Veterinarians working with many laminitic horses are well acquainted with the problem but others may be unfamiliar with it. It’s a laminitis-like syndrome triggered by cold weather.
Horses normally have a very high tolerance for cold. In all species, cold causes a reflex shunting of blood away from the extremities and toward the core to limit loss of body heat. Healthy horses prevent the hoof tissue from being damaged from low blood/oxygen supply by using local arteriovenous shunts – pathways which allow them to divert blood quickly back to the veins for return or to send it to the local tissues. When low blood supply reaches a critical level, the arteriovenous shunts to that part of the hoof can close, perfusing the tissue.
The only adverse effect of cold weather and reduced blood flow to the hoof in healthy horses is slower hoof wall growth. In horses with metabolic issues that result in high insulin levels, it may be a different story.
We don’t know all the details of the mechanism but it is clear from research that high insulin can cause laminitis. We also know that even if they have never had a full blown laminitis episode there are similar abnormalities in the structure of their laminae. One thing we do know about it is that levels of endothelin-1 are greatly elevated. This is a chemical in the body which causes blood vessels to contract down. It has also been shown that the vessels in the hoof become more sensitive to other messengers that cause contraction. These changes may interact with cold induced blood vessel constriction to cause a critical interruption of blood supply to the hooves of those horses.
Horses with cold induced hoof pain show obvious lameness and often typical laminitis stance but without bounding pulses or heat in their feet. In milder cases it may be mistaken for the sensitivity to moving over frozen uneven ground that all horses show. However, it doesn’t go away on level surfaces. There is variability in individual sensitivity to cold but signs may appear beginning at 40F [4.4C].
Even horses that have their insulin usually well controlled by a low carbohydrate balanced diet can be susceptible. This may be because cold weather has also been observed to often cause wide swings in insulin levels and/or because of previous damage to the circulation in the feet.
The first step in helping these horses is protecting their extremities from the cold. Leg wraps such as lined shipping boots work well and are safe to leave on because they won’t slip out of place and cause uneven pressure on the tendons [aka “bandage bows”]. Boots with pads and socks or fleece lining are essential.
The equine can be supported nutritionally by supplements which encourage the production of nitric oxide. Nitric oxide is a vessel dilating messenger that is the natural counterbalance to endothelin-1. The herb Gynostemma pentaphyllum (Jiaogulan) is a powerful support for nitric oxide. This is helped by providing the precursors for nitric oxide in the form of L-arginine and L-citrulline. Antioxidants also combat oxidative stress which inhibits the activity of the enzyme that produces nitric oxide inside blood vessels [eNOS – endothelial nitric oxide synthesis].
Winter laminitis has historically been regarded as very difficult to manage but understanding the vascular mechanism has led to significant strides in helping these horses balance the forces affecting the blood supply to their feet.
Eleanor Kellon, VMD