Shows a good parasite control program can reap rewards for commercial breedersIn September, 1982, I had the good fortune to be asked by Dr. Al Gabel, director of Equine Research at Ohio State University, to visit Woodburn Farm in response to a request by George Smith and Dr. Wilbur Johnston for advice on parasite control. That visit was the start of a long and fruitful association that resulted in some unique parasitology research and a mass of practical information of value to commercial horse breeding farms in northern latitudes. Most of this work was subsequently published in veterinary journals, but the main points of practical importance to horse owners are summarized here.
The studies were done for 12 years (1983-94) with support from Dr. Gabel, Ohio Equine Research Funds, the Morris Animal Foundation, and several pharmaceutical companies. Throughout this period, most stallions, mares, yearlings, weanlings, and foals on the farm were monitored regularly for parasite eggs, cysts, and oocysts, while each pasture was carefully sampled and checked for infective worm larvae. Body weights, body scores, and general health of horses were also assessed, and any horses taken to surgery for colic were checked for parasitic lesions. The effects of various management and parasite control strategies on these parameters were assessed.
Regular lunches with Smith and Johnston became an important part of this research program, providing a relaxed opportunity for discussion of new research ideas. Smith and Johnston always brought a wealth of experience, common sense, and intellectual curiosity to these discussions, unshackled by blind adherence to old traditions.
The following are some of the Woodburn Farm findings that may be of interest to other horse owners and breeders.

Drug resistance problem
It quickly became apparent after checking pre- and post-treatment fecal egg counts of horses that Woodburn Farm, in Centerville, Ohio, had a serious benzimidazole drug-resistance problem. An immediate change in dewormers was made to drugs that passed the fecal egg count reduction test with at least a 90% drop in egg counts following treatment.
Resistance is a worldwide problem on horse farms that have a history of repeated dewormings over several years. It often goes undetected because worm control programs are not monitored by proper fecal egg counts (giving a result in eggs per gram of feces) and money is wasted on ineffective dewormers.

Pastures perpetuate parasites
Studies over the years at Woodburn Farm amply confirmed the old adage that pastures perpetuate parasites. Even when horses were treated and grazed on a new Woodburn pasture that had not seen horses for at least 20 years, the pasture quickly became contaminated from encysted worms in the horses, the stage of parasite generally unaffected by deworming. The pasture was then infective to all horses that grazed it, in spite of its initial freedom from worms. Pasture larval counts at Woodburn Farm provided an exact measure of the safety or potential danger of each pasture and highlighted the fact that consideration of the pasture worm load is just as important as consideration of the horses worm load.

Small strongyle takeover
Contrary to popular opinion about horse worms, it was quickly discovered that the small strongyles (cyathostomes) that migrate and encyst in the gut wall were much more important than the large strongyles that migrate in the arteries and elsewhere.
From 1982-84 at Woodburn Farm, the small strongyles accounted for over 95% of the pasture contamination with worm eggs and pasture infectivity with worm larvae, while the large strongyles made up the rest. From 1985 until the present, there was a 100% small strongyle takeover and an apparent disappearance of the large strongyles (Strongylus vulgaris, S.edentaius). This is because most dewormers have been effective against the large strongyles, but the small strongyles have developed resistance. This finding is not unique to Woodburn Farm, but is part of a general trend in many areas.

Value of strategic treatments
One of the first control strategies initiated and monitored at Woodburn Farm was the use of strategic treatments in spring and summer, with no other treatments for worm control of adult horses. This was done to combat the spring/summer rise in worm egg counts exhibited by all horses in the northern United States and Europe.
The strategic and less frequent treatment program for adult horses at Woodburn Farm proved very successful over many years, as confirmed by horse health and pasture larval counts. Clearly, treatments year-round are not necessary for mature horses if they are treated strategically. Even the autumn bot treatment was suspended at Woodburn Farm because bots were no longer a problem.

Evaluation of Strongid C
The daily administration of Strongid C in feed was also evaluated at Woodburn Farm. Studies showed that strategic feeding of this drug to foaling mares in spring and summer only was just as effective as prolonged feeding. Furthermore, foals of mares on the strategic schedule did not need deworming with any drug until weaning. This meant that mares were treated for only half the year and foals were untreated until weaning.
In yearlings there was no difference in groups given daily Strongid C or monthly Strongid P. In these studies, there was no indication of resistance to Strongid C, but, as with most dewormers, there was an age effect, so that they were less effective in yearlings than in adult horses.

