HORSES
Additional info bytes to explain what to do when, on the parasites that can infect your horse and more
KEY PARASITES OF VETERINARY CONCERN
CYATHOSTOMINS
Small Redworm
These are the most common parasite species present in 70-100% of horses.
A key part of their lifecycle is that the migrating larval stages can burrow into your horse’s gut lining and encyst. This happens throughout the year and the cysts normally persist for around 10 days, after which the larvae re-emerges and re-enters the gut and mature into adults.
During the winter however, it is currently believed that a drop in the environmental temperature causes these stages to encyst for longer. This means the numbers of encysted larvae can increase over time within your horse’s gut wall and can go undetected as they haven’t matured into egg laying adults.
Consequently, in the Spring when the temperature increases, any encysted larvae emerge from the gut lining back into the intestines. If there are a large number of encysted stages present, this can cause damage to the gut lining and a disease called cyathostominosis.
PARASCARIS EQUORUM
Equine Roundworm
Ascarids are of particular concern in foals and young horses < 4 years of age due to their immature immune system. These are white worms that kind of look like beansprouts. Clinical symptoms of roundworm infestation include: coughing, nasal discharge, unthriftiness and colic.
ANOPLOCEPHALA PERFOLIATA
Equine Tapeworm
Tapeworms can grow to 8cm in length and a width of 1.5cm. They form into clusters at the junction between the small and large intestines where they can cause digestive disturbances, loss of condition, colic and fatal blockages. Horses become infected when they eat forage or grass contaminated with the infected forage mite
OTHER PARASITES TO KEEP AN EYE OUT FOR
EQUINE PINWORM
Adult female pinworm lay their eggs around the horse's anus. These eggs are coated in a sticky layer which can make control of this species tricky as the eggs can remain in the environment and allow for reinfection. They also cause irritation around the anus, causing the horse to scratch it's behind.
BOTS
Bots aren't a type of worm, but instead are a fly. Bot fly lay their eggs around the horse's mouth and front legs and the horse ingests these eggs when they groom themselves. It is the Bot Fly Larvae that develop in the horse's gut.
A FEW LESS COMMON PARASITES OF HORSES
STRONGYLOIDES WESTERI
Threadworm
Threadworms often remain dormant in adult horses but transfer to newborn foals via the mare’s milk. This leaves the foal weak and susceptible to diarrhoea and anaemia. The foal’s growth rate may also be affected. Foals should be wormed against threadworms as early as 4 weeks old and worming the mare during pregnancy will help reduce numbers transferring to the udder. Natural immunity usually develops by 6 months of age.
DICTYOCAULUS ARNFIELDI
Lungworm
Donkeys are the lungworm's natural host, therefore horses that are grazed with donkeys are at increased risk of infection.
CONTROL IN ADULT HORSES
Underpinned by Routine Testing
WINTER
December-February
During the winter all horses should be dosed with a suitable wormer for the inhibited encysted stages of small redworm. This is because current thinking is that when the environmental temperature drops over winter, this causes the larval stages of small redworm to encyst for a longer period of time. As time goes on, the number of larvae that are encysted can increase. When spring arrives and the temperature increases, if there are a large number of encysted larvae, they can emerge en masse, which can cause severe gut issues and in very serious cases, death.
When the larvae are encysted, they do not mature into adults which are capable of laying eggs. This means we cannot detect encysted stages through worm egg counts. Presently, there is no test available to detect encysted redworm. Therefore, a ‘better safe than sorry’ approach is recommended.
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14-17 days after administration of the wormer you should carry out a follow-up Resistance Test to check the efficacy of the product used.
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Checklist
Dose with either Moxidectin or a 5 day course of Fenbendazole
Resistance Test
SPRING
March-May
As the environmental temperature increases, the small redworm will be encysting and maturing into egg laying adults at the normal rate. It is recommended that you carry out a worm egg count to identify individuals that are shedding higher numbers of redworm eggs and treat appropriately.
If your horse requires treatment based on their worm egg count result, it is recommended that you carry out a Resistance Test 14-17 days after you’ve administered the wormer to check that it has worked to expected efficacy.
It is also recommended that you test for tapeworm using the EquiSal saliva tapeworm test.
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Checklist
Worm Egg Count
EquiSal Tapeworm Test
SUMMER
June-August
Carry out a worm egg count as in the spring. If your horse requires treatment based on their worm egg count result, it is recommended that you carry out a Resistance Test 14-17 days after you’ve administered the wormer to check that it has worked to expected efficacy.
If your horse is scratching it’s backside, also carry out a Pinworm Test to rule out Pinworm infection.
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Check List
Worm Egg Count
Pinworm Test
AUTUMN
September-November
It is recommended that you carry out a worm egg count to identify individuals that are shedding higher numbers of redworm eggs and treat appropriately.
If your horse requires treatment based on their worm egg count result, it is recommended that you carry out a Resistance Test 14-17 days after you’ve administered the wormer to check that it has worked to expected efficacy.
It is also recommended that you test for tapeworm using the EquiSal saliva tapeworm test, as it will have been 6 months since you last monitored for tapeworm infection.
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Checklist
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CONTROL IN FOALS AND YOUNGSTERS
Currently underpinned by Routine Treatments and Assessment of what parasites are present.
Young horses have an immature immune system so are more susceptible to parasitic infections. We don't want to eradicate parasites as they are important for the development of the horse's immune system so they can tolerate parasitic challenge when they are adults. Roundworm are of most interest in young horses so control is paramount.
DURING THE FIRST YEAR
Targeted treatment programmes aren’t recommended for foals as they can harbour more migrating larval stages than egg laying adults and there is a distinct lack of studies that have been carried out evaluating this approach in foals. Below outlines the currently recommended approach for foals during their first year of life.
2-3 months
1st de-worming treatment with a product that contains Fenbendazole (i.e. Panacur).
4-6 months (just before weaning)
At weaning
Carry out a Worm Egg Count to identify whether worm burdens are primarily redworm or roundworm.
9 months
De-worm mainly targeting redworm species. Also dose for tapeworm.
12 months
De-worm mainly targeting redworm species.
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Following each treatment, you should consider carrying out a Resistance Test to ensure the product used has been effective.
YEARLINGS AND YOUNGSTERS
As the horse matures, their immune system will also be maturing. Between Spring and Autumn you can carry out worm egg counts for horses aged between 1 and 4 years of age at a more frequent rate of 6-8 weeks. This is to ensure that parasite burdens are sufficiently monitored to prevent them spiralling out of control.
MUMS TO BE
Ensure control of Strongyloides westeri
PRIOR TO FOALING
If your mare wasn’t treated with the wormer that contains Moxidectin in the Spring prior to foaling, it is advised that you dose with either Moxidectin, Ivermectin, Fenbendazole or Mebendazole to control Strongyloides westeri. This prevents the passing of this present, if present in the mare, to the foal through the mum’s milk.
In addition, make sure that you check the wormer you are going to use on your pregnant or lactating mare is safe to be used. Some products have been more thoroughly tested than others. If in doubt, ask your SQP or vet for advice.