The aim of this study was to determine whether the effects of selected human-canine interaction/environmental factors on bites occurring when the victim was and was not playing with the dog differed from each other. A veterinary clinic-based retrospective cohort study was conducted in Kingston, Jamaica (709), and San Francisco, USA (513) to compare the effects of selected exposures on non-play bites (161) relative to bites preceded by play with the dog (110) as reported by veterinary clients. Additionally, 951 non-biting dogs were used for a risk factor analysis of bites occurring during play. Using directed acyclic graphs and the change-in-estimate procedure to select and adjust for confounders, modified Poisson regression was used to estimate (a) the ratios of proportions of non-play bites out of all bites comparing exposed to unexposed dogs (proportionate bite ratios) and (b) risk ratios for bites occurring during play for each factor of interest. Proportionate bite ratios ranged from 0.84 to 1.29, with most 95% confidence intervals including one, thus implying a lack of specificity of effects of the examined factors on non-play bites relative to bites occurring during play with the dog. Consistent with this lack of specificity, risk ratios for bites occurring during play were similar in magnitude and direction to risk ratios previously published for non-play bites using the same non-biting dogs as a reference group. No country-specific differences in proportionate bite ratios were detected. Each human-canine environmental factor showed similar levels of association with both types of bites. One possible explanation is that both types of bites have a common causal pathway leading from each factor up to the point of human-canine contact. If the human-canine contact then leads to either play or non-play interactions with dogs and subsequently to both types of bites, the presence of such a common pathway would make the factor non-specific to either type of bite. As some of the examined factors are associated with increased frequencies of both types of bites, this could explain high percentages of bites occurring during play with the dog as reported in various case series of dog bites. If so, dog bite prevention strategies targeting these factors will simultaneously reduce the incidence of both types of bites.
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