Sex with animals tied to penile cancer: What explains link?

(CBS) Doctors have identified several risk factors for cancer of the penis, including poor hygiene and infection with human papillomavirus, or HPV. And now urologists from Brazil have fingered another potential cause of penile cancer:

Having sex with animals.

To reach that rather off-color conclusion, the urologists used questionnaires to study 492 Brazilian men between the ages of 18 and 80, including 118 who had penile cancer. One hundred seventy-one of the men acknowledged having sex with animals, including 44.9 percent of penile cancer patients and 31.6 percent of the healthy men.

The study found no association between penile cancer and the number of animals the men had "used," according to LiveScience.com. Nor was there any link between penile cancer and the species of the animals, which included mares, pigs, chickens, and cows.

The research - published in the Journal of Sexual Medicine - didn't prove that sex with animals causes penile cancer, only that "SWA" was associated with the malignancy. But the study's lead author suggested that abrasions of the penis sustained during SWA might explain the link.

"We think that the intense and long-term SWA practice could produce micro-traumas in the human penile tissue," Stenio de Cassio Zequi, a urologist in Sao Paolo, told LiveScience. "The genital mucous membranes of animals could have different characteristics from human genitalia, and the animals' secretions are probably different from human fluids. Perhaps animal tissues are less soft than ours, and non-human secretions would be toxic for us."

But that's not the only possible explanation. The study also fingered smoking, having sex with prostitutes, precancerous conditions of the penis, being nonwhite, and phimosis (a condition in which the foreskin cannot be fully retracted) as risk factors for penile cancer.

The men in the study were from rural areas. But if you assume sex with animals, a.k.a. zoophilia, is something only poor country bumpkins do, you're in for a surprise. According to Live Science, a 2003 study published looked at 114 zoophiles in the U.S., Australia, New Zealand, and Europe showed that 36 percent lived in large cities and 83 percent had completed at least some college - and many reported high incomes.

The National Cancer Institute has more on penile cancer.