Is it safe for the foal, and would they receive enough through this method for it to have any effect?
Michael, Wairarapa, New Zealand
A: Hi Michael. A good principle for deworming is that there has to be worms present before you treat them.
A newborn foal does not harbour any parasites, and there is no benefit from treating them ahead of time. Depending on the dewormer, the mare may pass metabolites in her milk, but it is unlikely to have any efficacy against parasites in the foal.
The reason why some people practice to deworm mares around the time of foaling, is because of the threadworm Strongyloides westeri.
This is a relatively benign parasite that infects foals. It uses several different routes to infect the foals, but one of them is through the mare’s milk during the first couple of weeks. By treating the mare, this route of infection can be impaired.
However, the most important parasite for foals is the large roundworm, Parascaris equorum, and treatments of foals against this parasite are recommended at the age about two months and onwards.