|IODINE IN THE HORSE
TOO MUCH OR TOO LITTLE
|Iodine is an essential nutrient for reproduction and
normal physiological function in the horse. Thyroxine contains iodine, and this hormone
along with triiodothyronine (T3) has powerful effects on the overall health of the
horse. These hormones influence nearly every process in the body, from heat
regulation and feed utilization to proper bone growth and maturation.
Nearly 75% of the iodine in an animal's body is in the thyroid gland. Iodine deficiency may result in goiter as the thyroid gland enlarges in an attempt to produce thyroxine. In the horse, goiters often occur in the foal at birth. Foal goiter may result from a deficiency in iodine in the mare's ration during pregnancy or it may be caused by a goitrogenic substance. Symptoms of iodine deficiency may be a stillborn foal or a very weak foal at birth that cannot stand and nurse. The foal may also have a rough haircoat, contracted tendons, angular limb deformities or other abnormal bone development. A Russian study (Kruzkova, 1968) indicated that mares which had shown anovulatory cycles responded to iodine supplementation.
The horses most sensitive to high iodine levels are foals from mares who are supplemented with high levels of iodine. Iodine is concentrated across the placenta and in milk so that the fetus and nursing foal receive much higher concentrations than are present in the mare's ration. Therefore, goiters may be present in newborn foals while sparing the mother A dietary intake of 83 mg I/day is the lowest level reported to have caused goiter in a horse more mature than a suckling foal (Drew et al, 1975). Baker and Lindsey (1968) reported that goitrous foals were born on three farms which were feeding mares high levels of iodine. The incidence of goiter was proportional to the level of iodine fed and equaled 3% on one farm feeding 48-55 mg I/day, 10% on a farm feeding 56-69 mg I/day and 50% on another farm feeding 288-432 mg I/day. A neighboring farm which did not have any goitrous foals fed iodine at a rate of 6.3-7 mg I/day.
Sipple (1969) reviewed a case in which 11% of the foals born on a farm had goiters. Analysis of the diet revealed that the mares received between 160-400 mg I/day. Coincidentally, the author discovered that the manager of this farm was the brother of the manager of one of the farms in Baker's study in Florida. apparently, the Florida horseman had prescribed the same iodine supplement for his brother's horses 1,000 miles away. Drew et al (1975) reported that on one stud farm in England four foals were born with greatly enlarged thyroids and leg weakness. One mare also had an enlarged thyroid. Feed analysis showed that the mares had received 83 mg I/day from a proprietary feed during pregnancy. The year before the introduction of this proprietary feed, the mares received a vitamin / mineral supplement which supplied about 12 mg I/day and there was no problem with goiter on the farm. The results of these studies are summarized in Figure 1. It appears from these reports, that around 50 mg of dietary iodine is required in the daily rations of mares to produce any incidence of goiters in their foals. One other study (Driscoll et al, 1978) reported goitrous foals from mares receiving 35 mg I/day. There is some question, however, about what levels of iodine the mares in this study actually received. The authors reported that the mares were given 12 ounces per day of a supplement which was reported to contain 58 PPM iodine. The guaranteed analysis on the product's label stated that it contained 340 PPM iodine and independent analyses of the same product revealed that it contained at least 580 PPM iodine, a level 10 fold higher than reported in the paper. Using the manufacturer's guarantee, the mares would have received a total of 131 mg I/day and according to the independent analyses, a total of 212 mg I/day. These levels are within the ranges reported to produce goitrous foals in other studies.
Baker, H.J. and J.R. Lindsey. 1968. Equine goiter due to excess dietary iodine. J. Am. Vet. Med. Assoc. 153:1618.
Drew, B., W.P. Barber, and D.G. Williams. 1975. The effect of excess iodine on pregnant mares and foals. Vet Rec. 97:93
Driscoll, J. et al. 1978. Goiter in foals caused by excess iodine. J. Am. Vet., Med. Assoc. 173:858
Kruzkova, E. 1968. Mikroelementy i vos proizvoditel-'naja funkeija kobyl. Tr. Vses. Inst. Konevod-stvo. 2:28 (as cited in Nutr. Abst. Rev. 39:807, 1968).
National Research Council. 1989. Nutrient Require-ments of Horses. Washington D.C.: National Academy Press.
National Research Council. 1980. Mineral Tolerance of Domestic Animals. Washington D.C.: National Academy Press.
Sipple, W.L. 1969. A Veterinarian's Approach to Stud Farm Nutrition. Eq. Vet. J. 1:203
A: 6-7 mg I/day. No
suggestion of iodine toxicity in mares or foals. Baker and Lindsey (1968).
|"IODINE IN THE HORSE
TOO MUCH OR TOO LITTLE"