The taxonomic family Trichostrongylidae contains most of the important gastro-intestinal nematodes of cattle around the world.  Abomasal genera include Ostertagia (probably the most important trichostrongyle of cattle worldwide), Trichostrongylus, and Haemonchus; and small intestinal genera include Trichostrongylus (species different from those in the small intestine), Cooperia and Nematodirus.  Non-trichostrongyle gastro-intestinal nematodes of cattle include Strongyloides (occurs commonly only in some parts of Canada and usually in young calves) and Bunostomum (hookworm - very rare in Canada) in the small intestine, and Oesophagostomum (occurs in Canada) and Trichuris (common in Canada) in the large intestine.

The life cycle of the trichostrongyles is direct.  For most species first-stage larvae develop in, and hatch from, the eggs passed in faeces.  These larvae moult twice to the ensheathed third-stage, which are infective and are ingested by the cattle.  Under ideal environmental conditions this translation takes approximately one week, but the rate of development is temperature-dependent.  With Nematodirus, the eggs do not hatch until the third-stage larva has developed; which takes approximately three weeks, and these larvae are then ingested.  The infective larvae continue their development in the mucosa of that part of the gut in which the adults live, then emerge into the lumen and become adults.  The pre-patent period for most of the trichostrongyles is approximately three weeks.  Under some circumstances the development of larval Ostertagia in the abomasal mucosa is delayed, taking several months.  This delay is referred to as inhibited development and tends to occur during parts of the year when conditions on the pasture are not supportive of egg and larval development - in temperate regions during the cold winters and in arid regions during the dry summers.

In cattle trichostrongyles and the other nematodes may have few detectable adverse effects, they may hinder production of meat and milk, or they may cause obvious clinical disease.  Among the trichostrongyles and other gastro-intestinal nematodes of cattle, some (e.g., Ostertagia and rarely Trichuris) can act as soloists and cause distinct clinical disease, but often several genera act together as an orchestra.  In Canada most problems in cattle are caused by orchestras.  The most important of the trichostrongyle-associated diseases worldwide are associated with the larvae of Ostertagia (acting as a soloist) in the abomasal mucosa, where they can seriously disrupt normal structure and function.  Disease associated with the development of this parasite on a normal time scale is referred to as Type I ostertagiasis and is characterized by loss of appetite and body condition, and diarrhea, and usually occurs in younger animals towards the end of their first grazing season.  Morbidity is usually high but deaths are unusual.  Disease associated with inhibited development of Ostertagia larvae is referred to as Type II ostertagiasis and usually occurs in younger animals several months after the end of their first grazing season (in the late winter and spring in temperate regions), although it is sometimes seen in older animals.  Morbidity is usually low, but mortality can be high.  While the occurrence of Type I ostertagiasis is unknown in Canada (the parasite is usually part of an orchestra), Type II disease seems to be very rare in this country.

The gastro-intestinal nematodes of cattle are generally considered non-zoonotic, although rare human infections with trichostrongyles have been reported, but not in Canada.

In sheep

The taxonomic family Trichostrongylidae contains most of the important gastro-intestinal nematodes of sheep around the world. Abomasal genera include Haemonchus (species different from cattle and probably the most important trichostrongyle of sheep worldwide), Teladorsagia (species different from cattle) and Trichostrongylus (species same as cattle); and small intestinal genera include Trichostrongylus (species different from those in the small intestine and some shared with cattle), and Cooperia and Nematodirus (some species of these genera also infect cattle). Non-trichostrongyle gastro-intestinal nematodes of sheep include Strongyloides (same species as in cattle and seems to be rare in sheep in Canada) and Bunostomum (hookworm - different species from cattle and very rare in Canada) in the small intestine, and Chabertia (common in Canada and does not infect cattle), Oesophagostomum (species different from cattle) and Trichuris (species different from cattle) in the large intestine.

The life cycle of the trichostrongyles is direct. For most species first-stage larvae develop in, and hatch from the eggs passed in faeces. These larvae moult twice to the ensheathed third-stage, which are infective and are ingested by the cattle. Under ideal environmental conditions this translation takes approximately one week, but the rate of development is temperature dependent. With Nematodirus, the eggs do not hatch until the third-stage larva has developed; which takes approximately three weeks, and these larvae are then ingested. The infective larvae continue their development in the mucosa of that part of the gut in which the adults live, then emerge into the lumen and become adults. The pre-patent period for most of the trichostrongyles is approximately three weeks. Inhibited development by larval trichostrongyles occurs in both sheep and cattle.

