The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.
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28 TheHorse.com THE HORSE February 2016 Does a Canine Lyme Vaccine Elicit a Response in Horses? We have at our disposal vaccines designed specifically for horses to help prevent a number of common ailments, from West Nile virus to influenza. But an equine vaccine that remains elusive is one protecting against Lyme disease—a tick- borne disease veterinarians are diagnos- ing more and more frequently. There is, however, a canine vaccine that many veterinarians use off-label for horses in Lyme-endemic areas. But to date, there have been no studies assessing this product's impact on horses' humoral (blood) immune responses. So Kathryn Slaughter, a veterinary student at Western University of Health Sciences, in Pomona, California, and colleagues recently evalu- ated how 42 healthy horses reacted to it. Lyme disease is caused by the bacteria Borrelia burgdorferi. Horses are inciden- tal dead-end hosts (meaning they can contract but can't transmit the disease). B. burgdorferi transmission from Ixodes tick to horse takes 24 to 48 hours, mean- ing the tick must be embedded in a horse for that time to pass along the bacteria—a fleeting bite won't do it. Clinical signs generally appear two to five months or more after disease transmission. Clinical signs of Lyme disease are nonspecific in horses and include chronic weight loss, low-grade fever, sporadic or shifting leg lameness, muscle tenderness, and arthritis. Some horses also develop poor performance, sensitivity to touch, behavioral changes, or neurologic signs. The canine product is an outer-surface protein A (OspA) vaccine designed to pro- vide protection against B. burgdorferi by prompting OspA antibody production. Slaughter and colleagues selected study horses in a desert habitat (making it im- probable that they'd had previous contact with B. burgdorferi) and tested all the ani- mals' baseline OspA, C, and F levels. Then they split the horses into three groups: ■ Group TD (20 horses) The team admin- istered ½ mL of the vaccine transder- mally (through the skin) over each chest muscle, for a total of 1 mL; ■ Group SQ (19 horses) The team admin- istered 2 mL of the vaccine subcutane- ously (beneath the skin) in the left side of the neck; and ■ Group C (3 horses) These animals served as unvaccinated controls. The team inoculated the vaccination groups on Days 0, 22, and 226, and an examiner monitored all study horses for reactions for 24 hours following ad- ministration. The team collected blood samples throughout the study to compare the horses' antibody levels with baseline measurements and found that: ■ Both vaccinated groups showed an increase in OspA antibodies on Days 42 and 244 following the initial vaccine administration; ■ There was no evidence of variations in OspA levels between age, sex, or breed; ■ There was also no difference in OspA levels based on vaccine administra- tion route, suggesting that the smaller transdermal dose is equally as effective as the higher subcutaneous one; and ■ Minor vaccine reactions (i.e., swelling at the injection site) occurred (47% in TD and 20% in SQ), most commonly in the same horses throughout the study. Also, OspC and OspF levels decreased following vaccination, both of which develop in horses after natural or experi- mental B. burgdorferi exposure. "The reductions seen in OspC and OspF antibodies post-vaccination were unexpected, but interesting," Slaughter said. "The protective nature of this vac- cine response is yet to be determined and requires further studies." Based on their results, the team reported that vaccine administration followed three weeks later by a booster would conceivably afford the most effec- tive protection against Lyme disease, and the OspA antibody titer levels suggest that vaccination could provide protection for about six months. "These data may aid in strategic vac- cination protocols and the development of a USDA-approved vaccine for horses in the prevention of Lyme disease using the OspA vaccine," Slaughter said. Managing Limb Lacerations If your first instinct after discovering a hemorrhaging leg laceration is to call your veterinarian, you're correct (this is preferable to fainting). Chances are, you'll need his or her help to manage a severe leg wound properly. Still, it never hurts to know the steps involved so you can care for your horse until your veterinarian arrives and have an idea of what will hap- pen when treatment begins. Southwood listed steps veterinarians should take with emergency lacerations: ■ With any wound, she said, the first priority should be to stop the hemor- rhaging. How easy this will be depends on the severity of the laceration. Com- mon ways to slow bleeding include applying pressure to the affected area with towels or a pressure bandage. "Keep in mind that while stopping hemorrhage is critical, horses do have a lot of blood—about 40 liters (10.5 Both vaccinated groups (SQ and TD) showed an increase in OspA antibodies following the initial vaccination compared to controls (C). 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 0 22 42 93 179 244 313 355 FIGURE 1 Mean OspA antibody levels in study horses post-vaccination COURTESY DR. KATHRYN SLAUGHTER DAYS OspA Levels C SQ TD