Ivermectin

Abstract

Foal heat diarrhea (FHD) is a major factor in the management of foals between 2 and 3 wk of age. The objective of this on-farm study was to determine the efficacy of postpartum anthelmintic treatment of mares for reducing the incidence and/or the severity of FHD in foals caused by parasitic infestation. Twenty-four Quarter Horse mares and their foals were alternately assigned to a treated or non-treated group according to expected foaling date. Treatment consisted of oral administration of Zimectrin® (Farnam Companies, Inc., Phoenix, AZ) to the dam within 12 h post-foaling. From d 1 through 25, foals were weighed, and vital signs and severity or absence of diarrhea were recorded. Incidence scores of 0 to 3 were assigned to each foal based on severity of diarrhea: 0 = no affliction and 3 = severe, watery diarrhea. There was no difference in incidence scores because of treatment; however, there was a trend for foals from treated mares to experience more severe diarrhea with a shorter duration. There was no difference in BW gain related to treatment or non-treatment of mares. Postpartum administration of Zimectrin to mares did not significantly decrease the incidence or severity of diarrhea in foals. Additionally, it did not affect growth rate of foals. The trend for foals from treated mares to have a shorter duration of FHD may be beneficial to farm managers who spend time and money treating and cleaning foals.

(Key Words: Foal Heat Diarrhea, Ivermectin, Zimectrin®.)

Introduction

Foal heat diarrhea (FHD) is a significant problem that affects the breeding industry. It is the most common cause of non-infectious, self-limiting diarrhea in foals

Foal heat diarrhea is characterized by semi-soft to soft watery stool that is not malodorous and occurs in the first 6 to 10 d of life (Merck, 1998). The foal remains active and alert and has normal vital signs and a good appetite. The only outward sign of FHD, other than soft stool, is a scalded or hairless perineal area. The mechanism of diarrhea in these foals appears to be small intestinal in nature, but it is unclear whether diarrhea is due to increased permeability, hypersecretion, hypermotility, maldigestion, or malabsorption within the small intestine. There are usually no detrimental effects of FHD, but if it is prolonged, or leads to an infective type of diarrhea, dehydration, acidosis, electrolyte imbalances, impaired growth, and death may occur.

Numerous theories, spanning many decades, have been introduced regarding the causative agent of FHD, but no one has determined a definitive cause. Possible etiologies include hormone changes associated with maternal estrus; changes in milk composition; ingestion of gastrointestinal irritants or forage; carbohydrate intolerance; overeating; viral, bacterial, or parasitic infection; and physiologic changes in the foal's developing gastrointestinal tract (Johnston et al., 1970; Martens and Scrutchfield, 1982; Masri et al, 1986; Koterba and Brewer, 1987; Lyons et al., 1993; Vaala, 1996).

The purpose of this study was to evaluate the efficacy of ivermectin administered to the dams of the observed foals within 12 h postpartum to curtail the symptoms of FHD caused by possible parasitic infestation.

Materials and Methods

For this field study, conducted at Texas A&M, 24 multiparous Quarter Horse mares, with an average age of 9.7 yr, in the last trimester of gestation were utilized. These mares were housed and owned by Texas A&M and were routinely de-wormed every 60 d. Mares and foals were maintained on pasture and group-fed a pelleted concentrate to meet NRC (1989) requirements. For this project, mares were assigned to either a treated or non-treated group based on alternating selection according to expected foaling date in an attempt to eliminate any seasonal effects. Substitutions were made when the approximate time of foaling could not be deduced. In this instance, that mare's treatment was switched with the next mare due.

Although a number of anthelmintics are effective against milk-borne parasites, an ivermectin-based product was utilized because of availability and efficacy against milk-borne strongyloides infestation. Treatment consisted of oral administration of Zimectrin® paste (Farnam Companies, INC., Phoenix, AZ) to the dam within 12 h post-foaling. The dose was determined by the mare's post-foaling BW and package labeling at a dose of 0.2 mg/kg BW. Beginning on d 1 and continuing through d 25, each foal was weighed, vital signs were recorded, and the foal was evaluated for the severity or absence of diarrhea. Incidence scores (severity of diarrhea) were assigned objectively, without knowledge of treatment group, based on a O- to 3-numbering system: O = absence of symptoms and 3 = extremely profuse diarrhea. Data were collected daily between 0800 and 1200 h.

all data were analyzed for treatment, day, and treatment by day effects by an ANOVA table. The mean incidence scores and daily weight were plotted for each day, and the significance was evaluated at P

Results and Discussion

Twenty-three of 24 foals that were evaluated in this field study experienced FHD. Eleven of 12 foals with treated dams and all of the foals with untreated dams demonstrated signs of diarrhea. Only two foals, both from the non-treated group, attained an incidence score of 3. Foals from treated mares had symptoms of diarrhea from d 2 through 21, and those from non-treated mares had symptoms from d 2 through 25 (Figure 1). The greatest number of foals from treated mares showed symptoms from d 8 through 13; those from non-treated mares showed symptoms from d 9 through 18 (Figure 1). Overall, the incidence of FHD occurred from d 8 to 14, which is closely correlated with results from other studies (Ludwig et al., 1983; Masri et al, 1986).

