Does pond scum cause ALS? Maybe – but the evidence is far from clear.
Recent media reports have raised the question of a possible link between an increased risk of developing amyotrophic lateral sclerosis (ALS) and living near Lake Mascoma in Western New Hampshire.
The Union Leader in New Hampshire and other news outlets have reported that the risk of developing ALS is 25 times higher than average for people living around Lake Mascoma, located in Enfield and Lebanon, N.H. The source of this statistic was not explained.
The Union Leader article says researchers at Dartmouth-Hitchcock Medical Center in Lebanon have proposed that the higher incidence of ALS in the lake area results from a combination of bacterial toxins in the lake and a genetic predisposition to develop ALS.
Researchers say cyanobacteria, also known as blue-green algae, are the putative toxin. Collections of millions of these bacteria look like blue-green scum on the surface of water.
Elijah Stommel, a neurologist at Dartmouth-Hitchcock who's involved with the study, said there's evidence for a link between living near Lake Mascoma and an increased risk of developing ALS, but that it's "circumstantial to some degree at this point." Stommel said his group is working on "better establishing the link" and that a scientific publication is in preparation.
|Collections of millions of cyanobacteria look like blue-green scum on the water's surface.|
Tracie Caller, in her second year of postgraduate training in neurology and preventive medicine at Dartmouth-Hitchcock, is also involved in the study. "The publicity was not something we had anticipated," she said, noting that it occurred following a workshop at Lake Mascoma where she presented information on cyanobacteria, which have health implications in general. (Different cyanobacterial toxins can cause cause skin and eye irritation and damage to the liver or nervous system.)
"Our purpose there was merely to inform everyone about our project, since a lot of the lake volunteers are helping us to collect samples," she said.
She noted during her presentation that there is no scientific proof of a correlation between cyanobacteria toxin in lake water and ALS.
Lorene Nelson, an epidemiologist at Stanford (Calif.) University who has MDA funding to study genetic and environmental interactions in ALS, noted that many reported "clusters" of ALS "have not withstood scientific scrutiny." She said a certain number of apparent ALS clusters can be expected to occur on the basis of chance alone. Others occur not because of anything in the environment but because of factors such as a skewed age distribution, as ALS is a disease that strikes mainly older people.
She noted that the town of Enfield, in which the lake is partly located, has six housing projects for elderly residents, while most other towns in New Hampshire have one or two.
Nelson said cyanobacteria are extremely common in soil, as well as in sea water and fresh water, and are believed to cause damage to the liver after acute exposure. "The possible long-term effects on the risk of later-life cancer or neurological diseases is not known," she said.
Supporting an ALS-cyanobacteria link, she said, is another ALS “cluster” on the Pacific island of Guam. It’s hypothesized that the higher-than-average incidence of ALS on Guam is related to a cyanobacterial toxin called BMAA, which is ingested when people eat bats in which the toxin has accumulated.
For more about ALS on Guam, see Guamanian ALS: a Tough Nut Finally Cracked?
An exploration of ALS clusters can be found in the July-August 2009 issue of the MDA ALS Newsmagazine.