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.
The preservative benzyl alcohol has been associated with serious adverse events, including the "gasping syndrome", and death in pediatric patients. Although normal therapeutic doses of this product ordinarily deliver amounts of benzyl alcohol that are substantially lower than those reported in association with the "gasping syndrome", the minimum amount of benzyl alcohol at which toxicity may occur is not known. The risk of benzyl alcohol toxicity depends on the quantity administered and the liver and kidneys' capacity to detoxify the chemical. Premature and low-birth weight infants may be more likely to develop toxicity.