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Re: Authors' Reply

Dear Editor

We would like to thank Dr Preece for his letter.[1]

He raises the issue that loss of symptomatic workers during follow-up does not explain the absence of a decline in lung function in workers who worked with laboratory animals for more than 4 years, and concludes that lung function decline in short-term employed workers is not sustained. We think that this interpretation of our data is somewhat overoptimistic and is based on the assumption that lung function changes are similar in those lost to follow-up and those available for follow-up and that short- term and long-term employed workers can be compared.

In our paper we analysed lung function decline separately for long- term and short-term employed workers as there were strong indications that those working with laboratory animals for more than 4 years were less likely to experience adverse health effects due to contact with laboratory animals. Workers lost to follow-up were more often sensitised to laboratory animals and more often reported allergic and chronic respiratory symptoms. In addition, long-term employed workers were less likely to become sensitised to laboratory animals during follow- up (unpublished data). Long-term effects in the recently employed workers should therefore not be inferred from (the lack of) effects in this survivor population.

We also do not agree that an excess decline in FEV1 in the order of 80 ml/yr for exposed and sensitised short-term employed workers can be qualified as a 'small' effect. From a public health point of view such a decline implies more than a tripling of the normal decline due to aging, and is larger than the excess decline found in populations of heavy smokers.[2-4] The estimated decline in FEV1 for symptomatic short-term employed workers was -22 ml/yr, which is similar in magnitude to the normal decline due to aging. Although this estimate was not statistically significant, we think this is mainly an issue of statistical power.

The fact that we found no additional lung function decline due to allergen exposure in sensitised long-term employed workers may also be related to qualitative or quantitative aspects of exposure. The exposure measure we used to characterize exposure during follow-up was rather crude and might have failed to capture relevant differences in exposure level among the long-term employed workers.

On a final note, we would like to point out that the models we used are best suited to identify etiological factors and should not be used for predictive purposes. An article on diagnostic modelling, explicitly dealing with laboratory animal allergy has been published in this journal quite recently,[5] and is recommended reading for the interested occupational physician.

References

(1) RM Preece. Lung function decline in laboratory animal workers [electronis response to Portengen et al. Lung function decline in laboratory animal workers: the role of sensitisation and exposure] occenvmed.com 2003 http://oem.bmjjournals.com/cgi/eletters/60/11/870#90

(2) Xu X, Dockery DW, Ware JH, Speizer FE, Ferris BG, Jr. Effects of cigarette smoking on rate of loss of pulmonary function in adults: a longitudinal assessment. Am Rev Respir Dis 1992; 146:1345-8.

(3) Burchfiel CM, Marcus EB, Curb JD, Maclean CJ, Vollmer WM, Johnson LR, et al. Effects of smoking and smoking cessation on longitudinal decline in pulmonary function. Am J Respir Crit Care Med 1995; 151:1778-85.

(4) Anthonisen NR, Connett JE, Murray RP. Smoking and lung function of Lung Health Study participants after 11 years. Am J Respir Crit Care Med 2002; 166:675-9.

(5) Meijer E, Grobbee DE, Heederik D. Detection of workers sensitised to high molecular weight allergens: a diagnostic study in laboratory animal workers. Occup Environ Med 2002; 59:189-95.

Send letter to journal:
Re: Lung function decline in laboratory animal workers

Dear Editor

In their recent paper Portengen et al.[1] have made an important contribution to our understanding of laboratory animal allergy. However, they have omitted to draw attention to an observation of clinical importance to occupational physicians.

They have suggested that the lack of decline in lung function in "experienced" workers may be due to the healthy worker effect. Their suggestion is not supported by their own data: The decline in lung function over two years amongst newly exposed workers with symptoms of LAA was not significant and surprisingly there was a significant increase in function amongst the symptomatic experienced workers. This being the case there seems little reason to conclude that the loss of symptomatic workers (due to a healthy worker effect) would adequately explain the absence of a decline in function. An equally valid conclusion is that the effect observed in newly exposed workers is small and may not be sustained in the long term.

Physicians are wise not to preclude sensitised workers and those with symptoms of LAA from work with animals solely on the basis of concern that this may have a deleterious effect on health.[2-4] Portengen et al. have provided new evidence that supports this.

I agree that the results and conclusions should be interpreted with caution and that further work is needed. However, this is a reassuring study and with important implications to current animal workers and their health providers.

References

(1) Portengen L, Hollander A, Doekes G, de Meer G, Heederik D. lung function decline in laboratory animal workers: the role of sensitisation and exposure Occup Environ Med 2003; 60: 870-875

(2) Botham PA, Lamb CT, Teasdale EL, Bonner SM, Tomenson JA. Allergy to laboratory animals: A follow-up study of its incidence and of the influence of atopy and pre-existing sensitization on its development. Occup Environ Med 1995; 52:129-133.

(3) Newill CA, Evans R III. Khoury M. Preemployment screening for allergy to laboratory animals: Epidemiologic evaluation of its potential usefulness. J Occup Med 1986; 28:1158-1164.

(4) Preece R, Renstrom A. Laboratory animal allergy In: Handbook of Laboratory Animal Science, Second Edn: Essential Principles and Practices, Vol I Hau van Hoosier GL(Eds) Boca Raton: CRC Press 2002