Poor responses of yearlings to dewormers
One of the most striking finds at Woodburn Farm observed over many years is that modern dewormers are significantly less effective in yearling horses than in adult horses. This is probably related to the lack of immunity to worms in the youngsters with a consequent accumulation of large numbers of encysted small strongyles in the gut wall.
A new drug, moxidectin (American Cyanamid), which is not yet marketed in the U.S., is the only dewormer to show significant activity at normal dosages against the encysted small strongyles. The poor response of yearlings to most dewormers is a serious problem and highlights the limitations of chemical control and the need for non-chemical approaches to parasite control.

Value of pasture hygiene
During a six-month sabbatical leave at the Equine Research Station in Newmarket, England, in 1984, I showed that twice-weekly pickup of manure from horse pastures provided better worm and colic control than regular deworming, and it increased the grazing area by 100% through elimination of pasture fouling.
We subsequently confirmed these findings with pasture sweeping studies at Woodburn Farm using a Jacobsen sweeper to pick up horse manure. This approach offers excellent parasite and colic control under intensive grazing conditions, and it eliminates drug-related problems like resistance and environmental toxicity. There is now a need for studies to develop an improved and economical pasture sweeper or vacuum for horse farms.

Importance of immunity
Throughout the studies at Woodburn Farm, special care was taken to ensure that foals and weanlings were exposed to a low level of parasites to stimulate their immunity to infection, but not enough to harm them. Thus, although we generally kept mean fecal egg counts below 100 eggs per gram (epg), we rarely let them get close to zero. Likewise, we liked to keep pasture larval counts low, but above zero, to ensure continued stimulation of the immune system.
If young horses are given an over-aggressive treatment program (e.g., 12 times a year), there is a high probability that they will fail to develop anti-worm immunity and later experience severe parasitism and colic if moved to a farm with even moderately contaminated pastures.

Presence of Cryptosporidium and Giardia
In a series of studies on Woodburn Farm and Thoroughbred farms in Kentucky, Dr. Lihua Xiao and myself found that both of the above protozoal infections were common in horses. Cryptosporidium infection was prevalent in foals, whereas Giardia infection was found in horses of all ages. There is no cause for alarm with these infections, as most horses develop a strong immunity and do not show any clinical signs of infection. However, the potential exists for clinical disease with diarrhea and ill thrift in a small percentage of foals that do not develop a good immunity to them. There is also a possibility of infection to humans. These infections do not respond to conventional treatments for diarrhea, and there is no effective treatment for Cryptosporidium infection. Every effort should be made to enhance foal immunity by avoiding stresses such as malnutrition, overcrowding, early weaning, and other infections.

Environmental risks of dewormers
After ivermectin was used repeatedly at Woodburn Farm, George Smith noticed that the usual meltdown or degradation of dung in the fields was not occurring. This was due to the fact that treated animals excrete ivermectin in the dung, which is toxic to a large number of dung-breeding insects of benefit to the environment. These insects not only help in the removal of the dung, but they play a key role in nutrient recycling and soil fertility.
Controlled experiments at the OSU Veterinary Research Farm confirmed Smiths observations about the delayed rate of dung degradation in ivermectin-treated horses. An international workshop on the environmental impact of ivermectin was subsequently sponsored by OSU in Columbus in 1992 and helped stimulate worldwide interest in this issue.

Monitoring parasite control programs
One of the most important points generated by the Woodburn Farm studies was that, unless you closely monitor your parasite control program and use of dewormers, you have very little idea of what is actually happening on your farm. This means monitoring each different horse group and pasture and checking the efficacy of your dewormers, the presence of drug resistance, and the safety or danger of your pastures. It is also very important to use reliable techniques like the McMaster slide for fecal egg counts and the direct immunofluorescence assay for the detection of Cryptosporidium and Giardia infections.
The superb collaboration between OSU and Woodburn Farm made it possible to obtain results under real-life conditions on a commercial horse breeding farm with a high density of horses and a constant flow of horses to and from other farms and racetracks.
Changes in the horse-parasite-pasture relationship were monitored, which could not have been observed in an artificial university setting. This led to results and recommendations that were immediately applicable to other commercial horse farms that face similar problems.
During this research period, horses raised at Woodburn Farm performed far above normal expectations, with a steady stream of stakes winners and a noteworthy percentage of two-year-old winners. As George Smith commented, An excellent parasite control program undoubtedly helped in this success.