The 10-year-old dog was brought in last week to East Port Veterinary Clinic without symptoms, but needing some basic health checks.

It was much to the surprise of veterinarian Adriarne Heaton and her team when the animal tested positive for heartworm.

"The dog was completely asymtomatic, just here for a routine check-up," Dr Heaton said.

Neither she nor her colleagues could recall treating an animal for heartworm, and certainly not as far south as Port Macquarie.

The parasite tends to flourish in temperate climates, passed between animals by mosquito.

There is no way to tell if the mosquito is carrying the bug.

If untreated, the parasitic condition can lead to death by congestive heart failure.

Dr Heaton said owners should be aware dogs are more likely to get heartworm, though other animals including cats can also be targeted.

The variety of symptoms means there are numerous steps in diagnosis.

Blood is taken, urine samples are collected, an ecocardiogram can be done and scans of the chest viewed to check for pulmonary damage.

Even after all that testing, Dr Heaton said it still takes keen diagnostic skills to identify heartworm because "it often doesn't show in the early stages".

The dog brought into her clinic has since been transferred elsewhere for further treatment, Dr Heaton said.

The rare diagnosis should not alarm the community, but it should serve as a reminder.

"I know prevention is not cheap, particularly if you've got a larger dog" Dr Heaton said.

"But it really is the best way to keep your pets safe."

She said ensuring doses were up to date was particularly important for those animals whose treatment may have lapsed for more than four months.