Understanding thrush: An overview

Introduction

Thrush is a bacterial infection of frog and its sulci characterized by accumulation of dead keratin tissue which is reportedly seen as thick, foul smelling black and necro-malodorous discharge originating within the central or collateral sulci of frog (Baxter and Stashak, 2011; Dabareiner et al., 2011) [4,5] . In worst condition, according to Stashak (1996) [18] , when left untreated cause involvement of deeper structure of foot viz., digital cushion, heel bulb region and even hoof wall causing sloughing off of the structure leading to permanent lameness.

Etiology and Pathophysiology

Any deviation affecting the overall health of frog are the elements forming etiology of thrush. Thrush is the destruction of a portion of the horse's hoof called the frog. Various notorious noncontagious keratolytic anaerobic bacteria and fungi has be ill reputed for the cause of thrush but among all Fusobacterium necrophorum and Bacteroides sp. are often isolated (Petrov and Dicks, 2013; Spencer and Dwight, 1994) [13,17] . Thrush is a degenerative condition of the epidermal tissue of the frog generally caused by a bacterial infection. Horses with a long heel conformation that results in deep, narrow frog sulci when exposed to damp and unclean environmental conditions provides best environment for thriving of anerobic bacteria (Kumar et al., 2014) [10] . Since bacteria penetrates the outer horn and enter deep into the dermis, accumulation of debris cuts off the oxygen supply making the most suitable home for the growth of anaerobic bacteria. Evidence of the disease commonly begins in the central sulcus of the frog and eventually spreads to the collateral sulci and main portion of the frog. Since animals fail to exhibit any sign of discomfort most of the time, it goes unnoticed and when left untreated, the infection can damage the sensitive structures of the hoof, including the frog and the digital cushion below it. This can cause prolonged infection and lead to lameness.

Potential Cause of Thrush

Hoof diseases are common in equines. A study conducted by Holzhauer et al. (2017) [8] revealed that 85% of the horses under observation had a foot disease of some kind when they were checked during routine hoof trimming. Equines are well shaped with hoof's natural self-cleaning mechanism. Frog is the widest part of the hoof. A large, healthy frog helps absorb concussions, shares load-bearing duties with the other palmar/plantar hoof structures, and functions as an expansion joint to keep the heels apart. Weight bearing ensures the frog's prolonged health by stimulating its natural physiology under normal circumstances. When the foot is loaded, a well-formed frog that occupies the region between the heels also facilitates a natural self-cleaning mechanism by expanding and clearing any collected dirt or debris from the frog sulci. An unhealthy frog is markedly smaller in size and recessed below the level of the ground surface of the hoof, thus creating a void in which debris can accumulate. Whether an animal is at a walk or a canter, its foot changes form when pressure is applied to the heel, causing it to expand and sink even further. When the sole flattens and loses its concave shape, the frog expands along with it until the unshod hoof touches the ground. Horses with imbalanced feet, improper trims (Abidina et al., 2013) [2] , and deep clefts linked to constricted heels are more likely to have thrush because these conditions facilitate the accumulation of debris and make it challenging to remove (Holzhauer et al., 2017) [8] . Moreover, equine kept with restricted movement in confined boundaries reduces the natural mechanism of cleaning. Proper shoe care can mitigate the impact of debris on the hoof capsule's normal biomechanics, which in turn promotes bacterial growth. While shoes may not have a significant effect on this mechanism in well-shod hooves, it is still important to may more concern while using wide-web or heart-bar shoes as they interfere with natural ventilation. Other contributing factors are poor stable hygiene management, long standing on soiled bedding, poor flooring with pits and poor ventilation system.

Clinical Signs and Diagnosis of Thrush

Lameness is typically imperceptible, but when it is, it can vary in intensity. Animals exhibiting sudden mild lameness accompanied by moist, dark colour discharge, smelling feet although not every smelly foot has thrush, blackish discharge. On visually assessing by farrier shallow depression (< 1cm) in central sulcus, presence of degenerative malformation in frog sulci, hoof tenderness and exhibition of painful response during cleaning of affected areas are evident. The surface of the horn of the frog usually has a rough appearance, and bleeding often occurs when the frog tissue is incautiously touched with a hoof pick. Evident of lameness in severe form is mainly due to structural damage of sensitive tissue causing pain on manipulation. Depending on the severity of the infection, swelling in the lower limb is also reported.

Treatment of Thrush

Anaerobic bacterial infection is most predominant and these bacteria commonly rife in faeces and soil. The most crucial factor in the persistence of the anaerobic bacteria causing this illness in horses is their living environment.