In sheep trichostrongyles and the other nematodes may have few detectable adverse effects, they may hinder production of meat, milk and wool, or they may cause obvious clinical disease. Among the trichostrongyles and other gastro-intestinal nematodes of sheep, some (e.g., Haemonchus and rarely Teladorsagia) can act as soloists and cause distinct clinical disease, but often several genera act together as an orchestra. In Canada most problems in sheep are caused by orchestras. The most important of the trichostrongyle-associated diseases worldwide are caused by pre-adult and especially adult Haemonchus contortus in the abomasum, where they suck blood from the host causing primarily anaemia and often death. Clinical haemonchosis is an occasional but significant problem in sheep in Canada.

Key features of the epidemiology of trichostrongyles of sheep and cattle are the mechanisms by which young animals become infected. For most trichostrongyles of cattle in non-tropical regions it is the dams and other older animals that seed the pasture with eggs at the beginning of the grazing season. These eggs give rise to infective larvae that are available to the calves as they graze. If winter conditions are not too severe eggs and infective larvae can survive on the pasture over the winter and also be available to the grazing calves. The system in sheep is very similar, except that often the ewes will have an increase in egg abundance in the faeces around the time of lambing (the periparturient rise). This serves to increase the number of infective larvae subsequently available to the lambs. The exception to this pattern is Nematodirus, which in cattle in western Canada seems to be rare in adult animals and to be transmitted from calf crop to calf crop through eggs and larvae that overwinter on the pasture.

The gastro-intestinal nematodes of sheep are generally considered non-zoonotic.

Phylum: Nematoda
Class: Rhabditea
Subclass: Rhabditia
Order: Strongylida
Superfamily: Trichostrongyloidea
Family: Trichostrongylidae

In veterinary parasitology, the term "trichostrongyles" usually refers to a group of related nematodes that are parasites of the GI system of ruminants. The trichostrongyles of ruminants include several genera and a wide range of species. Adults of most trichostrongyle species infecting ruminants are found within a single part of the GI system. All the trichostrongyles of ruminants have a similar basic structure and generally similar life cycle.

In Canada, the most common trichostrongyles in cattle are:

In Canada, other (non-trichostrongyle) GI nematodes include:

For details of the taxonomy of S. papillosus and Trichuris species, see the notes on related parasites in dogs. Oesophagostomum species are within the Family Strongylidae of the Superfamily Strongyloidea of the Order Strongylidae.

In general, adult trichostrongyles are small and slender, up to 15 mm (males) and 25 mm (females) in length, depending on the species. Parts of the alimentary and reproductive systems are often clearly visible in specimens recovered at post-mortem. Each genus can be identified on the basis of a few morphological features that can be seen microscopically in cleared specimens. These include modifications to the cuticle, the structure of the female reproductive system (eg. Haemonchus vs. Trichostrongylus axei), and of the copulatory bursa in the male (eg. Haemonchus vs.Trichostrongylus axei).

Eggs of the various species of trichostrongyles of ruminants (other than Nematodirus), and of Oesophagostomum species, are microscopically indistinguishable. They are oval, with a thin, smooth shell and measure up to approximately 50 µm by 85 µm. When passed in the feces each egg contains a small clump of cells – a "morula". The eggs of Nematodirus species are larger, measuring up to approximately 120 µm by 230 µm, but have a similar basic structure.

The eggs of Strongyloides papillosus eggs of Strongyloides papillosus measure approximately 20 µm by 50 µm and contain a first-stage rhabditiform larva when passed in the feces.

The eggs of Trichuris  are lemon-shaped (typical of the genus) and measure up to approximately 40 µm by 80 µm.

Many of the genera and some of the species of trichostrongyles of ruminants are shared by cattle and sheep and others are specific to one or the other host. Horses and pigs can also harbour trichostrongyles. In horses, the most common species, Trichostrongylus axei, is shared with ruminants, while that in pigs, Hyostrongylus rubidus, is host-specific.

