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 manufacturer's recommendations for use. Halazone® and Emergency Drinking Water Germicidal Tablets® were effective under all conditions, while the Globaline®, bleach, and I₂ methods suffered some shortcomings. Pentaiodide resin was superior to triiodide resin in cysticidal activity.