Diseases & Solutions
Gastrointestinal Worms in Sheep
Gastrointestinal worm infections have the potential to reduce lamb growth rates by up to 40%, leading to delayed drafting for sale, increased feed bills, strain on labour where clinical symptoms are presented and obvious welfare implications for the animals. Certain species of gastrointestinal worms can also affect adult sheep.
The typical life cycle of a gastrointestinal worm described below (applies to Teladorsagia, Haemonchus and Trichostrongylus) varies in length depending on prevailing weather conditions.
Within the sheep itself, there is no multiplication. Adult female worms lay eggs that pass into faeces and hatch into L1 larvae, which develop into L2 and feed on bacteria in the faeces. The development of L2 into L3 larvae, the stage that can infect the sheep when ingested, is temperature dependent. Rainfall is critical for L3 larvae too. It facilitates movement away from faecal pellets and on to pasture.
In the springtime, development from and L2 to infective L3 larvae can take up to three months, while warmer temperatures in the summertime sees L3 development in less than a fortnight.
An ingested L3 larvae embeds into the wall of the intestine or stomach and eventually develops into an L4 and then an adult worm. The period from ingestion to adulthood (appearance of eggs in faeces) takes 16-21 days. The lifespan of an adult worm is typically around 3 months.
Gastrointestinal worm infections have the potential to reduce lamb growth rates by up to 40%, leading to delayed drafting for sale, increased feed bills, strain on labour where clinical symptoms are presented and obvious welfare implications for the animals. Certain species of gastrointestinal worms can also affect adult sheep.
The typical life cycle of a gastrointestinal worm described below (applies to Teladorsagia, Haemonchus and Trichostrongylus) varies in length depending on prevailing weather conditions.
Within the sheep itself, there is no multiplication. Adult female worms lay eggs that pass into faeces and hatch into L1 larvae, which develop into L2 and feed on bacteria in the faeces. The development of L2 into L3 larvae, the stage that can infect the sheep when ingested, is temperature dependent. Rainfall is critical for L3 larvae too. It facilitates movement away from faecal pellets and on to pasture.
In the springtime, development from and L2 to infective L3 larvae can take up to three months, while warmer temperatures in the summertime sees L3 development in less than a fortnight.
An ingested L3 larvae embeds into the wall of the intestine or stomach and eventually develops into an L4 and then an adult worm. The period from ingestion to adulthood (appearance of eggs in faeces) takes 16-21 days. The lifespan of an adult worm is typically around 3 months.
Carryover & Immunity
Warm weather increases the rate of activity and metabolism in L3 larvae on pasture. If they are not ingested during the summer, a lot will die off. The larvae fare better during colder weather and some deposited late in the year can overwinter on pasture. However, there are differences between the species in this regard.
Lambs typically develop immunity to gastrointestinal worms (Teladorsagia, Haemonchus and Trichostrongylus) after continuously ingesting L3 larvae for a 3-4 month period. Immune animals will still carry small populations of worms; however the development of these worms will be retarded by the sheep’s own immune system and they will thus be smaller and reproduce less frequently. There is also an energy cost associated with this immunity, with research suggesting that its maintenance incurs a 15% loss in productivity.
It is widely accepted that most sheep will have a high level of natural immunity after one full year of grazing. However, very high burdens can still be dangerous, particularly in younger animals. In addition, changes to a ewe’s metabolic state around lambing causes the ‘periparturient relaxation in immunity’ (PPRI) – or spring rise. The PPRI commences between 2-4 weeks pre-lambing and persists for 6-8 weeks after. During this time, parasites already in the sheep are not expelled, adult worms produce more eggs and an increased number of ingested larvae will proliferate.
As a result, ewes’ faecal egg output drastically increases and they become a source of contamination on pasture, increasing egg and larvae burdens for other sheep later in the season. The spring rise will affect thinner ewes and ewes carrying twins or triplets to the greatest extent. Its effects can be curbed by supplementation with rumen-undegradable protein (e.g. soya bean meal).
According to SCOPS, there are 15 species of gastrointestinal nematode worms affecting British sheep. Of these, there are four of concern for farmers, vets and SQPs when considering worm control measures.
Thin-necked Intestinal Worm
Nematodirus battus
Nematodirus is unique in several ways. Unlike other gastrointestinal worms, much of the clinical damage is caused by the larval stages of Nematodirus. The parasite’s life cycle is also extremely long, and eggs can survive through long cold periods on pasture. A mass hatch of these eggs occurs when a cool period of weather is followed by a sudden increase in temperature bringing averages above 10°C. Lambs will then ingest the hatched larvae. With a heavy burden, the intestine will become compromised, preventing the absorption of nutrients including water. Infected lambs become rapidly dehydrated and a tell-tale sign of an infection is when lambs are seen to be gathered around water troughs.