Trichostrongyles are distributed around the world, although there is geographic variation in the genera and species present in the various hosts. These nematodes do best where climatic conditions, especially warmth and moisture, support development and survival of the free-living stages.

In many parts of the world, these parasites are considered major causes of reduced production and clinical disease in both cattle and sheep. In Canada, their significance varies with climate and management practices.

All trichostrongyles of ruminants, except Nematodirus, have a similar life cycle. Adult parasites live in the GI tract and the females produce eggs which pass in the feces. A first-stage larva develops within each egg, which then hatches. The released larva then develops into the infective, ensheathed third stage which retains the old second-stage sheath. In Canada, under ideal environmental conditions, the development from egg to infective larva takes approximately one week. Infection of cattle is by ingestion of the third-stage larvae. After ingestion, the larvae exsheath in the rumen and then migrate into and out of the mucosa in the area of the GI tract in which the adults will establish (eg abomasum for O. ostertagi). The pre-patent period is approximately three weeks, although this may be prolonged during inhibited development (see below).

Nematodirus species differ only in that the egg does not hatch until the third-stage, infective larva has developed, and this process takes at least three weeks.

The basic pre-patent period for the trichostrongyles, including Nematodirus, is approximately three weeks, although this may be prolonged during inhibited development, in which pre-adult larvae remain in the GI mucosa for longer periods than occur with a normal pre-patent period. Inhibited development is particularly important with Ostertagia ostertagi in cattle and with cyathostomes in horses.

Strongloides papillosus has a life cycle similar to that for S. stercoralis in dogs, except that it is larvated eggs, rather than first-stage larvae, that are passed in the feces.

Oesphagostomum species in cattle develop similarly to the trichostrongyles in the environment, but following ingestion the larvae of some species penetrate into the muscle layers of the wall of the large intestine.

Trichuris species in cattle has a life cycle similar to that for Trichuris in dogs.

 Life Cyle; Trichostrongyles

Life Cycle: Nematodirus species

Trichostrongyles other than Nematodirus

There is now a good understanding of the way in which trichostrongyles spread among cattle at pasture in Canada, specifically in cow-calf systems. Less is known about the situation in feedlots or on dairy farms. It is believed that the trichostrongyles and Oesophagostomum usually require pasture for significant transmission, while Strongyloides and Trichuris are able to transmit at pasture, or in paddocks and yards where there is little or no grass.

At the beginning of the grazing season in many parts of Canada, including the prairies, pastures are essentially parasite-free, except for Nematodirus (see below). Any free-living stages of the other trichostrongyles remaining on the pasture at the end of the previous summer have died over the winter or during the early spring, before the cattle are put out to graze. In other areas of the country, for example southern Ontario, southern Quebec, and parts of the Maritimes and British Columbia, the winter climate means that the free-living stages of some trichostrongyles could overwinter on the pasture in sufficient numbers to infect grazing cattle in the spring, and perhaps cause disease.

Where there is essentially no overwinter survival of the parasites on pasture, it is the eggs in the feces of the cows, and other older animals, that introduce trichostrongyle infection to the pastures in the spring. The rate of development and hatching of these eggs increases with seasonal warming of the environmental temperatures, and when the calves begin to graze significantly the pasture contains plentiful infective larvae. These larvae establish infections in the calves, which are more susceptible to infection than are older animals, sometimes at high levels. The adult parasites in the calves then produce more eggs which, if environmental temperatures are sufficiently warm, develop into a second generation of infective larvae. As temperatures cool in the fall, egg hatching and larval development on the pasture slow then stop. As indicated above, in many parts of Canada, these residual free-living stages die over the winter. Thus for cows and calves where there is no over-winter survival by the trichostrongyles (other than Nematodirus) on pasture, during the grazing season the parasites move from the cows through the pasture to the calves. Where any free-living stages of the parasites do survive over-winter on the pastures, these can act as the initial source of infection for the calves, and perhaps older animals.

Nematodirus species

For Nematodirus in cattle in Canada, it seems that cows and other older animals play little role in the transmission of the parasite. Instead, eggs, and perhaps hatched infective larvae, over-winter successfully on pastures and serve as the source of infection for calves in the spring. The adult parasites derived from these larvae than produce eggs from the feces of the calves which develop on the pasture and survive over the following winter. Thus Nematodirus is transmitted from one year’s calves to the next by over-winter pasture survival of the free-living stages.

