Northland dog owners are being warned to be extra vigilant with their pets due to an outbreak of the deadly parvovirus.

Canine parvovirus, also known as parvo, is highly contagious and especially lethal for puppies.

Vet Rob Knott, of Bay of Islands Veterinary Services, said he normally treated about three cases a year but that had jumped to about three a month. He was convinced that was only the tip of the iceberg.

''It's a horrible, fatal disease. It's death by diarrhoea.''

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Parvo could affect any dog but it was more prevalent in puppies because their immune systems were not fully developed. Some owners believed keeping their puppy confined to their property was enough to keep it safe but that wasn't necessarily the case.

Parvovirus was so contagious, and so hardy, it could survive in the environment for a year or more and be carried on to a property on shoes or car tyres, or caught from objects such as bowls and bedding. It could also be caught from the street, especially in urban areas where there were lots of dogs.

So far all the puppies brought in to Bay of Islands Vets had survived but only though intensive treatment in an isolation unit.

It was far better to prevent the disease in the first place by vaccination, Knott said.

The first shot could be given at 6-8 weeks with one or two boosters by the age of 14-16 weeks.

While it was hard for some families to afford vaccination, typically around $60 a shot, the cost of treating a critically ill puppy could easily top $1000.

The virus could affect any type of dog but larger breeds such as staffies, pitbulls and bull terriers seemed to be worst hit.

Parvo was more common in Northland than many other parts of the country, which Knott put down to higher numbers of unwanted or homeless dogs making it easier for the virus to replicate and spread.

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Parvovirus attacks a puppy's white blood cells, severely weakening their immune systems, and the lining of their intestines. The symptoms can vary but typically start with lethargy and loss of appetite, later progressing to severe vomiting and bloody, foul-smelling diarrhoea, which can cause life-threatening dehydration.

Treatment is by administering lots of fluids, antibiotics and anti-inflammatories. Once a puppy is infected it's too late to vaccinate.