Benji, how old is your bitch?
Recent studies have found that the ovaries in bitches provide protective effects for the bitch, resulting in increased longevity (in the same way that women tend to live longer than men, in our species). No one knows exactly how the ovaries function in this way; more research is needed - but findings indicate that bitches who have remained intact until age 6/7 yrs are much more likely to live an extremely long life than bitches spayed below 4 yrs.
The research for this is here:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2805875/?tool=pubmedHere are some significant quotes:
Like women, female dogs were more likely than males to achieve exceptional longevity (OR, 95% CI = 2.0, 1.2-3.3; P= 0.006). However, removal of ovaries during the first 4 years of life (i.e. median age at ovariectomy) erased the female survival advantage over males (OR, 95% CI = 1.2, 0.7-2.2; P= 0.55). In females that retained their ovaries for more than 4 years, likelihood of exceptional longevity increased to more than three times that of males (OR, 95% CI = 3.2, 1.8-5.7; P< 0.0001).
...dogs with the longest ovary exposure (6.1-8.0 years) were 3.2 times more likely to reach exceptional longevity than dogs with shortest exposure (P= 0.002) (Table 2; Supporting Fig. S1). ...
...After excluding all cancer deaths, females who kept their ovaries during the first 7 years of life (i.e. highest tertile of ovary exposure) were more than nine times more likely to reach exceptional longevity than females with shortest ovary exposure (P= 0.001)...
... In a population of 237 female Rottweiler dogs who died at 1.3-12.9 years, females that had intact ovaries for the first 4.5 years of life had 37% lower mortality than females that underwent elective ovariectomy before 4.5 years, i.e., median age at ovariectomy (hazard ratio, 95% CI = 0.63, 0.49-0.82; P < 0.0001 log-rank test). ...
...we observed a robust ovarian association with longevity that was independent of cause of death, suggesting that a network of processes regulating the intrinsic rate of aging is under ovarian control. ... "
What I draw from this paper is that we are not yet clear exactly when is the 'optimal' time to spay, but it appears to be some time between 4.5 yrs and 8 yrs. The longer you can wait within that time-frame, the more protective 'ovary exposure' your dog will receive, and the better her chances of living a long life.
The risk of pyo, although it exists, is not massive for a bitch under the age of 8yrs. Personally, I would want to spay at age 6/7 yrs to prevent pyo, whilst also ensuring maximum exposure to ovaries for the bitch concerned.