By Esco Buff, PhD, APF-I, CF
Horses have two different types of sweat glands: apocrine and merocrine (or eccrine). 1-3,6 The majority being apocrine sweat glands which are distributed across the entire body. 1-3 All sweat seen on a horse can be regarded as being of apocrine origin since these glands open into the hair follicles. 1-3 Merocrine or eccrine glands are restricted to the hairless areas, like the frog.1-3,6 The key difference between merocrine and apocrine sweat glands is that merocrine sweat glands excrete sweat directly onto the surface of hairless areas, like the frog. 1-4,6 Many horse owners as well as equine professionals have never even heard of a merocrine gland before.8,9
Merocrine sweat glands carry out several important functions in the human body. They are for thermoregulation, protection, and excretion.4-7 Moreover, merocrine sweat glands produce thin clear watery sweat while apocrine sweat glands produce a thick fluid which nourishes bacteria on the skin.4-6
When trimming the frog, occasionally the farrier will encounter a small pocket of whitish fluid that will drain and dry up very quickly. A merocrine gland release is not an abscess and there is no sensitivity or lameness associated with it.8,9 Nothing needs to be done with the merocrine gland and most times, it only happens once and is never seen again. 8,9
There is limited research into equine merocrine glands of the frog as to why they occur and exactly what they specifically do. Research with dogs and cats show merocrine glands located in their pads of their feet help function to cool the animal.10,11
In summary, don’t panic as they are a normal function even though they are rarely seen. Quickly take a picture as the merocrine gland secretion dries up quickly.
References and Resources
1. Potts, S., Thatcher, R., Jones, A. W., Warren, L. K., Tenbroeck, S. H., Nottage, F., & McEwan, N. R. (2015). Sweat osmolarity shows intra-animal regional variation in the horse. Veterinary dermatology, 26(5), 374–e85. Retrieved from: https://doi.org/10.1111/vde.12225
2. Watanabe A, Kanemaki N, Matsuura K. Distribution Densities of Hair Follicles in Racehorses. Jap. J. Equine Sci. 1993; 4: 55-60
3. Eldredge, D.M. (2015). Anatomy of Your Horse’s Skin. Retrieved from: Anatomy of Your Horse’s Skin (horsehealthproducts.com)
4. Udayangani, S. (2019). Difference Between Merocrine and Apocrine Sweat Glands. Retrieved from: Difference Between Merocrine and Apocrine Sweat Glands | Compare the Difference Between Similar Terms
5. Hodge BD, Sanvictores T, Brodell RT. Anatomy, Skin Sweat Glands. [Updated 2021 Oct 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan. Retrieved from: https://www.ncbi.nlm.nih.gov/books/NBK482278/
6. Professional Equine Grooms. (2022). Weird Horse Anatomy – Hoof and Leg Edition. Retrieved from: Weird Horse Anatomy - Hoof and Leg Edition - Pro Equine Grooms
7. Parks, A. (2017). Anatomy and Function of the Equine Digit. In Equine Laminitis (eds J.K. Belknap and R.J. Geor). https://doi.org/10.1002/9781119169239.ch3
8. Thomas, H. Smith. (2018). Don’t Panic When Encountering Milky Frog Fluid. American Farriers Journal. Retrieved from: Don’t Panic When Encountering Milky Frog Fluid (americanfarriers.com)
9. Buff, E. (2021). Merocrine Glands in the Frog. Retrieved from: www.facebook.com/media/set/?vanity=EscoBuffsProfessionalFarrierServiceLLC&set=a. 2874818479442244
10. Burke, A. (2022). Do Dogs Sweat? American Kennel Club. Retrieved from: Do Dogs Sweat? How Do Dogs Sweat? — American Kennel Club (akc.org)
11. Pendergrass, J. (2018). How Do Dogs Sweat? PetMD. Retrieved from: How Do Dogs Sweat? | PetMD
