For the most part, each livestock species harbors its own species of parasites. However, sheep and goats share some of the same parasites. Only one worm species, Trichostrongylus axei, affects all livestock species; it is a minor stomach worm usually of little concern. Cattle and goats can be grazed together, each consuming the other's parasites and reducing the number of available infective larvae for the preferred host species. If co-grazing is not preferred, cattle and goats can be grazed alternately on the same pastures. Again, each consumes the other's parasites and when returned to the same pasture, numbers of available infective larvae have been reduced. Both livestock species should gain from this over time. The one situation that requires some care with this strategy is the presence of young calves. Calves can become infected with Haemonchus contortus, but problems in calves are generally much less than those in goats.
Pasture rotation or rotational grazing has been used for years to break parasite cycles. The main reason to use pasture rotation is not for parasite control but to provide the most nutritious forage for growth and development; indeed, higher protein diets help animals at risk be more resilient to parasite loads. If animals are grazed correctly, most forages reach the next most nutritious stage in about 30 days. Therefore, many rotation schemes have animals returning to pastures at around 30-day intervals. Unfortunately, this 30-day interval is also about the same time necessary to ensure that the previous worm parasite contamination has now been converted into the highest level of infectiousness for the next grazing group. Thus, 30-day rotation schemes may actually lead to increased worm parasite problems. In fact, heavy exposure over a short period of time can lead to disastrous clinical disease and losses. Rotation schemes of two to three months have been shown to have some effect on reducing pasture infectiousness in tropical and subtropical environments in the southeastern United States, but in more temperate environments, infectiousness can extend out to eight to 12 months, depending on conditions. For the most part, it is impractical to leave pastures ungrazed for such extended periods of time. Some success at reducing infectiousness can be achieved by cutting pasture for hay between grazing periods and by not grazing pastures below 3 inches, where the majority of parasite larvae reside. When rotation schemes are used, stocking rates are usually high, and the resultant increase in contamination may make problems worse.
Copper oxide wire particles (COWP) have been marketed for years as a supplement for livestock being managed in copper-deficient areas. COWP come in adult cattle, calf, and ewe boluses—25, 12.5, and 4 grams, respectively. Only cattle boluses are available in the United States. Due to potential toxicity in sheep, only one dose per year is recommended. It is also well known that copper has some anthelmintic activity against abomasal worms but not other gastrointestinal worms. That makes it a very narrow-spectrum product. But, in view of the potentially devastating problem of anthelmintic resistance by H. contortus, recent work has revisited the possibility of using COWP to specifically target H. contortus. Such work has shown that as little as a gram or less (lambs) and two grams (ewes) may remove substantial numbers of H. contortus. Similar work in goats has not been tested adequately to establish what is needed, but similar doses may be appropriate. As mentioned, copper has to be used cautiously in sheep because toxicity can develop due to accumulation in the liver. Toxicity may not be an issue in goats because they are less sensitive to excess copper intake. Thus, higher doses and/or more treatments during haemonchosis season may be useful in goats.
An approach to parasite control that has not been adequately explored in the United States is the use of medicinal plants with anthelmintic properties. There is growing evidence in work from New Zealand and Europe that grazing or feeding of plants containing condensed tannins (CT) can reduce fecal egg counts (FEC), larval development in feces, and adult worm numbers in the abomasum and small intestine. A number of CT-containing forages grow well throughout the southern United States, but most of these have not been tested for their potential anthelmintic properties. Preliminary tests with sericea lespedeza (SL, Lespedeza cuneata), a CT-containing, perennial warm-season legume, have shown positive effects of reduced FEC in grazing goats and in sheep and goats in confinement when forage was fed as hay. In addition, an effect on reducing worm burden has also been reported. Similar results have been observed using CT-containing quebracho extract for small intestinal worms but not abomasal worms. In addition to its potential use in controlling worms, SL is a useful crop for limited-resource producers in the southern United States. It is adapted to hot, droughty climatic conditions and acid, infertile soils not suitable for crop production or growth of high-input forages such as alfalfa. It can be overseeded on existing pasture or grown in pure stands for grazing or hay. Farmers could increase profits by marketing SL anthelmintic hay or using it themselves and reducing their deworming costs. In South Africa, SL has been reported to increase profits with rangeland farmers by bringing poor, drought-prone, infertile land into useful production for sheep; any anthelmintic uses would increase the value of SL even further. The same is true in the southern United States, which has a climate and soils ideal for growth of this plant. In addition to hay, SL is being evaluated in the form of meal, pellets, and cubes to be fed as a supplement to grazing animals or as a deworming method under temporary short-term confinement. SL-processed products are expected to become available in the near future.
There is considerable evidence that part of the variation in host resistance to worm infection is under genetic control in goats and sheep. Resistance is most likely based on inheritance of genes that play a primary role in expression of host immunity. Based on survival of the fittest management conditions, several goat and sheep breeds are known to be relatively resistant to infection. Such breeds include: goat: Small East African, West African Dwarf, and Thai Native; sheep: Scottish Blackface, Red Maasai, Romanov, St. Croix, Barbados Blackbelly, and the Gulf Coast Native. Katahdin sheep have been considered as being more parasite resistant, but studies to document this are few and not conclusive. Using resistant breeds exclusively or in crossbreeding programs would certainly lead to improved resistance to worm infection, but some level of production might be sacrificed. While such a strategy may be acceptable to some, selection for resistant animals within a breed is also a viable option. Selection for resistant lines within a breed has been demonstrated with goats (Scottish Cashmere) and sheep (Merino and Romney). Within breed, animals become more resistant to infection with age as their immune system becomes more competent to combat infection. However, some animals within such a population do not respond very well and remain relatively susceptible to disease. This means that the majority of the worm population resides in a minority of the animal population. It would make sense to encourage culling practices (based on FEC, packed cell volume, FAMACHA©, etc.) where these minority "parasitized" animals were eliminated, thus retaining more resistant stock. Identifying sires that throw relatively resistant offspring would speed up this process. This approach has been used successfully in goats (Scotland) and sheep (New Zealand and Australia), but it may take eight to 10 years to achieve satisfactory results. Heritabilities for FEC, a common measurement for assessing parasite burden, range from 0.17 to 0.40, which is quite good. Thus, selection for resistance and/or selection against susceptibility using a measurement such as FEC has been moderately successful. The real benefit to this approach is that reliance on dewormer intervention for control can be reduced, thus conserving the activity of such dewormers for when they are truly needed to save an animal's life.
