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PGE in Sheep

Introduction

Gastrointestinal nematode (roundworm) infections are one of the most important health issues for sheep production in the UK. The most important nematode infections of sheep are:

Telodorsagia circumcincta and Trichostrongylus species - these worms are present in all UK sheep flocks and cause parasitic gastroenteritis (PGE). This disease is typically seen in growing lambs, although disease may also be seen in older animals where control measures are inadequate or where there is concurrent, immunosuppressive disease e.g. Johne’s disease.
Haemonchus contortus – the “barber’s pole worm” is less common in the UK, but can cause sudden and severe disease (haemonchosis). This parasite can affect both lambs and adult sheep.
Nematodirus battus – a parasite which exclusively affects lambs causing nematodirosis. Discussed in detail on a separate sheet

Cause

All sheep carry gut roundworm infections, but adult animals mostly have a good immunity and rarely show signs of disease. These worms all share a similar life-cycle. Eggs are shed in the faeces, develop into infective larvae and emerge onto pastures where they are then consumed by grazing sheep. Development time from egg to the infective stage on pastures typically takes around 2-3 weeks, although this development time is variable and can be shorter or longer depending on the parasite and conditions present. Different worm species live in different locations of the gut (abomasum or intestines), but all go through further stages of development in the gut wall, emerging as adults to start producing eggs 2-3 weeks after initial infection.

With traditional management of sheep on permanent pasture in the UK, PGE results from the build-up and ingestion of very large numbers of infective larvae as the season progresses. Pasture larval burdens typically originate from two sources:

An increase in egg production by ewes around lambing known as the periparturient rise (PPR). Reduced immunity in ewes can lead to a significant increase in the egg production of infected animals, typically over the last two weeks of pregnancy up to eight weeks post-lambing.
Over-wintered infective larvae which have survived on pastures grazed by infected animals (typically lambs) the previous season.

Lambs grazing pastures contaminated through either of the above sources will become infected early in the season. Whilst infections at this stage can be light with no signs of disease, these animals will start to shed eggs onto pastures in increasingly large numbers, acquiring further infection as the season progresses. Without adequate control measures the resulting level of pasture contamination resulting from this process later in the grazing season, usually from mid-summer onwards, can lead to overwhelming levels of infection and clinical disease. The brown stomach worm, Teladorsagia circumcincta, is typically associated with a profuse watery diarrhoea in mid- to late-summer, whilst species of Trichostrongylus (black scour worm) are frequently associated with disease into the autumn and winter, when eight to ten-month-old lambs present with dark coloured, foul-smelling diarrhoea.

Conditions on pastures can play an important role in severity and onset of disease. Larvae require relatively warm and wet conditions to develop and disperse onto pastures. Under dry conditions, as may be experienced during hot summers, larvae present on pastures may die off quickly. However, if lambs were already infected and shedding eggs by this time, new infective larvae may have developed and survived within the faeces to emerge once conditions become favourable. Consequently, acute outbreaks of PGE may be seen in lambs in the weeks following dry periods ended by rainfall as large numbers of new infective larvae emerge onto pastures. Additionally, towards the end of the grazing season, the larvae of both Teladorsagia and Trichostrongylus have the ability to encyst or hypobiose – effectively hibernating in the wall of the abomasum (stomach) or intestine until conditions become favourable the following year. Triggered mass emergence in animals carrying heavy burdens of arrested larval stages the following spring can cause severe clinical cause disease similar to type-II ostertagiosis in cattle.

Figure 1: PGE affecting a growing lamb

Figure 2: Telodorsagia typically causes disease in autumn and winter

Haemonchus contortus, or the barber’s pole worm, is much less common than Teladorsagia and Trichostrongylus, but is of importance due to the acute and severe nature of the disease it causes (haemonchosis). This parasite lives in the abomasum (stomach) and, unlike the other PGE-causing roundworms, feeds on blood. Outbreaks of haemonchosis can also affect animals of all ages. Females worms produce large numbers of eggs (up to 10,000 eggs per worm per day) which may develop to infective stage larvae on pastures in as little as 5 days under optimal conditions. The result is that animals grazing contaminated pastures can accumulate very high burdens of these blood-feeding parasites very rapidly leading to widespread acute and severe clinical disease and deaths.