Effects of climate

In the Canadian prairies the climate, especially the long, cold winters, and the hot, dry summers, coupled with the large land areas and low stocking densities commonly used for cow-calf operations, are not particularly conducive to GI nematode transmission. The free-living stages have a hard time developing and surviving, and hosts may be few and far between. In British Columbia, however, especially in the lower mainland and areas of Vancouver Island, and in southern Ontario, southern Quebec and areas of the maritime provinces, a gentler climate and greater livestock densities are more supportive of the parasites, and greater burdens and more significant effects are likely.

Epidemiology of the Trichostrongyles

In sheep

There is some understanding of the way in which trichostrongyles spread among sheep at pasture in Canada, specifically among ewes and lambs. Less is known about the situation in drylots and feedlots. It is believed that the trichostrongyles, Chabertia and Oesophagostomum usually require pasture for significant transmission, while Strongyloides and Trichuris are able to transmit at pasture, or in paddocks and yards where there is little or no grass.

At the beginning of the grazing season in many parts of Canada, including the prairies, pastures are essentially parasite-free, except for Nematodirus in some areas of the country. Any free-living stages of the other trichostrongyles remaining on the pasture at the end of the previous summer have died over the winter or during the early spring, before the sheep are put out to graze.

Where there is essentially no overwinter survival of the parasites on pasture, it is the eggs in the feces of the ewes, and other older animals, that introduce trichostrongyle infection to the pastures in the spring. The rate of development and hatching of these eggs increases with seasonal warming of the environmental temperatures, and when the lambs begin to graze significantly, the pasture contains plentiful infective larvae. These larvae establish infections in the lambs, sometimes at high levels. These adult parasites in the lambs then produce eggs which, if environmental temperatures are sufficiently warm, develop into a second generation of infective larvae. As temperatures cool in the fall, egg hatching and larval development on the pasture slow then stop. As indicated above, in many parts of Canada, these residual free-living stages die over the winter. In some warmer areas of the country, however, free-living stages can over-winter on pasture, and may establish infection, and sometimes disease, in young animals introduced in the spring.

A key feature of the epidemiology of trichostrongyles in sheep is the periparturient (or spring) rise in egg numbers in the feces of ewes that begins in late pregnancy, peaks in early lactation, and then declines. This increase in egg output, which is a flock-wide phenomenon, is thought to result from some combination of the maturation of inhibited larvae in the abomasal and intestinal mucosa, the development to maturity of larvae newly acquired during winter pregnancy (unlikely in most areas of Canada because of climate), and life span prolongation and increased fecundity of the adult parasites in hosts immunosuppressed by the stress of pregnancy. The periparturient rise "seeds’ the pastures very effectively with eggs, and ultimately infective larvae, just as the lambs are beginning to graze.

For Nematodirus in sheep in Canada, it seems that ewes and other older animals play little role in the transmission of the parasite. Instead, eggs, and perhaps hatched infective larvae, over-winter successfully on pastures and serve as the source of infection for lambs in the spring. Eggs produced by adult parasites in the lambs then develop on the pasture and survive over the following winter. Thus Nematodirus is transmitted from one year’s lambs to the next by over-winter pasture survival of the free-living stages. Nematodirus battus, perhaps the most pathogenic species of Nematodirus infecting sheep, appears to be limited in Canada to a few foci in the maritimes.

In the Canadian prairies the climate, especially the long, cold winters, and the hot, dry summers, coupled with the large land areas and low stocking densities often used for ewes and lambs, are not particularly conducive to GI nematode transmission. The free-living stages have a hard time developing and surviving, and hosts may be few and far between. In British Columbia, however, especially in the lower mainland and areas of Vancouver Island, and in southern Ontario, southern Quebec and areas of the maritime provinces, a gentler climate and greater livestock densities are more supportive of the parasites, and greater burdens and more significant effects are likely. Interestingly, some of the outbreaks of clinical haemonchosis in sheep in Saskatchewan have been associated with very high stocking densities and overgrazing.

Epidemiology: Trichostrongyles

Generally in Canada, trichostrongyles and the other GI nematodes are present in cattle as mixed infections. These can have a range of effects on the cattle, from no effect, through "sub-clinical" loss of production (particularly of meat and milk), to obvious clinical disease, often characterized by loss of appetite, weight loss and diarrhea.