Research with nematode-trapping fungi under both experimental and natural conditions in Denmark with beef cattle, horses, and pigs has demonstrated the potential of nematode-trapping fungi as a biological control agent against free-living larval stages of livestock parasites. The concept of using microfungi as a biological control agent against worms was introduced in the late 1930s and early 1940s. These fungi occur ubiquitously in the soil/rhizosphere throughout the world where they feed on a variety of free-living soil nematodes. These fungi capture nematodes by producing sticky, sophisticated traps on their growing hyphae. Of the various fungi tested, Duddingtonia flagrans possesses the greatest potential for survival in the gastrointestinal tract of ruminants. After passing through the gastrointestinal tract, spores of this fungus are able to trap the developing larval stages of the parasitic worms in a fecal environment. This technology has been successfully applied under field conditions with cattle, sheep, and goats. This is an environmentally safe biological approach for control of worms in goats under sustainable, forage-based feeding systems. To date, the only delivery system is incorporating the fungal spores into supplement feedstuffs that have to be fed daily. This requires a management system that can accommodate daily feeding to ensure that all animals consume an equivalent amount of feed. To achieve adequate control of larvae in the feces during the transmission season, spores have to be fed for a period of no shorter than 60 days. This can be expensive and time-consuming. A bolus prototype is being developed that would allow a single administration where spores would then be slowly released over a 60-day period. This product is not commercially available at this time.
As a consequence of drug resistance among worms of grazing ruminants, efforts have increased in recent years to develop functional vaccines. This has been made possible by newer technologies in gene discovery and antigen identification, characterization, and production. Successful vaccines have been developed for lungworms in cattle and tapeworms in sheep. The most promising vaccine for nematodes has been what is called a "hidden gut" antigen and it specifically targets H. contortus. This antigen is derived from the gut of the worm and when administered to the animal, antibodies are made. When the worm ingests blood during feeding, it also ingests these antibodies. The antibodies then attack the targeted gut cells of the worm and disrupt the worm’s ability to process nutrients necessary for proper growth and maintenance, causing worms to die. This vaccine has been tested successfully in sheep under experimental conditions but has had limited success under field conditions. Reasons for this are unclear. Effect of this vaccine on H. contortus in goats has not been evaluated. The one drawback to this vaccine is that the antigen is normally "hidden" from the host and a number of vaccinations may be required to maintain antibody levels high enough to combat infection. This may be quite expensive. In addition, massive numbers of whole worms are necessary to extract limited amounts of antigen; therefore, this will only be practical when methods are derived to artificially make the antigen so it can be mass produced at a lower cost. Vaccines for other worms that do not feed on blood have focused on using antigens found in worm secretory and excretory products. These antigens do have contact with the host and should stimulate continuous antibody production. However, protection has been quite variable, and marketing such products has not been pursued. Vaccines are not available at this time.
The control of worms traditionally relies on grazing management and/or dewormer treatment. However, grazing-management schemes are often impractical due to the expense and the hardiness of infective larvae on pasture. Currently in the United States, only three dewormers are approved for use in sheep and two in goats. The three for sheep are levamisole (Levasol and Tramisol, oral drench), albendazole (Valbazen, oral drench), and ivermectin (Ivomec for sheep, oral drench). The two for goats are fenbendazole (Safeguard/Panacur, oral drench) and morantel tartrate (Rumatel, feed additive). Use of any other dewormers or other methods of administration are not approved and constitute extra-label use. There are FDA rules and regulations governing use of such drugs where extra-label use may be necessary. The evolution of dewormer resistance in worm populations is recognized globally and threatens the success of drug treatment programs. In South America, South Africa, and the southeastern United States, prevalence of resistance to dewormers has reached alarming proportions and threatens future viability of small ruminant production. In the only comprehensive study in the United States on prevalence of dewormer resistance in goats, 90% of all farms had resistance to two of three drug classes, and 30% of farms had worms resistant to all three drug classes. Fortunately, the one dewormer that may still remain effective in some circumstances is moxidectin (Cydectin). However, there are now several reports of moxidectin resistance.
There is an urgent and increasing need to develop alternative strategies that could constitute major components in a sustainable parasite-control program. The most promising of these methods that are immediately applicable are smart drenching, copper oxide wire particles, and FAMACHA©. An integrated approach using these current methods should have an immediate impact on productivity and profitability of small ruminant production systems in the southeastern United States and other regions where H. contortus and/or other worms can be a problem. Producers will be able to reduce overall dewormer usage by integrating an alternative compound (copper oxide wire particles) with identification of animals in need of treatment (FAMACHA©) and adopting smart drenching procedures, thereby reducing cost of production while improving animal health and productivity. Lower frequency of deworming will also reduce potential environmental impact of excreted anthelmintics and will decrease the development of resistance, thereby prolonging the usefulness of available dewormers. This integrated approach will provide a cornerstone for inclusion of future environmentally sound worm-prevention and control technologies to secure a sustainable, growing small ruminant industry. Integration of other methodology/technology certainly will be instituted when evaluation is complete and ready for use.