Figure 3: Haemonchosis does not cause diarrhoea; in chronic infections the most obvious clinical sign is weight loss

Welfare and Economics

Even relatively low worm burdens can affect lambs, resulting in reduced growth rates and subtle but cumulative production losses. High numbers of worms cause more obvious clinical signs and may result in death of badly affected individuals. PGE is estimated to cost the UK around £84 million per year.

Poorly controlled PGE can also lead to heavily soiled fleece and an increased risk of flystrike.

Ewes mostly develop a strong immunity to Trichostrongylus and Telodorsagia unless they have underlying health problems such as Johne’s disease. Haemonchosis, however can affect sheep of any age, and can cause significant losses in previously uninfected flocks.

In addition to the welfare and economic costs directly associated with disease, it is extremely important to note that control of PGE is complicated by the emergence of wormer resistance, which is a serious and growing problem for UK sheep farmers. Recent studies have identified multiple drug-resistant worms in flocks across the UK. Ineffective and repeat treatments add considerable cost to and production system without any guarantee of improving animal productivity. Consequently, steps must be taken to slow the development of further resistance and prevent the introduction of drug-resistant roundworms into flocks through appropriate quarantine measures. And to work around problems of drug resistance where it is identified.

Clinical signs and diagnosis

The severity and range of clinical signs associated with PGE depend on a number of factors including age, breed genetics, nutritional status, particularly protein and trace element intake, immune status and concurrent diseases.

Figure 4: Clinical signs of scour and significant weight loss are seen if worm burdens are high or resilience is low.

Subclinical infections may cause only mild signs, with inappetence, reduced growth rates, poor wool quality and reduced milk production common features in ewes. Larger worm burdens and/or poor resilience against infection, such as in growing lambs grazing contaminated pastures, will lead to more obvious signs including diarrhoea, acute weight loss, dehydration and death.

Haemonchosis presents differently. Unlike the other PGE-causing roundworms diarrhoea is not usually a feature of this disease. Severity of clinical signs depends largely on parasite burden, with a rapid accumulation of large numbers of blood feeding worms leading to acute disease with severe anaemia (gums or tissue around the eyes may be pale to white), lethargy, weakness, and sometimes collapse and death. Ingestion of smaller numbers of infective larvae over a longer time period (weeks to months) results in a more progressive loss of condition leading to emaciation, anaemia, wool break and fluid accumulation such as bottle jaw.

Figure 5: Haemonchus infection can result in severe anaemia

Whilst diagnosis of clinical disease can be made based on age, type of animals affected, grazing history and clinical signs, good control strategies should aim to prevent clinical disease from occurring. Routine diagnostic testing, and in particular faecal worm egg counts (FECs) play a central role in this. Patent roundworm infections can be identified easily by faecal worm egg counts (FECs), are relatively quick and easy to perform and can be used on individual or pooled samples to determine infection status. Whilst high worm egg counts indicate a heavy worm burden, the opposite is not always true with negative or low egg counts, meaning interpretation of results can be challenging in some cases. FECs are also of limited value for assessing encysted larval burdens, and in very acute cases of haemonchosis, where clinical signs may be observed before eggs are being produced. Similarly, where clinical signs of haemonchosis are suspected veterinary advice should be sought, as these can appear similar to those seen with liver fluke infection.

Necropsy of lost stock remains a valuable diagnostic tool in outbreaks of clinical disease to determining what parasites are present. For more information about this seek veterinary advice.

Additional diagnostic tools for PGE include a blood test to identify and assess abomasal damage caused by Teladorsagia infections, and a special staining technique which allows distinction of Haemonchus eggs from those of other roundworms. Antibody ELISAs are also available for some gut roundworm species.

