Question: I have a five-year-old mare that I purchased in January 2009. In June I took her to a barrel-racing clinic in Texas. She did great hauling. While waiting for our turn at the clinic, she would always seem to fall asleep. I didn't think much of it because it was hot and humid, and she barely had her winter coat off. I gave her electrolytes and made sure she was hydrated. After I got home she would do well some days, but other days she would get irritated, buck and then rear. She even started pulling back at the trailer. After she either bucked or reared, I'd really work her, and she would do fine. As soon as I would tie her up to the trailer, though, she would literally fall asleep, hang her lip and be out. One week later I could not get her into a lope. I took her to the vet, where he tested for tick-borne diseases (all negative). He put her on 10 days of steroids, and she seemed to get more energy. I took her to a weekend clinic this past weekend, and she was extremely lethargic again. I put her in her stall and within 30 minutes she was lying down, completely sound asleep. Then I rode her that night, and she bucked me off. Any suggestions? She doesn't have the signs of EPM.
Answer: There are quite a few things that could be going on here with your mare. And it may take some detective work to get it all figured out, since the obvious signs of disease or lameness are not present.
Is your horse sleep-deprived? Here is an article about sleep deprivation--it may be more common than we think. You have a young horse who is working hard, still growing at the age of five and learning to travel. She may need more rest than she is getting. You do not say if you keep her at a boarding stable or at home, but that can play a role in sleep deprivation. The activity in a boarding stable, especially a large one, can keep sensitive horses from resting properly. Some horses do not care--they can rest with all sorts of activity--while others may be quiet and well-behaved, but never really rest.
Is there evidence your mare lays down in the bedding at night? Do you ever see scrapes on her front ankles? These could be signs she is not resting properly or could be falling asleep at other times.
Horses can have a true case of narcolepsy, a condition where they uncontrollably fall asleep. This seems to be less common in young horses. Here are some other articles about equine sleep patterns. Your mare may be behaving badly because she is so overtired that working hurts her. Or there may be other sources of pain behind the behavior.
I am glad your veterinarian checked for Lyme and other tick-borne diseases as well as doing a neurologic exam for EPM (I assume that was done, since you said there were no signs of EPM). The tick diseases are new, and veterinarians are still learning about them since many parts of the country have had very few cases. Lyme, in particular, is very difficult to diagnose because the symptoms are variable and may come on slowly. Although equine Lyme tests are more useful than human ones, they still are not totally reliable in my experience.
Lyme can cause behavior changes that range from mild or severe fatigue to extremely bad-tempered behavior, along with pain located anywhere in the body. It is important to recognize that your mare does not feel well right now, and you should not push her and force her to work until you figure out what is going on. I have written several EquiSearch.com "Ask the Vets" about pain and how to figure it out. Contrary to what many people think, making a horse work through pain does not teach him anything positive, except that people can be painful. Respecting a horse's complaint and stopping your ride actually teaches your horse that you are a good person, and he/she will be less likely to behave badly in the future.
Since the Lyme test is not totally reliable, after investigating the sleep deprivation aspect, I would retest for Lyme disease. Sometimes the second sample shows a higher titer level than the first, which means the immune system is reacting to a fresh disease, but it takes awhile to build up. I frequently see evidence of liver enzymes being a bit high with Lyme disease, so a recheck of the chemistry would also be in order.