Determining whether an exposure causes a disease in an individual is difficult, but such determination can be supported by demonstrating biological plausibility. Unfortunately, the mechanisms suggested by Kohlmeier et al. are not consistent with all available information. Although inhalation of avian antigens may cause hypersensitivity pneumonitis, neither hypersensitivity pneumonitis nor pulmonary fibrosis, which occasionally results, is associated with lung cancer. In addition, avian particulates, owing to their size, are not likely to reach the alveoli, nor have they been proven to be carcinogenic. Finally a mycologic pathway is unlikely, given that pet birds seldom are a source of Cryptococcus neoformans, even among immunosuppressed individuals, because few birds shed this organism and there is little aerosolization from feces.

Some members of the birdkeeping community have expressed concern over reports that scientific studies have shown birdkeeping to be a risk factor for respiratory cancers. Since I live with an unruly mob of conures and have easy access to the epidemiological literature, I decided to have a look at the studies. A very brief summary is shown below; more detail follows.

Odds ratio = odds of exposure in patients with a disease divided by odds of exposure in controls (those without the disease). For example, if the odds of exposure in the patients is .5 and in the controls is .25, the odds ratio is 2.0. An OR of 2.0 for a group of patients means that people in that group are two times as likely to have been exposed to birdkeeping as controls. If the OR is .50, they are half as like to have been exposed.

Author,     Bird      Smoking       Diet        Diet Odds    # of cases
          keeping                                Ratio

Kohlmeier   2.14      16.08     daily carrot     0.24           239
et al.                          consumption

Gardiner    1.29      19.69     beta carotene    0.56           143
et al.                           2000 mg/wk

Holst       6.7       10.00     vitamin C        0.23            49
                                50 mg/day

All three of these studies were case-control studies. In a case-control study, each case is compared with one or more non-cases. In general, a study with more cases is less likely to produce results solely due to chance. Errors due to study design can occur in a study of any size, and someone can always be found to disagree with a study design. It has been said that an epidemiologist is someone whose job is to disagree with other epidemiologists.

Following are full citations, abstracts, and some additional data on each of the three studies. All articles and the editorial appeared in the British Medical Journal (BMJ).

BMJ 305: 989-992, 24 October 1992. 3 references

ABSTRACT:

Objective

To investigate the association between birdkeeping and risk of lung cancer.

Design

Case-control study asking detailed questions on exposure to domestic birds and other pets, smoking, and various demographic and potentially confounding variables.

Setting

District general hospital; current admissions interviews in hospital or recent admissions interviewed at home.

Patients

143 patients with lung cancer, 143 controls with heart disease, and 143 controls with orthopedic conditions individually matched for age, sex, date of admission, and current or past admission.

Main outcome measures

Odds ratios for lung cancer in relation to various aspects of birdkeeping, after adjustment for smoking and other relevant confounding variables.

Results

Risk of lung cancer was not significantly associated with household exposure to pet birds at any time or at various specific periods in life, or to keeping large numbers of birds. For specific types of birds no association was seen for living in households with budgerigars or canaries but risk was significantly associated with keeping pigeons (odds ratio 3.53, 95% confidence interval 1.56 to 7.98). This remained significant after regression analysis to account for confounding variables (3.9, 1.2 to 12.62) in both sexes and all age groups.

Conclusion

Bird keeping may confer some risk of lung cancer, but the relation is not as strong as previously reported.

Ever kept bird in household          1.29

Type of pet

   Budgerigar                        1.14
   Canary                            0.54
   Pigeon                            3.53
   Finch                             1.28
   Cockatiel, parakeet,
     lovebird, parrot                2.03
   Dog                               1.07
   Cat                               0.77
   Other pets                        1.22

Ever worked
   In pet store                      2.22
   In greenhouse w/chicken manure   20.01
Current smoker                      19.69
Beta carotene >2000mg/wk             0.56

BMJ 305: 986-989, 24 Oct. 1992. 37 references

ABSTRACT:

Objective

To test the hypothesis that exposure to pet birds increases risk of developing lung cancer.

