Controlling the Worm Cycle
As horses graze, they can swallow about 65 species of nematode parasites (worms) lying in the grass. As many as 150,000 parasites can live in the wall of a horse's large intestine. Parasitic worms live in the intestines of horses, ponies, donkeys, cows and sheep. Small numbers of worms can be tolerated, causing no effect on well being. Larger worm burdens can cause a range of problems including ill thrift, diarrhoea, colic and death. Pasture management and drug administration assist the horse’s immune system in keeping the intestinal population of worms under control. The immune response to worms is better in some horses and ponies than others and as such, some are more susceptible to worm infestation.
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Worm Resistance |
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If worms survive a drenching they get to breed and pass on that genetic advantage. The numbers of those with this advantage grow while those with no special resistance diminish. Eventually, the drenches we use become less effective because more resistant worms are surviving. We know that we can slow this process by using different drench families, which means that we can kill some of the resistant worms with a different chemical, but it is little more than a delaying tactic.
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What types of worms are there and what problems do they cause? |
Small red worms (Cyathostomins)
These are now by far the most common equine intestinal parasite. As the horse grazes it swallows the larvae of the small red worms, which can be found on blades of grass in contaminated pastures. The larvae colonise the lining of the large intestine where they develop into adults that lay eggs which are passed with the horse’s droppings onto the pasture and develop into larvae to complete the life cycle. Larvae picked up in the autumn can go into hibernation in cysts in the intestine wall. These ‘encysted larvae’ can survive in this state for a considerable length of time, to develop into adult worms when the conditions become right for them, typically in the spring. They are fairly resistant to most wormers whilst in this 'hibernating' state. It is the emergence from the bowel wall of large numbers of encysted larvae that can result in severe diarrhoea and weight loss - a disease known as cyathostominosis. 
Large red worms (Strongyles)
Once the scourge of horses, these worms are now very rare. Again, larvae are picked up and swallowed as the horse eats contaminated pasture. The lifecycle of these worms involves the migration of larvae through the blood vessels of the intestine and liver, where they can cause inflammation and obstruction of the blood vessels resulting in damage to the intestine supplied by the damaged vessel. Symptoms of infestation can include colic, ill thrift, and diarrhoea. 
Tapeworms (Anoplocephala perfoliata)
Probably the second most common equine intestinal parasite (after cyathostomes). These typically live at the junction between the small intestine and caecum. A mite, present on the pasture, acts as the intermediate host for the transmission of tapeworms, allowing it to complete its lifecycle. Recently it has become apparent that tapeworms are important causes of spasmodic colic in horses. Also, it can cause some other forms of more serious colic. 
Bots (Gasterophilus spp.)
These are common but generally harmless parasites. They are the larvae of the adult bot fly. The larvae develop in the stomach of horses and are passed in the droppings. They hatch and the adult flies lay eggs on the horse's coat, usually on the legs. The eggs are then swallowed when the horse licks its legs. Bots can result in mild ulceration of the stomach wall and have been reported to have caused stomach ruptures. Our treatment recommendation:
Ascarids (Parascaris equorum; adult and immature)
Adult roundworms can grow to 50cm in length and are particularly dangerous to foals and young horses as older horses develop immunity. When ingested from the pasture the larvae transfer through the gut wall, to the liver and then to the lungs. The larvae are then coughed up and swallowed where they mature to egg laying adults within the intestine. Heavy infestation can cause respiratory signs, such as a cough and nasal discharge, as the larvae journey through the lungs, or it can cause intestinal signs such as weight loss, a pot-bellied appearance and diarrhoea. 
Pinworms (Oxyuris equi; adult and immature)
Pinworms lay their eggs around the outside of the anus causing intense itching and irritation. Persistent scratching will result in hair loss and open sores around the tail head which can become infected. 
Intestinal Threadworms (Strongylus westeri; adult)
Natural immunity to threadworms usually develops by six months of age and threadworms often remain dormant in adult horses. Threadworms transfer to newborn foals via the mare’s milk. Young foals have no immunity to this worm and infection leaves the foal weak and susceptible to diarrhoea and anaemia. The foal’s growth rate may also be affected by heavy infestation. Foals should be wormed against threadworms as early as four weeks old and worming the mare during pregnancy will help reduce numbers transferring to the teat. 
