November 2017 - AAEP is on Stall Rest
AAEP is taking the month of November off from "Ask the Vet", but will be back in December to answer your equine health questions concerning winter weather care for your horse with expert, Dr. Christine Tuma.
Click here to read this month's questions and answers.
Can I vaccinate my pregnant horse? I purchased her already bred in October and she is due in April. I currently live in Florida.(View Answer)
Most vaccines are approved for use in pregnant mares. Live virus vaccines should be avoided in pregnant mares. For more information, visit AAEP's Vaccination Guidelines. Manuel Himenes, DVM, Kailua, HI
I have a large number of miniature horses and they have always been vaccinated on the old schedule starting with two vaccinations of tetanus and flu a month apart, then yearly. I have heard that I should now be giving them a 3rd booster after 6 months. If I start this, what should I do about the many horses I have that have not followed this schedule? Can I just give them their normal booster then another after 6 months and thereafter yearly?(View Answer)
Tetanus vaccines are only needed annually after the initial series. Influenza vaccinations have a short duration of immunity. Antibody titers to influenza decline after only a few months and therefore it is recommended to booster influenza virus vaccinations at 6 month intervals. You can put your other horses on a 6 month influenza schedule as you have already done the initial series. Again, tetanus is only needed once per year. Manuel Himenes, DVM, Kailua, HI
My 21-year-old Thoroughbred gelding's indented area over the eyes suddenly became full and puffy. Eyelids are also puffy, but no color discharge. The attending veterinarian cannot determine the cause. The boarding facility recently switched to pelleted shavings about the same time the puffiness began so now they are back to regular shavings for 12 days with no change. This was the only change my horse has expierenced. Big head shaking from time to time. He likes the warm compress on the puffy areas and light massage on them.(View Answer)
The recess over your horse's eyes can become enlarged for a few reasons. One might be that you are investigating is allergies and if an environmental charge or dose of steroids does not resolve the issue then there are other possibilities. Trauma could be a cause but it would be unlikely due to both sides being affected an there are no abrasions or signs of trauma to his face. This swelling can also be fat deposits and may be a sign of Cushing's disease. Does he drink water excessively and flood his stall with urine? Does he have a pot belly appearance? Is his hair coat shaggy and does not shed normally?
If the puffiness does not resolve in the next week I would consider testing him for Cushing's disease. Your veterinarian can advise you on the best test to use for your area and conditions. Manuel Himenes, DVM, Kailua, HI
I suspect at least one of my horses is eating thistle - not the big mature plant, but the immature rosette stage of the plant. Will this harm them (especially due to the prickly parts of the leaves)? I have been trying to eliminate thistle from the pasture and am making headway but am unable to completely eliminate the plants. My pastures are not ideal due to the drought over the past few years, but I only let my horses graze a couple of hours a day; otherwise they are kept in a dry lot where I can better control their food intake as they are easy keepers.(View Answer)
Please remove your horses from the thistle infested area immediately! Yellow star thistle (Russian Knapweed) contains a potent neurotoxin that will cause irreversible damage to your horse's brain. Paralysis of the lips and tongue are the first signs seen. Keep your horses on a dry lot until you can remove all of the thistle. Manuel Himenes, DVM, Kailua, HI
I have a 14-year-old dutch gelding. He injured his front suspensory and received the PRP injections once. His leg thickened up substantially, and has never gone down. We did the full program of rehab and it's now one year later. The horse is now returning to work. His leg is still thick and lumpy, although the last ultrasound showed that the ligament has healed. There is quite a bit of scar tissue in the area of treatment and I am wondering if there are any remedies to lessen it? The leg is not hot, but goes up and down with puffiness at random times. Can you suggest any treatments for this condition?(View Answer)
I am glad to hear that your horse is back to work and doing well. Scar tissue along a ligament has to be reduced by internal remodeling. This remodeling will rarely remove all of the scar tissue. The puffiness that you report is due to the scar tissue interrupting the drainage of fluid from the leg. If the puffiness goes down with work you could try more exercise, turn out or move him to a paddock. The leg may continue to be thick and lumpy but if he is sound, he can continue work. I'm sorry I do not have any treatments for reduction of this type of scar tissue. Manuel Himenes, DVM, Kailua, HI
Does improper hoof trimming (long toes, short heels) contribute to white line disease?(View Answer)
Unbalanced hooves result in uneven forces along the ground surface of the foot. This unevenness create a shearing force along the laminae of the hoof creating separation of the white line. This separation allows a secondary infection (white line disease) to occur. Manuel Himenes, DVM, Kailua, HI
My horse had a disgusting discharge coming from his right nostril the other day. The smell was the most offensive thing I have ever smelled in my life. I had the veterinarian examine him and she thinks it's a sinus infection. She put him on SMZ's and he has been on the medication for about three days. He still has a large amount of discharge from his nose and while the smell isn't quite as offensive, it is still awful. I've asked my instructor if he could have tooth root rot in which she said no since he is eating fine. The dentist isn't coming out until March. Does this sound like a problem with the teeth? I'd like to get it taken care of as soon as possible. I can send pics if needed. Please advise.(View Answer)
Your veterinarian may be correct that it is a sinus infection and may need to be surgically drained. It could also be a tooth as those smell really bad. Radiographs or an endoscopy may be necessary to identify the source of the discharge. Once the source of the drainage is identified then a treatment plan can be started.