Infection with trichostrongyles, especially in Canada, appears to be much more common than is any detectable disease, either sub-clinical or clinical. In general, calves are the most susceptible to the parasites’ adverse effects, and adult cattle the least, although there are exceptions. Yearlings and adult cattle seem to develop some sort of "resistance" to the establishment of heavy trichostrongyle infections and to the damage that they cause.

In many instances of sub-clinical or clinical trichostrongyle disease in cattle, it is very difficult to determine which species of parasite is/are the most important. Where several are involved, the situation can be compared to hearing the music of an orchestra; whereas with a single species, only a soloist is heard.

Ostertagia ostertagi and ostertagiasis

Around the world, probably the most significant soloist among GI nematodes of cattle is Ostertagia ostertagi, a common parasite recognized as the cause of two distinct clinical disease syndromes:

Type I ostertagiasis is usually seen (in temperate areas) in young animals towards the end of the grazing season, with clinical signs including mild to moderate anorexia, dehydration, weight loss, and diarrhea, and high morbidity but little or no mortality. Type I ostertagiasis is associated with the tissue damage resulting from the development and gradual emergence of large numbers of Ostertagia larvae from the abomasal mucosa on a normal time scale (pre-patent period of approximately three weeks). In Canada, Type I ostertagiasis probably occurs, although it is often difficult to tease it apart from mixed infections.

Type II ostertagiasis is usually seen in late winter and spring (in temperate areas), with signs including acute anorexia, dehydration, weight loss, oedema and diarrhea, with low morbidity but with some deaths. Type II ostertagiasis is associated with the inhibited development and simultaneous emergence of large numbers of Ostertagia larvae from the mucosa, resulting in very significant disruption to the structure and function of the abomasums. Larvae of O. ostertagi undergoing Inhibited development at the early fourth stage (EL4), and remain quiescent in the abomasal mucosa for prolonged periods (up to several months) and then emerge and resume their development to adults. The factors inducing larvae to inhibit and then to resume development are not fully understood. Experimentally, cooling larvae to 4C for several weeks results in inhibition when the larvae are administered to calves. Type II ostertagiasis has been reported rarely in eastern Canada.

The pathology of ostertagiasis, particularly Type II, has been extensively studied. Developing larvae destroy the epithelial cells of the abomasum, especially those that produce hydrochloric acid. This results in an increased abomasal pH, reduced conversion of pepsingen to pepsin, increased bacterial growth, and leakage of proteins from the plasma into the abomasal lumen, leading to hypoproteinemia. The Ostertagia larvae and the damage they cause result in a typical nodular swelling of the abomasal mucosa, which is described as having the appearance of Morocco leather. In general, the pathology of Type I ostertagiasis is similar but less acute and less severe.

Other trichostrongyles

Most problems in cattle in Canada associated with GI nematodes result from mixed infections. Other trichostrongyles of cattle also have inhibited development, but in Canada at least, it is probably of little epidemiological or clinical significance. Where the effects of trichostrongyles are in cattle are seen as an "orchestra", which occurs most commonly in calves in the late summer and fall, clinical signs include anorexia, dehydration, weight loss, and diarrhea, and often most of the animals in a group will be affected to some degree.

Relatively little is known of the means by which the other trichostrongyles and the other GI nematodes affect GI structure and function. Perhaps the most significant overall effect of all the GI nematodes, including O. ostertagi, is that they suppress appetite, leading to reduced feed intake. This is particularly damaging for young, growing animals.

Production studies

For many of the GI parasites of cattle, there is considerable information on the production effects in experimental infections. Also, numerous studies in North America and elsewhere have compared production parameters of cattle treated with an anti-parasitic product to those for untreated cattle. Many of these studies have shown a beneficial effect of the treatment, but this is sometimes not statistically significant. Also, the design of some of the studies makes interpretation of results difficult, although the situation is improving. One of the problems is that limited tools are available to measure some of the parasites’ more subtle effects, particularly in field situations.

That said, two interesting and well designed studies from eastern Canada, reported in 2002 and 2005, found significantly improved milk yields following treatment around calving in dairy cattle that spent some time at pasture, but not in those kept in confinement.

