Worming CPD January 2016


Management of gastrointestinal parasites on stud farms

By Douglas Palmer BVMS MRCVS, Norbrook veterinary advisor for northern UK.

The challenge of managing parasites on a stud farm is four-fold:

• High infection pressures due to a relatively high proportion of the equine population being youngstock: they are more likely to be high shedders of eggs onto pasture, and more likely to succumb to clinical disease
• Increased resistance to multiple drugs in parasites of interest
• Increased animal movements on and off the stud farm increase risk of importation of parasites, with or without drug resistance, necessitating stringent quarantine policy
• Limited number of products licensed for use in pregnant mares and young foals

Traditionally, interval dosing was used to meet the needs of the stud farm i.e. all horses were routinely wormed at set intervals throughout the year. However, this system promoted over use of anthelmintics and increased selection for worms possessing anthelmintic resistant genes. Due to the development of resistance in worm populations affecting equids, an interval-based approach is no longer sufficient to control internal parasites and a targeted regimen is recommended. 

The targeted approach to worm control

In a targeted plan only those horses showing a high worm burden (demonstrated by a faecal egg count) are treated with an anthelmintic product. This ensures that a proportion of worms on the stud farm are left unexposed to anthelmintics, either because they are on the pasture, or residing within untreated animals. 

This slows down the selection for resistant worms, because the higher the proportion of worms which are not exposed to anthelmintic, the more dilution there is of the population of resistant worms on the stud premises. The result of this is that the majority of worms remain susceptible to anthelmintic treatment and the proportion of resistant worms is kept to a minimum. This concept is termed ‘in refugia’.

On the stud farm a targeted regimen needs careful management as parasites can build up quickly in the environment and young foals have no natural immunity to worm infections. In severe cases of worm infestation foals can develop diarrhoea, weight loss and sometimes colic which can be fatal. The main objectives of a targeted plan concern negating the likelihood of young susceptible animals suffering clinical disease, whilst also trying to slow the development of resistant populations of worms on the premises. Resistance is a one-way street; once the worm population on farm is resistant to a class of anthelmintics, there is no going back!

In this article we look at the main worm species of concern on the stud farm, the control measures that can be put into place and where anthelmintics can fit into this regime. 

Parasite species affecting the stud

Strongyloides westeri (threadworms)

These are found in the small intestine and are less than 1cm in length. Only female worms are found in the intestine where they lay eggs produced by asexual reproduction. Parasitic larvae enter horses and foals by ingestion from the pasture or skin penetration; foals can also become infected via the mares’ milk.

Once ingestion or penetration of the skin has occurred, the larvae migrate to the lungs via the blood. They travel to the larger airways and trachea, from where they are coughed up into the mouth, then swallowed back down to the gastrointestinal tract. They mature to adults once in the small intestine and start producing eggs. 

In older animals after penetrating the skin, they can accumulate in the subcutaneous tissue and then move to the mammary glands when lactation starts. Foals can therefore become infected through drinking the milk from the mother. Adult worms can start producing eggs in the small intestine of the foal from about one week after foaling. 

Clinical signs of Strongyloides infection can be subclinical, but high burdens in foals cause diarrhoea and weight loss. Occasionally dermatitis can also be seen after larvae have penetrated the skin. 

Foals not showing any symptoms of disease may also shed high numbers of eggs in their faeces in the first weeks of life, before they have established a solid immune response. Environmental contamination can therefore build up over a breeding season on a stud farm exposing later-born foals to a greater risk. Most anthelmintics, including macrocyclic lactones, e.g. ivermectin or moxidectin are effective against this parasite. 

Parascaris equorum (ascarids)

This is thought to be the most significant worm for young foals. The adult worms are long (up to 40 cm) and white in colour. They live in the small intestine and produce large numbers of eggs in the foals’ faeces. These eggs are not infective until the larvae develop inside. For this to happen they need a warm humid environment and it usually takes several weeks for the larvae to develop. These eggs are sticky, very thick walled and can persist in the correct (cool and moist) environment for up to five years. They are also resistant to many disinfectants. 

When the infective eggs are ingested they hatch and the larvae burrow through the intestinal wall and then travel to the liver. In the liver they cross into the blood circulation and travel to the lungs. After migrating through the lungs to the trachea they are coughed up and swallowed. Once in the small intestine they develop into adults and start producing eggs. Newborn foals can start shedding eggs in faeces within 12-13 weeks. High numbers of eggs can be found on pastures that are heavily stocked and used for youngstock for many consecutive years. Inadequate cleaning of stables can also lead to a build-up of eggs in the housing. 

In older animals, particularly yearlings, no clinical signs are seen but they can continue to shed eggs in faeces which contaminate the environment and are an important link in the transmission of these worms. 

Heavy infestations cause generalised malaise, poor growth, poor coat, diarrhoea, lowered resistance to other diseases and colic can also be seen. In severe infestations intestinal obstruction and perforation can occur resulting in the need for surgery. Lung damage caused by migrating larvae may result in fever, coughing and a nasal discharge. 

Treatment is possible with macrocyclic lactones, benzimidazoles or pyrantel although resistance to several groups has been reported in the UK. 