Equine dental specialty in veterinary medicine is just beginning and I would be sure that the "dentist" that you mention is a veterinarian. There are many fly by night "equine dentists" that are not veterinarians and have no formal training. Manuel Himenes, DVM, Kailua, HI
I am interested in buying a 7-year-old buckskin Quarter Horse gelding. During the vet check, it was pointed out to me, by the vet, that he has a very small corneal scar in his left eye. He said that most of the time this will go away. This horse has been out on a pasture for a couple of years. I don't think the initial injury was recognized and treated. Could this cause a vision problem now and as the horse ages?(View Answer)
Many corneal scars will eventually go away or become very small. Theses scars are like a spot on a windshield they are a minor nuisance but does not significantly affect vision. I would not be concerned about the scar as long as the est of the eye is clear and non painful. Manuel Himenes, DVM, Kailua, HI
I have a 2-year-old 16h AQHA gelding that is showing signs of physitis in his left knee. Besides balancing his diet and light exercise, I wanted to see if there is anything else I could do to help him recover faster such as wrapping to help with circulation or what not? I also wanted to know the time duration it takes for bone to fully regenerate in horses? Also, what time duration should I expect to see changes or progress?(View Answer)
Physitis is a frustrating condition because all you can do is wait. Proper nutrition and weight loss are the keys. Some horses need to be stall rested with no exercise until the inflammation calms down. Wrapping and increasing circulation will not speed up his recovery. Pain relief, as need, is important along with proper hoof trimming. I wish I could give you a rapid miracle but bone takes time (months) to remodel and recover.
Bone regeneration is an ongoing process and time for bone remodeling is variable. Most fracture repairs take 8 to 12 weeks and I would expect a physitis to take at least 12 weeks to heal once the diet is balanced and the weight is reduced. Manuel Himenes, DVM, Kailua, HI
My horse has a lump on the near side rear fetlock, which the veterinarian has x-rayed and scanned. The diagnosis was a ligament injury and 6 months off work with no other suggestions. My horse is not lame but just has the lump. Do you have any other suggestions?(View Answer)
If your horse has a ligament strain with no lameness and only a lump, you are very lucky. Your veterinarian probably made the rest recommendation based on the changes they saw on the radiographs or scans. Once the ligament is fully healed you can try an address the lump if it has not gone away. Topical liniments or poultice can be put over the lump to try and reduce the size. I would be cautious about doing any invasive therapies such as steroid injections as this may delay the healing of the ligament. Manuel Himenes, DVM, Kailua, HI
I have concerns for over vaccinating my horse. Does checking titers tell a complete story? Is Mercury used in all vaccines?(View Answer)
The question of vaccine titers is a controversial one. There is conflicting data on the protective titer level. Vaccine titers are available for the following diseases: Equine herpes III (rhino), Potomac horse fever, Equine encephalitis (EEE, WEE, VEE), Equine viral arteritis, Equine influenza, Rabies titer (RFFIT: non export), West Nile virus antibody titer, Strangles and Lyme disease. Titers for influenza and equine herpes virus drop off very rapidly and will often result in revaccination. It is often best to determine the disease risk for your horse then vaccinate accordingly. For example, if your are in a rabies free area, rabies vaccine may not be necessary.
