New pastures in production medicine

The goal of the program committee for the American Association of Bovine Practitioners' 2006 annual conference, Sept. 21-23 in Saint Paul, Minn., was to arrange a lineup that would give attendees at least one good idea to implement in their practice.

Summarized here are two of the many presentations with practical take-home messages.

Novel approaches to parasite control

The emergence of multiple-drug-resistant gastrointestinal nematode parasites in sheep and goats underscores the need to move away from control through frequent administration of anthelmintic drugs. Another dynamic that is driving producer interest in reducing use of anthelmintics is the rapidly growing area of organic production, including wool.

Two speakers at a near-capacity joint session of the AABP and American Association of Small Ruminant Practitioners discussed aspects of "smart drenching," the new approach to parasite control that emphasizes maximizing the effectiveness of treatments while decreasing the development of drug resistance.

Dr. Joe Snyder—a practitioner and sheep rancher in Myrtle Point, Ore., and AASRP incoming president—said grazing management is the most important factor in reducing the need for anthelmintics.

Management must address the increased susceptibility to parasites that results from raising species that are inappropriate for the climate or terrain. Goats and sheep that originated from central Asia are raised on flat, lowland pastures in the United States, and llamas and alpacas that live above the timberline in a high-altitude desert in native lands are raised in lush, lowland U.S. pastures.

Utilizing alternate-species grazing programs with access to "safe pasture" is probably the best tool for limiting parasite exposure, Dr. Snyder said.

The traditional "treat and move strategy" of turning animals into a "clean" pasture has to be abandoned, he said. Only parasites resistant to treatment remain in these animals, which colonize the next pasture exclusively. He said, "Now we believe it's best to turn out on a pasture with a small population of internal parasites that haven't been constantly hammered with anthelmintics so that we slow the development of resistance."

Veterinarians must advise clients to test and quarantine new animals to avoid introducing resistant parasites into the flock or herd, Dr. Snyder said.

Turning to the nematode life cycle, Dr. Snyder said, "Our traditional focus has been on attacking adult parasites in the animal with medications. We need to expand our thinking to the larval stages in the environment." Understanding the details of life cycles enables people to plan effective pasture management schemes to limit parasite exposure.

He noted that heat and desiccation are useful, because infective larvae barely survive 24 hours at high heat and low humidity. Freezing is helpful but less so.

"Nothing is going to affect parasites like ivermectin once did," Dr. Snyder acknowledged, "(but) some things might reduce your parasite load 20 percent to 50 percent—(if) you start adding those things up, you could make a significant dent in parasite load."

Dr. Snyder described some parasite-inhibiting plants and other agents. Over time and as part of integrated management, these can reduce shedding significantly, he said. Plants include the legumes Serecia lespedeza, Lotus corniculatus (birdsfoot trefoil), and Hedysarum coronarium (Sulla), and the herb Plantago (plantain).

Well-nourished animals are more resistant to and resilient against parasite infestation. Protein depletion resulting from parasites needs to be replenished. Of minerals, selenium is the most important in parasite control. Chronic parasite infestation could be exacerbated by copper deficiency. To test a flock or herd for copper concentrations, Dr. Snyder recommended saving the livers of several dead or stillborn animals and submitting them. Administration of copper oxide wire particles by a veterinarian provides at least a month of protection for lambs against Haemonchus contortus—the most problematic parasite—without creating toxic concentrations in the liver, he said, and should also work in goats.

Despite some anecdotal success stories, there are no controlled studies documenting the efficacy of diatomaceous earth in inhibiting parasites.

"Parasites have predators," Dr. Snyder said—nematophagous fungi, which attack and kill larval nematodes. It may be possible to grow them for use in parasite control.

"I like to think of this as biological star wars," he said.

Dr. Ray Kaplan, associate professor in the Department of Infectious Diseases, University of Georgia, said anthelmintic resistance has become a worldwide phenomenon. "We're at risk of having no effective anthelmintics in the near future."

New drug classes were introduced every decade from the 1950s through the 1980s, he said, but no new drug classes have become available since then, except for cyclodepsipeptide (emodepside), which Bayer introduced in 2005 for cats (but not yet in the United States). It is not clear whether an emodepside product will be developed for livestock.

