Implications of biofilm-associated waterborne Cryptosporidium oocysts for the water industry.

Angles, M.L., Chandy, J.P., Cox, P.T., Fisher, I.H. and Warnecke, M.R. (2007) Trends in Parasitology, 23 (8); 352-6.  

Cryptosporidium has been responsible for a number of drinking water disease outbreaks in developed countries. Current management practices to prevent Cryptosporidium from entering drinking water supplies are focused on source-water management and water treatment as Cryptosporidium is resistant to chlorination. To properly assess the risk of waterborne Cryptosporidium , a more detailed understanding of the fate of Cryptosporidium oocysts in water distribution systems is required with particular emphasis on the interactions of Cryptosporidium in biofilms.  

The main aetiological agents of cryptosporidiosis in humans are the species Cryptosporidium parvum (predominantly zoonotic) and Cryptosporidium hominis (predominantly anthroponotic). The primary hosts for C. parvum and C. hominis are therefore restricted mostly to cattle, sheep or human faecal sources. Determining the point sources and loads, genotypes and transport of Cryptosporidium in source-water catchments is therefore an important step in establishing and managing the risk to the quality of drinking water and to public health. Water treatment is the main physiochemical barrier to preventing Cryptosporidium from entering the drinking water distribution system. Treatments such as coagulation, flocculation and sand filtration all reduce Cryptosporidium concentrations. Disinfection methods such as membrane filtration and ultraviolet irradiation can further reduce concentrations. Cryptosporidium however can still enter drinking water supplies and result in outbreaks of cryptosporidiosis.  

Biofilms are an integral part of the water distribution system and form whenever a surface is in contact with water. Both bacterial and viral pathogens can become incorporated in biofilms and it possible that Cryptosporidium oocysts may also become incorporated. There is little information in the literature that considers the interaction of oocysts with biofilms. One recent study showed that C. parvum oocysts had greater surface deposition on biofilms formed from a monoculture of bacterium Pseudomonas aeruginosa , than on clean surfaces with oocysts retained in the biofilms for over 24 h. A study using a mixed population of microorganisms isolated from reservoir water showed that the majority of oocysts introduced to a laboratory-scale reactor attached in clusters in the denser regions of biofilm with a large amount of the surface-associated oocysts remaining viable for at least 15 days. This highlights the possibility that detachment of biofilm-associated viable oocysts may occur at concentrations that represent an infectious dose.  

Another study found that the majority of oocysts introduced to a pipe-rig constructed from exhumed 70 year-old drinking water pipes became incorporated into the existing biofilm and were sporadically released from the biofilm. Furthermore oocysts were detected in the water phase up to two weeks post-inoculation. Free chlorine was applied at concentrations typically experienced in a drinking water distribution system downstream of a chlorinated reservoir with little impact on oocyst detachment. Outcomes from this study were confirmed by another set of studies undertaken in laboratory-scale biofilm reactors. An increase in oocyst detachment was found with increased linear flow velocity as might occur during mains flushing or hydrant sampling and oocysts were detected up to 58 days following inoculation; this has implication for distribution system management. The results from the experimental studies and observations during distribution system contamination events do not support hydrant sampling as the sampling procedure itself may cause resuspension of oocysts and lead to a confusing view of system recontamination. Neither the pipe-rig study nor the biofilm-reactor studies assessed the infectivity of oocysts.  

It has been proposed that the presence of oocysts in biofilms is of no concern because source-water loading and treatment removal efficiency determine the number of oocysts in a distribution system with biofilms not contributing to the total oocyst loading in a system. Under such circumstances, effective risk management involves understanding the genotypes of Cryptosporidium that are present in source water , their loads and transport and the effectiveness of treatment processes in removing them . However during unusual conditions that contribute to distribution system contamination, oocysts may accumulate in distribution system biofilms, persist past the initial contamination events in an infectious state and subsequently detach from the biofilms. Under such conditions the ability of oocysts to incorporate into biofilms and sporadically detach raises new risk issues that need to be considered so that more vigorous risk assessments can be undertaken and system managers can make more informed operational decisions. Of great importance is the determination of the factors that contribute to the loss of oocysts infectivity, how long oocysts remain infectious in biofilms and how long the risk remains following a contamination event. Infectivity may be lost rapidly as biofilms may actually reduce the risk from oocyst contamination by incorporation of oocysts into the biofilm. It is also important to quantify the degree of oocyst resuspension and understand the system parameters that contribute the most to oocyst detachment. Further research needs to consider these issues so that effective management of drinking water distribution systems occurs particularly during contamination incidents.