Winter worming 

10th Januray 2018

Rosie Naylor BVetMed MVetMed DipACVIM MRCVS, RCVS Specialist in Equine Internal Medicine

Parasite control in horses differs over winter from other times of the year. This is due to the lifecycle of the worms and the effect that climatic conditions can have on their development. Winter is the time to think specifically about bots and small strongyles in addition to the year round threat of tapeworm and roundworms.

Adult large and small strongyles are detected by faecal egg counts (FECs), however FECs do not detect larval stages of bots and small strongyles (also known as Cyathostomins) or tapeworm. 


Stomach bots are larvae of flies from Gasterophilus species. The flies lay their eggs on the haircoat of the horse in late spring through to the autumn. Grooming stimulates the eggs to hatch into larvae which are then ingested by the horse. Eventually the larvae reach the stomach and attach to the stomach lining, where they are frequently seen during gastroscopy. As fly activity ceases after regular frosts, bots over-winter as larvae inside the horse. Worming at this time with an effective wormer such as ivermectin or moxidectin should allow bots to be eliminated, without the chance of reinfection from flies in the environment.

Figure 1: Bot fly larvae in a horse’s stomach


Tapeworm have a 6 month lifecycle, therefore horses are traditionally treated for these worms in the spring, prior to the grazing season and again in the autumn. Tapeworm are not reliably detected with faecal egg counts, although tests have recently been developed to measure antibodies to tapeworm to diagnose infection. These tests can be performed on saliva or blood samples. If a horse has not been treated or tested for tapeworm in the last 6 months, this should be incorporated into the winter worming regime.

Small Strongyles

Small strongyle larvae are ingested from the pasture by the horse and some of these larvae undergo arrested development within the wall of the intestines, particularly during the winter months. The risk of larvae encysting within the intestinal wall is greater in horses grazing heavily infected pastures throughout the year, overstocking and short grazing. Encysted larvae can remain in the intestinal wall for 2 years but they often emerge in the spring. Encysted larvae are a normal part of the lifecycle of the small strongyles, but when large numbers of encysted larvae emerge simultaneously from the intestinal wall, this can lead to intestinal damage and a condition known as larval cyathostominosis. This is characterised by diarrhoea and weight loss. Younger horses less than 5 years of age are most susceptible. 


Figure 2: Small redworm larvae can occasionally be seen in the droppings, but there is currently no test for the encysted stages.

It is recommended that all horses receive a treatment for encysted small strongyles once a year. Traditionally this has been performed over winter. Two drugs treat encysted small strongyles, these are moxidectin and fenbendazole. However, resistance to fenbendazole is widespread. If fenbendazole is used, it is recommended to perform a faecal egg count reduction test to confirm that treatment has been effective. As moxidectin remains the only effective treatment for encysted strongyles in many areas, many experts recommend that this drug should be reserved for this use and only given once a year to minimise the development of small redworm resistance to this vital product (Coles 2009). Throughout the rest of the year other wormer drugs should be used.

The colder weather can make it more challenging to maintain good paddock care during the winter months, but it remains important to stay on top of poo-picking and pasture management during this time.


Speak to your vet or SQP for direction on worming over winter


Regardless of the wormer used, ensure that the correct dose is given for your horse’s bodyweight. As bodyweight can fluctuate throughout the year use a weigh tape or scales to determine this accurately. A winter coat can make visual estimation even harder! Remember that not all syringes contain the same amount of drug, for larger horses make sure there is enough drug in the syringe, under-dosing will lead to ineffective treatment and is a major risk factor for the development of resistance.


Ensure that your horse receives all of the correct dose. Worming tablets are an alternative for horses difficult to worm with a syringe.



Coles, G. (2009) Anthelmintic resistance in equine worms Vet Times.