Hair Testing - Yay or Neigh?
Hair testing in horses has many uses; however, identifying mineral deficiencies or diagnosing veterinary conditions is not one of them.
Hair Analysis and Mineral Status
Attempts have been made to use hair analysis as an indicator of whole‑body mineral status, including macrominerals such as calcium and phosphorus, and trace elements such as copper, molybdenum, zinc, selenium, and iron. However, hair mineral content does not correlate reliably with whole‑body mineral levels, and the validity of this approach in horses has not been confirmed.
At best, hair analysis may provide a broad snapshot of dietary mineral intake over the previous six months. Results from a single horse cannot be extrapolated to other horses or used to make general recommendations. Numerous factors influence hair mineral concentrations; some can be identified, but many cannot.
Use of Hair Analysis for Heavy Metals
Hair analysis can be useful for measuring exposure to heavy metals, including selenium, mercury, arsenic, lead, and cadmium. Samples are easy to collect, store, and transport. That said, several factors significantly affect the reliability and interpretation of hair analysis results.
Factors Affecting Hair Mineral Levels
Hair absorbs minerals from the environment, meaning hair mineral concentrations are often not indicative of body mineral status. For example, a mild zinc deficiency may reduce hair zinc levels without affecting hair growth rate, whereas a severe zinc deficiency slows hair growth, resulting in zinc concentrations that appear normal or even elevated.
In addition, mineral levels in hair are influenced by many variables, including:
External and biological factors: shampoo use, coat conditioners, age, hair thickness, colour, breed, gender, sire, body location, season, and rate of hair growth
Hair colour: coloured hair contains higher concentrations of calcium and selenium than white hair from the same horse; similar patterns are seen in dogs and cattle
Grey hair: higher levels of copper, titanium, and zinc, and lower levels of boron, calcium, selenium, and strontium compared with other hair colours
Age: hair from young animals is often lower in zinc, manganese, and iron, but higher in sodium, calcium, copper, and potassium than hair from older animals
Pigmentation: pigmented hair contains more calcium, magnesium, potassium, and sodium than unpigmented hair
Black hair: contains more sodium, phosphorus, potassium, magnesium, and calcium than chestnut hair
Body location: differences may reflect surface contamination, hair growth cycles, or hair texture
Seasonal variation: some studies report lower mineral levels in winter hair (except copper), while others have found higher mineral concentrations in winter samples
Mineral exchange: hair absorbs minerals by exchanging them for others, increasing the absorbed mineral while decreasing the exchanged one
Laboratory methods: results vary due to differences in decontamination procedures, digestion methods, trace element recovery rates, and minimum detection limits
Appropriate Uses of Hair Testing
Hair testing is most useful for detecting:
Heavy metal toxicosis (arsenic, cadmium, lead, mercury, selenium). For selenium, the hair sample must reflect growth during the period of excess intake, and results depend on the chemical form of the consumed selenium.
Anabolic steroids
Drug misuse - unlike blood or urine testing, hair analysis can detect and quantify drugs weeks, months, or even years after administration.
Hair Testing in Pre‑Purchase Examinations
Hair testing can be valuable during pre‑purchase veterinary examinations to identify misuse of anti‑inflammatory or sedative drugs. It provides a historical record of drug or chemical exposure, although some loss may occur due to chemical changes or leaching. Hair root analysis may also indicate acute exposure to substances such as codeine.
Limitations and Practical Considerations
Due to the many factors influencing hair mineral content, hair analysis is not a precise indicator of mineral status in horses. If hair testing is undertaken, reference values should come from horses of similar breed, sex, age, colour, sire, and season.
As a quality check, it may be useful to submit multiple samples from the same horse, or the same sample to multiple laboratories, and compare the results.
Preferred Methods for Assessing Mineral Intake
Blood testing is generally unreliable for assessing most minerals. In my experience, diet analysis provides the most reliable basis for evaluating mineral intake and guiding supplementation decisions.
Dr Jennifer Stewart
BVSc BSc PhD Equine Veterinarian and Consultant Nutritionist
