Tobacco smokers are more likely to test positive for a type of oral human papillomavirus, HPV-16, than non-smokers, a study has found.
As few as three daily cigarettes is enough to increase the HPV infection risk by one third, or 31%, because of the elevated levels of cotinine – an alkaloid found in tobacco.
Human papillomavirus is the term for a group of viruses that affect the skin and mucous membranes affecting the cervix, anus, mouth and throat. Oral HPV-16 is a sexually transmitted virus that can trigger mouth and throat cancers. In recent years, the US has experienced an increase in the number of throat cancer cases caused by HPV-16.
"We saw a very strong association between higher levels of tobacco use and increased oral HPV prevalence across each of the biomarkers we evaluated," said senior author Gypsyamber D'Souza, of the Johns Hopkins Bloomberg School of Public Health.
D'Souza added that the researchers also found low levels of tobacco use, such as casual use cigarettes or second-hand smoke, can increase risk of HPV-16.
The researchers studied data on 6,887 adults aged between 18 and 59 who had been tested for HPV infection, alongside their recent nicotine use and blood and urine samples. The data had been taken as part of a national survey from 2009 to 2012.
Just under one third of the group were current tobacco users, who were more likely that non-users to be younger, less-educated and male, with a higher number of lifetime oral sexual partners.
Around 2% of the tobacco-users had the infection, compared to less than 1% of former tobacco users, or those who had never smoked.
Although infection with oral HPV-16 virus does not necessarily mean an individual will develop cancer, D'Souza said the increase in infection risk represented a significant difference.
The researchers emphasised that the link between HPV-16 and tobacco use was correlative, not causative.
Researcher Dr Carole Fakhry said the cause for the link is not yet known, but it is possible that tobacco may suppress the immune system and make it harder for the body to fight off the infection.
"This suggests the tobacco may have a role in in why they may be unlucky enough not to have cleared the infection," Fakhry said.
HPV is the most common sexually-transmitted infection in the US. In the UK, around half of the population will be infected at some point in their lives.
Xavier Bosch, a cancer epidemiology expert at the Catalan Institute of Oncology in Barcelona, who was not part of the study, told Reuters that smokers tend to have more sexual partners and risky sexual practices than non-smokers.
Yet D'Souza said this theory does not fully explain the connection, as there was still a link between tobacco and HPV-16 infection when sexual behaviour was accounted for.
The research was published in the online journal JAMA.