Vet's Perspective

Jul 5, 2013
by WLJ

Equine Herpes Virus and West Nile Virus

Equine Herpes Virus (EHV) has been noted intermittently with outbreaks occurring in shelters, stables, and organized events throughout the U.S. These episodes have brought the disease to the attention of horse owners throughout the country. EHV has not been known to infect humans, but some literature hints at the potential of disease in camellids.

EHV is a viral infection seen in horse populations worldwide, causing respiratory and neurological disease as well as abortion storms and neonatal death. Sporadic occurrence of disease has been noted especially during times of stress for the horse, such as surgery or shows and transportation. Infections may remain latent in the horse’s immune system and only show their effects when the horse is placed under these noted stressful situations.

The virus affects the inner cells of blood vessels and ultimately acts upon the respiratory and central nervous systems.

The incubation, or time from infection to onset of clinical signs, is similar in nature as that of other viral causes, usually between three to 10 days. One of the first signs of disease is an elevated rectal temperature, often in a range of 103 to 105 degrees Fahrenheit. Horses may recover from illness but still shed virus for three weeks after recovery.

There is no cure for Herpes Virus infections, and thus treatment is directed towards preventing secondary bacterial infections and keeping the horse comfortable with anti-inflammatory agents and stall rest. Vaccination is the gold standard in preventing infection of your herd. During outbreaks, transport of horses should be minimized and quarantine parameters put into effect accordingly.

West Nile Virus (WNV) is a preventable disease caused by a virus transmitted in the saliva when mosquitoes bite a horse, human, or several other mammals. Clinical signs include fever, lethargy, ataxic or wobbly gait, alterations from normal behavior, blindness, and muscle tremors. Approximately one-third of horses that develop clinical signs of disease are likely to die from the virus; survivors often will retain mental deficits.

In 2002, WNV made a big impact on the horse industry, with more than 15,000 equine cases reported that year. Since the virus was first identified in the U.S. in 1999, over 25,000 equine cases have been reported. As of late October 2012, the incidence of (veterinary) reported WNV cases had risen fourfold in comparison to those recorded in 2011. Humans may also be affected by the virus; in 2012, the Centers for Disease Control and Prevention reported approximately 4,891 human illness cases and 223 mortalities.

As ambient temperatures rise, mosquito activity also increases significantly.

In 2013 thus far, positive equine and test mosquito cases for WNV have been identified in Boulder, Larimer, and Mesa counties in Colorado. Concerns for ranch horses are legitimate; in many cases, disease can be preventable. For more information regarding vaccination against these important viruses, please contact your local veterinarian. — Dr. Genevieve Grammer

[Dr. Genevieve Grammer is a mixed-species veterinarian practicing in eastern Colorado. Please direct correspondence to drgigi19@].