The potential risk of fluoride-supplemented public water to horses is a topic that periodically arises, especially on the Internet. A casual internet search of this topic can uncover alarming reports of purporting fluoride poisoning in horses from fluoridated municipal water. These reports typically are published in non-peer reviewed sources and are missing important information necessary to confirm the diagnosis, to rule out exposure to other fluoride sources and to eliminate other potential causes. A careful review of the peer-reviewed literature in reputable scientific journals showed no published reports documenting fluoride poisoning in horses due to ingestion of fluoridated public water.
Fluoride is one of the most common elements in the environment and is found naturally in soil, rock, water, air, plants and animal tissues. Volcanic rock and ash and water from deep wells or hot springs in some regions are naturally high in fluoride. Low concentrations of dietary fluoride can be beneficial to animals; excessive amounts can cause fluoride poisoning (fluorosis).
Fluorosis can occur in any species, including horses. In the past, fluorosis occurred more commonly due to ingestion of forages or waters contaminated with fluoride-containing industrial waste, high-fluorine rock-phosphate supplements in animal feeds, and fluoride-containing rodenticides, insecticides, and other chemicals. Regulations restricting the amount of fluoride in industrial pollution, requiring de-fluoridation of rock-phosphate feed ingredients, and banning many fluoride-containing pesticides have greatly decreased the occurrence of fluorosis. Fluoride poisoning still occasionally occurs in areas with high volcanic activity or secondary to ingestion of fluoride-containing medications or contaminated water.
Acute, high-dose intoxications result in severe signs and rapid death. Chronic, lower dose intoxication causes predominantly tooth and bone abnormalities. While small amounts of fluoride improve tooth and bone strength, excessive amounts can cause lameness, stiffness, bone thickening, pain and difficulty eating, weight loss, poor growth rates and poor health. Teeth are affected during the period of tooth development, which in horses is complete before 4-5 years of age. Fluorotic dental lesions will not develop if animals are exposed to excessive fluoride after permanent teeth have erupted.
Public water sources in Kentucky and nationwide often are supplemented with fluoride to help prevent dental disease in humans. Fluoride supplementation in public water is targeted to achieve fluoride concentrations of 0.8 to 1.3 mg/L. The maximum fluoride concentration permitted in public water sources by the national Safe Drinking Water Act is 4 mg/L. The maximum safe level of fluoride in water for horses has not been established. Published guidelines for horses are based on extrapolations from other species. In the USA, the EPA recommends a maximum fluoride concentration of 2 mg/L in water intended for livestock.
In Kentucky, the majority of horses drink fluoridated public water as their major water source, and fluorosis is not seen in this horse population. Studies are needed to determine safe limits of fluoride in feed and water for horses; however, evidence to date indicates that fluoride concentrations allowable in U.S. public water systems are well tolerated by horses and do not cause fluorosis.
Reprinted from Equine Disease Quarterly.