Cats do not ‘spread brain tumours’

No need to worry

As anyone who has lost a loved one to a brain tumour knows, the disease can be devastating.

So headlines claiming that cats may be responsible for brain tumours – based on limited and tentative evidence – are not only misleading but also deeply insensitive to patients and their families.

Despite recent advances in treating brain tumours, we’re still largely in the dark about the causes of these cancers. This is partly because brain tumours are relatively rare, so studying their causes is much harder than for more common cancers, such as lung, bowel or breast.

Investigating the cause of a disease often involves delving into large sets of data and look for patterns. Sometimes these patterns turn out to be the cause itself – such as the well-known link between smoking and lung cancer. Other times it’s something else – a relationship between two things that simply hints at an underlying cause, such as the link between the age a woman starts her periods and her risk of breast cancer (which is probably related to overall levels of hormones over her lifetime).

Either way, finding a link is just the first step on a long road: a promising clue, which may – or may not – yield further clues on further investigation. History is littered with examples of ideas that don’t hold up once further evidence is gathered. But this doesn’t mean doing this research is pointless – rather, that’s exactly how science works.

It was looking for these sorts of patterns – these ghostly signals of causation or correlation – that led a team of French scientists to investigate links between rates of a common infection, Toxoplasma gondii – which is spread by eating infected meat or by handling cat faeces – and rates of brain tumours.

Publishing their latest results in the Royal Society’s journal, Biology Letters, they found that countries that reported higher brain tumour rates also tended to have higher rates of T. gondii infections.

This is an intriguing morsel of evidence. But it certainly doesn’t mean, as many newspapers have unfortunately stated, that ‘cats may be spreading brain cancer to their owners’, nor that infected cats ‘could almost double their owner’s chance of developing brain cancer’.

So what’s the real story behind the headlines? And should cat lovers be concerned?

What do we know about brain tumours?

There are many different types of brain tumour, often with long, tongue-twisting names like astrocytomas, oligodendrogliomas, or primitive neuroectodermal tumours.

With all these different forms, it’s impossible to assign one over-arching cause to ‘brain cancer’, and its likely that they’re triggered by different things. But there are a few things we do know – increasing age, certain inherited gene faults, and increased exposure to X-rays can all increase the risk.

There’s also some evidence to suggest that people with weak immune systems are more likely to develop brain tumours. This is important and we’ll come back to it in a minute.

As well as these definite risks, there have also been a number of long-running, high-profile controversies over other causes of brain cancer; most notably mobile phones, but the spotlight has also shone on hair dyes, cured meats, and power lines. None of these has been conclusively proven, and they’re unlikely to play a significant role in the disease.

And finally, there have been hints of a link between certain brain cancers and infections.  Several studies have found a link between glioblastoma multiforme and a common virus called cytomegalovirus , but others have found no link.

So what about this cat parasite?

Toxoplasma gondii is a parasitic infection that reproduces in cats, but can infect other mammals, including humans, causing a condition called toxoplasmosis. According to the NHS website:

Up to half of the UK population will have a toxoplasmosis infection at some point in their lives. Once infected, a person is then immune from further infection for life.

Toxoplasmosis can cause mild flu-like symptoms, such as a high temperature and muscle aches, but these will usually pass without treatment after a few weeks.

The NHS link above is well worth a read for more info about the parasite and the disease it causes.

What about the new research?

Our former colleague Ed Yong has written an excellent and comprehensive article about the new research on his blog, Not Exactly Rocket Science. As he explains:

[The researchers] compared national figures from 37 countries and found that brain cancers are more common in countries where T.gondii infections are more common. Brain cancer was 1.8 times more common in countries where T.gondii was most common, than in those where it was virtually absent.

However, these countries also differ in other important ways. Wealthier countries have more sophisticated technology that is better able to detect brain cancer, as well as more accurate cancer registries. To account for that, [the researchers] adjusted their results for national wealth, along with latitude (T.gondii is more common in the tropics), and cell phone use (which has been spuriously linked to brain cancer). None of these adjustments changed the link between T.gondii infections and brain cancer rates.

But, as Ed goes on to point out, the big problem with drawing conclusions from this is that the study is an ‘ecological’ study, “one of the weakest designs in the hierarchy of medical research”, and says nothing about the risks of individual people living in the countries examined in the study.

So why did they find a link?

One obvious thing to say is that it’s equally plausible that brain tumours cause an increase in T. gondii infections – i.e. the exact opposite of what the media reported.

As we said above, brain tumours are more common in people with weakened immune systems. These people could just be more susceptible to parasitic infections, which would neatly explain the link.

It’s also possible that, as with reproductive history and breast cancer, the infection is a ‘marker’ for something else. After all, millions of people in the UK have been infected with T. gondii, yet fewer than 5,000 people each year develop a brain tumour. Even if the link stands up to future research there would have to be another factor at play – possibly genetics, or maybe another infection.

A word about the headlines

As we’ve seen, this is a preliminary expedition into the relatively uncharted territory of brain tumour risk. Understanding what causes this varied set of cancers is a problem that needs urgent attention, but because of the complexities of the cancers – and their relative rarity – progress is slow and painstaking.

This research paper builds on previous studies, but is categorically not a definite link by any means. But reading many of the news reports, you would be forgiven for thinking the science was more certain than it is.

For example, the media’s claim that infected cats ‘could almost double the risk’ of brain tumours is plain wrong. The ‘doubling’ (actually a 1.8-fold increase) refers to the difference between nations, not between people with and without cats.

But more importantly, pointing the finger at the family pet over something as heart-wrenching as a brain tumour, in our view, fundamentally misrepresents the state of the evidence, and does a grave disservice to people worried about why they or their loved ones have the disease.

For the record, here’s the statement we issued to one media outlet (at their request) last night, but which didn’t even make the “19^th paragraph”.

“As the researchers admit themselves, this study does not prove a link between T. gondii infection and brain cancer – it merely highlights a statistical link that requires further investigation. Cat lovers should not be at all alarmed by these findings.”

The idea of ‘cancer-causing cats’ makes a great headline – but we question the decision to cover the story in this way, and fail to see how this helps cancer patients and their friends and relatives make sense of this terrible disease.

Henry

Reference:

Thomas, F., Lafferty, K., Brodeur, J., Elguero, E., Gauthier-Clerc, M., & Misse, D. (2011). Incidence of adult brain cancers is higher in countries where the protozoan parasite Toxoplasma gondii is common Biology Letters DOI: 10.1098/rsbl.2011.0588