• Cause: ingestion of fluorine via contaminated feed or water, exposure to environmental fluorine or born to fluorotic dam. 
• Signs:  
  • Acute: pain, multiple organ failure signs, collapse, seizures, sudden death. Oral ingestion of corrosive substance (hydrofluoric acid) may cause lesions to mucous membranes and abdominal pain. 
  • Chronic: abnormal dental wear, particularly to molars. Skeletal exostoses may cause lameness. Teeth may stain from food. Enamel may appear chalky. 

• Diagnosis: clinical signs, history of fluorine exposure, tissue and feed fluorine analysis. 
• Treatment: remove from exposure; pain relief.
• Prognosis: dependent on severity of signs: poor.

• Worldwide: largest known natural deposits are in US, UK and Germany. 
• Africa, Argentina, India and Australia have high fluoride concentrations in water. 
• Particularly grazed areas where ground rock high in phosphate/presence of geothermal springs. 
• Countries without robust regulations regarding disposal of industrial effluent. 

• Younger animals may be more susceptible than older animals due to developing teeth and bones. 

• Presence of fluorosis in horses may have human health implication depending on source of fluorides and whether humans are suffering similar environmental exposure.  
• Fluoridated water can be a source of fluorosis in horses. 
• If horse meat is going into the food chain, then it will need to be checked to ensure safe levels of fluorine.  

• Cost of diagnostics, treatment and loss of affected animal(s). 

• Acute: 
  • Rapid ingestion of fluorine in feed or water. 
  • Contact with fluorine containing compounds. 
  • Inhalation of fluorine containing compounds. 

• Chronic: chronic ingestion of fluorine leading to dental/bone lesions.  

General

• The form of ingested fluoride can be significant with absorption from the intestinal tract being greater for fluoride ingested in feed crops than fluoride ingested from raw rock or from feed supplements. 

Specific

• Water or feed contamination with fluorine such as:
  • Proximity to fluorine producing industries (metal smelting, particularly aluminum, brickworks, glass works, petrochemicals, medical and chemical factories, etc).
  • Proximity to natural mineral deposits.
  • Water with naturally high fluoride content such as geothermal springs.
  • Contamination by volcanic ash.
  • Contaminated soil.
  • Contaminated feed crops.
  • Superphosphate fertilizers.
  • Phosphate feed supplements. 
• Fluorine-containing pesticides such as sodium fluoride, sodium fluoroacetate (compound 1080) and sodium fluorosilicate are less widely used than in the past. 

• Fluorine is an incredibly reactive and unstable element that forms compounds with nearly all other elements. 
• In its pure form it is a highly toxic gas.  
• In nature it does not exist as a pure element but as fluoride compounds such as fluorspar (CaF2), fluorapatite (Ca5(PO4)3F) and cryolite (Na3AIF6). 
• Fluoride is believed to be an essential element and may be involved in many biochemical pathways, but evidence is lacking to determine whether fluorine truly is an essential dietary element in horses.  
• Fluoride is primarily absorbed in the stomach and small intestine following ingestion or via the respiratory tract if inhaled.  
• It is then sequestered into bones and teeth where it interferes with bone homeostasis processes. Research indicates that fluorides may mediate their actions through the Mitogen-activated protein kinases (MAPK) signaling pathway and may result in changes to gene expression, cell stress, and cell death. 
• Fluorine is excreted via urine. Bone and teeth stores may become saturated. The pH of urine affects excretion rates. 
• Scientific data is lacking to be able to accurately state the dietary requirements of fluorine in equines or to accurately state the toxic concentrations of fluorine for equines. The long-term maximum tolerable dietary concentrations of high availability fluoride for horses has been estimated at 40 mgF/kg dry matter. 
• In utero, foals may be exposed to high fluoride levels from their dam’s blood, but it is believed that the placenta does limit exposure to some degree. 
• Both enamel and dentin may be affected during tooth development. Enamel concentrations of fluorine will not increase once the tooth erupts, as mature enamel does not remodel or change. Dentin may continue to remodel and therefore have fluorine concentrations similar to bone.  

• Acute exposure to large overdose may result in rapid clinical signs. 
• Chronic exposure to lower doses of fluorine is required for dental and bony changes. This may occur over a prolonged period. 

• Mild dental fluorosis is of little clinical significance and so may be underreported. 
• Severe skeletal and dental fluorosis is not common but is clinically significant. 

Refereed papers

• Recent references from PubMed and VetMedResource. 
• Choubisa S L (2023) Chronic fluoride poisoning in domestic equines, horses (Equus caballus) and donkeys (Equus asinus). J Biomed Res 4 (1), 29-32.  Probiologists.
• Kelly L H, Uzal F A, Poppenga R H et al (2020) Equine dental and skeletal fluorosis induced by well water consumption J Vet Diagn Invest 32 (6), 942-947 PubMed.
• Livesey C & Payne J (2011) Diagnosis and investigation of fluorosis in livestock and horses. In Pract 33 (9), 454-461 WileyOnline.
• Everett E T (2011) Fluoride’s effects on the formation of teeth and bones, and the influence of genetics. J Dent Res 90 (5), 552-560 PubMed.
• Choubisa S (2010) Osteo-dental fluorosis in domestic horse and donkeys in Rajasthan, India. Fluoride 43, 5-12 FluorideResearch.
• Macicek P & Krook L P (2008) Fluorosis in horses drinking artificially fluoridated water. Fluoride 41, 177-183 FluorideResearch.

Other sources of information

• National Research Council (2005) Fluorine. In: Mineral Tolerance of Animals. 2nd edn. National Academies Press, USA. pp 154–181.
• World Health Organization (WHO) Preventing Disease Through Gealthy Environments. Website: www.who.int.
• Legislation.gov.uk (2022) Directive 2002/32/EC of the European Parliament and of the Council of 7 May 2002 on Undesirable Substances in Animal Feed. Website: www.legislation.gov.uk.

• European Parliament. Website: www.europarl.europa.eu.
• World Health Organization (WHO). Website: www.who.int.