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, streams 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.
Hawke's Bay Vet Services' Stuart Badger warns that dogs are particularly susceptible to the toxins, compared with 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.
- 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 via 834-1815.
