Potentially toxic cyanobacteria health warnings

Friday November 21, 2014

​Health warnings have been put in place at two Canterbury rivers following the detection of potentially toxic algae (benthic cyanobacteria), while a health warning has been lifted for potentially toxic algae (planktonic cyanobacteria) for Lake Forsyth//Wairewa.

Mats of potentially toxic Phormidium species have been found in the Selwyn/Waikirikiri River at the Whitecliffs Domain and the Ashley/Rakahuri River near State Highway One.

People and animals, particularly dogs, should avoid these areas until the health warnings have been lifted.

Canterbury Medical Officer of Health Dr Alistair Humphrey says the algae look like dark brown to black mats and can produce toxins harmful to people and animals.

"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips," Dr Humphrey says.

"If you experience any of these symptoms, visit your doctor immediately, also let your doctor know if you've had contact with dark brown/black algal mats or water in these areas."

Reticulated town water supplies are currently safe but no one should drink the water from the river at any time.

"Even after boiling the water from the river, it does not remove the toxin, therefore it should not be consumed," Dr Humphrey says.

Pets should be taken to a vet immediately if they are showing signs of illness after coming into contact with algal mats.

Meanwhile recent water testing at Lake Forsyth/Wairewa has shown the quantity of potentially toxic algae (planktonic cyanobacteria) in the lake has decreased and concentrations are now below the levels that are of concern to public health.

Dr Humphrey says the Canterbury District Heath Board has lifted the algal bloom health warning issued at Lake Forsyth / Wairewa in October this year.

He says Environment Canterbury's (Ecan's) sampling of Lake Forsyth will continue on a monthly basis.

"The public will be informed if testing shows that concentrations have increased and there is a risk to public health again," Dr Humphrey says.

Facts about cyanobacteria:

  • In rivers; appears as dark brown/black mats attached to rocks along the riverbed and often has a strong musty smell

  • In lakes; if the water is cloudy, discoloured, or has small globules suspended in it, avoid all contact

  • The algae occur naturally but can increase rapidly during warmer months

  • Algal toxin concentrations can vary over short periods with changing environmental conditions

  • Although high river levels will remove the algal bloom, detached mats can accumulate along the shore and increase the risk of exposure to toxins

  • If a health warning is in place avoid contact with the water

  • Although district or city councils may place warning signs, these may not be seen at the numerous river access points, hence the need for people/ dog-walkers to treat every low-flowing river cautiously.

For further information visit http://ecan.govt.nz/services/online-services/monitoring/swimming-water-quality/Pages/Potentially-Toxic-Cyanobacteria.aspx

Or contact Community and Public Health on (03) 364 1777.


Page last reviewed: 25 November 2014