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Pet owners and lymphoma

Thursday 9 October 2008

“Owning a pet can reduce the chances developing a form of cancer by nearly a third, researchers claim,” the Daily Mail reported. It said a study of 4,000 US patients found that those who owned a pet were less likely to develop non-Hodgkin’s lymphoma, a cancer of the immune system. It also claimed that the longer families owned a pet, the lower the risk. It said that the scientists behind the study believe that pets help protect against the cancer by boosting the immune system.

This study supports the theory that altered immune function could lie behind this particular cancer. The authors call for further investigation into a possible link, including pooling the results of all known studies that have measured exposure to pets. As the researchers point out, there are likely to be several contributing factors to developing non-Hodgkin’s lymphoma, and studies of this of this type cannot prove causes of conditions like non-Hodgkin’s lymphoma. Based solely on this study it is not possible to say conclusively that pet ownership cuts cancer risk, as has been reported.

Where did the story come from?

Dr Gregory Tranah from the California Pacific Medical Center Research Institute, along with colleagues from university departments in San Francisco, California carried out this research. The study was supported by grants from the National Cancer Institute and the National Institutes for Health. The study was published in the peer-reviewed medical journal, Cancer Epidemiology, Biomarkers and Prevention.

What kind of scientific study was this?

This was a population-based case control study in which the researchers aimed to assess the link between animal exposure and non-Hodgkin’s lymphoma (NHL).

To date, there have been few risk factors identified for this type of cancer, and previous research has focused on environmental factors that may have contributed to the increase in NHL rates. These studies have examined relationships between NHL and viral, chemical, lifestyle, and occupational exposure.

Using data from a previous study of people diagnosed with NHL living in the San Francisco Bay area between 1988 and 1993, the researchers conducted interviews with 1591 patients (cases). This represents 72% of eligible patients. They then used random telephone dialling to identify 2,515 controls, i.e. people from the same county of residence who were matched for sex and age with the cases (78% of those contacted agreed to participate).

They then conducted personal interviews in the volunteers’ homes and asked them about a range of factors, including their occupation, use of therapeutic drugs, immunisations, allergies, viral infections (including HIV), and lifestyle. They asked specific questions about farming, agricultural work with animals, and pet ownership. These queries related to activities up until one year before diagnosis in the cases, or one year before the interview for the controls. The participants were also tested for HIV.

The researchers excluded those known or found to be HIV positive leaving 1,262 cases and 2,094 controls for analysis. They used statistical techniques to adjust for such additional factors that may also have affected the link, such as race/ethnicity, education level, self-report of animal and plant allergies, number of brothers or sisters, and age at first pet or farm exposure. The results were reported as odds ratios (OR), which can be interpreted as a ratio of the “risk” of having NHL in exposed people versus unexposed people.

What were the results of the study?

The researchers reported several risks. Current pet owners had a significantly reduced risk of NHL (OR 0.71, 95% CI 0.52 to 0.97) compared with those who never had owned a pet. This represents the 29% reduction that has been reported in the papers. Those who responded that they had “at some time” owned dogs and/or cats, showed similarly reduced risk (OR 0.71, 95% CI 0.54 to 0.94).

Longer duration of cat ownership, dog ownership and owning both was “inversely associated” with risk of NHL, meaning the longer the duration of ownership the lower the risk.

Ownership of pets other than cats and dogs was also associated with a reduced risk of NHL. However, exposure to cattle for five years or more was associated with an increased risk of NHL (OR 1.6, 95% CI 1.0 to 2.5), as was exposure to pigs (OR 1.8, 95% CI 1.2 to 2.6).

What interpretations did the researchers draw from these results?

The researchers conclude that, “the association between animal exposure and NHL warrants further investigation in pooled analyses”.

What does the NHS Knowledge Service make of this study?

Overall, this well-conducted study goes some way to showing the effect that pet ownership may have on the rates of NHL but, as the researchers state, case-control studies are observational studies, and cannot themselves prove a cause.

As noted in the study, further research will be needed to establish whether pets themselves or some associated activity or risk factor contribute to the disease. 

Within investigations of this type, researchers can limit the effect of bias in their results by giving careful attention to how the participants are selected, careful measurement of exposures and outcomes, and taking steps such as statistical adjustment to reduce the influence of other known risk factors. In this study researchers reduced bias by taking the following steps:

  • Reducing the possibility of selection bias by matching subjects with lymphoma with control subjects as closely as possible (except for their diagnosis of NHL).
  • Carefully measuring exposures by personal interview, and doing separate analysis for different subsets of lymphoma type.
  • Acknowledging that assessing the link between NHL risk and exposure to farm animals is complicated, as farm workers may also be exposed to other possible causes of lymphoma, such as animal viruses or pesticides. These types of possible causes were not measured by their interview.
  • Acknowledging that not measuring these risk factors associated with farming could confound the results between those exposed to farm animals and those who had pets.

As noted in the study, further research, such as pooling the results of similar studies, will be needed to establish a links between pet ownership and lymphoma, as they were reported in the press. 

Analysis by Bazian
Edited by NHS Website

Links to the headlines

Owning a pet 'cuts lymphatic cancer risk'.

The Daily Mail, 9 October 2008

Owning a pet reduces risk of developing type of cancer.

The Daily Telegraph, 9 October 2008

Links to the science

Tranah GJ, Bracci PM and Holly EA.

Domestic and Farm-Animal Exposures and Risk of Non-Hodgkin's Lymphoma in a Population-Based Study in the San Francisco Bay Area.

Cancer Epidemiol Biomarkers Prev 2008; 17: 2382-2387