Worming Foals

Horses of all ages are affected to one degree or another by internal parasites. Whilst foals are born free of parasites, they are often exposed to parasites within the first few days of life. The immature immune system of foals makes them very susceptible to worms. Some worm species are very common in young horses but rarely seen in older horses, as their immune system develops with age. Foals are also more susceptible to damage caused by worms due to their smaller size.

There are four major types of worm parasites that are of concern when it comes to foals:

  1. Strongyloides
  2. Ascarids
  3. Large strongyles
  4. Cyathostomes

Two of the major parasites affecting foals, Strongyloides and Ascarids, are much less of a problem in adult horses because immunity to these worms develops as the horse ages.  However, in young foals, they can have quite devastating effects, which is why it is important they are addressed. Heavy worm infestations can lead to health problems, including diarrhoea, colic, anaemia, ill thrift and poor or reduced growth rates.

When should I worm my foal and what should I use?

Unfortunately, there is no simple, one-wormer solution to treating foals. Some worming medications are very good for one type of worm but less effective for another type. Many of the wormers available today are ivermectin-based, which are usually very good but have very little effect against the ascarids. Fenbendazol (FBZ) has very good effectiveness against the Ascarids but is ineffective against the Strongyloides worms. Therefore, a combination of wormers is usually needed to achieve the desired result. The process of worming foals usually entails:

  • Worming the mare regularly during pregnancy and just prior to foaling with an ivermectin-based product. This will greatly reduce the number of Strongyloides larvae in the milk and faeces of the mare.
  • Worming the foal at around eight weeks with a FBZ or BZ product.
  • You can continue to worm the foal every six to eight weeks with a FBZ or BZ product.
  • Once the foal is 4-5 months of age, it can be wormed it with an ivermectin-based product.
  • From weaning to 12 months, it is best practice to start doing Faecal egg counts (FEC) to determine the best wormer for your foals situation.
  • Once the foal is 12 months old it should be treated regularly under an adult worming schedule, which will include regular FEC’s.

Remember there is no single deworming program or strategy that suits all foals. The geographic location, climate, horse density, housing conditions, and other factors need to be considered. The parasite load of individual horses and the effectiveness of a parasite control program can be evaluated by determining faecal egg counts prior to, and after, deworming. It is also recommended to begin a FEC monitoring program for yearlings to help guide treatment frequency.


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