Background
Since first being identified in March 2024, infections of highly pathogenic avian influenza (H5N1) have been reported in over one thousand dairy herds in the United States. Canadian dairy cattle have remained free from H5N1 infection, due in part to the testing regulations for interstate and international dairy cow movements. To better understand the trends in historical dairy cow importing patterns and the subsequent potential for H5N1 introduction into Ontario, Canada, we present a descriptive analysis of dairy cow imports from the United States into Ontario dairy herds enrolled in Lactanet Canada services.

Results
Between 2009 and 2019, a total of 6,145 dairy cows were imported into 735 Lactanet-enrolled herds. Imports peaked during summer and early fall, with imports spiking at the beginning of each month. Annually, most Lactanet dairy herds did not import any dairy cows. While the total annual number of dairy cow movements decreased between 2014 and 2018, the proportion of dairy cow movements within Ontario made up of imports increased over time. The regional distribution of imported cows was similar to the distribution of Lactanet herds, mostly being concentrated in western and eastern Ontario. However, when considering the density of Lactanet herds, there were disproportionately more imports per herd in southern Ontario. Outgoing infection chains (OIC) of importing herds were mostly short, suggesting that for most herds that imported dairy cows, there were no subsequent domestic movements to other Ontario dairy herds in the calendar year. Mean OIC values were skewed by several importing herds with very large OICs, connecting a large number of herds across all regions of the province.

Conclusions
Relative to the network of domestic dairy cow movements, imports from the United States were relatively uncommon and largely clustered in southwest and eastern Ontario. Importing appeared to be seasonal, peaking during late summer and early fall. Outgoing infection chains identified that most herds that import dairy cows do not subsequently move cows to other Ontario-based dairy herds. However, for a few herds, the chain formed by time- and direction-respecting movements had the potential to reach many non-importing herds across the province.