Should we curb our appetite for beef?

ResearchBlogging.orgPolyomaviruses in beef: virology of new virusesIn 2012, Nobel Prize winner Harald zur Hausen noted the volume of research suggesting a causal link between beef consumption and colorectal cancer. This disease predominantly occurs in high-income countries where Western-style diets incorporate large amounts of red meat. Meanwhile, colon cancer is almost completely absent in India, where beef is largely avoided for religious reasons. Japan and Korea only began importing significant amounts of beef after World War II and the Korean War, at which point a significant increase in colon cancer rates occurred.

On the other hand, other studies have found that the risk of colon cancer is actually slightly higher for vegetarians than those of us who eat meat. What, then, is the secret behind the relationship between beef and the third most common type of cancer in the UK? We spoke to Dr Christopher Buck from the US National Institutes of Health in Bethesda, Maryland to find out more.

Despite his self-professed love for a good steak – as he says, “I’m 5% Neanderthal…I swear it’s in my genes, this love for large ruminants” – Dr Buck is leading studies to understand how consuming beef might be harmful to us. His research focuses on polyomaviruses, a group of small, DNA-based viruses that infect a range of animals. Some polyomaviruses, such as raccoon polyomavirus and the recently discovered Merkel Cell Polyomavirus, which infects humans, are known to cause cancer in their native hosts.

In a new paper recently published in the Journal of General Virology, Dr Buck and his colleagues describe three polyomavirus species that they isolated from ground beef bought at American supermarkets. One of these viruses, bovine polyomavirus1 (BoPyV1), was previously known to science: it is present in foetal calf serum, a material widely used in laboratories to grow cell cultures. The presence of BoPyV1 in foetal calf serum was noted as early as the 1970s, but Dr Buck suggests that this fact may be widely disregarded today despite the risk that many cell cultures could be contaminated with the virus as a result.

Bovine polyomaviruses type 2 and 3 (BoPyV2 and BoPyV3, respectively), meanwhile, are both newly discovered viruses. BoPyV2 is closely related to Merkel Cell Polyomavirus, which was only discovered in 2008 and is carcinogenic in humans. It is also a close relative of raccoon polyomavirus, which causes brain cancer in raccoons. However, we do not yet know whether BoPyV2 is carcinogenic to cattle – or to humans.

BoPyV3 is related to two polyomaviruses named human polyomavirus 6 and 7, respectively, which had been discovered by Dr Buck and his team in an earlier study.

Are these viruses carcinogenic?
The first question that comes to mind when thinking about potentially carcinogenic viruses is ‘Can these viruses cause cancer?’ In the case of BoPyV1, 2 and 3, the short answer is: ‘We’re not entirely sure.’ Currently, no formal research has been undertaken to understand the exact relationships between these viruses and bowel cancer risk.

However, there is a chance that the new viruses might increase the risk of bowel cancer. Like all polyomaviruses, these viruses share a gene called large T antigen, where the T stands for ‘tumour’. Dr Buck describes this gene as the ‘Swiss Army Knife of oncogenes (genes that cause cancer)’ because it can turn a range of different cell types cancerous.

The reason polyomaviruses are dangerous is that they are so well adapted to their native host species. To understand why this is, let’s take a step backwards and look at the evolution of polyomaviruses. They replicate with very low rates of mutation, meaning that they evolve slowly – at a similar rate to their native host species – despite the fact that viruses generally evolve and change more quickly than larger organisms such as mammals. This slow process of evolution in polyomaviruses makes them poorly suited to transmission between different species of host animals. Instead of killing cells in a non-native host outright, polyomaviruses attack these cells ineffectively, which may turn the cells in the non-native host cancerous. Therefore, even if bovine polyomaviruses do not cause cancer in cattle, they might nevertheless be considered a risk factor for humans.

Raccoon polyomaviruses (RacPyV1) may lend further credence to the hypothesis that polyomaviruses are simply not suited to infecting new species. Unlike most other polyomaviruses, RacPyV1 frequently causes brain cancer in its native host. Dr Buck suspects that RacPyV1 may have been transmitted to raccoons only very recently from a different species of carnivore. The virus does not yet appear to have become specialised enough to consistently kill raccoon cells, causing cancer in the animal instead. Indeed, RacPyV has only been found in California, an area where raccoons have been present for just a few decades. During their migration from the southwestern USA, raccoon populations may have been exposed to a polyomavirus from a closely related species such as the coatimundi.

So while no studies have yet been conducted to assess whether BoPyV1, 2 and 3 are carcinogenic to humans, there is a chance that they might be.

How can we protect ourselves?
Heating polyomaviruses to 85˚C in the laboratory has been shown to lower their infectivity by up to 1,000 times, considerably reducing the risk of infection. However, even ground-meat products and well-done steaks aren’t usually prepared at such temperatures.

Despite the fact that the three polyomaviruses Dr Buck and his colleagues discovered in beef could be carcinogenic, he does not see this as a reason to stop eating beef or to change how we cook it. Overall, bowel cancer ranks among the top three types of cancer in terms of lifetime risk for both men and women. There are also several types of colon cancer tests that can help ensure that any malignancies are discovered as early as possible.

Colonoscopies are the traditional method of bowel cancer screening, and they continue to work well – but in the USA, this form of bowel cancer screening is only routinely available to people over 50 years of age. ‘Occult blood tests’, which can discover trace amounts of blood in stool, are a common alternative screening method.

More recently, studies have found that bowel cancer causes a characteristic pattern of genetic ‘switches’, and state-of-the-art tests have been developed to detect bowel cancer these changes.

Dr Buck notes that the ideal scenario for humans would be if one of the bovine polyomaviruses turned out to cause disease in cattle rather than humans. This would create an incentive for farmers to implement large-scale vaccination schemes for their cattle, which would drastically lower the likelihood of the viruses reaching humans.

Before focusing on polyomaviruses, Dr Buck’s research focused on a different group of viruses called papillomaviruses. He was involved in the development of tools to isolate and analyse papillomaviruses, and found that these methods could easily be adapted to work for polyomaviruses as well.

While Dr Buck and his colleagues have not yet formally described or cultured the two new polyomaviruses, they aim to do so in the near future. It is important to stress that no formal recommendations can be made based on the researchers’ findings, and neither Dr Buck nor I intend to stop eating beef. Much more research is needed to better understand how beef consumption affects our risk of getting bowel cancer, and how prevalent polyomaviruses are in beef more generally. It is also important to address whether these viruses are carcinogenic in cattle – at which point they would become a veterinary hazard and large-scale cattle vaccination schemes could be considered. This would benefit human consumers, as well, since the virus would not make it into the meat we eat.

Jon Fuhrmann

Peretti, A., FitzGerald, P., Bliskovsky, V., Buck, C., & Pastrana, D. (2015). Hamburger polyomaviruses Journal of General Virology DOI: 10.1099/vir.0.000033

Image credit: Kiran Foster on Flickr under CC BY 2.0
This entry was posted in Epidemiology, Food Microbiology, Virology and tagged , , , , , , , . Bookmark the permalink.

One Response to Should we curb our appetite for beef?

  1. Pingback: Links to go viral – 11th January 2015 | about the little things

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