I received my copy of Veterinary Practice News today. It is the February issue, which is not yet available online, but one of the front page articles claims that a newly published study links increased longevity to dog's which retained their ovaries for at least 6 years. The actual study is available here. To be fair, Veterinary Practice News is not peer reviewed in any way, and is mostly an excuse to sell advertising. It is sent out for free to practicing veterinarians and is published in a tabloid format with articles of interest to veterinarians. The article in question is titled Study Links Ovaries and Longevity, and claims that their canine model compares well with humans in relation to longevity. The Skept Vet has an excellent summary of the benefits and risks of neutering (click through to download the PDF) which relates to this post, and helps to explain some of the problems I have with the article and the study.
the article itself is a typically hyperbolic report that suggests veterinarians should rethink their spay/neuter practices based on the results of this one study. Unfortunately, the paper draws nearly the same conclusions.
There are a couple of confounding factors which the authors claim to have accounted for, but may not have as well as they could have.
The main problem I have with the study is that it involves only one breed, and has relatively small sample sizes. The breed use for the study was Rottweilers, and the total sample size was 100 dogs of usual longevity and 83 of exceptional longevity. Rottweilers are a breed that has a high incidence of cancer, including osteosarcoma, the incidence of which may be strongly affected by the age at which a dog is spayed or neutered (see The Skept Vet summary linked above). Other cancers in rottweilers may also be affected by reproductive hormones in this way. Indeed, when all the dogs who died of cancer (73% of the usual longevity group and 32% of the exceptional longevity group) were removed from the study, the total sample sizes decreased to 27 and 53 respectively. with these small numbers, the authors still claim that retaining ovaries for six years or longer provided a protective effect.
It would appear that retaining ovaries may protect rottweilers from certain types of cancer that are very common in that breed, but it is not clear to me that simply keeping their ovaries was the only factor which resulted in increased longevity. The exceptional longevity group also averaged 5lbs lighter and a half an inch shorter than the usual group. This could reflect a protective effect of smaller size, or a difference in breeding which resulted in smaller size and also reduced susceptibility to certain diseases. The authors do not address these potential confounders, and present no data to suggest that the difference in size and weight could have been related to the age of spaying or genetic factors.
Before any claims such as those made in this paper, and especially in the Veterinary Practice News story are taken too seriously, they should be replicated in other breeds and larger populations. At the very least, the most common cancers and other causes of death are much different in rottweilers and humans. That alone should make the authors think the rottweiler model for human longevity may have some problems.