A SHORT GUIDE TO PARASITES OF HORSES
Lifecycles, clinical signs, diagnosis and control of the most common horse worms in Australia is discussed below.
PDF fact sheets for each parasite is avaliable to download at the end of each section.
Small strongyles make up over 95% of worm infections in adult horses in Australia. Deworming regimes should target small strongyles first, and all other worm types second.
Synonyms: trichonemes, cyathostomes, cyathostomins, Trichonema spp., Cyathostomum spp., Culicocyclus spp., Cylicodontophorus spp., Cylicostephanus spp.
This group of worms encompasses over 40 different species, with 15 species being most commonly found in horses. All these species of small strongyles are very similar in morphology, behaviour and lifecycle so are treated as a general group of parasites when diagnosing, treating and managing infection. Small strongyles make up over 95% of all horse worm infections
Lifecycle: Small strongyles have a direct lifecycle. Adult worms reside in the large intestine and produce eggs that pass out in the manure. These eggs hatch into larvae on the pasture, where they develop through larval stages to become infective larvae (L3). This can take as little as two weeks during warm, damp weather, however larvae can survive on pasture for up to 6 months. Horses will ingest larvae from pasture. The L3 larvae then invade the wall of the ileum and large intestine before developing into L4 larvae, which leave the intestinal wall and become adults in the gut lumen. Adult worms feed on the mucosa of the intestine wall. Occasionally larvae will encyst in the intestine wall at L3 stage. Mass emergences of L4 larvae can occur, which leads to either acute or chronic inflammatory disease that can resemble colic. This is known as larval cyathostomosis.
Clinical signs: Most adult horses carry small strongyle burdens without any obvious signs of ill health. Heavy burdens can lead to ill-thrift, poor condition and sometimes diarrhoea.
Diagnosis: Small strongyles can easily be diagnosed via FEC. Unfortunately no diagnostic is available for encysted larvae. Larval cyathostomosis is usually diagnosed via clinical signs and environmental history.
Control and Management: Small strongyles are highly prevalent – these worms require ongoing management. Pasture management, to limit infective larvae on pasture is a key element. Manure should be collected at least twice a week, and horses should be rotated between paddocks. If possible, cross-grazing with ruminants will help remove infective larvae.
Chemical treatment of worms should only take place after positive FEC results. In order to prevent drug resistance occurring, horses should only be wormed if their FEC results is above 200 EPG (eggs per gram manure). Using FECs, chemical deworming can be decreased to half of a herd to reduce worm egg shedding by 95%.
Also referred to as Ascarids, P. equorum is most common in young horses (under 5 years old). P. equorum infection can be easily detected via FEC due to the their distinct egg morphology. Young horses should be routinely checked for P. equorum due to the high pathogenicity of this worm. As the adult worms are very large, blockages can also occur, causing colic: FECs will allow diagnosis and appropriate deworming treatment if required.
Synonyms: Ascaris, Ascarids, large roundworm
Ascarids are the largest roundworm that infects horses. Female worms can reach 40cm in length while male worms 15cm. Most horses develop immunity by the time they are 5 years old, with the majority of heavy infections confined to foals and yearlings. Therefore, these worms are mainly problematic on stud farms and other properties where there are always young animals present to allow the worms lifecycle to continue.
Lifecycle: Unlike the majority of other horse worms which reside solely somewhere in the gastrointestinal tract for their entire lifecycle, ascarids have a migratory lifecycle that includes the liver and lungs. Eggs are ingested from the environment, and hatch in the intestine. Larvae migrate through the intestinal wall and make their way to the liver. Two weeks after this, they will have migrated to the lungs, where the larvae then migrate up the trachea and are swallowed. Once swallowed, the larvae finish their maturation into adult worms before beginning to produce eggs that pass out in the manure. This entire cycle will take a minimum of 10 weeks. Once worms are adults, they can live for up to 2 years in the intestines.
Clinical Signs: While the larvae are migrating through the liver and lungs of young horses, horses may display symptoms such as coughing and nasal discharge, otherwise foals will remain in apparent good health. At these stages the infection will not be detected on a FEC. The adult intestinal stage can be well tolerated in low burdens, but moderate or heavy infections will lead to ill health. The worms themselves do not cause significant damage to the intestine – loss of condition in the horse is likely due to the worms competing for nutrients. Heavy infections can also lead to impaction colic. Adult worms can grow up to 40cm in length, and so their effects on gut movement may be significant.
Diagnosis: Infections with adult worms can easily be identified due to the unique morphology of ascarid eggs from other worm species present in horses.
Control and Management: It is wise to keep young horses on a regular FEC regime prior to worming to check for ascarid infection and to ensure proper treatment and paddock management can occur to limit infection. Although ascarids can be treated using either BZ or avermectin dewormers, it is recommended to use a BZ (such as oxfendazole), due to mode of action. Avermectins kill worms through paralysis: paralysed worms are more likely to cause gut blockages. Unfortunately, ascarids can be very difficult to control in the environment. The eggs are resistant to external factors and can survive in the environment for several years. In addition to this, female worms lay very high numbers of eggs – one infected foal may pass millions of eggs per day. If ascarids are present on a stud farm, it is recommended to swap between foal paddocks each year, to a prevent yearly transmission between young animals.
