I’ve written before about infectious disease concerns associated with animal smuggling. It’s usually focused on the large-scale smuggling of reptiles, birds and other small critters, but it can happen in larger animals as well.
Some parts of the US/Mexico border appear to be rather popular smuggling routes for cattle and horses. US Border Patrol agents recently seized 14 horses that were being smuggled into the US across the Rio Grande River, south of El Paso, Texas. USDA officials tested them for a range of diseases and found that all 10 adult horses were positive for equine piroplasmosis, a potentially fatal bloodborne parasitic disease. This is an important disease that’s common in Mexico but has been considered a foreign disease in the US (although recent recurrent outbreaks make it clear that it is established in some parts of the country). Regardless, smuggled horses come in with no testing, no documentation, not contact tracing and no controls, so they represent a great way to either bring in new diseases or spread existing ones around.
In 2011, approximately 280 cattle and around 160 horses (including donkeys and mules) were seized along the Rio Grande. I’m not sure what percentage of smuggled animals get caught. However, it’s probably a minority, so it’s likely safe to assume that lots of horses and cattle make their way into the US as illegal aliens and potential disease vectors every year.