In general, sheep farmers treat against Nematodirus in the month of April/early May, with timing of treatment dependent on prevailing weather conditions. Often, agricultural media organizations will release Nematodirus warnings when weather conditions are favourable for a mass hatch. It must be noted that symptoms of a Nematodirus infection are typically concurrent with coccidiosis symptoms and as the latter is caused by protozoa, faecal egg/oocyst analysis is crucial for accurate diagnosis.
Benzimidazoles (white wormers) and levamisoles (yellow wormers) are typically used to treat Nematodirus. While benzimidazole resistance is an issue on certain sheep farms, it is rare in Nematodirus populations at present. The gastrointestinal worm species that have shown resistance to white wormers thus far tend to proliferate later in the season.
Lambs typically develop their own natural immunity to Nematodirus within one month of first infection.
As prevailing temperatures rise during the season, the risk of Nematodirus symptoms decreases and Teladorsagia and Haemonchus become the main worm species of concern, with Trichostrongylus coming into the fold later in the season.
Small brown stomach worm
Teladorsagia circumcincta
Teladorsagia is more commonly known as ostertagia and the clinical form manifests itself in two forms, known as Type I (occurring during the summer) and Type II ostertagiosis (during the winter months). With Type II ostertagiosis, the parasite’s development in the animal becomes temporarily arrested at the L4 stage during the autumn, resuming during the winter month and often causing clinical disease in yearling sheep.
An infection with Teladorsagia will yield the classic gastrointestinal worm symptoms of profuse scour and loss of thrive. Animals appear empty as a result of inappetence and reduced feed intake. Teladorsagia L3 larvae develop into L4 and adults in the stomach glands and the process of emergence from these is responsible for mechanical damage to the organ’s mucosal layer.
Barber’s Pole Worm
Haemonchus contortus
Haemonchus c. is a sinister parasite in that its symptoms don’t become obvious until a relatively advanced stage of infection. Scour is not a symptom of Haemonchus burden in an animal. Both larval and adult stages of the parasite feed on blood and an adult can remove 0.05ml of blood daily. A heavy infestation could see up to 5000 worms per animal – equating to potential blood loss of 250ml in a single day. Expectedly, anaemia is the principle symptom of Haemonchus and first clinical signs can involve rapid weakening of animals that appear in good body condition. Indeed, infected animals may collapse if driven. Inspection of the animal’s conjunctivae (eyes) and gum is a common method for diagnosis. If a burden is high enough, sudden death is possible. Haemonchus risk is not confined to lambs and can affect adults too. Eggs will develop into larvae in warm, wet conditions and though they do not survive a over winter well, a female worm can lay 10,000 eggs daily so pastures can become infective very quickly. The main way that Haemonchus overwinter is in arrested L4 state in the animal, similar to Teladorsagia.
Black Scour Worm
Trichostrongylus spp.
Symptoms of Trichostrongylus are generally seen later in the year and, as the name suggests, the principle sign is soft, dark faeces. It is often a disease of purchased lambs or replacements. Weight loss and death, where a burden is high enough, can follow. The parasite generally affects younger sheep. Of the four worm species mentioned here, Trichostrongylus would be considered the least pathogenetic. However, this is assuming a farm has equal burdens of all four parasites. A high enough burden of any of the gastrointestinal worms cited here on a holding can be lethal.
Use the solutions below with our FAQ and Best Practice sections to make good decisions on gastrointestinal worm control in sheep.
Carryover & Immunity
Warm weather increases the rate of activity and metabolism in L3 larvae on pasture. If they are not ingested during the summer, a lot will die off. The larvae fare better during colder weather and some deposited late in the year can overwinter on pasture. However, there are differences between the species in this regard.
Lambs typically develop immunity to gastrointestinal worms (Teladorsagia, Haemonchus and Trichostrongylus) after continuously ingesting L3 larvae for a 3-4 month period. Immune animals will still carry small populations of worms; however the development of these worms will be retarded by the sheep’s own immune system and they will thus be smaller and reproduce less frequently. There is also an energy cost associated with this immunity, with research suggesting that its maintenance incurs a 15% loss in productivity.
It is widely accepted that most sheep will have a high level of natural immunity after one full year of grazing. However, very high burdens can still be dangerous, particularly in younger animals. In addition, changes to a ewe’s metabolic state around lambing causes the ‘periparturient relaxation in immunity’ (PPRI) – or spring rise. The PPRI commences between 2-4 weeks pre-lambing and persists for 6-8 weeks after. During this time, parasites already in the sheep are not expelled, adult worms produce more eggs and an increased number of ingested larvae will proliferate.