In sheep

Generally in Canada, trichostrongyles and the other GI nematodes are present in sheep as mixed infections. These can have a range of effects on the sheep, from no effect, through "sub-clinical" loss of production (particularly of meat and milk), to obvious clinical disease, often characterized by loss of appetite, weight loss and diarrhea, and occasionally death. Thus infection with trichostrongyles appears to be much more common than is any detectable disease, either sub-clinical or clinical. In general, lambs are the most susceptible to the parasites’ adverse effects, and adult sheep the least, although there are exceptions. As with cattle, older animals seem to develop some sort of "resistance" to the establishment of heavy trichostrongyle infections and to the damage that they cause.

In many instances of sub-clinical or clinical trichostrongyle disease in sheep, it is very difficult to determine which species of parasite is/are the most important. Where several are involved, the situation can be compared to hearing the music of an orchestra; whereas with a single species, only a soloist is heard.

Haemonchus contortus

Around the world, probably the most significant soloist among GI nematodes of sheep is Haemonchus contortus. The fourth stage larvae and adults of this parasite attach to the mucosa of the abomasum and voraciously suck blood. They secrete an anticoagulant and when the parasites detach to seek another feeding site in the mucosa, the original site continues to bleed. The clinical signs are typical of a blood loss anemia. Haemonchus contortus can cause significant morbidity and mortality in sheep flocks, including in Saskatchewan and elsewhere in Canada.

Nematodirus species

Nematodirus species have also been seen as a cause of clinical problems in sheep in Saskatchewan, usually in wet years and in sheep with inadequate grazing. Nematodirus battus is a significant pathogen in sheep in other parts of the world, but in Canada this species is uncommon and has a very patchy geographic distribution, seemingly not including Saskatchewan.

Other trichostrongyles

Where the effects of trichostrongyles in sheep are seen as an "orchestra", which occurs most commonly in lambs in the late summer and fall, clinical signs include anorexia, dehydration, weight loss, and diarrhea, and often most of the animals in a group will be affected to some degree.

Relatively little is known of the means by which the other trichostrongyles and the other GI nematodes affect GI structure and function. Perhaps the most significant overall effect of all the GI nematodes, other than H. contortus, is that they suppress appetite, leading to reduced feed intake. This is particularly damaging for young, growing animals.

Diagnosis of clinical disease in cattle associated with trichostrongyles and the other GI nematodes usually depends on an assessment of the individual or individuals affected together with the other animals in the population at risk, even though these may be showing no abnormalities when first examined. The key parameters to be assessed are the history, particularly age, season and management and treatment histories, and the clinical signs shown by the affected animals.

The numbers of GI nematode eggs in the feces is also helpful, although the eggs of the trichostrongyles of cattle, other than Nematodirus, are microscopically indistinguishable. Also, it is usually very difficult to reliably relate the numbers of eggs in the feces (eggs per gram or epg) either to the number of parasites in the GI system or to any sub-clinical or clinical effects. That said, it is sometimes helpful to examine fecal samples from animals in a group across the range of severity of clinical signs, and large numbers of eggs (hundreds or thousands per gram) are likely to be more indicative of a parasite problem than are lower numbers.

For assessing GI nematode burdens in lactating dairy cattle on a herd basis, the quantification of antibodies to trichostrongyles, particularly O. ostertagi, in samples from bulk milk tanks is showing some promise. This approach has now been extended to Dictocaulus viviparus, the lungworm of cattle, and the common liver fluke, Fasciola hepatica.

In many parts of the world, anthelmintic resistance is a very significant complication in the control of several trichostrongyle nematodes of sheep, especially Haemonchus contortus, and the problem is becoming more common among trichostrongyles of cattle in several countries. Although resistance has not been reported for cattle in Canada, recent published data indicate that at least in parts of the United States there are populations of Ostertagi ostertagi, Haemonchus placei and Cooperia species in cattle that are resistant to one or more of the currently marketed avermectins (doramectin, eprinomectin, ivermectin), and/or milbemycins (moxidectin), and of H. contortus (primarily a sheep parasite) resistant to avermectins and benzimidazoles (specifically albendazole). Also reported, in cattle in New Zealand, are Cooperia species resistant to albendazole The overall extent and significance of anthelmintic resistance among cattle nematodes in the United States and Canada are not yet fully known.

