Emily Steele, DVM

February 2007

West Nile

West Nile Encephalitis is a viral disease that can be transmitted to people and horses through the bite of a mosquito.  West Nile has also been found in Africa and Europe.  In 1999 the first North American West Nile case was reported in New York City.  Since then, it has spread rapidly across the United States.  Mosquitoes acquire the virus by biting birds infected with the virus.  Birds are the primary carrier of this disease and humans and horses are accidental hosts of West Nile.  Although they can show signs of the disease and even die from West Nile, an infected horse or human is not contagious.  West Nile Encephalitis affects the neurological system in horses and signs of the disease include:  weakness, incoordination, depression, paralysis, blindness, and/or fever.  Horses can develop one or all of these signs, and unfortunately no cure is available.  Some horses will recover with supportive care and others will not.  In 2005, 22 horses in Minnesota were reported to have West Nile and 38% of those died or had to be euthanized.  At the time of publishing, there are currently 10 reported cases of West Nile in Minnesota horses for 2006.  What does this means for your horse?  West Nile is definitely present in Minnesota and we need to vaccinate against the disease.  Several brands of very effective vaccines are currently available through your veterinarian. 

Anthrax

Anthrax is a disease that can infect many species, including horses and humans.  The disease is caused by the bacterium Bacillus anthracis.  This bacterium produces a toxin that in horses can cause fever, depression, convulsions and difficulty breathing.  Most animals die within 24 hours of contracting the disease.  B. anthracis is found in the soil and has the ability to form spores which can survive for decades.  Horses can ingest the bacteria by drinking from natural water sources or by grazing in areas that have been contaminated by flooding.  In addition, the bacteria can enter the body through open wounds.  B. anthracis is present throughout the United States and there are normally a few cases in Minnesota every year.  There is a vaccine against anthrax available. However, there is an increased risk of side effects associated with this vaccine, and it is recommended that horses be vaccinated only if going to an area that has recently had anthrax cases.  In addition, when traveling to different areas with your horse, it is wise to prevent your horse from drinking from unfamiliar natural water sources such as rivers, puddles, or lakes. 

Equine Infectious Anemia

Equine Infectious Anemia (EIA), commonly referred to as Coggins disease, is a viral disease carried in the blood of horses.  It is transmitted when blood from an infected horse is transferred to another horse, most often via biting insects.  The disease has two forms, acute and chronic.  In acute cases, horses developed a fever and become very depressed.  Some horses will die from this acute infection; others that survive may develop the chronic form of EIA.  In the chronic form, horses will have recurring bouts of fever and depression.  Over time, these horses develop severe weight loss and anemia.  In between episodes, these horses often appear completely normal.  The disease can be transmitted from these horses even when they appear normal, making undiagnosed horses particularly dangerous.  There is no cure or vaccine available for EIA.  Although the disease is rare, it is present in Minnesota and in 2006, 66 new positive horses were reported.  Therefore, it is important to test horses regularly for the disease.  This test is often referred to as a Coggins test.  While requirements vary, most states require yearly testing, especially if you are traveling with your horse.  Because affected horses can transmit the disease for the rest of their lives, positive horses must be euthanized or permanently quarantined to protect other horses from the disease. 

Vesicular Stomatitis

Vesicular Stomatitis is a viral disease that affects horses, cows and pigs.  The disease can be transmitted through biting insects or via direct contact.  Affected animals develop very painful sores on the mouth and lips.  They may also develop sores on the teats, prepuce or hooves.  There is no vaccine available to prevent the disease.  Most animals survive this disease, but the painful sores can cause weight loss and increased salivation.  In addition, the disease produces identical symptoms to another important virus:  Foot and Mouth Disease.  Therefore, it is a disease that must be reported to the USDA when found.  This disease has not yet occurred in Minnesota and the only positive cases of the disease this year have been in Wyoming.  However, when traveling to or from Wyoming and nearby states, many states require a health certificate specifically stating that the horses have not been exposed to Vesicular Stomatitis. 

Equine Herpes Virus (EHV)

Also referred to as Rhinopneumonitis, EHV is a herpes virus that affects the horse.  There are 5 different strains of EHV that affect domestic horses.  Like all herpes viruses, EHV has the ability to become latent and "hide" in nervous tissue.  After the initial infection subsides, the disease can return during times of stress.  Once infected, horses will carry the virus for the rest of their lives.  EHV 2 and 5 are highly prevalent in horse populations, but rarely cause disease.  EHV 3 causes a venereal disease.  Affected horses often develop red papules on their vulva or penis.  These papules then grow in size and rupture.  The infection can be very uncomfortable and breeding should be delayed until the papules heal, as they are painful and can spread the disease to other animals.  Most horses recover well from this virus on their own and EHV 3 has not been shown to affect fertility.  EHV 1 and 4 can cause respiratory disease, foal deaths, neurologic signs and abortions.  Although EHV 4 can cause all of these symptoms, EHV 1 is most often found to be the culprit.  On breeding farms, EHV 1 often affects many mares at once, leading to "abortion storms".  The disease is spread easily by contact between horses, and is therefore particularly devastating to large barns.  Often, a new, stressed horse will actively shed the disease, triggering symptoms to occur in many other horses.  In addition, pregnancy is a well known stress, making mares in foal particularly susceptible to break-outs of the disease.  The majority of horses have been exposed to EHV 1 and/or 4 at some point in their lives and symptoms can recur when stressed.  However, vaccination can prevent these symptoms from re-emerging and current recommendations include vaccinating horses 2-4 times per year depending on their exposure to other horses.  In addition, pregnant mares should be vaccinated at months 5, 7, and 9 of their pregnancy to prevent abortion and new horses should be quarantined for 3-4 weeks.  The most recent outbreak of EHV occurred in Florida in December 2006. Several horses developed neurological signs and some of the affected horses died or had to be euthanized.  Although the available vaccines protect against EHV 1, 4 and have been licensed to protect against abortion and respiratory disease, these vaccines are not licensed to protect against the neurological form of the disease.