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Horse Owners have other Reasons to Vaccinate for West Nile Virus

Dr. Frederick Harper, Extension Horse Specialist, University of Tennessee

Vaccination against West Nile Virus (WNV) is recommended to prevent the disease in horses. There were 103 positive WNV horse cases in Tennessee in 2003 and 150 cases in 2002. Seventeen horses died or were euthanized in 2003 for a death rate of 16.5 percent. In 2002, 43 horses died or were euthanized from WNV for a death rate of 28.5 percent.

Now these numbers make a good case for vaccinating one's horse to prevent WNV. No one wants to lose their horse to this or any other disease which can be prevented.

Researchers from the University of Minnesota surveyed horse owners who had horses which had survived WNV. The horses were described as pleasure riding, breeding animals or both. They made some startling discoveries that horse owners need to know about the after effects of WNV.

They found that 40 percent of the horses that had recovered from WNV still experienced some negative effects. These horses had either some gait abnormalities or behavioral changes or both.

In WNV horses that the owners felt had fully recovered, the duration of clinical abnormalities was 35 days with a range of 1-180 days.

Some of the horses that were considered fully recovered had some residual deficits such as behavioral changes, loss of muscle mass, more frequent stumbling, less energy, weak hind legs or abnormal gaits.

In addition to abnormal gaits, weakness in either the front and/or hind legs and more frequent stumbling was observed in horses that were classified as having incomplete recovery. Behavioral changes, loss of muscle mass and diminished energy along with vision loss and difficulty in swallowing were also reported.

One owner had ceased riding her horse after it fell on her. Severe and persistence gait defects resulted in two horses being euthanatized six months after being diagnosed with WNV.

Relapses of clinical signs were reported in horses that were classified as fully recovered, having residual defects or an incompletely recovered group. Relapse occurred from 2 weeks to 5 months after the initial diagnosis of WNV. Most of the relapses were mild and did not require veterinarian care but some cases did.

Behavioral changes were observed to occur in the demeanor, mental abilities and abnormal behavior patterns. Five horses were noted as becoming very quiet and calm while a similar number became more easily spooked or startled. Three horses were described as being more irritable. Owners stated that five horses had loss of memory or vision. Episodes of sudden sleepiness, suggestive of narcolepsy, were described in two horses.

Based on this research, not all horses that recover from WNV return to normal at within the next few months. About 40 percent will have some changes in their behavior and/or gaits about 6 months after being diagnosed with WNV.

It was concluded that the majority of behavioral changes only had a minimum impacted on the horse's riding ability. However, owners need to be cautious about the potential risks of increased spookiness, irritability, seizures or narcolepsy which has an increased risk for human injury.