Products for Trichostrongyles of Cattle

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

Equally important to the choice of product is the design of the parasite control programs, particularly linking them appropriately to the epidemiology and effects of the parasites and to the management of the cattle. In Canada, the use of antiparasitic products needs to be considered in four groups of cattle: cow-calf; feedlot; dairy replacement stock; and adult dairy cows.

Cow-Calf

The most common time to treat for parasites in commercial cow-calf operations in western Canada is in the fall, as the animals are leaving pasture or entering wintering accommodation. An endectocide may be used, or an anthelmintic (for nematodes), or a treatment for lice, or an anthelmintic and a lice treatment. The objectives of the treatment are to remove any parasites that are present, to prevent the spread of parasites among the cattle, to enhance production, particularly the growth rate of the animals, and to remove a stressor for over-wintering following fall weaning of the calves. Sometimes the cows, and rarely the calves, will be treated in the spring at around the time they go to pasture. These treatments are designed to neutralize the major source of pasture contamination in the spring (cows – other than for Nematodirus), and to increase calf productivity, perhaps by increasing milk production by the cows (see below). Purebred producers may treat in the spring and fall more commonly, and at other times of the year, in an effort to improve the health status and the appearance of their animals.

Because in the prairies at least the cows are the only source of trichostrongyles (other than Nematodirus) for the grazing calves, treatment of the cows before they are put on the pasture might be expected to be beneficial. Also, considering the epidemiology of the trichostrongyles, fall treatment of the calves may occur after nematode burdens in the calves have reached their peak. That said, fall treatments probably do have a beneficial effect in that they remove the trichostrongyles, they are helpful for lice control, and they fit well with common management practices.

While many studies have demonstrated a significant positive effect of various antiparasitic treatment regimes for cow-calf systems, others have not. In addition, it is difficult or impossible to make any assessment of the cattle to reliably determine whether or not treatment on a particular farm in a particular year will have a significant positive effect on production.

Feedlot

In western Canada most animals entering feedlots are treated once with an antiparasitic product (at least until the discovery of BSE in Alberta in 2004). The objectives of the treatment are broadly similar to those for cow-calf operations, with increased rates of gain and feed conversion efficiencies among the production goals. As for the cow-calf sector, while many studies have demonstrated positive economic effects, others have not, and there are still no reliable assessment tools to predict the benefits of treatment for an individual producer.

Dairy Replacement Stock

Several studies have demonstrated beneficial effects, in terms of growth rates and age at first calving, following treatment of dairy replacement stock for GI nematodes. Again, however, the results are inconsistent and a prior assessment of the likely benefits essentially impossible.

Adult Dairy Cows

The possibility of a positive and significant relationship between treatment for trichostrongyles (and the other GI nematodes) and milk production has been the subject of numerous studies over several decades. Despite this, there is still no definitive answer as to whether these treatments are effective, primarily because of the high level of variability in the results of the studies. Also, as with the other groups of cattle, there are currently no reliable pre-treatment assessment techniques, although there is some progress with the use of antibody levels in milk to assess trichostrongyle infection levels on a herd basis.

A major meta-analysis published in 2004 reviewed 75 studies from a range of countries (completed between 1972 and 2002) and showed an overall increase in milk production following anthelmintic treatment of approximately 3.5 kg/cow/day. The results showed considerable variation between studies and were influenced by several variables, for example the size of the study population, the choice of anthelmintic product, and the timing and frequency of treatments in relation to lactation. The authors concluded that the meta-analysis could not serve as the basis for guidelines for the treatment of adult dairy cattle to increase milk production but they suggested that if a reliable method could be developed for the diagnosis of Gi parasitism, "a positive treatment effect could be expected in herds classified as having parasite problems."

Overview

In western Canada, the control of trichostrongyles in cattle relies to some extent on the use of antiparasitic products. The choice of product and the timing of treatments are crucial to the success of this approach. Additional control measures include attention to parasites when planning pasture management strategies, particularly to prevent overgrazing, thinking about what might accompany new introductions into the herd, and maintaining a high level of overall health in the herd.

