| The Risk of Infection from Polychlorinated Biphenyl Exposure in the
Harbor Porpoise (Phocoena phocoena): A Case-Control Approach Ailsa J. Hall,1 Kelly Hugunin,2 Robert Deaville,3 Robin
J. Law,4 Colin R. Allchin,4 and Paul D. Jepson3 1Sea Mammal Research Unit, Gatty Marine Laboratory, University of
St. Andrews, St. Andrews, Fife, United Kingdom; 2North Carolina
State College of Veterinary Medicine, Raleigh, North Carolina, USA; 3Institute
of Zoology, Zoological Society of London, London, United Kingdom; 4Centre
for Environment, Fisheries and Aquaculture Science, Burnham Laboratory, Burnham-on-Crouch,
Essex, United Kingdom Abstract
The objective of this study was to determine whether the risk of mortality from infectious disease in harbor porpoise in U.K. waters increased with high exposure to polychlorinated biphenyls (PCBs) , using a case-control study design. This is the first time that data from a long-term marine mammal strandings scheme have been used to estimate any increase in risk. The exposure odds ratio (OR) from a logistic regression model with infectious disease deaths as cases and physical trauma deaths as controls, after controlling for the effect of confounding factors, was 1.048 [95% confidence interval (CI) , 1.02-1.07]. To further adjust for the difference in energetic status between cases and controls and account for the negative relationship between PCBs (sum of 25 chlorobiphenyl congeners) and blubber mass, we also “standardized” the blubber PCBs to an optimal blubber mass. This lowered the OR to 1.02 (95% CI, 1.00-1.03) . Thus, for each 1 mg/kg increase in blubber PCBs, the average increase in risk of infectious disease mortality was 2%. A doubling of risk occurred at approximately 45 mg/kg lipid. In this study, we have endeavored to avoid selection bias by using controls that died of physical trauma as representative of the exposure prevalence in the population that gave rise to the cases. In addition, we controlled for the effect of variation in energetic status among the cases and controls. However, as with case-control studies in human and veterinary epidemiology, unforeseen misclassification errors may result in biased risk estimates in either direction. Key words: cetaceans, dose response, immunosuppression, PCBs, risk assessment. Environ Health Perspect 114:704-711 (2006) . doi:10.1289/ehp.8222 available via http://dx.doi.org/ [Online 13 January 2006]
Address correspondence to A.J. Hall, Sea Mammal Research Unit, Gatty Marine Laboratory, University of St. Andrews, St. Andrews, Fife, KY16 8LB, UK. Telephone: 01334-462630. Fax: 01334-462632. E-mail: ajh7@st-andrews.ac.uk We thank J. Baker, T. Kuiken, T. Patterson, and H. Ross for conducting porpoise necropsies and J. Learmonth, C. Lockyer, S. Northridge, and E. Rogan for age determination analyses of porpoise teeth.
Postmortem investigations and analyses of blubber contaminants were conducted under contract to the U.K. Department for Environment, Food and Rural Affairs. The authors declare they have no competing financial interests. Received 19 April 2005 ; accepted 12 January 2006.
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