Many vaccines do contain mercury as a preservative. The modified live vaccines usually do not. Most tetanus vaccines do have mercury. You can find out if a vaccine has mercury by looking at the company web sight for the technical information they must list contents there by law. Manuel Himenes, DVM, Kailua, HI
You say, "The organism of EPM, Sarcocystis neurona, can lie dormant in the horse's central nervous system for years before a bout of illness, stress, or time alone results in disease symptoms." Please, can you provide me with the reference for this statement?(View Answer)
Equine Internal Medicine 3rd edition pp. 609, Merk Veterinary Manual, Purdue Summer Newsletter 2004. There are several other references that can be located. Also, veterinary researchers that have looked at EPM will tell you that the interval from infection to clinical signs is unknown. I am currently treating a horse for EPM that would have to have been infected 8 years ago. Manuel Himenes, DVM, Kailua, HI
I administer my own vaccinations to my Arabian horses. I purchased vaccines last year and kept them in the fridge. Unfortunately, there were some family emergencies and basically I forgot about the vaccines, which expired in November 2014. I was going to throw them away but a friend said they might still be effective. The vaccines are 4-way (eastern, western, encephalitis-influenza, tetanus). It's now early January. Should I still use them, or dispose of them?(View Answer)
An expired vaccine has no guarantee of potency or sterility. I would recommend that the vaccine not be used and disposed. The cost of the vaccine is minimal compared to the cost of disease treatment or a possible abscess from an expired vaccine. Manuel Himenes, DVM, Kailua, HI
What would cause my 7-year-old Quarter horse mare to have her back ankle pop out of socket? I've only had her about 6 months. Her back feet had not been trimmed in a long time since her previous owner said she wouldn't let the farrier trim them. My farrier was able to trim her back feet and when he did he trimmed them very short. Could that have caused her to develop the unusual dislocation/popping out of place?(View Answer)
The popping of the fetlock can often be a response to foot or pastern pain. The flexor tendon has a reflex response to pain. If the foot is not lifted off the ground the fetlock will try to flex with the foot still flat on the ground. This results in a popping back and forth of the fetlock. Your veterinarian can examine your mare and try to determine the source of the pain. Good luck with your horse. Manuel Himenes, DVM, Kailua, HI
My 9-year-old Arabian gelding has some yellowing in the whites of his eyes. It was suggested that his liver enzymes may be off and blood required. I researched this issue and find that blood panels will not give an answer to this. What should I ask my vet to do? Is there a supplement I could include to help?(View Answer)
Yellowing of the mucus membranes and the sclera (whites) of the eyes is called icterus, jaundice is the common term. This is caused by a buildup of pigments in the body called billirubin. Billirubin can be elevated for many reasons one of which is liver disease. There are specific blood tests for liver disease that are very accurate. Liver disease is very serious and should be screened for as the sooner this is treated the better the outcome.
Blood chemistries and a CBC should give you the answer as to the yellowing of his eyes. Manuel Himenes, DVM, Kailua, HI
Our donkey has scratched his rearend until it is almost raw. What could be causing this? I have noticed he has sores on the outer side between his belly and back hind quarter.(View Answer)
Sensitivity to flies, mites and allergies could all cause these signs. If you look closely at his skin can you see "walking dandruff", which would indicate mites. Pinworms can cause the rear end rubbing so that also may need to be checked. If after checking there is no evidence of mites or flies, a trial treatment with steroids may be necessary. If the lesions clear up with the steroids, an allergy is likely. Determining the allergen is difficult and may require a veterinarian to do a skin sensitivity or serum allergy test. Manuel Himenes, DVM, Kailua, HI
My horse has recurrent lameness in the winter months, which appears in the shoulder. Could a blanket that constantly rubs his shoulder cause this? He gets adequate turnout, slow warmups, plenty of water and hay. The only difference is the cold and the blankets.(View Answer)
Shoulder lameness in the horse is rare so I would be sure that the pain is coming from the shoulder. Cold weather can exacerbate lameness due to many causes. I would suggest a lameness exam to determine the source of the pain. It would be unlikely that a blanket would cause any lameness without some significant skin disease. Skin disease that severe should be visible. Manuel Himenes, DVM, Kailua, HI
What supplement is good for loss of pigmentation? I had been using clovite but have not been able to find it lately.(View Answer)
Loss of pigmentation may be due to a disease process so be sure that your horse does not have healthy skin that has lost it's color. Copper supplementation has often been used to recover pigmentation, however this has not been proven. I have placed a piece of copper tubing in the water troughs of horses to see if that improved their pigment. You may want to try this until clovite becomes available. Manuel Himenes, DVM, Kailua, HI
