The research I’ve read shows onchecerca microfilaraie in Canada, USA, Australia, France, Poland… simply because those are the places the researchers lived. The incidence in their samples varied enormously. In Canada (post mortem skin samples from in front of the umbilicus), they found that horses from Canada had very little OM, while the horses that had moved over from the border from the US had more. Likewise in France, the French horses showed low incidence, while the Polish imports showed more. Clearly, a horse who has neck threadworms (and may not be reacting) can travel to a new area, where an obliging midge does the honour and carries the larvae to another horse.