A bulletin for the Australian Food Industry    November 1998

Contents: Listeria in food - is zero tolerance necessary? | Safe Food Production New South Wales (Safe Food) | Parasites in water supplies | Comparison of ATP luminometry systems | Foods produced using gene technology | Hand washing in the food service industry | Thank you

Available issues | Search for articles | Index to articles

Parasites in water supplies

In a timely article in the Medical Journal of Australia 169 (21 September) 1998 research workers at the Department of Epidemiology and Preventive Medicine and the Cooperative Research Centre for Water Quality at Monash University discuss some of the issues facing water authorities in the control of waterborne protozoan parasites and the restoration of public confidence in Sydney and possibly other Australian water supplies.

These workers believe that any attempt to write into legislation standards for Giardia and Cryptosporidium would be both unwise and unworkable at present. They base this position on the grounds that the magnitude of risk to public health is far from clear and there are many limitation in the understanding of these organisms and the methods used for their detection in drinking water.

While authorities in Britain are apparently proposing tight restrictions on Cryptosporidium levels in drinking water - no more than 10 oocysts per 10 litres of water (Sydney Morning Herald, September 16, 1998) - the Monash workers point out that very little is known about what constitutes an infective dose of these organisms. Experimental studies have shown that as few as 10 cysts of Giardia lamblia could establish infection in adult males but none of 40 subjects developed giardiasis symptoms despite ingesting up to one million cysts during the trial (American Journal of Hygiene 59 1954 209-220; The New England Journal of Medicine 332 (1) 1995 855-858 ). In human experiments with Cryptosporidium parvum, of five seronegative volunteers receiving 30 oocysts, one became infected but experienced no symptoms. At the next dose level of 100 oocysts, three of eight volunteers became infected and developed symptoms.

Figures reported during the Sydney incidents exceeded 1,000 oocysts per 10 litres of water but not on a continuous basis. Waterborne outbreaks, and none has occurred in Sydney, are usually attributed to 'spikes' of contamination from increases in protozoan numbers in the source water or failure in water treatment processes.

The trial is due to be completed in March 1999 and may provide additional data on which a risk assessment can be made.

No recommendation is made regarding an active level of oocyte concentration in drinking water.

The Inquiry's final report is expected before the end of the year.