Ask the Vet: Lameness

Answered by, Michael St. Blanc, DVM, Louisiana State University Veterinary Teaching Hospital, Baton Rouge, LA

Question: What scientific data links laminitis to grass consumption? In my region, people are locking their horses up or putting them in grazing muzzles for fear of laminitis. If horses have evolved eating grass, why is grass now poison to them? Is it the grass, the fertilizers and pesticides applied to the grass, or something else? I truly would like links to scientific studies if you can provide them. 

Answer: This is a great question. I will preface by saying that laminitis is an incredibly complex disease process that remains incompletely understood. While we have a much better grasp of risk factors for and pathogenesis of the disease, there are still many unknowns. As you alluded to, laminitis can be induced by exposure to grasses in certain instances. In these cases, laminitis is proposed to occur due to the carbohydrate content of the grass. It is thought that an increase in highly fermentable carbohydrates causes a so-called “carbohydrate overload” in the gastrointestinal tract, which results in the release of inflammatory mediators leading to the development of laminitis.

In some studies, horses that previously had no pasture access that were suddenly exposed to grass were much more likely to become laminitic. Another study showed an increased risk of laminitis with exposure to “high quality” grasses. However, it can be difficult to ascertain the exact cause, as there are numerous other factors that may contribute to the development of laminitis, such as metabolic disease and body condition.

To answer your question, I am not aware of any data linking pesticides or fertilizers to laminitis. I’m also unaware of data suggesting a higher prevalence in horses today compared to many years ago. Some have speculated that in certain areas of the country, native grasses were overgrazed and were replaced with grasses higher in carbohydrate content. It is possible that this phenomenon, combined with an increased ability to recognize the condition and a heightened awareness of its presence, may contribute to the number of cases observed. Additionally, current management practices incorporating feeding higher volumes of concentrates as well as increased body condition scores are also suggested to have an effect on the prevalence of laminitis in today’s horse population. This is definitely a “hot topic,” and it will be interesting to see where the current research lead us in the near future.

The following articles may be useful for your reference:

  • Case-control study of risk factors for pasture and endocrinopathy-associated laminitis in North American horses. Coleman et al. Journal of the American Veterinary Medical Association. Vol 253, No. 4. August 15, 2018
  • Current Concepts on the Pathophysiology of Pasture-Associated Laminitis. Raymond J. Geor, BVSc, MVSc, PhD. Veterinary Clinics of North America, Equine Practice. Volume 26, Issue 2, August 2010, Pages 265-276.
  • Forage and Pasture Management for Laminitic Horses. Watts, K.A. Clinical Techniques in Equine Practice. 2004.

Question: I have a 4-year-old gelding with a suspensory ligament injury – suspected to be caused by being turned out with 1 and 2-year-olds in the pasture. After four months, he is much improved but still showing some shortening of the hind leg. If this resolves in another 2-3 months, what are the chances of problems with soundness in the future?

Answer: This is another great question, as disease of the suspensory ligament is a relatively common cause of lameness and can be frustrating to treat. The short answer to your inquiry is…it depends. The first step would be classifying the injury according to several factors. The suspensory ligament runs along the back of the cannon bone in the hind limb, arising just below the hock and coursing down towards the fetlock, at which point extensor branches become evident on the inside and outside of the leg. Injuries can occur at any length along the ligament but are commonly grouped into one of three locations: the top of the ligament (proximal suspensory ligament), the middle (body) of the ligament, and the extensor branches.

Suspensory ligament pathology is often diagnosed via ultrasound. This allows one to more accurately determine the type of abnormality present. In some cases, the ligament becomes inflamed/enlarged without obvious structural changes. Other times, the fibers within the ligament become damaged or torn. In some cases, radiographs may even reveal bony changes where the ligament inserts on the back of the leg.

Some horses will respond to conservative treatment consisting of rest and anti-inflammatories. Regenerative therapies such as injection of platelet rich plasma (PRP) or stem cells may be employed, along with modalities such as shockwave treatment. It is good that your gelding has shown some improvement since the initial injury; however, it is very difficult to estimate the chance of future soundness issues. Some horses will heal and go on to perform without difficulties. Others will experience a transient improvement that gradually regresses, and some will never experience full resolution of lameness. It is difficult to say where your horse will fall along this spectrum. There are many factors that can affect the outcome, including the nature, location, and severity of the initial injury. Even in cases where all of these are known, it is difficult to predict the likelihood of issues down the road.

I would strongly encourage you to have your veterinarian examine the horse. This will be important to monitor progress and to tailor a treatment/rehabilitation program that is most appropriate for your horse and his specific injury, as well as for ruling out other causes of lameness in that limb. Thanks for your question, and best of luck to you and your gelding.