Health Stream Literature Summary - Issue 56 - December 2009

Occurrence of Naegleria fowleri in Arizona drinking water supply wells.
Bright KR, Marciano-Cabral F and Gerba CP. (2009) Journal / American Water Works Association, 101(11); 43-50.

Naegleria fowleri is a protozoa which can cause a rare but fatal disease known as primary amoebic meningoencephalitis. N. fowleri mainly affects healthy children and young adults with a history of swimming and/or diving in freshwater lakes and ponds. Infections have generally been associated with forcible entry of water into the upper nasal passages, from which the organism may infect the olfactory nerves and then spread to the brain. In 2002, two fatalities occurred in children in Phoenix, Arizona who had no known exposure to lakes or ponds. Investigations showed the presence of N. fowleri in the undisinfected tap water supply which was drawn from deep groundwater, and it was concluded the children had most probably been infected from tap water. The presence of N. fowleri in drinking or bathing water is not currently regulated in the United States. This study investigated the presence of N. fowleri in high-volume public drinking water supply production wells operated by municipal utilities or private water companies in the greater Phoenix and Tucson Arizona metropolitan areas.

The study was carried out in two phases. Phase 1 was an initial survey (n=58) to identify wells testing positive for the presence of N. fowleri (or N. fowleri DNA). A total of 58 wells were sampled for Naegleria during September (1 litre grab samples from at or near the wellhead, prior to disinfection, and collected after flushing to reduce turbidity). Phase 2 involved sampling every other month (April, May, June, August and October the following year) in a subset of wells previously identified as either positive (n=5) or negative (n=2) for N. fowleri. A number of samples were also collected from other wells (n=55) not previously tested. In Phase 2, samples were initially collected as water was turned on ('initial') and then after an approximate three borehole volumes had flushed through the system ('purged'). Well water samples were also tested for a number of water quality parameters. Each well water sample (phase 1 and phase 2) was tested for the presence of N. fowleri DNA via polymerase chain reaction (PCR). Positive and negative PCR products were coded to prevent bias and confirmed by gene cloning and sequencing. During phase 2 of the study, each well water sample was also tested for the presence of viable amoebae.

In phase 1 of the project, of N. fowleri was detected in five of 58 wells tested. In phase 2 there were 14 of the 113 wells that were tested multiple times. Five of the 14 wells always tested positive for N. fowleri. During the whole of the phase 1 and phase 2 sampling periods, N. fowleri was detected in 12 of 113 wells (10.6%), with 29 of 185 samples (15.7%) testing positive. Analysis for the detection of live amoebae during phase 2 found live flagellates of undetermined species in 20 of 62 (32.3%) wells and in 26 of 125 samples. Viable N. fowleri was only confirmed by PCR in one well which tested positive for the presence of N. fowleri DNA on five separate occasions. This may be because of the low occurrence of the infectious form of the organism in water, because the organisms detected by PCR were no longer viable or because of the differences in the assay volume for detection of live trophozoites (0.75 mL) versus PCR (30 mL equivalent volume of non-concentrated sample). Coliform bacteria were present in 8.9% of the wells tested, E. coli were not detected in any wells and heterotrophic bacteria exceeded 500/mL (for at least one sample) in 51.3% (58 of 113) of the wells. There were no correlations found between heterotrophic bacteria, coliforms, E. coli, temperature, specific conductance or turbidity in relation to the presence of N. fowleri in wells. N. fowleri was most frequently detected after the well was purged (86.2% of all positive samples) suggesting that the organism was present in the aquifer or was released from the well casing or pump column during pumping.

Chlorination or other forms of disinfection may be effective for controlling N. fowleri transmission via drinking water but there is no guidance on the required dosages and contact time for drinking water storage tanks or distribution systems. Laboratory studies suggest that both cysts and trophozoites are fairly resistant to free chlorine. It is not known why some wells are positive for the presence of N. fowleri while others are only transiently positive. Further research needs to consider the maintenance of wells and other characteristics that may influence the occurrence of N. fowleri and also the effectiveness of well borehole chlorination or other practices in eliminating the organism from wells.

Comment The investigation into the Naegleria deaths in Phoenix, Arizona was reported in Health Stream Issue 28. Australian experience has shown chloramination to be effective in eliminating this organism from pipelines and distribution systems.

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