quantity water disinfection methods have been tested against Giardia cysts following the manufacturers recommendations for use. Halazone® and Emergency Drinking Water Germicidal Tablets® were effective under all conditions, while the Globaline®, bleach, and I2 methods suff...

Contact and demand‐release disinfectants

A new class of solid‐phase disinfectants has emerged in recent years. Certain types of these disinfectants, the iodinated strong base resins, release iodine upon demand to microorganisms in water. These triiodide and pentaiodide resins have proven effective against a large number of bacterial genera, viruses, and representative parasites such as Giardia lamblia. Although federal regulatory policies have prevented their use for large scale water purification, they have been selectively applied in the U.S. Space Program and in situations for short‐term water disinfection. They appear to hold promise as components of hybrid devices utilizing filtration and halogen scavenging to produce high quality, potable water.

Resistance of cysts to disinfection agents.

Soon after the realization that many infectious diseases could be spread via food and water came attempts by numerous investigators to devise means by which to prevent this spread. As a result, many types of disinfection methods have been developed which when properly applied can virtually eliminate the possibility of infection by many disease-causing microorganisms which enter the host through this route.