In sheep

Antiparasitic drugs for sheep 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 disease and to improve production (eg. milk production, rate of gain)
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

—Many trichostrongyle species are common in sheep in western Canada, and can be associated with clinical disease either as mixed infections (orchestra) or when Haemonchus is present in large numbers (soloist). As with cattle, it is sometimes difficult to precisely identify the effects of these parasites in sheep when there is no clinical disease. In general, the gentler the climate, the smaller the amount of grazing space per animal, and the poorer the nutrition, the more likely the parasites will be significant

Only oral and injectable ivermectin (VARIOUS) are approved in Canada for the treatment of trichostrongyles in sheep. Ivermectin also has a label claim for the mucosal larvae of some of the parasite species. Additionally, even though not approved, the benzimidazole fenbendazole is used commonly to treat trichostrongyles in sheep, including in Canada. The consequences of any such extra-label use are the responsibility of the veterinarian, not the manufacturer. The scheduling of treatment of sheep 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.

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

In many parts of the world trichostrongyles of sheep, particularly Haemonchus contortus, have developed resistance to several of the commonly used anthelmintics, notably the benzimidazoles and ivermectin. In some sheep-producing regions this resistance is a very major problem. The situation in Canada is more or less unknown. A recent study in Ontario included 47 sheep farms, many of which were investigated in detail for the occurrence of resistance using the Faecal Egg Count Reduction Test (FECRT). Resistance to ivermectin was detected on 28/29 farms examined, to fenbendazole on 19/20 farms, and to levamisole on 1/17 farms. On many farms there was resistance to both ivermectin and fenbendazole. Faecal culture results indicated that Haemonchus contortus was the major resistant species. The occurrence of resistance in sheep in other regions of Canada is not known.

An interesting recent development is the launch of two new anthelmintics for sheep: monepantel (Zolvix; Novartis) in New Zealand, Australia, Europe and South America; and derquantel (Startect; Zoetis) in New Zealand, the United Kingdom, and Ireland. Monepantel is an amino acetonitril derivate (AAD), and derquantel is a spiroindol, new classes of drugs with modes of action different from those of ivermectin and other macrocyclic lactones and fenbendazole and the other benzimidazoles. These features provide hope that resistance to the new drugs, if it occurs, will be slow to develop and that there will be no cross-resistance between the new drugs and older drug classes. Neither of these new drugs is currently marketed in Canada.

The presence of anthelmintic resistance on a farm or in a region presents significant challenges for parasite control. Key elements are an assessment of the extent of the resistance, selection of the most effective drug(s), and adoption of a treatment program based on the epidemiology of the parasites and the management system for the sheep. For example, for grazing ewes and lambs the pre-parturient rise in egg counts can be prevented or minimized by treating the ewes in late pregnancy or around the time of lambing. If resistance is not yet an important factor, the treatment program should be designed to minimize both its emergence. One approach currently being explored is targeted (selective) treatments, in which faecal egg counts are used to identify the most heavily infected sheep in a flock, and only these animals are treated. The goal of these targeted treatments, which are based on the usually aggregated distribution of GI nematodes in a host population (most of the parasites in few of the hosts), is to reduce exposure of the parasites to antiparasitic drugs and thus delay, reduce or prevent resistance.

Trichostrongyles of cattle are not considered zoonotic, but rare, low-level human infections with some have been reported from various areas of the world, not including Canada.

In sheep

Trichostrongyles of sheep are not considered zoonotic, but rare, low-level human infections with some have been reported from various areas of the world, not including Canada.

Sutherland IA and Leathwick DM (2010) Anthelmintic resistance in nematode parasites of cattle: a global issue? Trends in Parasitology 27: 176-181.

Gasbarre LC et al. (2009) The identification of cattle nematode parasites resistant to multiple classes of anthelmintics in a commercial cattle population in the US.
Veterinary Parasitology 166: 281-285.

In sheep

Taylor MA (2013) Parasite control in sheep: a risky business. Small Ruminant Research 110: 88-92.

Falzon LC et al. (2013) Anthelmintic resistance in sheep flocks in Ontario. Veterinary Parasitology 193: 150-162.

Kaplan RM and Vidyashankar AN (2012) An inconvenient truth: global warming and anthelmintic resistance. Veterinary Parasitology 186: 70-78.

O Connor LJ et al. (2006) Ecology of free-living stages of major trichostrongylid parasites of sheep. Veterinary Parasitology 142: 1-15.