Cyathostomes (small strongyles or small redworm)

These form part of the strongyle group of worms. There are ten common species which have a direct non-migratory life cycle. Eggs pass into the environment via the faeces and under optimum conditions (shade, moisture and moderate temperature) these will hatch and become infective L3 larvae on the pasture. These larvae are then consumed and will enter the walls of the large intestine where they become encysted and remain in a small nodule for between 7-18 weeks before breaking back out into the intestinal lumen. Clinical signs associated with adult cyathostomins include ill-thrift, diarrhoea and anaemia. Resistance to all classes of anthelmintics in adult cyathostomes has been reported in the UK, particularly to benzimidazoles.

Mares are initially the main source of infection as they can carry appreciable burdens and pass on large numbers of eggs whilst potentially not showing any clinical signs. In areas such as the United Kingdom where the winters are cold and the summers are mild, large numbers of worms will build up over the spring and summer so that massive contamination of pastures with infective larvae occur in late summer and early autumn when young susceptible horses are present.

In heavy infections, emergence of large numbers of larvae over a short period causes inflammation of the large intestine with small ulcers where larvae have emerged, haemorrhaging and excess mucous production. Clinically, rapid weight loss, diarrhoea and colic can be seen. Unless treated early the prognosis is guarded. 

Outbreaks of disease due to simultaneous emergence of cyathostomes commonly occurs in late winter or early spring, however this can happen at any time of year and may also be triggered by anthelmintic treatment. This disease process is known as larval cyathostomosis and can be seen in horses of all ages, most commonly in those less than 5 years old and older foals. 

Larval cyathostomosis can be challenging to diagnose as faecal egg counts can be zero as it is the emergence of the encysted immature stages that cause disease. The inhibited mucosal stages are insusceptible to many treatments. All foals should receive treatment with moxidectin or five day benzimidazole at the end of the grazing season to reduce levels of infection with cyathostome larvae².

Anoplocephala perfoliata (tapeworm)

This parasite’s life cycle is more complex. The eggs are produced in the horses’ faeces within a tapeworm segment which disintegrates quickly releasing the eggs. These are consumed by forage mites and within the mite they develop into a cysticercoid larval stage in 2-4 months. These mites are then ingested by the horse and 1-2 months later the adult tapeworms are found in the intestines usually around the ileo-caecal junction and can cause ulceration and thickening here. 

Heavy infestation may interfere with gut motility and cause colic. Diagnosing infections can be tricky as the eggs can be difficult to find in faeces. There are antibody tests that can be used on either blood or saliva samples. Treatment for tapeworm is usually with a pyrantel or praziquantel based anthelmintic. All horses and foals should be treated as a minimum at the end of the grazing season and again in the spring if blood or saliva testing has come back positive. 

Additional strategies to reduce reliance on anthelmintics

In summary there is no definitive protocol that will be appropriate for every stud farm so stud managers, SQPs and vets should work together to develop a suitable and sustainable protocol. 

Minimising infection pressure from the environment is paramount. Disinfection and regular cleaning of all stables and foaling boxes throughout the breeding season is essential. There are limited options for grazing management however cutting the roughs which offer protection for the larvae, or mixed or alternated grazing with ruminant species can help reduce contamination as the parasites are host specific. 

Use clean paddocks for the youngest foals and avoid moving them onto pasture where older foals and yearlings have been. Removal of faeces twice weekly from all paddocks and daily from nursery paddocks is very effective provided heavy rainfall does not disperse the faeces first. This can be by hand or on tractor or quad-mounted mechanical devices. 

New arrivals should not be mixed before having a quarantine period of three days and treatment with moxidectin. 

Consider grazing management strategies, where possible, such as removing faeces from the paddocks and alternate or co-grazing with ruminants. 

Appropriate treatment should be given to all foals for encysted cyathostomes and tapeworms at the end of the grazing season with a larvicidal dose of wormer and tapeworm treatment. Treatment for Strongyloides westeri and Parascaris equorum should be discussed with the animal health advisor (vet or SQP) as infection pressures will differ in separate management systems. 

Regular faecal egg counts every 1-2 months from all stock¹ and antibody testing for tapeworm is essential to identify those shedding significant numbers of worms who should therefore be treated with anthelmintics. This will maintain a proportion of worms ‘in refugia’ who retain susceptibility to anthelmintics. Treated horses should not be moved onto clean pasture immediately as this will allow any surviving resistant worms to contaminate the fresh field. 

When using an anthelmintic follow the manufacturers’ guidelines closely. Check that the product is licensed specifically for the age, sex and reproductive status of the individual animal that you are treating. 

Use a weigh bridge or a weigh tape to ensure an accurate dose is given. Ensure the whole dose is taken. 

If resistance is suspected in the population of parasites on a stud, the veterinary surgeon should be contacted to discuss performing a faecal egg count reduction test (FECRT).

In conclusion, traditional interval-based dosing previously adopted for all animals on stud farms, whilst simple and uncomplicated to administrate, is unsustainable, and has contributed to increasing reports of anthelmintic resistance in equine gastrointestinal parasites across the UK. Special attention must be paid to stud farm health planning, to develop a strategic worm control policy, to preserve susceptibility in worms on premises, whilst preventing outbreaks of clinical disease in youngstock. 

About the author: DOUGLAS PALMER BVMS MRCVS qualified from the University of Glasgow Veterinary School in 2002 and has since worked in mixed practice in the north east of England. Douglas joined Norbrook in July this year as veterinary advisor for northern UK.

1. Practical management of parasites on breeding premises, Proceedings from BEVA Congress 2015.