Design

Case-control study. Computerized interviews were used to assess previous exposure to pets and other risk factors for lung cancer.

Setting

Three major hospitals treating respiratory disease in former West Berlin.

Subjects

All people newly diagnosed as having primary malignant neoplasm of the trachea, bronchi, or lung who were 65 or younger and control subjects matched for age and sex from the general population of former West Berlin. 279 cases and 635 controls qualified for the study; 239 cases and 429 controls participated.

Main outcome measure

Odds ratio of developing lung cancer according to whether or not pet birds were kept and the duration of keeping pet birds.

Results

In addition to the risk of lung cancer imposed by smoking, passive smoking and occupational exposure to carcinogens, an increased relative risk of 2.14 (95% confidence interval 1.35 to 3.40) was found among people exposed to pet birds. The adjusted odds ratio for exposures longer than 10 years was 3.19 (1.48 to 8.21).

Conclusions

Avian exposure seems to carry a risk of lung cancer. Until the pathogenesis is understood, long term exposure to pet birds in living areas should be avoided, especially among people at high risk of developing lung cancer.

NOTE: 95.4% of cases and 55% of controls were smokers.

BMJ 305: 970-971, 24 Oct. 1992.

Final paragraph: Despite the possible sources of error in these studies it is essential to give the work credit. If valid, the association with pet birds would not only identify an easily avoidable cause of disease but also open new avenues for pathogenic research. The immediate priority is to build on these findings by conducting investigations that control properly for the effects of smoking. One solution would be to study lung cancer only in lifetime non-smokers, perhaps by combining data on non-smokers from these three studies. The finding in the two papers in this issue of an association with intake of vitamin A also highlights the potential value of investigating dietary intervention in the prevention of lung cancer. Smoking may remain the most important cause of lung cancer, but we should not ignore other possible causes.

BMJ 297: 1319-1321. 19 November 1988. 15 references

ABSTRACT: To find out whether keeping birds in the home is an independent risk factor for lung cancer, a case-control study was carried out in four main hospitals in The Hague, The Netherlands. Forty nine patients under 65 years of age with lung cancer were matched for age and sex with two control subjects who attended the same general practice. Data were collected on social class, cigarette smoking, intake of beta carotene and vitamin C, and alcohol consumption. It was found that smoking, birdkeeping, and a low intake of vitamin C were significantly and independently related to the incidence of lung cancer. The odds ratio for lung cancer among people who kept birds as pets was estimated to be 6.7 after adjusting for smoking and vitamin C intake. The results of this study suggest that keeping pet birds is an independent risk factor for lung cancer.

NOTE: 98% of cases and 84% of controls were smokers.

• Holst has also published a book: Birdkeeping as a source of lung cancer and other human diseases: a need for higher hygienic standards. Heidelberg: Springer-Verlag, 1988.
• American Cancer Society: 1-800-ACS-2345
• At the Cancer Response System the public could get information about cancer (every state should have one). The system has a print out about pet birds and cancer. It may be easier to understand than the abstracts.

Personally, I do not intend to give up my conures, but I do intend to install an air filter in the Fall; since my fans run continuously in the summer, the filter would not have much chance to do its work before the air is whisked away to the outdoors (and polluted outdoor air is whisked in). I have never been (or lived with) a smoker, and had no notable occupational exposure to respiratory carcinogens, so I would seem to be at lower risk than most of the cases in this study, but there are no guarantees. Even persons who are not in the known high risk categories (smoking, passive smoking, occupational exposure) occasionally get a disease, and not everyone in the high risk categories gets the disease.

The principal question as I see it is: does living with birds enhance my life? The answer is: Yes.

Comments or Suggestions should go to the current maintainers of the FAQ files:

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This FAQ is by no means meant to replace the many wonderful and informative books, breeders, magazines, and veterinarians that are out there.

Last Revised: Friday, 18-Jul-2008 19:46:25 MDT ( Damian )