Cyathostomins

The cyathostomins are a large group of genera and species of closely related nematodes that live as adults in the large intestine of horses around the world, including Canada.  Globally the cyathostomins are the most abundant of nematode parasites of horses.  The life cycle is direct with eggs passed in the faeces and infection of horses by ingestion of infective third-stage larvae from the environment.  Under ideal conditions this larval development can occur in a few days. Parasitic development involves only mucosal (or rarely sub-mucosal) larval migration.  The normal prepatent period varies with species and for most is two to three months.  Under some circumstances cyathostomin larvae can undergo inhibited development in the gut mucosa, remaining as early third-stage larvae - and emerge in response to a variety of stimuli.  This pattern of development is important in the pathology, epidemiology and treatment of the parasites.

Although cyathostomins can occur in very large numbers in individual horses (sometimes hundreds of thousands) until relatively recently their pathological and clinical significance were unclear.  In the early 1970s, however, a distinct disease syndrome was recognized in Europe associated with the simultaneous emergence of large numbers of previously inhibited larvae in the spring.  Clinical signs included diarrhea, dehydration, inappetance, wasting, oedema, and sometimes death.  Diagnosis in the living animal can be problematic.  This syndrome - larval cyathostomiasis (or larval cyathostominosis) - has many similarities to Type II ostertagiasis in cattle, and has now been observed at other times of the year and in several countries, including rarely in Canada.  Links between the cyathostomins and various forms of colic have not been fully established.

The treatment and control of cyathostomins and of larval cyathostomiasis can be difficult because in many parts of the world, including some areas of Canada, they have developed resistance to the benzimidazoles (fenbendazole etc.), and elsewhere in the world and to a lesser degree, to pyrantel, ivermectin, and possibly moxidectin.  Also, of the anti-parasitic drugs approved in Canada, at normal dose levels only moxidectin is usefully effective against the inhibited larvae in the mucosa.  In the absence of this drug these larvae act as a reservoir of infection that can repeatedly re-populate the gut lumen with parasites following spontaneous loss or removal by treatment.

Cyathostomins of horses are not known to be zoonotic.

Phylum: Nematoda
Class: Rhabditea
Order: Strongylida
Superfamily: Strongyloidea
Family: Strongylidae

The cyathostomins are also known as the cyathostomes, small strongyles or trichonemes. Cyathostome genera include Cylicocyclus, Cylicodontophorus, Cylicostephanus, Poteriostomum and Gyalocephalus. The taxonomy of the group is not yet fully defined. Closely related to the cyathostomes are the migratory large (Strongylus species) and the non-migratory large strongyles (Triodontophorus, Craterostomum and Oesophagodontus).

 Adult cyathostomins measure up to approximately 25 mm in length, depending on the species. They have a small but prominent  buccal capsule with small teeth around its opening, a dorsal gutter, but no teeth at the base. The copulatory bursa of the male is also prominent, and some elements of the alimentary and reproductive systems are usually obvious microscopically.

Eggs of the cyathostomins are typical "strongyle" eggs, oval, with a thin, smooth shell and measure approximately 90 µ by 50 µ. In fresh feces each egg contains a small clump of cells (a "morula"). Eggs of the various species of cyathostomins cannot be differentiated microscopically from each other or from those of the species of Strongylus, nor from those of the non-migratory large strongyles.

The cyathostomins occur in horses and other equids throughout the world. A very heavily infected horse may harbour tens of thousands of these parasites.

Adult cyathostomins live in the large intestine. Eggs pass in the feces. A larva develops within each egg, which then hatches. The released larva develops and, in 7 days or more, moults twice to the infective, third stage. Infection of the horse is by ingestion of these larvae, which follow a mucosal migration, moving into and out of the mucosa (or for some species the sub-mucosa). The pre-patent period of cyathostomins is ten weeks or longer, depending on the species.

Sometimes, larval development in the intestinal mucosa is delayed (they undergo inhibited development as early third-stage larvae), and these larvae can act as a reservoir to replace adult parasites lost from the lumen as a result of senescence or treatment. Experimental evidence indicates that the mucosal reservoir can repeatedly re-populate the lumen, even in the absence of re-infection.

