Reports of resistance and re-emergence in the worm world mean that targeted treatment is more important than ever. Cara Hallowell-Evans MRCVS debunks some of the myths of modern worm control

Myth one

Using ivermectin in spring will keep small redworm under control.

Fact

Confusion sometimes surrounds the significance of cyathostomins (also known as small redworm or strongyles), a group of parasites affecting almost all grazing horses. It is the immature larvae of the small redworm that causes disease, rather than the adult worms.

Larvae are the juvenile stage of gut worms. The larvae go through
a period of development within the wall of the intestine to enlarge and shed their outer sheath within the gut wall.

Horses become infected by ingesting infective larvae, which have developed on pasture from eggs passed in faeces. The warmer the weather, the faster the larvae develop to the infective stage.

Larvae embed themselves (become encysted) within the wall of the intestine, where they develop. This development can be arrested within the wall for months or even years, however, and we still don’t know the precise factors that trigger subsequent reactivation.

Clinical disease, termed larval cyathostomosis, ensues when large numbers of larvae emerge and cause damage to the interior intestine wall. This results from the accumulation of large burdens of larvae in the mucosal lining and is common in youngstock, or as a consequence of poor management in adult horses.

Treating a horse with ivermectin during spring is a known risk factor for this simultaneous emergence — possibly due to the decimation of the adult population that encourages larvae to emerge
and “fill the gap”.

Larval cyathostomosis is most common in spring, although it can occur at any time. Signs include worms in the faeces, severe diarrhoea and oedema (pouches of fluid under the abdomen and jaw).

Sadly, severe cases have a survival rate of just 50%.

Myth two

Large redworm rarely cause a major problem.

Fact

Large redworm can also cause serious disease. As with small strongyles, eggs passed in the faeces develop into infective larvae on pasture and are ingested by the grazing horse. Clinical disease occurs when even small numbers of large redworm larvae migrate to major abdominal blood vessels, causing clots and obstruction that can lead to severe colic.

Large redworm infection is uncommon in the UK in well-managed horses, due to decades of intensive worming. A single annual dose of moxidectin is thought to prevent establishment of infection on farms, but it is important to be vigilant to ensure this parasite does not re-emerge as the reduced use of worm-control drugs is encouraged.

A cyathostomin,  or small redworm: this parasite — here at the infective larval stage which would be ingested by the horse from pasture — affects almost all grazing horses and is most common in spring

A cyathostomin, or small redworm: this parasite — here at the infective larval stage which would be ingested by the horse from pasture — affects almost all grazing horses and is most common in spring

Myth three

Regular treatment is better than diagnostic-based treatment.

Fact

One of the most important and frequently overlooked facts about worm egg counts is that there is little relationship between the eggs counted and the number of adult worms present. So what’s the point in performing the test?

Egg counts are really useful to identify and target treatment to those horses excreting large numbers of eggs onto pasture — the ones who are therefore contributing most to parasite transmission.

Targeting treatments in spring and summer aims to reduce pasture contamination, decreasing the chance of ingestion of large numbers of larvae and lessening risk of disease. As a method of worm control, targeting is both cost-effective and sustainable.

In addition, it is impossible to identify horses with heavy infection by eye. The only way to establish the presence of parasitic infection is with diagnostic testing, which is more effective for “wormy” poor doers than repeated treatment.

Myth four

A negative tapeworm test means a horse is free from infection.

Fact

Awareness of the impact that high tapeworm burdens can have on equine health is now better than ever, yet testing for these parasites can be challenging.

Tapeworm transmission takes place when segments of the mature worm containing eggs break off sporadically, passing onto pasture in faeces. These eggs are then consumed by a forage mite, which is ingested and digested by the horse.

An infective tapeworm is then released into the gut, where it attaches itself at the junction of the ileum (the end of the small intestine) and caecum (a pouch at the beginning of the large intestine). This causes irritation and potentially colic.

While faecal testing for tapeworm is available, intermittent egg release means that results cannot be taken at face value. A positive result gives a definite indication of active infection, because eggs aren’t possible without the presence of adult parasites. Negative results can never be relied upon to indicate freedom from infection, however, as testing relies on egg packets being shed into the examined portion of faeces.

The ELISA method, which examines equine blood or saliva for antibodies produced in response to tapeworm infection, can be used for individuals but is best used for co-grazing horses to examine herd infection and transmission levels. Interpretation is challenging and it is important to consider that antibodies may persist for months after clearance of tapeworms by treatment.

If test results are moderate or high, it indicates that biannual treatment in spring and autumn is needed. Annual treatment is often sufficient where results are low or negative, but biannual testing is still recommended to ensure control measures are effective.

Although worm  egg counts do not predict how many adult worms will emerge, they are  a useful tool for identifying and targeting treatment to those horses contaminating pasture

Although worm egg counts do not predict how many adult worms will emerge, they are a useful tool for identifying and targeting treatment to those horses contaminating pasture

Myth five

Picking up droppings is a waste of time.

Fact

Good pasture management involves regular dung removal and does make a huge difference to parasite burdens.

Traditional methods such as harrowing and rain washing (letting heavy downpours do the job) and even the more modern paddock sweeper, have been shown to be less useful control methods as they distribute larvae all over the pasture.

Methods that remove, rather than spread, manure, by hand or vacuum, are more effective. Weekly removal will prevent dispersal of dung, and therefore larvae, by rainfall or insects. Additional measures such as clipping rough patches to remove protective larval habitats, or grazing sheep/cattle after horses, can help.

Ref: Horse & Hound; 18 February 2016