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Review of the Epidemiology of Cancer in Dogs

by Todd Bessinger

(adapted from: Kelsey, JL, AS Moore, and LT Glickman. (1998). Epidemiologic studies of risk factors for cancer in pet dogs. Epidemiologic Reviews 20(2):204-217. Jennifer Kelsey is a well-known cancer epidemiologist from Johns Hopkins. This article appeared in a journal otherwise devoted exclusively to human epidemiology. It seems the author recently lost her own dog to cancer and dedicated the article to the dog.)

I. Introduction
II. General Overview
III. Female breast cancer
IV. Testicular cancer
V. Lymphoma
VI. Osetosarcoma (bone cancer)
VII. Bladder and Ureteral Cancer
VIII. Nose cancers
IX. Lung cancer
X. Prostate Cancer
XI. Conclusions

I. Introduction

Epidemiology is the study of diseases in populations rather than in individuals. Epidemiological studies examine factors that are associated with developing diseases. Since entire populations usually cannot be examined, epidemiology relies on sampling smaller parts of the overall population and, using statistics, generalizing those results to everyone. When you read that eating more fiber is (or is not) good for preventing heart disease, you are reading the outcome of an epidemiological study.

One of the most important things to remember whenever you read an epidemiological study is that it reports ASSOCIATIONS and not causes. It takes well-controlled, laboratory or clinical studies to prove CAUSATION.

For example, it is well-documented that increases in the murder rate in the United States are associated with increased ice cream sales. Does this mean that eating ice cream causes homicidal tendencies? Actually, increases in homicides and ice cream sales are both associated with higher daytime temperatures. Please keep this example in mind when reading this or any other epidemiological study.

II. General Overview

Almost two-thirds of American households include at least one dog. The studies described hereafter are almost all observational studies of dogs in their usual household environments--not lab animals.

The studies are all case-control studies. This type of study takes a groups of dogs with a particular disease (cases) and matches this group with a similar group of undiseased dogs (controls). Then, looking back through time, searches for significant differences in their lives (exposures). The degree to which these exposures influence the development of cancer is determined using statistical models and is expressed by an odds ratio. This gives an idea of the risk or protective benefit of a given exposure.

Because there are large differences between the breeds, changes due to aging, and gender in dogs, statistical models are used to try and compensate for these differences.

In the United States, there are approximately four dogs in every thousand which are diagnosed with cancer each year. The most commonly-diagnosed cancers in dogs are:

Dogs

Bitches

Type of Cancer

% of Total Cancers

Type of Cancer

% of Total Cancers

Connective tissue

17

Breast

51

Testis

16

Connective tissue

9

Skin (melanoma)

14

Skin (melanoma)

8

Mouth and throat

10

Lymphoma

6

Lymphoma

10

Mouth and throat

5

Bone

4

Liver and bile tracts

2

Stomach and intestines

3

Bone

2

Keep in mind, however, that these numbers are from the 1960s! There are no newer studies and the increased life expectancy of dogs since the 1960s has probably changed this order somewhat.

Interestingly, cancers of the ovaries and uterus are rare in dogs. This could be because most dogs do not reach the age at which these cancers become more common. Dogs do not have much lung cancer either, presumably because they do not smoke and have fewer occupational exposures to known carcinogens.

Colon and rectal cancer, the third most common tumor in humans, is extremely rare in dogs. This could be due to more rapid transit time of food through dogs' relatively short intestinal tract, more exercise than humans, and diet.

Because skin cancer (melanoma) and connective tissue cancer is largely unstudied in dogs, these two cancers are not included in the review article.

III. Female Breast Cancer

Ninety-seven percent of all breast cancer in dogs occurs in females. It is the most common cancer in bitches. About 76% of all these tumors are the same type as found in humans--adenocarcinomas (arising from the cells lining the ducts of the breast). The breasts in dogs that are closest to the rear of the body are more likely to have cancer than those closer to the head. The likelihood of cancer increases moving from head to rear breasts.

Purebred dogs are twice as likely to get breast cancer as are mixed breed dogs of the same age. The most important conclusion gained from examining studies of breast cancer in dogs is that early spaying protects against breast cancer.

