Monday, May 3, 2010

HPV, smoking and cervical cancer

Everybody knows that smoking causes cervical cancer; that scientific studies have demonstrated a statistical association. The anti-smoker crowd has pronounced the evidence clear and unequivocal; beyond dispute. And, Health Canada concurs.

Or, at least they did as recently as 2007 when they released their latest Smoking Attributable Mortality report (based on StatCan mortality data from 2002). At that time Health Canada was claiming that 126 of 382 deaths (34.7%) from cervical cancer were caused by smoking. And, that statistic, to the best of my knowledge, has yet to be updated.

But, multiple studies, including one titled “Human papillomavirus is a necessary cause of invasive cervical cancer worldwide”, tend to disagree.

The study was published in PubMed in September, 1999 and stated: “Combining the data from this and the previous study and excluding inadequate specimens, the worldwide HPV prevalence in cervical carcinomas is 99.7 per cent. The presence of HPV in virtually all cervical cancers implies the highest worldwide attributable fraction so far reported for a specific cause of any major human cancer.”

What the study is saying is that, although both smoking and non-smoking women get cervical cancer, HPV is present in virtually all (99.7%) women who contract cervical cancer. And, the fact that at least one high risk strain of HPV (there's over 100) is present in virtually all women who contract cervical cancer clearly points to HPV, not smoking, as the sole cause of cervical cancer.

Another study, published in the Journal of Clinical Pathology in 2002, states: “The causal role of human papillomavirus infections in cervical cancer has been documented beyond reasonable doubt. The association is present in virtually all cervical cancer cases worldwide. It is the right time for medical societies and public health regulators to consider this evidence and to define its preventive and clinical implications. A comprehensive review of key studies and results is presented.”

And, unlike lung cancer, the link between cancer and HPV is not limited to a statistical association. It has been shown to cause cancer in animal experiments. A July 1999 article in Journal of Virology, titled “HPV is Both a Necessary and a Sufficient Cause of Cervical Cancer”, claims: “The human papillomavirus type 16 E6 gene alone is sufficient to induce carcinomas in transgenic animals.”

In fact, there are literally dozens of studies linking HPV and cervical cancer. You'll find links to a good number of studies, available on-line, at smokershistory. But, despite the abundance of evidence, the association between HPV and cervical cancer is not well known in the general population.

Part of the reason for that may be studies which attempt to perpetuate the myth that smoking causes cervical cancer.

At least one such study which tries to show that smoking causes cervical cancer appears to show the opposite. It states: “[C]urrent smokers with a high HPV-16 viral load had an increased risk of 27.0 (95% CI, 6.5-114.2) compared with current smokers without HPV-16 infection.” Huh?

Women who have HPV and smoke are 27 times more likely to contract cervical cancer than those who smoke and don't have the virus? Forgive my confusion, I'm not a scientist; but doesn't that point to HPV as the real risk? After all if both groups smoke and the HPV infected group has 27 times more cervical cancer . . .

The same study claims: “Within nonsmokers, however, high HPV-16 load contributed to only a 6-fold increased risk compared with HPV-16-negative nonsmokers at time of first smear.”

“Only” a six-fold increased risk? That's over 5 times the relative risk (1.9) attributed to smoking.

Again, I admit my ignorance. But, doesn't this mean that even non-smoking women with HPV are at a much higher risk of developing cervical cancer than non-smoking women without an infection? Doesn't this also point to HPV as the cause of the cancer rather than smoking?

In addition, this study considered only HPV-16 when there are numerous other types, which are known to cause cervical cancer, which were not tested for and may have been present. Remember, other studies have found 99.7 cases of cervical cancer had one strain or another of the virus present. HPV 16, a high risk strain, was merely the most prevalent.

Which brings us to today's question.

Are the anti-smoker zealots callous enough to downplay evidence which might point to the sole cause of cervical cancer (HPV), and possibly other cancers, because it would undermine their case against smoking? After all, HPV has been implicated in a number of different cancers, including lung cancer.

I'll explain in my next post why I believe some public health agencies may downplay the evidence to appease their more rabid anti-smoker colleagues.

1 comment:

Anonymous said...

It seems a shame that no one apparently comments on your posts. The last few have been particularly interesting and informative.
I recall some self righteous twat telling me in the 1970s that smoking caused cervical cancer, I can't remember the whole conversation, but I did remark that it was a bloody funny place to put a cigarette.