Managemental Correction

Identification of predisposing cause and implementing the corrective majors to eliminate the cause should be the first step. Maintain good stable hygiene with clean dry bedding and mucking out well. Compromising with the maintenance of good Hygiene management leads to such alarming medical condition. Prevention of condition like thrush can be easily achieved by keeping the horse in clean and dry environment. It is recommended that the stable floor be constructed from non-slip concrete and even, provided that the shed is well cleaned and sanitized on a regular basis. Hooves need lots of firm and strong support, especially under the frog. They need a dry bedding but not so dry that it can cause cracks in the hoof wall where bacteria can cause infection. According to Holzhauer et al. (2017) [8] , hoof condition started improving with regular cleaning and trimming of hoof and proper barn cleaning. Above all, they need a bedding that offers hygienic protection from the harmful bacteria and fungi that cause diseases like thrush. Bedding of wood savings like pine wood savings which is scientifically proven natural antibacterial resistance (Elina et al., 2023) [6] , straw, wood pellets and hemp bedding. Removal of any source or object causing the dampness of the living environment of horse be eliminated. Few preventive measures viz., vigorously brushing off with stiff brush and routine application of astringent in hoof, air dying and liming of the floor at least for 3-4 h in a day can be a cost-effective measure.

Hoof Care

Good condition and soundness of hoof can be achieved by maintaining routine corrective trimming of hoof. Over grown hoof, hoof with long heel conformation and unbalanced hoof should be given extra care. In long standing animals, trimming off the extra growth avoids accumulation of dirt. In case of presence of heel instability, the palmar aspect of the foot can be stabilized by a bar shoe. When heel is trimmed to the same plane as the frog, wall perimeter of healthy hoof should be round when animal is left barefoot. Animals should be relocated to a dry, clean place after identification. On examination of foot, necrotic debris and undermined tissue are meticulously removed using a hoof knife and cleaned (Pollard et al., 2019) [14] . On early stage of infection, frog area is found to be sensitive with noticeable bleeding. The infected area of frog is washed with potassium permanganate solution (1:10000 dilutions), cleaning with disinfectants (iodine/tincture of iodine) and dried thereafter using clean cloth. If the infection is not deep-seated, topical disinfectant/astringent application along with regular wash with disinfectant may be sufficient to cure it in 7 to 14 days. Application of causative materials like as hydrogen peroxide, bleach, and copper sulphate should be avoided during the early stages of treatment as it may result in further tissue damage. Caustic products should be applied once the frog gradually starts to harden and keratinize. Historically, thrush was treated with amalgamation of several chemical substances viz., bleach, formalin or formaldehyde, copper sulphate, Stockholm tar and hydrogen peroxide have been used. Others have recommended foot soaks in chlorine bleach (30 mL of bleach in 5 L of water). Several other caustic materials have been suggested, including a combination of phenol, tincture of iodine, and 10% formalin, Kopertox solution (Fort Dodge, Fort Dodge, Iowa, United States), or methylene blue, however, they do carry the risk of causing additional harm to healthy tissue and delaying the healing process. Numerous studies have demonstrated the efficacy of treating advanced stages of thrush with readily accessible triple sulfate powder. Strong vascular endothelial growth factor (VEGF) is present in copper sulfate, which stimulates angiogenic responses at the site of injury (Voelker, 2000; Sen et al., 2002) [19,16] . Ferric sulfate has a potent astringent action in addition to being an efficient hemostatic agent (Azargoon et al., 2011) [3] . Epsom salt, or magnesium sulfate, has a strong hygroscopic salt that aided in the removal of debris and degenerative tissues. When combined these all properties synergistically act in curing the degenerative infection in hoof. Kumar et al. (2014) [10] reported successful healing of frog with total 8-10 application of triple phosphate powder containing equal amount of ferrous sulphate, copper sulphate and magnesium sulphate (triple phosphate) in packed bandage. The frog grooves were clearly visible after treatment. Salt containing magnesium sulfate has long been used to treat hoof infections. Since Epsom salts have a hygroscopic quality that aids in the debridement of dead tissue, they encourage blood flow to the hoof and speed up the healing process. Thrush in horses has been successfully treated with a variety of combinations of magnesium sulfate and supportive antibiotics, including gentamicin, amoxicillin, oxytetracycline, trimethoprim sulfa, sulphadimidine, enrofloxacin, and polymixin B. O'Grady (2018) [12] and Qaisar et al. (2024) [15] reported successful recovery of thrush with soaking the infected hoof in epsom salt solution till three weeks. Further packing and bandaging the cleft of frog with antiseptic or astringent soaked cotton gauze will help in eliminating the infection quicker. The application of Phythium oligandrum for the treatment of thrush has been the suggested from a recent study conducted by Načeradská et al. (2017) [11] and Qaisar et al. (2024) [15] , which reported a successful method for curing hoof-thrush. Phythium oligandrum, also known as Ecosin, is essentially a fungus that functions as a myco-parasite for over 50 species, including some bacterial activity. Gabrielova et al. (2018) [7] stated it releases hydrolytic enzymes to break the life cycle of bacteria and fungi. Ecosin tablet 3g was dissolved in 02 L of luke warm water. This was allowed for 20-30 minutes. The affected hoof was completely cleaned and disinfected before being submerged in the bucket with the ecosin solution for 20 minutes. The hoof was allowed to dry naturally. Treatment was repeated three times a week for three weeks. Animal treated with ecosin powder were found to be healed early as frog growth after 4-5 treatments were evident. In addition, a wide range of commercial products, such as Grooms Hand Deep Thrush Gel, Equiderma Thrush, Vetericyn Equine Hoof Car, Absorbine® Hooflex Thrush Remedy, Therazure, Kopertox (copper napthenate) ® Water-Resistant Horse Thrush Treatment, and others, are easily accessible for treating thrush.