Resistance is likely to outpace the introduction of new anthelmintics, and frequent application of dewormers is no longer a viable approach, Dr. Kaplan said. He cited a study published by his research group in JAVMA Aug. 15, 2003, that concluded that the high prevalence of resistance to multiple anthelmintics in gastrointestinal nematodes of goats in the southern United States emphasizes the need to re-examine control practices.

The Southern Consortium for Small Ruminant Parasite Control, at scsrpc.org, was formed in response to the critical state of the small ruminant industry associated with the emergence of anthelmintic-resistant worms.

Dr. Kaplan said few anthelmintics are approved for use in goats, but most are approved for sheep. In reviewing specific drugs, he said that moxidectin is the treatment of choice for severely ill sheep and goats on farms where it is known to still be highly effective.

Explaining why testing the efficacy of the drug is so important, Dr. Kaplan said that killing some worms will result in clinical improvement, making it appear that treatment was effective. In practical terms, that means when resistance is present in moderate degrees, parasite control will be inadequate, but it will not be noticed clinically. Without testing, resistance will not be observed until it reaches a high degree, resulting in severe production loss and death of some animals. Rotating anthelmintics on a schedule also can mask resistance and not slow the progression. "Resistance develops slowly to all drugs simultaneously; one effective drug will 'cover' for another," he said.

To treat infestation with H contortus, Dr. Kaplan recommends integrating FAMACHA into the parasite control program. FAMACHA is the new on-farm system that classifies an animal into a category based on its degree of anemia. This information, which can be found at scsrpc.org, is then used to determine whether the animal requires anthelmintic treatment.

Fetal sexing and sex-sorting semen

In an AABP beef session, Dr. Brad Stroud of Stroud Veterinary Embryo Services Inc., Weatherford, Texas, presented guidelines for ultrasound fetal sexing in cow-calf operations and strategies for sex-sorting semen to be inseminated in breeding stock.

Both technologies enable selection for offspring of the desired sex. After an ultrasound determines fetal sex, breeders of purebred beef cattle are likely to keep the undesired males for themselves and elect to sell females carrying female fetuses in consignment or production sales, because the female fetus makes the sale bring upward of $500 or more than their counterpart males.

Breeders often don't know that the technology exists to do ultrasound fetal sexing. Dr. Stroud conjectures it's because most bovine practitioners, not having the skill, don't offer the service. As reproductive technologies become more commercialized, the demand for this skill is growing, and it can be a practice builder.

Despite a steep learning curve, any veterinarian can learn the skill, Dr. Stroud said. The learning curve depends not only on factors such as palpation and ultrasound experience but commitment. "You've got to be a bulldog," he said.

A veterinarian must become proficient at obtaining an image with a transducer in the cow's rectum. This can be learned in one's practice or at an interactive lab. Dr. Stroud said the most common mistake is moving the transducer too quickly.

The window of opportunity for fetal sexing normally ranges from 60 to 90 days of gestation but can extend from 58 to 120 days, in some cases. Fetal sexing may not be possible when the fetus is carried too deeply or the fetus can't be reached with the ultrasound probe. In beef cattle, fetal sex can't be determined in about 1 of 40 cows because of these factors.

Before offering this service, veterinarians should achieve a field accuracy rate of at least 97 percent, Dr. Stroud said. The cost to a producer is volume-dependent and can run from $10 to $50 per cow.

"Fetal sexing is really an inefficient way to go about this," Dr. Stroud said. The newer technology of sex-sorting semen does not waste the eggs of ovulating cows, he said. Virgin heifers are inseminated during estrus with semen from sires that have high artificial insemination rates.

This leading-edge technology uses flow cytometry to sort X-bearing sperm from Y-bearing sperm. This is possible because a sperm bearing an X chromosome carries four percent more total DNA than a sperm carrying a Y chromosome.

The sorting speed is slow—5,000 sperm per second, with 90 percent accuracy. "If you look at the number (of sperm) you need to inseminate a cow, that's why it's costly," Dr. Stroud said. Also, conception rates are lower with sex-sorted sperm than traditional sperm, and there are other important differences.

Dr. Stroud underscored the need for veterinarians to advise clients on genetic, nutritional, and reproductive management before recommending use of sex-sorted semen, along with proper techniques for storage, handling, and insemination.