My 14-year old gelding has Equine Metabolic Syndrome and is very prone to ulcers. What is the best joint supplement I could give such a horse?(View Answer)
Joint supplements have been shown to be extremely variable in their content and effectiveness. If you would like to use a joint supplement I would look for a reputable company that has been in the feed supplement buisness for a while. You should look for a joint supplement that has very little added sugars or flavors that may contain sugar. There are many supplements to choose from and you may need to try more than one to find the correct one for your horse. Manuel Himenes, DVM, Kailua, HI
I have a 17-year-old Thoroughbred/Saddlebred horse. I just had the veterinarian out to evaluate some on and off lameness. The vet performed an ultrasound and found a tendon tear that is approximately 20-30% of the tendon on the front leg. He was and still is sound at walk, trot, and canter but would come up lame the day after jumping some cross rails and then would be fine a day later. The lameness was very sporadic and came and went quickly. Looking back, I think he probably has had the injury for at least a month. I am poulticing the leg everyday now for two weeks. He still gets turned out since he is arthritic and stocks up in his stall, but is quiet during turn out. My vet restricted our activity to walk/trot only, for 60 days (since he is actually sound) and can continue trotting over poles (although I am limiting the number of poles to 1 or 2 instead of our normal 5+). My horse has a weak stifle so we are trying to keep him built up. I have spent the last year strengthening him with lots of trotting ground poles and cavalletties. What exercises can I do at walk/trot to keep my horse in shape during his rehab without over pushing him? I want to give him the time and support he needs to heal but help him stay in shape.(View Answer)
It sounds like your veterinarian has prescribed a reasonable exercise program based on their findings. If you are concerned about his stifle, you could add some backing into your exercise regimen. However, the program you are following now should keep him in reasonable shape. Manuel Himenes, DVM, Kailua, HI
I have a 16-year-old miniature horse with severe Ataxia. After veterinary evaluation, she was started on Marquis today. Blood was drawn but of course, no results yet. The onset was very fast and she has difficulty rising. With this drug, when would it be reasonable to see an improvement if the clinical diagnosis is correct?(View Answer)
EPM is a difficult disease to deal with. It may take a few weeks to see results from Marquis and relapses can be common. Hopefully, you will get results soon although blood results only indicate exposure and not necessarily infection. Let your veterinarian know about her difficulty rising as there may be some additional treatment that can help her.
I hope she has a complete recovery soon. Manuel Himenes, DVM, Kailua, HI
Will Adequan IM work for an old kick injury on a knee? Will it prevent his knee from getting worse? The only thing I note at this time is some stiffness when his feet are trimmed or he is shod.(View Answer)
Adequan has been shown to reduce joint inflamation, which may help your horse. There is no evidence that adequan or any other drug can prevent arthritis, however reducing inflammation is beneficial to the joint. Is the stiffness due to holding the knee flexed when shod or trimmed? You may try and hold his hoof closer to the ground when trimming reducing the knee flexion. An injection into the knee may also be beneficial. You may want to have your veterinarian evaluate the joint and determine if an intra articular injection may be beneficial. Manuel Himenes, DVM, Kailua, HI
Can a horse get too high a dose of probiotics? My 23-year-old Thorougbred is having loose manure and it takes 3X the recommended dose to respond.(View Answer)
There is a lot of controversy about probiotics and how effective they are. There are no reports of a probiotic overdose so I would be concerned about that. Loose manure with no other signs of disease is more of a nuisance problem than a health issue. You may want to try and adjust your horse feed and add wheat bran. Manuel Himenes, DVM, Kailua, HI
I have a 10-year-old Connemara mare with Cushing's disease. Can you please tell me about the use of Pergolide when she is in foal? She is due to foal in May.(View Answer)
Pergolide is considered safe in pregnant mares. Uncontrolled Cushing's disease is far more dangerous to her pregnancy than any pergolide side affect. Manuel Himenes, DVM, Kailua, HI
I have found that a seller's non-disclosure of a horse's pertinent medical history can be easily made with very general statements of opinion. In an attempt to learn all I can about a horse's medical history prior to the pre-purchase exam, what are some very specific questions that I can ask?(View Answer)
Many veterinarians now request the official medical records for the horse prior to or at the time of the prepurchase exam. I would request from the seller all medical records for the horse being sold. If there are no medical records then I would simply ask if the horse has had any lameness or major medical problems such as colic in the past.