Conclusion

Many methods have been proposed for the treatment and resolution of thrush. Nevertheless, none of these will be very useful until the frog's health and structural integrity are enhanced. Horses diagnosed with thrush have a fair chance of making a full recovery if given the right care. It's critical to treat thrush infection as soon as possible to stop it from penetrating further into the hoof tissues. Health of the frog can't be improved and, consequently, the thrush can't be resolved without using appropriate farriery. Good farriery should be emphasized as the main strategy for preventing this condition, keeping in mind that prevention is always preferable than cure.

References

1. Department of Animal Husbandry and Dairying, Ministry of Fisheries, Animal Husbandry and Dairying, Government of India. 20th Livestock Census. New Delhi: Government of India; 2020. https://dahd.nic.in/documents/statistics/livestock-census
2. Abidina NSZ, Adamua L, Ahmada B. Incidence of equine hoof derangements in Malaysian horse population. IOSR Journal of Agriculture and Veterinary Science. 2013;5(2):26-32.
3. Azargoon H, Williams BJ, Solomon ES, Kessler HP, He J, Spears R. Assessment of hemostatic efficacy and osseous wound healing using HemCon dental dressing. Journal of Endodontics. 2011;37(6):807-811.
4. Baxter GM, Stashak TS. The foot. In: Baxter GM, Stashak TS, editors. Adams and Stashak's lameness in horses. 6th ed. West Sussex: Wiley-Blackwell; 2011. p. 521.
5. Dabareiner RM, Moyer WA, Carter GK. In: Ross MW, Dyson SJ, editors. Lameness in the horse. 2nd ed. St. Louis (MO): Elsevier; 2011. p. 311-312.
6. Elina K, Mika K, Veikko M, Anni H, Anna-Kaisa T, Anna-Maria V, et al. Touch the wood: antimicrobial properties of wooden and other solid material surfaces differ between dry and moist contamination in public and laboratory exposure. Environmental Advances. 2023;13:100416.
7. Gabrielova A, Mencl K, Suchanek M, Klimes R, Hubka V, Kolarik M. The oomycete Pythium oligandrum can suppress and kill the causative agents of dermatophytoses. Mycopathologia. 2018;183(5):751-764.
8. Holzhauer M, Bremer R, Santman-Berends I, Smink O, Janssens I, Back W. Cross-sectional study of the prevalence of and risk factors for hoof disorders in horses in The Netherlands. Preventive Veterinary Medicine. 2017;140:53-59.
9. Kettunen E, Kurkilahti M, Belt T, Möttönen V, Harju A, Tornberg A, et al. Touch the wood: antimicrobial properties of wooden and other solid material surfaces differ between dry and moist contamination in public and laboratory exposure. Environmental Advances. 2023;13:100416.
10. Kumar S, Deddar R, Singh J, Manuja A. Treatment of hoof thrush with triple sulphate in a pony. Online Journal of Veterinary Research. 2014;18(6):511-515.
11. Načeradská M, Fridrichová M, Kellnerová D, Peková S, Lány P. Antifungal effects of the biological agent Pythium oligandrum observed in vitro. Journal of Feline Medicine and Surgery. 2017;19(8):817-823.
12. O'Grady SE. Equine thrush: a closer look. American Farriers Journal. 2018;44(8):1-4.
13. Petrov KK, Dicks LM. Fusobacterium necrophorum, and not Dichelobacter nodosus, is associated with equine hoof thrush. Veterinary Microbiology. 2013;161(34):350-352.
14. Pollard D, Wylie CE, Verheyen KLP, Newton JR. Identification of modifiable factors associated with owner-reported equine laminitis in Britain using a webbased cohort study approach. BMC Veterinary Research. 2019;15(1):59.
15. Qaisar A, Fatima A, Hussain N, Jamali MN, Hassan S, Aziz A, Rashid HB. Comparative efficacy of Pythium oligandrum, thrush powder and magnesium sulphate to treat thrush in horses. Journal of Population Therapeutics and Clinical Pharmacology. 2024;31(5):295-321.
16. Sen CK, Khanna S, Venojarvi M, Trikha P, Ellison EC, Hunt TK, et al. Copper-induced vascular endothelial growth factor expression and wound healing. American Journal of Physiology-Heart and Circulatory Physiology. 2002;282(5).
17. Spencer SJ, Dwight CH. Characterization, distribution, and microbiological associations of Fusobacterium spp. in clinical specimens of animal origin. Journal of Clinical Microbiology. 1994;32(2):384-387.
18. Stashak TS. Practical guide to lameness in horses. 4th ed. Wiley-Blackwell; 1996. ISBN 0-683-07985-9.
19. Voelker R. Copper and cancer. Journal of the American Medical Association. 2000;283:994.