Effectiveness of breed-specific legislation in decreasing dog-bite injury hospitalizations in Manitoba--what it means to researchers, policy-makers and the public
Our population-based study (1) on the effectiveness of breed-specific legislation (BSL) targeting pit-bull (terrier) type dogs in the Canadian province of Manitoba generated some interest in the media and among policy -makers and the public in Canada and the United States (2-10). With this experience of listening to different stakeholders and communicating with some, we hope to elaborate on our findings in language that is accessible to all. The objective of the study was to determine trends in the frequency of dog-bite injury hospitalizations (DBIH) over time for jurisdictions with and without a ban on pit bull (terrier)-type dogs in Manitoba (1).
We reported that at the provincial level in Manitoba, there was a decrease in incidence of DBIH from 3.47 to 2.84 per 100,000 person-years associated with implementation of a ban on pit-bull terrier type dogs. That is, there was a decrease by 0.63 per 100,000 persons per year (an 18.1% decrease in DBIH rate) in 16 self-selected urban and rural jurisdictions. Correspondingly, in people aged 0 to < 20 years, there were 1.76 fewer DBIH per 100,000 person-years (a 25.5% decrease in DBIH rate) in Manitoba. This decrease in rates of DBIH may be a conservative finding because enforcement of legislation, which was not measured and is known to have varied across the jurisdictions and over the years, is assumed to be minimal, if at all. While the type of legislation studied was specifically a ban, no jurisdictions were known to have outlawed pit bulls overnight. As existing individual dogs were allowed to live out their lifetimes, no drastic reduction in numbers of pit bulls, and by extension, in numbers of DBIH, was expected in jurisdictions that implemented bans only gradually since 1990.
What does the change in incidence of DBIH at the provincial level mean? The Canadian province of Ontario, with a population about 11 times larger than Manitoba, has a province-wide ban on pit-bull terrier type dogs since 2005 (11). Assuming that Ontario's DBIH rate, rate of penetration of dog population (i.e., dogs per capita of human population) and dog-breed distributions are similar to those in Manitoba, we applied the decrease of 0.63 DBIH per 100,000 people per year to Ontario's population of 12.8 million in 2011 (12,13). (While Manitoba's rural population is considered to be 28%, Ontario's rural population is reported to be 15%.) We estimate that there may have been 81 fewer DBIH in 2011 alone in Ontario on account of the province-wide ban. As Ontario's population of those aged < 20 years was 3 million (13), 54 (66.7%) of the estimated decrease by 81 DBIH among all ages in 2011 would have been in people aged < 20 years.
When considering rate differences in post-legislation period compared with pre-legislation period in Winnipeg alone, our data do not indicate a change in DBIH rate. Therefore, it is natural to assume that BSL does not work. However, our study does not account for changes in overall number of dogs over the long period under study. Based on growth in number of pet dog populations in the United States over the last two decades (14,15), we propose that any hypothesized decrease in the number of DBIHs due to pit-bull attacks is likely masked, and the effect of legislation diluted, by a simultaneous increase in DBIHs due to attacks by dogs from other breeds or breed groups. Again, this explanation is quickly assumed to be evidence that breed bans do not work. After all, an argument against BSL is that breed composition in dog populations can change such that other dangerous dogs replace dogs from banned breeds. A limitation of the study was our inability to separate the proportion of DBIHs caused by dogs of banned breeds from the proportion caused by dogs of other breeds or breed groups. However, with the assumption that replacement is necessarily different from addition of more dangerous dogs to the existing numbers, we compared DBIH rates in jurisdictions with pit bull-specific ban (e.g., Winnipeg) to DBIH rates in jurisdictions without such bans (e.g., Brandon). The idea behind this analysis is that, unlike pit-bull specific bans, voluntary changes in breed popularity have no boundaries, and jurisdictions with bans are assumed to be similar to jurisdictions without bans in every respect other than the existence of the ban. Such an analytic approach is also an improvement over a pre/post analysis of data from a single jurisdiction adopting the ban.
We adopted a generalized estimating equations (GEE) model for this comparative analysis. This multivariate model allowed us to isolate the effect of legislation while modeling annual DBIH counts adjusted for human population counts, calendar year of DBIHs and baseline differences in underlying DBIH rates across jurisdictions with and without legislation. The model yielded an incidence rate ratio--i.e., the rate of DBIHs in jurisdictions with a ban relative to the rate in jurisdictions without a ban.
The results from the GEE model were not remarkable when data from all Manitoba jurisdictions were analyzed, but as control jurisdictions were more likely to be rural jurisdictions, there was a high inter-correlation among variables. One way of controlling for the confounding effects of rurality of jurisdictions is to stratify the dataset into rural and urban. Therefore, we restricted analyses to urban jurisdictions alone. The results indicated that for every one DBIH in Brandon, there were 1.29 DBIHs in Winnipeg before the pit-bull ban and 1.10 DBIHs after the ban. This is a 14.7% reduction in rate of DBIH in people of all ages. In people younger than 20 years old, for every one DBIH in Brandon, there were 1.28 DBIHs in Winnipeg before the ban and 0.92 DBIHs after the ban. This amounts to a 28.1% reduction in rate of DBIH. These findings were statistically significant. Other reasons for this decrease cannot be ruled out in this real-world, observational study which can be thought of as a non-randomized, self-selected community trial. However as far as we can ascertain, no other dog-control legislation is different between the two jurisdictions.
Going forward, researchers should compare DBIH rates temporally as well as geographically. Future (controlled) studies in other places where pit-bull specific bans have been in effect long-term are still necessary to conclusively understand if rates of DBIHs generally and gradually decline when pit-bulls are removed from the population. This is because effectiveness (or magnitude of rate decrease) may be variable depending on local conditions, even if everyone agreed that pit-bulls caused a disproportionate number of DBIHs. For example, if rate of pit-bull penetration is high, then magnitude of effectiveness of a pit-bull ban would likely be higher than observed in our study, if a cause-effect relationship truly exists. However, if rate of pit-bull penetration is zero (i.e., no pit bulls), then a ban that was proven to be 100% effective elsewhere (hypothetically speaking) would bring about little change to DBIH rate as, technically, there are no dogs to be banned. Furthermore, pit bulls in one region of the world may be less aggressive than pit bulls in another region owing to potentially different lineages and differences in dog-owning cultures. While the value inherent in local data should not be underestimated for the purposes of local policies, data from larger jurisdictions with bigger populations of dogs, including those from the banned breeds, and higher rates of DBIHs will further shed light on this public health topic that appears to attract a lot of public and stakeholder interest.
1. Raghavan M, Martens P, Chateau D, Burchill C. Effectiveness of breed-specific legislation in decreasing the incidence of dog-bite injury hospitalizations in people in the Canadian province of Manitoba. Injury Prevention doi:10.1136/injuryprev-2012-040389. E-pub ahead of print.
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10. Raghavan M. Invited presentation: Study on the effectiveness of breed-specific legislation in decreasing dog-bite injury hospitalizations in Manitoba--what it means to researchers, policy-makers and the public. Manitoba Agriculture, Food and Rural Initiatives (MAFRI) Lunch & Learn Session. August 13, 2012, Winnipeg, Manitoba.
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Conflict of Interest: