Worming young horses

The two parasites of concern for young horses are ascarids and small strongyles. This is largely due to their developing immunity which makes them more susceptible to parasite- related disease compared to adult horses. Immunity to parasites develops with time and some exposure is therefore beneficial to promote the development of immunity. Rotational or calendar-based worming is not recommended in any horse, however, a careful balance needs to be struck between reducing wormer use and preventing parasite disease.

Small strongyles (cyathostomins) are considered the most important parasite in horses due to their high prevalence and potential to cause serious disease. When a parasite management program is insufficient, large numbers of encysted, immature larvae may accumulate in the walls of the intestinal tract and emerge in mass numbers resulting in severe illness. Signs of disease include rapid and severe weight loss and diarrhoea. This disease is most common in horses under 6 years old but it can be seen in any age group. Mortality occurs in approximately 50% of cases. Moxidectin is usually considered effective against encysted larvae and should be administered yearly in young horses, regardless of the fecal egg count result. Adult horses (over six years of age develop robust immunity to cyathostomins and can tolerate large burdens without developing any clinical disease. It is important to note that moxidectin resistance has recently been reported in the USA, UK and Australia and careful use of this wormer is important to ensure it is effective in high-risk horses.

Ascarids are an important cause of colic in foals, but ascarid impactions can be seen in older horses. Adult ascarids may be up to 25cm long and 4mm in diameter and when present in large numbers, can obstruct the small intestine. This can result in colic symptoms and are usually seen in horses who have recently been wormed. Some cases may respond to medical treatment, however others require surgery. Resistance of ascarids to wormers such as ivermectin and moxidectin is common. If ascarids are present on a faecal egg count, then a benzimidazole wormer should be administered. Pasture management is critical where large numbers of foals and young horses reside.

Gold Standard (diagnostic-driven) worming plans are now the cornerstone of preventing the development of resistant parasites. In young horses, faecal egg counts will guide your choice of wormer use depending on what parasite you are targeting. Resistance of strongyles to benzimidazole wormers is now widespread, however this wormer is still effective against ascarids. Knowing which parasite you are targeting based on the results of a faecal egg count is very important in guiding your choice of wormer. As a rule of thumb, faecal egg counts should be performed every 2-3 months in young horses.

Young horses often return high faecal egg counts and caution should be exercised when interpreting these results. Remember, the results of your faecal egg count do not correlate with the worm burden in your horse. A high faecal egg count does not mean your horse has a higher parasite burden than a horse with a lower egg count. The faecal egg count is simply a tool to:

1. Detect which horse is shedding the most amount of parasite eggs onto pasture and;
2. What parasite you should target with your wormer

Pasture management is critical to any parasite management program to minimise exposure to the herd. If undertaken correctly, this will markedly reduce the need for wormer use. Exposure to infective larvae can be limited by regular poo-picking, co-grazing with other species, and reducing stocking density. To be effective, poo-picking should be performed as often as possible (ideally a minimum of twice per week). Dose-and-move strategies used to be recommended to remove exposure to parasites, however, this has inadvertently increased the spread of resistant parasites. Dose and hold strategies are now advised to prevent infection of paddocks with resistant larvae.

It is clear that when dealing with young horses, a balance needs to be struck between preventing colic and other parasite-related diseases while minimising the development of resistance. With a Gold Standard (diagnostic-driven) worming program and good pasture management, it should be possible to reduce the reliance on wormers without compromising the health of individual horses.

Young Horse Worming Recommendations

Take-home messages:

• Young horses have a lower immunity to parasites and require different management compared to adult horses.
• The majority of clinical disease associated with larval cyathostomin cases occur in horses under 3 years of age
• Faecal egg counts should be performed every 2-3 months in this age group to direct choice of wormer
• Annual treatment with moxidectin is recommended in young horses regardless of the faecal egg count
• Proactive grazing and pasture management is very important for young horses who are more susceptible to parasitic disease