Currently, information on best worming strategies for foals and yearlings remains largely anecdotal and it can be difficult to develop a clear plan for this age group. Zoetis Vet, Dr Wendy Talbot has reviewed the recent literature and pulled out key messages to help vets keep owners and breeders on the right course.
Foals and yearlings are usually more susceptible to worms than are adult horses because they have had little chance to develop any immunity. They are more vulnerable to related diseases and tend to have higher egg shedding, which increases the risk of infection. The main parasitic culprits in the UK for foals less than six months of age are large roundworms (ascarids). In older foals and weanlings, small and large strongyles, tapeworms (and pinworms) are the main considerations. Yearlings may also have a second wave of large roundworm infection at 8-10 months of age. Any control strategy will need to take into account the individual circumstances such as stocking density, pasture management and previous disease history.
Guidelines for parasite control in foals suggest a first treatment with fenbendazole at 2-3 months of age, targeting large roundworm, followed by another treatment at 5-6 months. Alternatively, pyrantel may be considered for these doses; however, there may be resistance in some areas and foals with large adult ascarid burdens may be at higher risk of intestinal rupture when this drug is used. At weaning (approximately six months of age), it is advisable to perform a faecal worm egg count (FWEC) to determine if treatment for strongyles is also required; the macrocyclic lactone anthelmintics (ivermectin or moxidectin) are expected to have the highest efficacy. At 9 and 12 months of age treatment for stronglyes and a larvicidal dose for encysted stages of small strongyles (moxidectin or a 5 day course of fenbendazole) are indicated. A tapeworm treatment should be included with one of these doses.
For yearlings, two methods have recently been proposed for worm control: the first involves using faecal worm egg counts (FWECs) at more frequent intervals than for adults to guide dosing for strongyles in the grazing season. The second suggests three baseline treatments targeting strongyles in spring, summer and late autumn, with FWECs in between to identify and treat any still shedding high numbers of eggs, up to a maximum of six treatments per year. Macrocyclic lactones have the highest expected efficacy against small strongyles and should form the basis of treatments; however, pyrantel can be considered for some treatments where it has been shown to have continued efficacy. Both strategies include a larvcidal treatment for encysted small redworm combined with testing or treating for tapeworm in late autumn. The inclusion of one or two doses of a macrocyclic lactone each year (usually already used for small strongyle control) should be sufficient to prevent large strongyle related disease.
Wendy said: “The wide differences in the circumstances of foals and yearlings, together with geographical variations in resistance patterns and a lack of consensus on best practice in this group means it is not possible to define a single approach to a worming plan. An important point to note is that FWECs may give surprising results because higher counts are generally seen in youngstock.”
To help you keep your clients on the right course Wendy has summarised the key points to consider when devising a worm control strategy for youngsters:
· Clean pasture is key to keeping foals and youngstock healthy. Regular removal of droppings is crucial to a successful worm control plan.
· All foals are considered to be susceptible and at risk of acquiring ascarid infection. Some degree of anthelmintic intervention should be considered and in most circumstances administration at 2-3 months of age and again at 5-6 months of age is advisable.
· Older foals are primarily at risk of strongyle infection; macrocyclic lactone anthelmintics have the highest expected efficacy.
· Worm egg counts are likely to be higher in yearlings compared to adults and interpretation of them requires good knowledge of all the circumstances including management, clinical history, previous worm control and test results. Cut-off values for treatment have been suggested to be between 100-500 epg.
· Moxidectin for the treatment of encysted small redworm is advised for all youngsters (> 6 months of age) in late autumn.
· Faecal worm egg counts are advisable in late winter/start of spring to identify high shedders. These should be treated as necessary with moxidectin particularly after a mild winter.
· New arrivals should be dosed with moxidectin and then quarantined for three days.
· A yearly faecal egg count reduction test is advised to check efficacy against strongyles and ascarids for all class of drugs used on the premises.
For further information on worming together with some equine specific CPD visit: https://blog.vetsupportplus.com/