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Keep clear of algae mats in rivers


With summer sun baking, river levels low and warm at this time of year, local rivers are fun and generally safe to swim in, but people should be aware of the risks to people and animals from toxic algae, warns Hawke’s Bay Regional Council and the Hawke’s Bay District Health Board.

River areas are also popular places to exercise dogs and owners need to be extra cautious in summer.  

While the specific cause of last week’s dog death at Pākōwhai Regional Park is unclear, it does emphasise the need for dog owners to look out for their four-legged family members.

During summer, Hawke’s Bay Regional Council staff monitor popular recreation sites weekly on the Ngaruroro, Tutaekurī and Tukituki rivers, but cannot check the whole length of rivers. People enjoying the rivers for recreation need to be aware of the variety of risks and take their own precautions.

In the height of summer, toxic blue-green algae can dry out at the side of rivers forming ‘mats’ as the water flows reduce. The toxin forming blue-green algae naturally occur in all rivers, stream and lakes.  Warm summer weather coupled with low river flows increases blue-green algal growth, sometimes making water unsafe for swimming in those areas and forming dark-coloured mats. Not all algae is toxic but blue-green algae, known as cyanobacteria, can be, even when dry.

These mats should not be touched, and people should avoid swimming where mats are present. The mats can vary in colour from brown/black when in the water to a pale brown/whitish colour when dry, and are identifiable by a strong musty smell which dogs will naturally want to investigate.  More detail on what the mats look like can be found at, search: #swimminginhb

Stuart Badger of Hawke’s Bay Vet Services in Hastings warns that dogs are particularly susceptible to the toxins compared to other stock.

“When you go near a river, take fresh water and a bowl so you can give your dog a good drink of water before they run around, so they are less tempted to drink from the river,” he says.

He says the main symptoms are gastro-intestinal, such as vomiting and diarrhoea with blood, or neurological, such as inability of the dog to use their back legs and shaking, although both ranges of symptoms can occur.  Any dogs or other animals that appear unwell or are sick after being in or by a river should get prompt veterinary attention.

Hawke’s Bay District Health Board Medical Officer of Health Rachel Eyre says people who come into contact with the toxic form of the mat may experience vomiting, diarrhoea, skin rashes and numbness or tingling around the mouth. Children are particularly at risk.  Anyone displaying signs of illness after being in a river bed should seek medical advice from a doctor immediately. 

The HBDHB Population Health services should be alerted in the event of health issues occurring after contact with river or lake water. The On Call Health Protection Officer can be contacted on 06 834 1815.


More information

Cyanobacteria in Hawke’s Bay – Information

Blue-Green algae/ Phormidium – Info sheet

11 January 2017

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