Prevention and control

Control of PGE is predominantly based on devising a safe grazing strategy for susceptible lambs and routine monitoring by FEC and/or liveweight gain. Due to the very real concerns over emerging anthelmintic resistance, dosing must be implemented in a careful, considered manner.

Efforts should be made from early in the season to reduce levels of infection and avoid excessive build-up of infective larvae on pastures later in the season. Lambs turned out to pastures with ewes will inevitably be exposed to roundworms due to the effect of PPR. However, the importance of good ewe nutrition during pregnancy and early lactation must not be underestimated, with provision of a good, high quality, high protein diet in the late stages of pregnancy allowing ewes to maintain body condition and supress egg output more effectively. The traditional practice of dosing all ewes ahead of turn-out to control increased egg production is no longer recommended due to the effect such widespread use of anthelmintics can have on selection for resistance. It has been shown targeting anthelmintic treatments in certain groups of animals, such as those in lower body condition, twin or triplet-bearing ewes and first season ewes, or animals identified as high egg shedders by FEC. As a minimum, 10-20% of ewes should be left untreated. Many producers that routinely monitor faecal worm egg counts are finding a much higher proportion of ewes can be left untreated if fit and healthy without any detriment to lamb performance. Use of long-acting group 3-ML products to control PPR in ewes should also be avoided where possible. Use of long-acting group 3-ML products for this purpose can lead to further unwanted selection for anthelmintic worm resistance, whilst minimising use of 2% injectable moxidectin is of particular importance in order to preserve their efficacy against sheep scab. Independent UK research carried out by the Animal and Plant Health Agency, funded by the Veterinary Medicines Directorate (VMD) in 2018 found no advantage in blanket worming ewes at lambing. Faecal worm egg counts from lambs reared on ewes that were wormed with either a short or long-acting (persistent) wormer were not lower than faecal worm egg counts taken from lambs reared on ewes not treated with a wormer. Specific advice and guidelines on dosing of ewes at lambing time has been published by the SCOPS group.

To further reduce disease risk in lambs later into the grazing season, avoid turn-out onto pastures grazed by last season’s lambs. These may still have relatively high infectivity due to presence of overwintered larvae. These pastures will become useable later in the grazing season as overwintered larvae die off.

Following turn-out, routine monitoring of growing lambs is an essential part of any parasite control plan. Monthly FECs and/or weight gain allows the level of infection and impact on productivity to be assessed at regular intervals and decisions on whether or not anthelmintic treatment is required before clinical disease occurs. Monitoring of individual worm egg counts and/or daily liveweight gain will also enable the practice of Targeted Selective Treatments (TST). TSTs provide a way to control PGE and prevent clinical disease whilst reducing the risk of selecting for resistant worms by only treating the animals within a group most affected by their worm burdens. By using this method, it should be possible to limit treatment to 40-60% of the group. In order to practice TST correctly and safely good record keeping is essential.

Selection and rotation of grazing over the grazing season is another way to limit exposure of lambs to high levels of infection later in the year. Since pasture infectivity and disease risk peaks around mid-summer, moving lambs “safe” pastures around this time can help avoid high worm burdens and outbreaks of clinical disease. Weaning often presents a good opportunity to move lambs onto safe grazing. Options considered safe at this time may include hay or silage aftermaths. Pastures grazed by lambs the previous season may also be considered safe at this time since the majority of overwintered larvae will have died off. On mixed farms it may be possible to rotate pastures annually between cattle and sheep and operate a "modified" two-year clean grazing system. Where animals are moving from heavily contaminated pastures, or where high levels of infection are suspected or identified through FEC monitoring and/or reduced liveweight gain, a “dose and move” strategy may be indicated. However, to avoid seeding the new safe grazing with populations dominated by resistant worms, it is advisable to:

Avoid use of long-acting group 3-ML products
Leave treated animals on dirty pastures for 2-3 days following treatment to allow them to become lightly re-infected with susceptible parasites
Use TSTs and aim to leave a minimum of 10% of animals untreated.