Lungworms (Dictyocaulus arnfieldi; adult and immature)
Lungworms prevail in pastures shared with donkeys – the lungworm’s natural host. These worms cause persistent coughing in horses as respiratory problems develop. Donkeys can tolerate very large worm burdens without showing any clinical signs. 
Hairworms (Trichostrongylus axei; adult)
Horsehair worms are slender and sometimes long animals that are notable for their writhing movement. They are also known as Gordian worms. They often appear to tie their body into knots as they squirm. Their larvae are parasites of arthropods, especially insects. When a larva has become an adult and is ready to leave its insect host, it takes control of the animal's behaviour. 
Large Mouth Stomach Worms (Habronema muscae; adult)
Habronema are vector-borne parasites using muscid flies as intermediate hosts. The adult parasites establish in the stomach upon ingestion of larvae deposited by flies around the mouth or by ingestion of dead flies carrying the larvae. Within the stomach, parasites become adults in about 8 weeks. Adult Habronema are found in close contact with the gastric mucosa, but cause no clinical problems. Equine stomach worms infect the mucosal lining of the stomach and cause catarrhal gastritis. In their gastric stage, Habronema parasites rarely cause clinical problems, and their most significant manifestation is the cutaneous condition often referred to as summer sores. 
Neck Threadworms (Onchocerca., spp.; microfilariae)
Adult Onchocerca cervicalis worms, also called equine neck threadworms, live in the large nuchal ligament that runs from the poll to the withers. Adult worms are very thin and may be several inches long. Most of the time, the adult stage causes no problems for the horse. Occasionally, radiographs of the neck can show mineralisations in the nuchal ligament because the adult worms can cause some inflammation and mineralisation results. Adult worms reproduce by releasing immature microfilaria, which are spread by biting insects such as no-see-ums and black flies. Microfilaria concentrate along the bottom skin of the horse’s abdomen and chest, where these flies prefer to feed. |
How are worm infestations diagnosed?Typical signs and symptoms (including ill-thrift, weight loss, colic, diarrhoea) and poor quality management (overcrowding, inadequate and contaminated grazing, and inadequate use of effective wormers) often suggest that intestinal parasitic problems are most likely.
Laboratory investigations are the definitive method of diagnosis. Two types of tests are performed: 1. Dropping samples are analysed to count the number of worm eggs per gram of faeces. This gives an indication of the types and number of adult worms present in the intestine that are producing eggs. False negative results can occur when the adult worms are not producing enough eggs to be detected and this can sometimes occur when the horse is very unwell. Dropping samples are most useful when collected from all or presentative batches of horses on a routine and regular basis to monitor the success of a worm control programme. Generally worm egg counts below 50 eggs per gram (50 epg) are not concerning. 2. Although blood samples are often suggested to help diagnose parasitism, the results of most tests are nonspecific and hard to interpret. There is, however, a specific blood test which has been developed to demonstrate tapeworm infestations. How can I make sure my horses do not suffer from parasitic worms?Whether you own one horse or a whole stud farm, you should develop a worm control program. This will be based upon many factors including your geographical location, the types and ages of horses that you have, your stocking density and the frequency with which horses come and go at your premises. Effective parasite control depends upon both management of grazing to minimise worm eggs and larval contamination and the use of wormers to remove parasites from the horses’ intestines. One cannot be adequately effective without the other.
How can I best manage my available grazing?Horses re-infest themselves by eating parasitic eggs or larvae by grazing on contaminated pasture. Therefore:-
1. All horses grazing on the pasture should be well wormed to reduce their output of parasitic eggs and larvae. 2. New arrivals should be treated with an effective wormer on arrival and should be separated and manure removed for twp to three days before being turned out onto clean pasture. 3. The pasture should not be overcrowded so horses can avoid eating contaminated grass. 4. Droppings should be frequently and regularly picked up if horses are living in crowded pastures (twice weekly should be sufficient) and removed from the pasture to minimise contamination. If you have sufficient acreage, mechanical ‘dropping pickers’ (vacuum-type devices powered by small tractors), are particularly effective. This approach is extremely effective at reducing parasitism (perhaps more so than worming!) and should always be encouraged. 5. If possible, paddocks should be regularly grazed with other species such as cattle or sheep, in rotation and then rested. These animals are not affected by equine parasites, their parasites do not affect horses, they graze more evenly than horses and their droppings stimulate good grass growth. Three months of horses not in the paddock will develop a 'clean' paddock. 
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