The greatest protection from spaying occurs if the dog is spayed before her first heat. The protective value of spaying drops steadily until age 2 1/2. If the bitch is spayed at or after age 2 1/2, the risk of getting breast cancer is statistically no different from a bitch which was never spayed. One study, however, found at least some protection from breast cancer when bitches were spayed up to five years of age. Clearly, however, the earlier a bitch is spayed, the less likely she is to get breast cancer.

As in humans, the presence of estrogen and progesterone receptors in breast cancer suggests a role for hormonal stimulation for development and aggressiveness of tumors. Tumors that have a greater number of estrogen and progesterone receptors are less malignant than those with fewer of these receptors.

Interestingly, one study showed that beagles who became pregnant at every heat cycle do not develop breast cancer. Another showed that high doses of oral contraceptives can actually induce breast cancer development in beagles.

Other studies have found associations between development of breast cancer and diet, obesity, and being underweight (the runt, if you will). One study showed that the amount of fat in the diet was not associated with breast cancer but that a low-fat and high-protein diet was associated with a better chance of surviving breast cancer. Unspayed bitches who were underweight as puppies have about half the risk of developing breast cancer as puppies who were of normal or above-normal weight. A related finding was that obese dogs with breast cancer were four times as likely to have more malignant, aggressive tumors than were dogs of normal weight.

IV. Testicular Cancer

While in human males one type of testicular cancer predominates (seminomas), dogs get any of three types of testicular tumors (Sertoli cell tumors, seminomas, and interstitial cell tumors). Also, there is no increase in the number of cases of testicular cancer in early adulthood in dogs as there is in humans.

Dogs with undescended testicles (i.e., the testicles do not properly migrate to the scrotum but remain in the body cavity) have a markedly higher risk than other dogs to develop this type of cancer. Dogs with inguinal hernias are also at increased risk. Obviously, neutering of dogs prevents the development of this type of cancer.

More testicular cancer was also found in working dogs from the Vietnam war who were exposed to parasitic infections, various treatments for these infections (tetracycline was singled out), and herbicides.

V. Lymphoma

Lymphoma is a cancer of the cells of the immune system. Canine lymphoma is very similar to non-Hodgkin type lymphomas in humans. Dogs are more likely to get this type of cancer as they get older. Males and females get this cancer at roughly the same rate. Neutering or spaying does not affect the development of this disease although purebreds are slightly more likely than mixed-breed dogs to get lymphoma.

There is a modest association between developing lymphoma and the use of herbicides and/or commercial lawn services to treat the dog's lawn. There is also an association between lymphoma and exposure to electromagnetic fields. Dogs living in homes with very high outdoor currents nearby were nearly seven times more likely to develop lymphomas than control dogs.

VI. Osteosarcoma

Bone tumors in dogs are very similar to those in humans. The small region between the shaft and ends of the long bones (the metaphysis, where growth occurs) is the most common site. These tumors are usually high-grade, aggressive, and usually spread to other parts of the body. The lung is most commonly involved.

Osteosarcoma tends to affect larger breeds with a slight increase in incidence with age. Males are more likely to be affected than are females. And since neutered dogs and bitches have twice the risk of developing the disease as compared to intact dogs, hormonal factors are thought to play a role.

Weight-bearing long bones of the legs are most frequently involved, especially the metaphysis of the radius. Breeds which weigh over 80 pounds are 61 times more likely to develop bone cancer than dogs weighing less than this amount. Also, the rates of developing bone cancer between breeds increases with standard height of the breed independent of the dogs' weight. This means, for example, that when you consider two breeds which weigh over 80 pounds, say an Irish Wolfhound and a mastiff, the wolfhound has a higher likelihood of getting osteosarcoma because he is taller at the withers. However, within a given breed, heavier animals are more likely to develop the disease.

As in human children, development of bone cancer in dogs is related to rapid bone growth. It is postulated that strenuous activity causing microscopic fractures of bones during periods of rapid growth induces cancer formation. Since taller dogs have a longer growth period than smaller ones, they are exposed to the risk of getting the cancer for a longer period of time. Likewise, heavier dogs are more likely to stress their developing bones leading to the microscopic fractures that start the tumor development process.

Ionizing radiation (as is given in radiation therapy) and having a metallic implant in the repair of a fracture are both associated with developing osteosarcoma. However, given their rarity in dogs, neither of these two factors is likely responsible for a significant number of bone tumors.

