Worming Breeding Stock

Pregnant Broodmares

A mare’s natural resistance to worms may be lower during pregnancy therefore it is vital that they are monitored closely throughout the gestational period and wormed when necessary, according to faecal egg counts. As with horses of all ages, treatment for encysted redworms should be administered during the winter months and appropriate tapeworm testing or treatment should be performed every 6 months, usually in the autumn and spring. Only wormers that contain praziquantel or pyrantel (which must be given at double the label dose) are effective against tapeworm.

In addition, broodmares are often wormed 2 – 4 weeks prior to foaling to ensure the mare is not carrying excessive worms and passing a significant number of eggs in her droppings when the foal arrives. The eggs that the mare passes can pose a significant risk of infection for the foal.

Not all equine wormers are licensed for use in broodmares - only a product that has been shown to be safe for the mare and the developing foal should be administered during this time.

Worming foals

Foals are born parasite free with little immunity to parasitic infection.  Worm burdens can lead to permanent intestinal damage and foals are especially vulnerable. Therefore it is essential that a correct and safe worming approach is in place to protect the foal, with treatment beginning from 6-8 weeks of age and continuing until 18 months, when adult worming protocols can be adopted.

Faecal egg counts should not be relied on in foals; they can harbour significant burdens of parasitic larvae that have not yet developed into egg laying adults. Foals should therefore be routinely treated with appropriate wormers at regular intervals during the first grazing season.

The importance of different gastrointestinal parasites varies with a horse’s age, the time of year and the parasites migratory pathway.  For young stock, there are several important parasites that need to be considered when designing worming regimes.

Ascarids

Ascarids are the major threat to foal health. They can cause stunted growth, airway inflammation, but most importantly the adult worms can result in blockage of the small intestine leading to severe colic. Drugs such as ivermectin and fenbendazole are often given to treat ascarids, however there is growing evidence of resistance emerging among ascarid populations in certain geographical areas. Local parasite sensitivity should be taken into consideration when selecting the most appropriate treatment to use.  

Small redworm

Small redworms or cyathostomins are considered one of the most important equine parasites worldwide. The larval stages of the redworm can encyst within the wall of the intestine and their emergence, often in the spring, can result in severe intestinal damage known as larval cyathostominosis. Affected horses lose protein across the damaged intestinal wall and can show signs of ill-thrift, colic and diarrhoea.  Weanlings and younger horses are more susceptible to infection. Throughout the year older animals may be shedding eggs and contaminating the pasture. Similar to adult horses, foals will also be exposed to large strongyles and tapeworm, during the first grazing season.

As foals are sensitive to wormer overdosing, due to their relatively low amount of body fat, it is imperative to accurately assess bodyweight before each dose to ensure the correct amount is administered. 

Mares with foals at foot

Regular faecal egg counts should be performed on broodmares from 3-4 weeks after foaling, to identify and treat those shedding large numbers of eggs and therefore minimise environmental contamination for the foal.

Treatments such as ivermectin, pyrantel or fenbendazole should be administered to those mares with greater than 200 eggs per gram of faeces. Rotating the type of wormer used will help to minimise resistance developing. An annual treatment of all stock with moxidectin or 5 days of fenbendazole during winter will target encysted larval redworm stages. As resistance to fenbendazole is widespread in the UK, moxidectin remains the only effective treatment against larval redworms in many areas. Many experts therefore recommend that moxidectin is reserved for a single annual use at this time, to slow the concerning development of worm resistance to this important drug and protect its efficacy for future use.

Good pasture management with low stocking density and regular ‘poo picking’ helps to reduce the risk of parasitic infection. Keeping stock in groups of similar ages and avoiding younger and older animals grazing the same paddocks is recommended.