Life Cycle: Cyathostomins

For significant transmission of cyathostomins to occur horses must have access to pasture. Horses can contain very large populations of these parasites, and faecal egg counts can easily be in the hundreds or thousands per gram. The annual pattern of development of cyathostomin eggs in the environment is generally similar to that for the trichostrongyles of ruminants, with temperature-dependent egg development rates, but the pre-patent periods of cyathostomins are longer (approximately three months) than those of the trichostrongyles (approximately three weeks).

Under ideal conditions the eggs of the cyathostomins can develop to infective third-stage larvae in approximately one week. This, combined with the three-month pre-patent periods, means that it is possible that these larvae can develop to adults the same year and begin to contaminate the pastures with eggs towards the end of summer.  Conversely, the longer pre-patent periods of Strongylus spp. means that they have an annual cycle, with infective larvae ingested one year becoming adults the next. Where there is very little or no over-winter survival of the free-living stages of the parasites on pastures the major source of cyathostomin infection for grazing foals is the mares and other older and adult horses with which they are pastured. Where there is some overwinter parasite survival, the pastures could act as the source of infection.

An intriguing feature of the parasitic development of the cyathostomins, but not Strongylus spp. or the non-migratory large strongyles, is that under some circumstances the larvae in the intestinal mucosa can undergo inhibited development and become a reservoir of infection, and in some cases the cause of larval cyathostomiasis.  Larvae in this reservoir can become mobilized to re-populate the intestinal lumen after natural loss of adult parasites from the GI lumen, or after they are removed by treatment.  Thus the inhibited development can be of considerable epidemiological significance.

Many horses with cyathostomins never show any clinical signs, although larval migrations and plug feeding by the adults may compromise health and occasionally cause colic and other GI disturbances. In some horses, however, a massive and sudden migration of larvae from the mucosa into the intestinal lumen occurs, and the resulting tissue damage leads to the disease larval cyathostominiasis. When this syndrome was first recognized in the UK in the early 1970s, it was most often reported in young horses in the late winter and spring. As more cases have been recognized in other areas of the world, however, it has become clear that larval cyathostominiasis can occur at other times of the year and in horses of all ages.

Larval cyathostominiasis is usually characterised by severe diarrhea, pyrexia, anorexia, colic, ventral abdominal oedema, weight loss, and sometimes death. At post-mortem, there is severe mucosal damage in the large intestine, together with large numbers of immature cyathostomins in the mucosa and in the lumen. Larval cyathostominiasis has been reported in Europe and the US, and recently in several horses in Ontario. A recent report described larval cyathostominiasis in a horse in Saskatchewan, but there are no other publications describing cases in western Canada.

Diagnosis of adult cyathostomin infection is based on the detection of eggs using a fecal flotation. Cyathostomin eggs cannot be distinguished from those of Strongylus species or of the non-migratory large strongyles.

The diagnosis of larval cyathostominiasis is complex and difficult, and is often based on the history and clinical signs, together with the presence of hypoalbuminaenia. Also, typically the faeces of affected horses contain large numbers of pre-adult and adult cyathostomins. Recently, a UK team has reported the isolation of a protein apparently specific to encysted (inhibited) cyathostomin larvae in the mucosa that can be detected in the blood of infected horses using an ELISA. There is also some indication that the ELISA titre correlates with larval burdens. This approach, that requires further development and validation, would provide a very useful diagnostic tool for larval cyathostominiasis, and for assessing larval burdens

Antiparasitic drugs for horses can be used to achieve one or more of four goals:

1) to prevent parasitic infection of the host

2) to remove parasites from the host in the absence of clinical parasitic

3) to treat parasitic disease in the host

4) to reduce contamination of the environment with parasite life cycle stages that can become infective for other hosts

 Each of the wide variety of products available in Canada is approved for use to achieve one or more of these goals. The scheduling of treatment of horses for parasites is very variable across western Canada, and even within a small geographic area.  To ensure that you are using a product appropriate for the parasite(s) of concern and for your goal, always consult the product information provided by the manufacturer.  In some instances, veterinarians may use products for purposes for which they are not approved.  The consequences of any such extra-label use are the responsibility of the veterinarian.

