[Note: this post was originally published at the lovely Julie Hecht's Dog Spies blog at Scientific American.]
The 2013 paper
looked at Golden Retrievers. The authors reviewed data from veterinary
hospitals, comparing Goldens who were diagnosed with various diseases,
those who were not, and the spay/neuter status of each group; they found
a correlation between spaying or neutering and cancers such as
osteosarcoma, hemangiosarcoma, and mast cell cancer. The 2014 paper
used a voluntary Internet-based survey to perform a similar
investigation in the Vizsla breed. They also found correlations between
spay/neuter status and mast cell cancer, hemangiosarcoma, and lymphoma.
These are scary results, but I caution that studying the causes of
multi-factorial diseases like cancer is incredibly challenging. Take the
Golden Retriever study, a retrospective study using data from a
veterinary referral hospital. This study was limited to dogs whose
owners chose to bring them to a relatively expensive referral hospital.
This is the kind of place where you take your pet when he has cancer and
you are willing to spend a fair amount of money to help him. As a
result, this hospital’s records probably provide a great source of data
on companion animals living with concerned owners, particularly owners
who have provided excellent medical care for much or all of the animal’s
life. However, this hospital’s records are less likely to provide data
on animals whose owners have provided sub-optimal care. This kind of
bias in sample selection can have a significant effect on the findings
drawn from the data.
The Vizsla study used an Internet-based survey instead of hospital
records. Like the Golden Retriever study, this study could have found
itself with a biased sample of very committed dog owners, in this case
owners who engaged in dog-focused communities online and who had enough
concern about the health of the breed to fill out a survey. This study
additionally suffered from a lack of verified data; owners were asked to
give medical details about their dogs and may have misremembered or
misinterpreted a past diagnosis.
Don’t get me wrong – these were both important studies, and they did
their best with the available resources. I applaud both sets of authors
for putting this information out there. But the studies both have their
limitations, which makes their findings difficult to trust or generalize
to other populations of dogs.
Meanwhile, another 2013 study
presented some other interesting results. This study drew data from
multiple referral hospitals to determine the causes of death in spayed
or neutered versus intact dogs – and they found that spayed and neutered dogs, on average, lived longer
than intact dogs. Intact dogs were more likely to die of infectious
disease or trauma, while spayed or neutered dogs were more likely to die
of immune-mediated diseases or (again) cancer. In other words, while
spayed or neutered dogs did get cancer, it didn’t seem to shorten their
This study shed a new light on the cancer question. It suggested that
perhaps spayed or neutered animals might be more likely to get cancer
simply because they were living long enough to get it. Intact animals were more likely to die younger, perhaps simply not aging into the time of life when the risk of cancer rises.
So where does that leave us? Is there a causal link between
spaying/neutering and cancer? I think the question is still wide open.
What we really need is a study that follows animals forward throughout
their lifetimes instead of using retrospective records or surveys to get
the data – and, thanks to Morris Animal Foundation’s groundbreaking Golden Retriever Lifetime Study,
we are getting just that. This study is enrolling Goldens as puppies
and following their health over the course of their lives. It will be
years before the study gives us answers, but it provides hope for more
solid data. (Of course, it still can’t address the issue of bias, in
that owners who enroll their puppies in this study could be highly
responsible dog owners who provide excellent medical care!)
We can, however, do something about cancer in dogs without waiting
for the results of that study. It is no coincidence that two of the
studies discussed here investigated Golden Retrievers. Sixty percent of
Golden Retrievers will die of cancer. That is indisputably a problem
with the genetics of the breed, and other breeds suffer from similar
problems. We should be attacking cancer on all fronts, and this is a
front we don’t have to study first. Golden Retriever breeders are
between a rock and a hard place, trying to breed for health in a gene
pool which doesn’t have enough genetic diversity to support it. The
solution is to bring in new blood from gene pools with much lower risk
of cancer, breeding dogs who don’t look like purebred Goldens for a few
generations to revitalize the breed as a whole. Genetics contribute far
more to risk of cancer than whether an animal is spayed or neutered. We
clearly have a strong desire as a society to reduce the incidence of
cancer in Golden Retrievers and other breeds. While we’re studying risk
from spaying and neutering, let’s address the genetics question that we
know we can fix.
Image: Rob Kleine, Golden Retriever, Flickr Creative Commons License.
Torres de la Riva G, Hart BL, Farver TB, et al. Neutering Dogs: Effects
on Joint Disorders and Cancers in Golden Retrievers. PLoS ONE 2013. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0055937
Zink MC, Farhoody P, Elser SE, et al. Evaluation of the risk and age
of onset of cancer and behavioral disorders in gonadectomized Vizslas.
Journal of the American Veterinary Medical Association 2014;244:309–319.
Hoffman JM, Creevy KE, Promislow DEL. Reproductive Capability Is
Associated with Lifespan and Cause of Death in Companion Dogs. PLoS ONE