This August 28, 2007 CNN report warns that oral cancers are associated with HPV-16, and that as the number of oral sex partners a man has increases, the risk increases:

There are several strains of HPV, which cause ordinary warts but also genital warts. These in turn can cause cancer in some cases. The researchers looked at various studies and concluded that HPV 16 was especially likely to be linked with certain cancers of the tonsil and base of tongue.

Smoking is a well known risk factor but rates of these cancers are staying fairly steady, despite declines in tobacco use.

In one study cited by Sturgis and Cinciripini, Dr. Maura Gillison of Johns Hopkins University in Baltimore and colleagues studied 100 patients with oral or throat cancer and compared them with 200 healthy people. They found those who had six or more oral sex partners had a high risk of the cancer.

They found evidence of HPV-16 in 72 percent of the tumors.

U.S. health officials estimate that more than a quarter of U.S. girls and women aged 14 to 59 are infected with HPV.

This report from Journal Watch gives details on a similar study:

The investigators collected demographic information and determined HPV infection status in 100 patients with OPC and 200 age- and sex-matched healthy controls. Most patients (86%) were male. Family history of cancer (especially squamous-cell carcinoma of the head and neck), a personal history of oral papillomas, poor long-term oral hygiene, consumption of 15 or more alcoholic drinks per week for more than 14 years, and a smoking history of more than 20 pack-years were all associated with OPC. In a multivariate analysis, the following factors were also significantly associated with OPC: having had more than 26 lifetime vaginal-sex partners or more than 6 lifetime oral-sex partners and presence of serum antibodies to HPV-16b L1 capsid protein. Of 60 patients with available frozen tissue samples, 72% had HPV-16 DNA in tumor cell nuclei. Neither high tobacco use nor high alcohol use was an additive factor in HPV-induced OPC. The authors speculate that alcohol- and tobacco-induced OPC may follow a different pathway than HPV-16–induced disease.

Comment: The incidence of tonsillar and base-of-the-tongue cancers has been increasing since 1973. Understanding the contributing factors beyond the well-established ones of heavy alcohol and tobacco use is important. Warts have been implicated in oral cancer, and this epidemiologic and virologic investigation confirms an association between such cancers and HPV. The number of sexual partners appears to increase the odds of having an oral HPV-16 infection. Because most patients with this disease are males, protection during oral-genital sex is not straightforward. With a vaccine now available and effective in preventing HPV-16–associated cervical carcinoma, seeing whether the vaccine also decreases the incidence of oral and pharyngeal carcinoma will be interesting.

My guess is that this increase in cancers since 1973 is because of:

1. Increased number of sexual partners increasing the chance of running into someone infected with HPV-16. Yes, I know that before 1973, every adult didn't stay celibate until marriage, and remain faithful for life. But I don't think anyone seriously believes that fear of pregnancy (which had risks for both woman and man back then) and the prevailing notions of sexual morality didn't act as something of a constraint. Remember that the percentage of a population that will be infected with an STD increases exponentially with the number of different sexual partners. Double the number of different sexual partners over a particular period, and you octuple the number of STD cases.

2. My impression from reading literature of the period is also that oral sex became a more mainstream activity in the 1970s, perhaps because of the increased exposure to it in pornography and sex manuals.