Stressing about your dog barking at the neighbours or your cat tearing up the couch might increase your blood pressure, suggests a new Australian study, challenging previous evidence that owning a pet lowers blood pressure.

The study, by Dr Ruth Parslow and Dr Anthony Jorm of the Australian National University in Canberra, is published in this week's issue of the Medical Journal of Australia (MJA).

"We found in this study pet ownership was associated with poor health," Parslow told ABC Online.

"We have no doubts that carefully selected and cared for pets can provide many emotional benefits for humans," she says. "Well trained pets going in to nursing homes can offer company and pleasure."

"But there is a whole base of other factors tied into having a pet, especially being responsible for a pet, and those factors may negate all the benefits you might get just being an aunt or uncle to a pet."

The researchers examined the relationship between pet ownership and blood pressure in 5079 adults in the 40-44 and 60-64 age groups in the Australian Capital Territory. Even after adjusting for factors known to increase risk to the heart, such as heavy drinking, smoking and lack of exercise, they found no evidence that pet ownership was associated with cardiovascular health benefits. They even found a significant increase in diastolic blood pressure - measured as the heart muscle relaxes after a contraction.

The researchers also found that pet owners had less education, higher body mass index and were more likely to be smokers. Pet owners in the 40-44 age group were more likely to undertake mild physical exercise, but this did not offset their higher level of blood pressure.

Parlsow said the increased health risk seen in the study was probably not directly linked to pet ownership, but could be due to differences in stress levels, diet or even genetic factors between pet owners and non-pet owners: "We're not saying that pet ownership causes it [high blood pressure]. It's reasonably unlikely that pet ownership makes your health worse, but it's not necessarily evident that is makes it better."

The findings contradict those of a previous 1993 Australian study by Dr Warwick Anderson and colleagues from the Baker Medical Research Institute. Anderson and team looked at 5741 voluntary participants in heart screening in Melbourne, and found that pet owners had lower systolic blood pressure - measured during a heart's contraction.

"These findings have been widely referenced in the lay media as support for the hypothesis that pet ownership per se reduces cardiovascular risk. We did not find such an association," say the researchers.

Parslow suggests that the different findings may be due to the fact that she used Medicare data to randomly select study participants whereas Anderson used participants who self-reported. She says the percentage of pet owners in Anderson's study and the percentage in the broader population were very different, and thus may not be representative of pet owners in the broader community.

In an accompanying editorial in the MJA Professor Bruce Headey of the University of Melbourne comments: "The state of the debate is that pets probably do confer health benefits, but we don't know precisely how."

"At a fundamental level, the benefits of pets appear linked to the human desire to be close to nature and other living creatures," writes Headey. "About 50% of adults and 70% of adolescents who own pets report that they confide in them. It is most unlikely that all this communication and companionship is wasted."