Bot flies are an insect (gastropod), rather than a 'worm'. However as part of their lifecycle takes place inside a horse, they are considered to be parasites. Adult flies deposit eggs onto the horse during late summer, these are ingested by the horse, where they then hatch and begin to develop into larvae. Bot larvae spend all of autumn and winter within the horses stomach, growing, before being passed out in spring/summer to complete their maturation into adults in the soil.
Unfortunately, bot flies cannot be seen on a FEC as they do not lay any eggs that can be detected in the manure. The best methods of managing bot flies is to 1) prevent bot flies laying eggs, 2)removing eggs once they are deposited on the horse and 3) treating all horses with a boticide dewormer once a year during autumn or winter.
Lifecycle: Bot flies are seasonal parasites for most of southern Australia, with adult flies present during summer and autumn. Female bots recognize horses by sight, and will lay eggs on horses’ legs, neck and body. Once ingested by the horse through grooming, the eggs will hatch into larvae and may burrow and migrate through the tissues in the mouth. Larvae will migrate to the stomach where they attach to the stomach lining. The larvae remain and develop for around 10 months, before passing out in the manure and pupating into adults. Adult flies do not feed and only live for a few days or weeks, to lay eggs and continue the lifecycle.
Clinical signs: Low level bot fly infections do not usually show any significant signs. Burrowing larvae in the mouth may cause pain and irritation in the mouth, while stomach-stage larvae can also result in inflammation and ulceration. Adult flies can cause significant irritation and avoidance behaviours in horses.
Diagnosis: As the bot larvae do not lay eggs within the horse, diagnosis cannot be achieved via a FEC. Occasionally, large burdens of stomach-stage larvae can be seen on gastroscopes.
Control and management: The best method of control is to prevent eggs being laid on horses, or to remove eggs before ingestion. As female bot flies use sight, chemical insect deterrents will not prevent egg-laying behaviour. Fly-rugs and boots may prevent bots from being able to deposit eggs and there is anecdotal evidence that paddock shelters will provide protection: it is theorised that the flies cannot recognize horses undercover of a shelter.
Chemical control can be achieved through washing off eggs with warm water and insecticide – the warmth stimulates the eggs to hatch and the insecticide then kills the larvae. During autumn/winter once the bot flies have disappeared, a single treatment with a boticide anthelmintic (ivermectin, abamectin, moxidectin) will also remove any stomach/mouth stages and break the lifecycle. If all horses in an area are consistently treated each year, fly populations will reduce over time.
Tapeworms are cestodes – a group of segmented flatworms. They are now rather rare, due to the frequent use of praziquantel, an anthelmintic that kills cestodes.
Lifecycle: Adult tapeworms reside in the small intestine. Rather than laying eggs, adults will drop segments that contain many eggs. These segments disintegrate and eggs are passed in the manure. Unlike most other parasites, tapeworms require an intermediate host: forage mites. These mites are free living in the pasture. The mites will ingest the tapeworm eggs, where the eggs hatch and develop into an intermediate stage which takes 2 to 4 months. Horses will then ingest mites along with pasture, and the intermediate tapeworms will then fully develop into adults in the horse, taking another 2 to 4 months to do so. The entire lifecycle of the tapeworm is between 4 and 8 months.
Clinical signs: Tapeworms are generally considered unpathogenic, however large burdens may produce the generic symptoms of unthrifitness and enteritis. Significant burdens may lead to colic or even perforation of the intestine, which is fatal.
Diagnosis: Tapeworm eggs are easily identifiable on faecal egg counts. However there are reports of tapeworms not being consistent egg layers, as such low burdens may be missed via a single FEC.
Control and management: Management on pasture is difficult, due to pasture mites act as a reservoir in their role as an intermediate host. A yearly treatment with a dewormer that contains praziquantel is usually enough to control infection.
Pinworms are relatively common parasites, and can often be seen passed in the manure. Adult females reach between 10-15cm in length, while males are far smaller at only 1.2cm long. While the presence of pinworms may appear dramatic, they are relatively unpathogenic with the major issue being the tail rubbing that accompanies infection
Lifecycle: Adult worms reside in the large intestine. When females are ready to lay eggs, they migrate to the anus, where eggs are laid in clumps on the skin outside the anus. These eggs then fall and contaminate the environment before being ingested again by horses. Following ingestion of larvae, they migrate to the large intestine before development through larval stages into the adult stage, a process that takes around 5 months. Adult worms can live for 6 months.
Clinical signs: Clinical signs are rarely observed in horses that are infected with pinworm. Extreme burdens may lead to an inflammatory response in the intestine. The main symptom of pinworm infection is the tail rubbing due to the itching caused by the egg laying worms.
Diagnosis: Pinworm infection can usually be identified by the horse owner, via the presence of the eggs laid around the anus of the horse (see picture below). Eggs sometimes appear on a FEC, however as they are not laid in the manure, they can be missed on a FEC. Tail rubbing alone should not be considered proof of pinworm infection, as horses may rub their tails for many reasons (e.g. skin infections etc).
Control and management: Pinworms are susceptible to most dewormers. Unless burdens or tail rubbing behaviour are significant, treatment of infection can wait until treatment for strongyles is also necessary.