In addition to lambs, rams are less resilient to roundworm infections than ewes, with high worm burdens having a detrimental affect their body condition, performance and fertility. Provision of safe grazing where possible, and monitoring of body condition and/or FECs ahead of tupping will help to avoid any unnecessary complications during this key period.

Figure 6: Bought-in sheep should be effectively quarantines and treated with a quarantine drench to reduce the risk of introducing resistant worms onto you farm.

Care must be taken when buying in stock from other farms. Without effective quarantine protocols it is very easy to introduce resistant worms into the flock. The SCOPS group have issued detailed guidelines on quarantine measures for bought-in animals. For roundworms, it is recommended to assume all animals brought onto the farm are carrying resistant worms and require treatment with at least 2 different worming group products and held away from pastures until 48 hours after treatment.

Figure 7: Targeted anthelmintic treatment can be used to prevent high egg output by the highest risk ewes

Figure 8: Blanket treatment of ewes with long-acting wormers is not recommended as it speeds up the development of resistant worm populations on farm

Treatment

Treatment of PGE involves the use of an effective anthelmintic. The choice and timing of treatments should be discussed with your vet or medicines advisor as part of a general parasite control programme that includes routine monitoring, TSTs and risk assessments to reduce their use where possible and minimise selection for resistant worms. Sheep should also be weighed, equipment cleaned and calibrated, and the correct technique employed to ensure accurate dosing.

There are five major anthelmintic groups effective against gut roundworms:

Wormer group	Colour	Examples of active ingredient
1-BZ	White	Albendazole, Fenbendazole
2-LV	Yellow	Levamisole
3-ML	Clear	Ivermectin, Moxidectin, Doramectin
4-AD	Orange	Monepantel
5-SI	Purple	Dual active (Derquantel and Abermectin)

These products and groups have ranging properties and levels of efficacy against different roundworm species. For example, some group 3-ML products (moxidectin) have a residual activity against certain roundworm species of several weeks whilst others do not. No wormers from the other 4 groups have any claim of residual activity. It is therefore essential to check the datasheets of any products for licence of efficacy and follow the manufacturer’s guidelines on dosing, administration and storage.

Resistance of roundworms to most major worming classes has been observed in the UK, with widespread reports in some cases. If treatment failure is suspected veterinary advice should be sought. FECs can be used to determine whether treatments have been effective by performing faecal egg count reduction tests (FECRTs) which involve performing FECs on 10-12 animals, or up to 10% of the affected group, before and 7-14 days after treatment depending upon the product used. A reduction in egg count of less than 90% indicates treatment failure and drug resistance.

Quarantine treatments are a valuable way of preserving efficacy of existing worming products on farms by ensuring no resistant parasites are introduced. To achieve this, SCOPS guidelines recommend consecutive treatments with at least two different worming groups on arrival before animals are turned out. The reasoning behind this method is that resistant parasites to one product will be treated by the other. Group 4-AD and 5-SI products are often used for this purpose since these are the most recent and resistance to them is much less widespread. Consequently, it is recommended their use to control roundworm burdens on farms more generally should be avoided wherever possible to preserve their efficacy for this important purpose.

To further reduce selection for anthelmintic resistance on farm, try to use treatments with a narrow spectrum of activity. This includes avoiding combination treatments where only one of the active products is required such as fluke-roundworm combination products, or products with a licence of efficacy against multiple parasites, such as certain injectable group 3-MLs that are used to treat both roundworms and sheep scab. Similarly, some flukicidal products, such as nitroxynil and closantel are effective against Haemonchus contortus and can be considered in cases where haemonchosis is the principle concern. Group 1-BZs are in most cases still effective against Nematodirus battus even if resistance has been identified in other roundworm species. Treatment to prevent nematodirosis early in the season can be performed effectively using a group 1-BZ product without selecting for resistance in other roundworm species to other drug classes. - seek advice from a vet or medicines advisor.

Figure 9: Dosing decisions should be made on the basis of faecal worm egg count results or weight monitoring. Not every animal in each group needs to be treated.