Why are foals different?


When breeding a foal, worming has always been, and remains to play an important role. From correctly and appropriately treating the mare whilst she is carrying her offspring, through to continual monitoring and treating the foal as it grows and develops in the field.

Foals are born with no immunity to infections, and gain relevant antibodies from their mother in her milk. But in order to develop tolerance to worms they will need to be exposed to parasite infections. Wormers are used to keep parasites at a level which will not cause the foal any long term damage.

Although initially prone to all equine worm species, there are two parasites that are of upmost importance in foals and youngstock. These are the intestinal threadworm and the large roundworm, or ascarid.

The intestinal threadworm (strongyloides westeri) can affect very young foals as the mare can release this parasite into her milk around five days after foaling, if she is infected. The larvae are ingested and migrate via the trachea to the lungs before they are swallowed. This worm, although very small, can breed very quickly and large numbers of adult worms can be found within the gut of the foal in just a matter weeks.

An infection of intestinal threadworms can cause diarrhoea and loss of weight, symptoms which may not sound too distressing. However, in a foal these minor symptoms can have huge detrimental effect upon the growth and development of the young animal.

Interestingly, and worryingly, this parasite can penetrate the horse’s skin and remain in the body tissue of the horse for many years. Adult worms can also survive outside of the horse on the pasture, and stresses the importance of careful pasture management.

Treatment is simple, through either a single dose of ivermectin or moxidectin (check all brands’ individual licenses) or a multiple dose of fenbendazole. Healthy foals should acquire a natural immunity or tolerance to this parasite at around six months of age.

The other parasite is the large roundworm, also known as the ascarid (parascaris equorum). Commonly found on pastures in the UK, rigid pasture management protocol is recommended to keep this parasite at bay, as resistance is developing to some drug families.

The egg of the large roundworm contains the larvae and survival for many years on the pasture is guaranteed as it has a thick sticky coating protecting it from extremes of hot and cold weather. Once the egg has been ingested by the foal, the larvae are released into the small intestine, but they do not finish their development here. The larvae penetrate the wall of the gut and travel to the liver. After two weeks they have reached the lungs. Here they migrate up the bronchi and trachea which causes the animal to cough. The larvae are then swallowed and develop into adults within the small intestine.

Symptoms of the ascarid are more distressing with migrating larvae causing coughing and pulmonary haemorrhaging. Due to the adult worm’s size, large numbers can cause fatal blockages and ruptures of the gut. Smaller numbers will still affect the foals growth and development. A typical visual symptom is the ‘pot belly’ look as well as a dull coat.  

Again, healthy young horses should develop natural immunity to this worm at around 18 months of age. It has been shown to affect older horses that have not been exposed to low levels of the parasite at a young age.

As well as regularly collecting the horses’ dung, foals and youngstock will require treatments for this parasite at frequencies dependent on the results of worm egg counts. The drugs available include ivermectin, moxidectin, pyrantel and fenbendazole, although lack of efficacy and resistance has been suspected or confirmed to all of these active ingredients. You should verify the resistance status of the parasite, in all foal programs, as permanent damage can be caused very quickly where wormer chemicals are not working any more.  

As the foal gets older and grazes more, the risk of other parasites such as the small and large redworm and tapeworms take over, and a more comprehensive programme will need to be drawn up on the risk of all parasites.

Intelligent Worming is the only company who can draw up individual worming programmes for your foal taking into consideration parasite risk, immune levels of the individual horse and the resistance to the wormers on the pasture. To get your foal on to an effective worming program call 01267 223322 today.