I Hope this helps. Manuel Himenes, DVM, Kailua, HI
Does a horse ever get over a stifle?(View Answer)
Stifle injuries in the horse range from minor to very severe. If your horse is locking its patella, the prognosis is very good with proper treatment. If your horse has ruptured ligaments or a torn meniscus, the prognosis is more guarded and will likely require surgery. OCD cysts of the stifle can be successfully treated if there is no arthritis present. Arthritis of the stifle has a poor prognosis for athletic soundness.
Your veterinarian will be able to determine the type of injury and prognosis. Manuel Himenes, DVM, Kailua, HI
I have a 17-year-old Saddlebred mare that is lame in her front left. She is sound cantering, but at the trot she shows as if it's a quick step. We have done X-rays from bottom to shoulder and have also had a nuclear scan in which nothing was shown. It seems to get worse as I work her out. However, after she has been in her stall for 20 minutes after her workout she goes back to the same lameness, which isn't that bad. Any idea of what it could be? It started 8 months ago with a bad dip step outside and she hasn't gotten better. No one can figure it out.(View Answer)
If diagnostic nerve blocks have not been done then I would start there. You may need to work her to increase the lameness for the exam. Your veterinarian may have already ruled that out but if not you may try to perform a dynamic ultrasound of the back of the fetlock to see if there are any issues with the annular ligament. Horses with annular ligament constriction will get worse with work and resolve with rest. If these tests are negative then try and lunge your horse. Does she also go lame without a rider? If not, check your saddle fit, your saddle for a broken tree, etc. Horses with withers, back, rib or chest injuries can also show similar signs.
Good luck and I hope you find a solution to your horse's lameness. Manuel Himenes, DVM, Kailua, HI
My mare went lame on her back left after being shod. We had radiographs done, which showed she had a old injury by her pastern. Is there anything I can do for her that isn't expensive?(View Answer)
If you want to find out where the lameness is coming from you will need to get a lameness exam that may require nerve blocks. Is the result of the old injury ringbone (pastern arthritis)? If she has an arthritic pastern, bute may be the least expense way to treat her.
You also stated that she went lame after being shod. Are the horseshoe nails OK? Was she cut short? Horses with muscle disease may also pull up sore after shoeing due to the limb being held up for a long time. These possibilities can be sorted out with a lameness exam. However if a few days of bute helps her that is the least expensive route. Manuel Himenes, DVM, Kailua, HI
I have recently opened a licensed boarding facility. We vaccinate our personal horses fully with veterinarian visits twice a year. I have one boarder (the only one right now) who has only vaccinated her horse for rabies. I can't convince her to get all the others. Can you give me the right words and information that might convince her to do so? She does not leave the property with the horse. Any other horses that would board at my facility, I am requiring them to be vaccinated before they come on the property and also have it set up to quarantine for 15 days....but I have her horse here.....any ideas?(View Answer)
I would start by explaining that vaccinations are cheap compared to the cost of treatment. In fact tetanus has a >90% fatality rate. Encephalitis viruses (Eastern, Western, West Nile) can also be fatal but the cost of care can be substantial and many horses never fully recover from these diseases. Equine herpes virus and influenza, while not as deadly, can cost hundreds to thousands to treat.
Since her horse does not leave the property she may feel her horse's risk is low. She is at a boarding facility and with new horses coming in, her horse will be exposed. By not vaccinating her horse, she also provides a window of opportunity creating an exposure risk for other horses on the property.
You have the right to require vaccinations of any horses on your property. Manuel Himenes, DVM, Kailua, HI
What are the chances a horse will get its sight back if treated for Equine Recurrent Uveitis (ERU)?(View Answer)
If there is no damage to the lens, retina or optic nerve, the prognosis for return to sight is good. If the cornea is completely white it may take some time for the cornea to clear and it may not clear completely. While the horses vision may not be perfect if the internal structures are not damaged the horse should have some sight.
One of the major complications of ERU is glaucoma. Glaucoma will result in blindness and a chronic painful eye.
Good luck with this horse. ERU is a difficult disease to treat and can be very frustrating. Persistence and patience are needed to have a successful outcome. Manuel Himenes, DVM, Kailua, HI
Should you deworm your horse and then do it again at a later date to kill the eggs as they mature or does the first dose kill both larvae and eggs?(View Answer)
Horses are infected eating larvae that have hatched out from the eggs. Ivermectin or Moxidectin will kill 95% of migrating larvae and adults in the horse. Larvae that have buried themselves into the wall of the intestine will not be killed with these dewormers. These larvae can be killed with a 5-day double dose of fenbendazole. Daily dewormers will also kill these larvae but are only effective as the larvae leave the intestinal wall and enter the gut.
