Reproductive factors are crucial in the aetiology of breast cancer.

Sir,Stewart et al (2000) propose that humans acquire mouse mammarytumour virus (MMTV) from mice. They advocate a viral aetiologyfor the high incidence of human breast cancer (HBC) and argueagainst other possible aetiological factors. However, their statis-tical analysis is simplistic and ignores social, cultural and demo-graphic variables that are known to affect the risk of breast cancer(Gilliland, 1997). Two countries, Algeria (low incidence rate andin the lands of M. domesticus) and Finland (high incidence rateand in the lands of M. musculus) contradict the viral theoryfor HBC. While the authors do not explain the findings forAlgeria they suggest cross-breeding between M. domesticusandM. musculusin Finland. We believe that socio-cultural and demo-graphic variables contribute significantly to the low incidence.We also note that in Finland, HBC was most common in highsocial classes throughout the period 1971–1995 (Pukkala andWeiderpass, 1999).It is well established that a woman’s reproductive history influ-ences her risk of HBC (Kelsey et al, 1993). Among reproductiveand hormonal factors, the most important known determinantsof breast cancer are late age at first birth and nulliparity, earlymenopause and use of hormone-replacement therapy. In Italy (aland of M. domesticus), the combination of risks associated with ahigh level of education, old age at first birth and nulliparity andolder age at menopause accounted for 51% of breast cancercases (Tavani et al, 1997). The lowest HBC incidence rate in theworld (South Korea, a land inhabited by other mice) was attributedto late age at menarche, early age at natural menopause, earlyage at first full-term pregnancy and larger number of full-termpregnancies (Suh et al, 1996).In Taiwan (a land inhabited by other mice) Chinese womenwere found to have lower incidence rates than white women of thesame area (Chie et al, 1995). A substantial increase in HBC risk inwomen who migrated from Asia to the USA was demonstrated,with the risk doubling during the first decade after migration.Increased use of contraceptive soon after migration to the USAcould possibly explain this rapid rise in risk (Ursin et al, 1995). Inthe USA (a high-incidence country) the incidence rates differedamong 25 counties in the San Francisco Bay area and correlatedwith the distribution of known risk factors (Robbins et al, 1997;Prehn and West, 1998). Moreover, Hispanic women living in theUSA have been shown to have the lowest incidence across mostgeographic regions of the USA (Jones et al, 1997).The incidence of breast cancer is increasing more rapidly insocieties that enjoyed a low incidence of the disease, such as mostAfrican countries. This is partly a result of the changing demo-graphic profile, acquisition of ‘Western’ lifestyle, and thechanging socioeconomic profile of the country (Adebamowo andAdekunle, 1999).The reported relation between M. domesticusand HBC suffersfrom ecological bias because it does not take into consideration thedensity of mice population and its correlation with HBC inci-dence-rate. It will be interesting to see whether this relation willhold after adjusting for both human reproductive factors and micepopulation density.SA Khuder, AB MutgiMedical College of Ohio, USA