As a result, ewes’ faecal egg output drastically increases and they become a source of contamination on pasture, increasing egg and larvae burdens for other sheep later in the season. The spring rise will affect thinner ewes and ewes carrying twins or triplets to the greatest extent. Its effects can be curbed by supplementation with rumen-undegradable protein (e.g. soya bean meal).
According to SCOPS, there are 15 species of gastrointestinal nematode worms affecting British Sheep. Of these, there are four of concern for farmers, vets and SQPs when considering worm control measures.
Thin-necked Intestinal Worm
Nematodirus battus
Nematodirus is unique in several ways. Unlike other gastrointestinal worms, much of the clinical damage is caused by the larval stages of Nematodirus. The parasite’s life cycle is also extremely long, and eggs can survive through long cold periods on pasture. A mass hatch of these eggs occurs when a cool period of weather is followed by a sudden increase in temperature bringing averages above 10°C. Lambs will then ingest the hatched larvae. With a heavy burden, the intestine will become compromised, preventing the absorption of nutrients including water. Infected lambs become rapidly dehydrated and a tell-tale sign of an infection is when lambs are seen to be gathered around water troughs.
In general, sheep farmers treat against Nematodirus in the month of April/early May, with timing of treatment dependent on prevailing weather conditions. Often, agricultural media organizations will release Nematodirus warnings when weather conditions are favourable for a mass hatch. It must be noted that symptoms of a Nematodirus infection are typically concurrent with coccidiosis symptoms and as the latter is caused by protozoa, faecal egg/oocyst analysis is crucial for accurate diagnosis.
Benzimidazoles (white wormers) and levamisoles (yellow wormers) are typically used to treat Nematodirus. While benzimidazole resistance is an issue on certain sheep farms, it is rare in Nematodirus populations at present. The gastrointestinal worm species that have shown resistance to white wormers thus far tend to proliferate later in the season.
Lambs typically develop their own natural immunity to Nematodirus within one month of first infection.
As prevailing temperatures rise during the season, the risk of Nematodirus symptoms decreases and Teladorsagia and Haemonchus become the main worm species of concern, with Trichostrongylus coming into the fold later in the season.
Small brown stomach worm
Teladorsagia circumcincta
Teladorsagia is more commonly known as ostertagia and the clinical form manifests itself in two forms, known as Type I (occurring during the summer) and Type II ostertagiosis (during the winter months). With Type II ostertagiosis, the parasite’s development in the animal becomes temporarily arrested at the L4 stage during the autumn, resuming during the winter month and often causing clinical disease in yearling sheep.
An infection with Teladorsagia will yield the classic gastrointestinal worm symptoms of profuse scour and loss of thrive. Animals appear empty as a result of inappetence and reduced feed intake. Teladorsagia L3 larvae develop into L4 and adults in the stomach glands and the process of emergence from these is responsible for mechanical damage to the organ’s mucosal layer.
Barber’s Pole Worm
Haemonchus contortus
Haemonchus c. is a sinister parasite in that its symptoms don’t become obvious until a relatively advanced stage of infection. Scour is not a symptom of Haemonchus burden in an animal. Both larval and adult stages of the parasite feed on blood and an adult can remove 0.05ml of blood daily. A heavy infestation could see up to 5000 worms per animal – equating to potential blood loss of 250ml in a single day. Expectedly, anaemia is the principle symptom of Haemonchus and first clinical signs can involve rapid weakening of animals that appear in good body condition. Indeed, infected animals may collapse if driven. Inspection of the animal’s conjunctivae (eyes) and gum is a common method for diagnosis. If a burden is high enough, sudden death is possible. Haemonchus risk is not confined to lambs and can affect adults too. Eggs will develop into larvae in warm, wet conditions and though they do not survive a over winter well, a female worm can lay 10,000 eggs daily so pastures can become infective very quickly. The main way that Haemonchus overwinter is in arrested L4 state in the animal, similar to Teladorsagia.
Black Scour Worm
Trichostrongylus spp.
Symptoms of Trichostrongylus are generally seen later in the year and, as the name suggests, the principle sign is soft, dark faeces. It is often a disease of purchased lambs or replacements. Weight loss and death, where a burden is high enough, can follow. The parasite generally affects younger sheep. Of the four worm species mentioned here, Trichostrongylus would be considered the least pathogenetic. However, this is assuming a farm has equal burdens of all four parasites. A high enough burden of any of the gastrointestinal worms cited here on a holding can be lethal.
Use the solutions below with our FAQ and Best Practice sections to make good decisions on gastrointestinal worm control in sheep.
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