The Truth About Dogs and Cats: Exposure to Animals and Risk of Lymphoma

Malignant lymphomas are of two types, Non-Hodgkin’s lymphoma and Hodgkin’s disease. The risk of Non-Hodgkin’s lymphoma has been rising, and over 60,000 people will develop Non-Hodgkin’s lymphoma in the United States this year. This is a life-threatening illness that requires treatment with surgery, radiation therapy, chemotherapy, and/or biotherapy to achieve control and often cure of the disease. Exposure to radiation has been a known risk factor in lymphomas, as well as some occupational exposures.

Pet ownership in the United States is very high. About 40 percent of households own a dog, and one-third of those have 2 or more. In addition, 30 percent of households have at least 1 cat. If you yourself do not have a pet, it is likely that at least one of your family members does. So exposure to dogs and cats is common.

A recent article by Dr. G. Tranah of the University of California at San Francisco and his colleagues (Cancer Epidemiology Biomarkers and Prevention Volume 17, Page 2382, 2008) has given us some added insight into impact of pet ownership. Individuals who have owned dogs and/or cats had a 29 percent reduced risk of developing Non-Hodgkin’s lymphoma. The longer the period of ownership, the less likely it was that lymphoma would be diagnosed.

In contrast, exposure to cattle for over five years was associated with an increased risk of Non-Hodgkin’s lymphoma of 60 percent. Surprisingly, exposure to pigs increased the risk even more, with an 80 percent increase in the risk of Non-Hodgkin’s lymphoma.

These results are important for each of us to consider. First, the reduced risk of lymphoma in dog and/or cat owners has not been known before, and is a good effect in addition to the social benefits provided by pet ownership in families.

But the increased risk of lymphoma in people who are in contact with cows or pigs is worrisome. It means that if you have lived on a farm or have had pet cows or pigs, you should be concerned if you develop a swollen lymph gland, which usually is felt in the neck, under the jaw, in the armpit, or around the groin. If you develop this, be sure to have it checked out by your physician to be sure it is not a lymphoma in the early stages. Detection of most lymphomas is the early stages results in higher chance of cure.

This study also reminds us to discuss our pet ownership with our primary care physician. Many types of pets harbor serious illnesses that can be spread to people. Turtles, for example, have given their owners certain infectious diseases. Owners of birds have sometimes developed respiratory infections.

The take-home lesson from this week’s column is that the psychological benefits of pet ownership can also be associated with health benefits, but also sometimes health risks. Consideration of pet ownership is important in the increasingly impersonalized society developing in America. The reduction in risk of Non-Hodgkin’s lymphoma is an example of one more benefit of dog or cat ownership.