There are several products approved in Canada for the treatment of adult cyathostomins, but only  moxidectin (QUEST, QUEST PLUS) is usefully effective for the larval stages in the intestinal mucosa.  Fenbendazole (PANACUR, SAFE-GUARD) would also be effective, but in southern Alberta and Ontario, and probably other areas of Canada, resistance to this drug is present among the cyathostomins.  Treatment of larval cyathostomiasis, the distinct disease associated with these larvae, is often complex and difficult.

Anthelmintic resistance is a  major issue in the treatment and control of the cyathostomins – the parasites have become resistant to a range of anthelmintic products and these products, therefore, have limited efficacy. Patterns of resistance of the cyathostomins vary around the world, and even locally. Products to which these parasites have become resistant are: 1) the benzimidazoles - resistance most widespread but varies with product and location; 2) pyrantel - resistance less widespread but increasing: and most recently 3) the macrocyclic lactones, specifically ivermectin and moxidectin (although uncertainty remains about the latter)- to date reported only in Europe and South America but possibly more widespread.  Also, with the benzimidazoles and possibly the macrocyclic lactones, resistance to one drug implies resistance to others in the group.

In Canada, cyathostomin resistance to the benzimidazoles has been reported in Ontario and in southern Alberta, and to pyrantel in Ontario, but resistance to these anthelmintics probably also occurs in other areas of the country, especially those with high densities of horses and where these drugs are commonly used, for example southern British Columbia.  Resistance of cyathostomins to ivermectin and/or moxidectin have not yet been reported in horses in Canada.

Drug(s)

Product(s)

Fenbendazole	PANACUR, SAFE-GUARD
Ivermectin	EQVALAN, BIMECTIN, EQUELL, PANOMEC
Ivermectin with Praziquantel	EQVALAN GOLD, EQUIMAX
Moxidectin	QUEST
Moxidectin with Praziquantel	QUEST PLUS
Oxibendazole	ANTHELCIDE
Piperazine	VARIOUS
Pyrantel pamoate	STRONGID T, STRONGID P, EXODUS

Additional information is on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

Successful control of the cyathostomins depends on the appropriate use of anthelmintics and the maintenance of an environment for the horses that minimizes the survival and development of the parasites’ free-living stages and/or contacts by horses with these infective stages.  An new approach for the optimal control of cyathostomins in horses depends on the aggregated distribution of the parasite population in a horse population - most of the parasites are in a few of the hosts.  The significance of this concept in parasitology was first articulated by Harry Crofton in 1971, and has since become one of the key concepts underlying helminth control programs for people in many areas of the world.  In horses the aggregated parasite distribution has led to the exploration of selective (targeted) treatments of only animals with large parasite burdens - usually assessed by faecal egg counts - to achieve optimal control and to minimize the occurrence and significance of anthelmintic resistance. It has recently been suggested, however, that these selective treatments - in which many horses are infrequently or never treated, could lead to the re-emergence of other parasites, for example Strongylus vulgaris. Whatever approach is used for the design and application of a control program for helminth parasites in horses, prerequisites for success include an understanding of the basic features of the parasites and the drugs, and of the management of the horses that are the subject of the program.

The cyathostomins are not known to be zoonotic.

Parasites within this group, comprising the genera Triodontophorus, Craterostomum and Oesophagodontus, have a life cycle with free living and parasitic development similar to the cyathostomes, involving only a mucosal (or sub-mucosal) migration. The eggs are indistinuishable from those of the cyathostomins and of Strongylus spp.. The large non-migratory strongyles are not associated with any particular pathology, and for treatment and control are considered along with the cyathostomins.

Kaplan RM et al. (2010) An evidence-based approach to equine parasite control: it ain t the 60s anymore. Equine Veterinary Education 22: 306-316.

Wobeser G et al. Cyathostominosis in a horse from Saskatchewan. Canadian Veterinary Journal 50: 1099-1100.

Peregrine RS et al. (2006) Larval cyathostomiasis in horses in Ontario: an emerging disease? Canadian Veterinary Journal 47: 80-82.