Cancer of the cervix and the papilloma viruses.

Squamous cancer of the cervix depends upon infection by Human Papilloma Virus (HPV), of which there are many strains. Some are more dangerous than others and they appear to compete with each other for "territory". The prospective use of vaccines and antiviral agents for HPV infection could disturb the balance of this ecosystem. This study derives the expressions for the prevalences of the different strains at equilibrium. The important parameters of these expressions are 1) the rate of change of sexual partner, 2) infectiousness on contact, and 3) the rates at which an infectious/immune phase decays to a non-infectious/immune phase, and then towards partial or full susceptibility to re-infection. The responses to changes in these parameters are investigated. Analysis shows that the balance between competing strains is locally and precariously stable; it can survive moderate inter-strain variations of the transmission and decay parameters; but larger differences, whether natural or artificially induced, can result in the rapid elimination of the disadvantaged strain. This might be exploited in a preventive programme. Conversely, if a harmless strain were eliminated, the harmful strains would fill the territorial gap, and the incidence of cervical cancer might increase.