About 40 per cent of breast cancer strains may be transmitted from human-to-human through saliva.
An analysis of studies done in 83 years across the world, including Kenya, concluded there is enough evidence to prove some breast cancers are caused by a virus linked to the domestic rat.
Scientists at the University of New South Wales, Australia also say the virus may be transmitted from human to humans through saliva as well as from dogs to humans.
“In our view, there is now sufficient evidence to conclude that a mouse mammary tumour-like virus (MMTV-like) is probably causal in a significant number of breast cancers,” says the review covering the period between 1936 and this year.
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Prevalence
MMTV was first discovered in the milk of rats in 1936 and found to cause tumours in pups suckled by cancerous mothers.
Since then studies have shown women with breast cancer are likely to have higher evidence of the virus in their body tissue compared to those without breast cancer.
Among studies that the authors have relied on to reach the conclusion that the mice virus is responsible for up to 40 per cent of breast cancers was one carried out in Kenya in 1981.
The study published in the Proceedings of the National Academy of Sciences of US had assessed the prevalence of MMTV in women with and without breast cancer in Kenya, Asia and the US.
The study found highest signs of the mouse virus in breast cancer patients from Kenya, 13 of 21 patients or about 62 per cent. Twenty seven out of 145 or 18.6 per cent of patients from the US had signs of the virus and 20 of 53 patients from Asia or about 38 per cent.
Among healthy women - without breast cancer - only seven of 26 or 27 per cent of Kenya women had evidence of the virus.
One out of 22 healthy women from Asia or 4.5 per cent had signs of the virus and one out of 36 healthy American women or 2.8 per cent.
Explaining the high levels of women found with the mouse virus in Kenya compared to women from the US or Asia, the authors suggest different strains of the disease, genetics or environment factors.
But they also say population in poor countries like Kenya may be more exposed to domestic rats compared to those in developed countries.
The house mouse is common in most households in Kenya, where they urinate and defecate on uncooked food.
Rats are also common in shops and supermarkets which stock cereals and other products that are eaten uncooked by humans.
Present in human milk
Because MMTV is endemic in many rodent populations, transmission by consumption of uncooked cereals and other foods is possible, says the study.
“It has been proposed that the higher the prevalence of the house mouse, Mus domesticus, the higher the prevalence of human breast cancer,” says the new study published in the journal Nature early this month (7th November 2019).
This, put together with other similar studies, the Australian team says leaves little doubt that many cancers are caused by the mice virus.
In women with breast cancer the presence of the mice virus is five times higher than in normal women.
Further the study says women with the virus can be identified between one and 11 years before the cancer develops, indicating a big opportunity for stopping the disease.
Evidence of the virus is also present in human milk from normal lactating women but at much higher prevalence in milk from women at risk of breast cancer.
“The evidence meets the classic causal criteria. The evidence with respect to MMTV being responsible for approximately 30–40 per cent of human breast cancers is compelling,” wrote James S Lawson and Wendy K Glenn.
The authors say that the virus is also present in dogs, cats, monkeys, mice and rats.
“Saliva has been identified as the most plausible means of transmission from human to human and possibly from dogs to humans,” says the study published in partnership with the Breast Cancer Research Foundation of Australia.