These data showed no significant difference in severity of diarrhea, as denned by the incidence scores of O to 3, because of treatment (Figure 2). However, there was a trend for the foals from treated dams to experience higher severity of diarrhea with a shorter duration. The foals from non-treated dams had a lesser overall severity of diarrhea, but it appeared to last 3 to 4 d longer than in the treated group. The length of each diarrheic episode ranged from O to 19 d in foals from treated dams and from 2 to 21 d in foals from non-treated dams (Table 1). This trend could be important for farm managers in light of the fact that treated foals do not have to be cleaned and have medications applied for as long as non-treated foals.

Mean foal birth BW for the treated and control groups were different; therefore, foal BW were normalized for comparison. There was no significant difference in growth rate of foals as indicated by BW change in the normalized data (Figure 3). Thus, it did not appear that the administration of Zimectrin® paste to the dams was detrimental to their foals. Previous studies utilizing ivermectin did not evaluate BW of foals. There was no difference in the observed frequency of FHD between the treated and non-treated groups.

Implications

Foal heat diarrhea may be unrelated to any of the causes discussed previously, or it may be multifactorial in origin and consist of complex interactions between any number of factors. This study has shown that the administration of ivermectin to broodmares within 12 h postpartum was safe for the foal, but had no significant effect on FHD. Fecal samples were not tested for the presence of parasites, but pretreatment and posttreatment fecal egg counts performed on the mares and foals would provide more conclusive information on the efficacy of ivermectin on parasite-induced diarrhea. In addition, results from a less intensively managed group of mares not receiving routine deworming would be useful. Currently, the most plausible causative agents are a change in intestinal morphology caused by normal development in the foal or parasitic infestation not affected by ivermectin administration to the dam in the first 12 h postpartum. Future research should focus on these areas.

Literature Cited

Johnston, R. H., L. D. Kamstra, and P. H. Kohler. 1970. Mares' milk composition as related to foal heat scours. J. Anim. Sei. 31:549.

Koterba, A. M., and B. D. Brewer. 1987. Intestinal disorders of the neonate. Proc. 9th Bain-Fallon Memorial Lectures: The Mare and Foal. p 185. Artemon, New South Wales.

Ludwig, K. G., T. M. Craig, J. M. Bowen, M. M. Ansari, and W. B. Ley. 1983. Efficacy of ivermectin in controlling Strongyloides westeri infections in foals. Am. J. Vet. Res. 44:314.

Lyons, E. T., S. C. Tolliver, J. H. Drudge, D. E. Granstrom, and S. S. Collins. 1993. Natural infections of Strongyloides westeri: Prevalence in horse foals on several farms in central Kentucky in 1992. Vet. Parasitol. 50:101.

Martens, R. J., and W. L. Scrutchfield. 1982. Foal diarrhea: Pathogenesis, etiology, and therapy. Comp. Cont. Educ. Pract. Vet. 4(4):S175.

Masri, M. D., A. M. Merritt, R. Gronwall, and C. F. Burrows. 1986. Fecal composition in foal heat diarrhea. Equine Vet. J. 18(4):301.

Merck and Co. 1998. Merck Veterinary Manual. Merck and Co. Inc., Whitehouse Station, NJ.

Palmer, J. E. 1985. Gastrointestinal diseases of foals. Vet. Clin. North Am. Equine Pract. 1:151.

National Research Council. 1989. Nutrient Requirements of Horses. NRC-NAS, Washington, DC.

Vaala, W. E. 1996. In Equine Stud Medicine. p 127. University of Sydney, Sydney, Austrailia.

S. E. HARRIS1, M. M. VOGELSANG, PAS, G. D. POTTER, PAS, and E. E. BASS

Department of Animal Science, Texas A&M University, College Station 77843

1 To whom correspondence should be addressed: sharris@txcyber.com

Copyright American Registry of Professional Animal Scientists Aug 2004
Provided by ProQuest Information and Learning Company. All rights Reserved