Dosing with ivermectin or moxidectin may also be used daily to control the habronema worm that causes summer sores.
Fecal egg count monitoring is the best way to decide on parasite control. If fecal egg counts are not done then bimonthly doses of ivermectin or quarterly doses of moxidectin. Manuel Himenes, DVM, Kailua, HI
My 10-year-old Thoroughbred/Percheron mare frequently loses her shoes. She is on a diet balanced by an equine nutritionist and my farrier is very competent. I have tried biotin supplements for over a year with no improvement. Can you recommend some other treatments?(View Answer)
In horses that fail to respond to biotin I will often try MSM (1500mg/day) with or without biotin. Are there other factors that are affecting your horse's hooves? Is she in a dry environment or are her feet constantly wet? If her feet are in a wet environment, you will need to take steps to allow her feet to dry for at least part of the day. Be sure that her shoes are fitted well and her hooves are balanced (unlikely as you are confident in your farrier).
I am currently trying a kevlar hoof dressing and this seems to help some of these brittle footed horses that have trouble holding shoes. I have only used it on a few horses but it does look promising.
Good luck with your horses feet and have a great new year. Manuel Himenes, DVM, Kailua, HI
How much of each Glucosamine, Chondroitin and MSM should I give my older horse per day?(View Answer)
The efficacy of these supplements is controversial but most research is done using 10g of glucosamine, 1.5g of chondroitin and 1.5g of MSM per day.
One of the concerns about these supplements is absorption and some researchers recommend that this dose be split into two feedings. There is no standard dose, therefore each company has their own proprietary dose. I recommend to my clients to use the product at the recommended dose and if improvement is not seen after 2-3 months, it is unlikely that your horse will be helped by that supplement. Manuel Himenes, DVM, Kailua, HI
My horse has arthritis in her stifle and is leaning on her handler and stamping her back foot. How do you know when enough is enough?(View Answer)
Each horse has a different tolerance for pain. Does you horse respond to a reasonable dose of phenylbutazone? I make the decision on quality of life based on attitude. If your horse is no longer eating, is not lying down to rest or seems to be in a chronic depressed state, in my opinion, he/she does not have a reasonable quality of life.
It is widely believed that if a horse cannot support at least 50% of their bodyweight on the affected leg the opposite leg is in danger of laminitis. If you cannot lift the opposite foot then the horse is in danger of support limb laminitis.
This is a very personal decision made by the people closest to the horse. My experience has been that people who have a love for their horses often make the correct decision about end of life. The fact that you are asking this question tells me that you will make the correct decision when the time comes. Manuel Himenes, DVM, Kailua, HI
Last year, my broodmare aborted three months early. She is doing it again this year. The foal is moving, but she is holding up her tail, winking with fluid coming out. Is there anything I can try to keep the foal from being aborted?(View Answer)
I would get an ultrasound examination of your mare immediately. She may have placentitis and infection of the placenta. This can be treated with antibiotics and other drugs to support the placenta and baby. The ultrasound can also access the health of the foal. With an ultrasound exam, the thickness of the placenta can be determined and if it is thickened, the sooner treatment begins the better. Best of luck and hope for a healthy foal. Manuel Himenes, DVM, Kailua, HI
My horse cannot see at night and has trouble with depth perception in the day. What might be her problem?(View Answer)
Vision in the horse is hard to access. I would first begin by having a veterinarian examine your horse's eyes and verify the the pupils can dilate. Sometimes if a horse has had an eye injury, the iris will stick to the cornea or lens and not allow the muscles of the iris to dilate the pupil. The pupil must dilate in low light conditions in order to allow more light into the eye. If one eye lost the ability to dilate this may cause the loss of telescopic vision required for depth perception. Also, be sure that your horse does not have any other diseases of the eye like uveitis, which can cause a loss of vision in one eye.
Careful examination of your horse's eyes can also rule out the eyes as a problem. Your horse may have other neurological or musculoskeletal problems that are not related to the eye itself. Good luck with your horse and I hope that the problem can be defined with a positive outcome. Manuel Himenes, DVM, Kailua, HI