VII. Bladder and Ureteral Cancer

Bladder cancers is dogs are more likely to occur in older dogs. Two studies found a one and one-half to threefold higher risk in females while a third study found no differences between the genders. This latter study did find that neutered dogs of both sexes seem to be at higher risk.

Bladder tumors have been experimentally shown to be induced by aromatic hydrocarbons including paraaminobiphenyl, paranitroliphenyl, betanapthylamine, and others. This is more than an association, these chemicals and other like them are KNOWN TO CAUSE bladder cancer in dogs.

The development of bladder cancer is associated with use of flea and tick dips, flea and tick shampoos, obesity a year before diagnosis, and proximity of the dog's home to a marsh. Dogs in one study who had flea and tick dips were 27 times as more likely than control dogs to develop bladder cancer.

Interestingly, the authors suggest that they believe it is not the active ingredients in the flea and tick products that cause bladder cancer but rather the "inert" or "carrier" ingredients. These carriers act as solvents for the active ingredients and generally account for 95% of the total product. They include known carcinogens such as benzene, toluene, xylene, and petroleum distillates.

One explanation of the gender differences in developing bladder cancer in dogs is that males urinate more frequently than females, giving the carcinogens more contact time with the bladder in bitches. Another explanation is that females have relatively more body fat and the chemicals which are known to cause bladder cancer are stored and relatively concentrated in fat. This also explains why obese dogs were more likely to develop the cancer.

VIII. Nose Cancer

Cancers of the nasal passage increase with increasing age in dogs. Males are slightly more likely to get this type of cancer than are females. Long-nosed breeds have the greatest risk of getting this type of cancer while short-nosed breeds have the lowest risk and mixed breeds and medium-nosed breeds have an intermediate risk. Judging from the examples given by the author, dalmatians would be considered a long-nosed breed.

Since the nose acts as an initial filter for incoming air, it is suggested by the authors that long-nosed breeds more efficiently filter out airborne carcinogens with their nose and deposit them in the nasal cavity. This leads to more contact of the carcinogen with the lining of the nose and induces formation of cancer.

Long-nosed breeds with a smoker in the house were twice as likely as control dogs to get nasal cancer. There was no increased risk for medium- or short-nosed dogs living with a smoker. Moreover, the risk for getting nose cancer in long-nosed breeds increased with increasing number of packs of cigarettes smoked in the home.

IX. Lung Cancer

Lung cancers are fairly rare in dogs though the number has been increasing. It is not known, however, whether the increase is real or a result of improved techniques to detect lung cancer in dogs.

Male and female dogs get this cancer at roughly the same rate. There may be a slight increase in risk associated with living in an urban area. Short-nosed breeds exposed to cigarette smoke in the home have twice the risk of getting lung cancer as medium- or long-nosed breeds exposed to a similar amount of cigarette smoke. This is, of course, the inverse of the nasal cancer findings and again speaks to the more efficient nasal filtration system in long-nosed breeds.

Cancer of the lining of the lungs (mesothelioma) is associated with exposure to asbestos in dogs just as it is in humans. The owners of dogs with this type of cancer were more likely to be exposed to asbestos at work or in their hobbies.

X. Prostate Cancer

Dogs are the only non-human species which also get any significant amount of prostate cancer. One in every 150 male dogs over the age of 8 was found to have prostate cancer. Unlike human disease, canine prostate cancer is an aggressive disease that spreads rapidly to lymph nodes, lungs, and bone. Usually, when dogs are diagnosed with prostate cancer, it is in its advanced stages. There were no data in the article describing the rates of prostate cancer in neutered versus intact dogs.

XI. Conclusions

The authors suggest several preventive measures for reducing the amount of cancer in dogs. They include:
1. Spaying before the first heat cycle.
2. Neutering dogs with undescended testicles
3. Limiting your dog's exposure to flea and tick dips, asbestos, and tobacco smoke.
4. Keeping dogs away from lawns which have recently been sprayed with herbicide.
5. Do not spend a great deal of time in areas with high levels of electromagnetic fields.

The authors point out that these last two suggestions need further study. They also hope that future studies of the causes of cancer in dogs will include larger sample sizes and quality of measurement. The article is dedicated to the late Sunny Kelsey.

(I would like to thank Todd Bessinger for generously allowing me to post this very informative review on Dalmatians.US.)

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