Vector-Borne Diseases – What You Should Know

Vector-Borne Diseases – What You Should KnowVector-borne diseases (i.e. any disease spread by a mosquito, tick, or other insect) always present a threat to horses as the weather gets warmer. As late summer hits, we can start to see a rise in the number of cases of vector-borne diseases. Here, we break down the most common causes of vector-borne illness in our area, and what you can do to take charge.

Eastern/Western Equine Encephalitis Virus (EEE/WEE)*

What They Are: Viruses spread by infected mosquitos (usually after feeding off an infected bird), causing neurologic clinical signs. EEE is primarily seen in the Southeastern US, though cases have been reported in all states east of the Mississippi River. WEE is typically seen in the Western states, though cases in the Midwest have been reported.

Clinical Signs: Variable. Most horses will have a high fever, a decreased appetite, and neurologic signs. These signs could include behavioral changes (decreased mentation or hyperexcitability), head pressing, circling, blindness, seizures, difficulty swallowing, facial nerve dysfunction, or even coma and death. Clinical signs typically appear a few days after infection, though some cases of WEE can take up to 3 weeks. EEE is more virulent than WEE, causing mortality in ~ 75-95% of horses, while WEE causes mortality in ~ 20-40% of horses.

Diagnosis: Serum virus testing and/or testing of spinal fluid.

Treatment: Once a horse is infected, treatment is supportive; horses that survive may show residual neurologic deficits.

Prevention: Vaccination. Vaccination is recommended annually prior to mosquito season, with re-vaccination at 4-6 month intervals in areas with a high mosquito population, or in horses that travel to warmer climates during the winter months. In addition, environmental mosquito control measures such as insect repellent (both in the barn and on the horse), minimizing standing water, and keeping horses in at night can be used to reduce the likelihood of mosquito bites.

West Nile Virus (WNV)*

What It Is: Virus transmitted by mosquitos that can cause infection in horses, humans, and birds, leading to neurologic clinical signs. An infected horse or human is not contagious to other horses or humans, as infection must come from an infected mosquito bite, and birds serve as the only known source of infection.

Clinical Signs: Depression/lethargy, decreased appetite, mild fever, colic signs, and sudden, progressive neurologic signs. These could include behavioral changes (decreased mentation, hyperexcitability), muscle fasciculations (twitches) of the face and neck, facial nerve paralysis (muzzle deviation), tongue weakness, head tilt, weakness or ataxia progressing to complete paralysis, inability to rise, and death. Clinical signs are often the most severe in very young or very old animals, and typically show up 7-10 days after being bitten by an infected mosquito.

Diagnosis: Serum virus testing and/or testing of spinal fluid.

Treatment: Once a horse is infected, treatment is supportive.

Prevention: Vaccination. Peak WNV infection is usually between July-October, though it can be seen year-round in southern climates. Annual vaccination before mosquito season is recommended, with re-vaccination in areas with a high mosquito population, or in horses that travel to warmer climates during winter months. In addition, environmental mosquito control measures such as insect repellent (both in the barn and on the horse), minimizing standing water, and keeping horses in at night can be used to reduce the likelihood of mosquito bites.

Lyme Disease

What It Is: Bacterial infection (Borrelia burgdorferi) spread by deer ticks, causing non-specific clinical signs. While typically seen in the Northeast, Lyme disease is becoming endemic in Wisconsin and Minnesota.

Clinical Signs: Clinical signs for Lyme disease are non-specific and can range from musculoskeletal issues (shifting lameness, swollen joints, stiffness, muscle soreness) to generalized illness (muscle wasting, fever, weight loss, lethargy). In some cases, the neurologic system can be affected (neuroborreliosis), causing behavioral changes, weakness, or ataxia.

Diagnosis: Serum antibody testing, looking for specific surface proteins to determine chronicity of disease. While testing is helpful, it is not 100% diagnostic, and should be considered along with clinical signs and ruling out other potential causes.

Treatment: Antibiotics in the tetracycline class—oxytetracycline, doxycycline, and minocycline are the most commonly used. Treatment often requires extended antibiotic usage, but even with appropriate medications, chronic cases can occur and can make it difficult to completely clear the infection.

Prevention: Environmental insect control (sprays/repellants), avoiding wooded areas or areas with long grass/brush, and frequent tick checks are the best options. No approved vaccination options for horses exist at this time.

Anaplasma

What It Is: Bacterial infection (Anaplasma phagocytophila) spread by deer ticks, causing infection of white blood cells (neutrophils and eosinophils) and leading to destruction of red blood cells, white blood cells, and platelets. It is often seen in fall to late winter.

Clinical Signs: Variable, though most horses have a moderate to high fever, decreased appetite, limb edema (swelling), lethargy, reluctance to move, jaundice, and occasionally petechiation (red spots on the gums, caused by damage to the blood cells and platelets). Occasional neurologic symptoms have been reported.

Diagnosis: Treatment is typically initiated before a definitive diagnosis is made, based on clinical signs and time of year. Diagnostics include microscopic evaluation of the blood cells, or blood testing for bacterial DNA or antibody titers.

Treatment: Antibiotics—oxytetracycline and doxycycline are most commonly used.

Prevention: Environmental insect control (sprays/repellants), avoiding wooded areas or areas with long grass/brush, and frequent tick checks are the best options. No approved vaccination options for horses exist at this time.

Potomac Horse Fever (PHF)

What It Is: Bacterial infection (Neorickettsia risticii) most commonly caused when horses ingest aquatic insects such as mayflies or dragonflies that contain infected flukes. Horses are not considered to be contagious to other horses by direct contact, though multiple horses on a farm may develop disease due to a contaminated water source.

Clinical Signs: Variable, though horses typically have a very high fever and colic signs, usually progressing to diarrhea (can be mild or severe) and laminitis if treatment is not initiated. Fever often occurs 7-14 days before diarrhea begins. Other clinical signs could include a decreased appetite, lethargy, or limb edema.

Diagnosis: Treatment is typically initiated before a definitive diagnosis is made, based on clinical signs, as delaying treatment while diagnostics are being performed can result in severe progression of clinical symptoms. Diagnostics include testing of feces, whole blood, or serum.

Treatment: Antibiotics—oxytetracycline and doxycycline are most commonly used

Prevention: Vaccination. Vaccination prior to peak insect season will help reduce the severity of clinical disease, but has not been shown to be 100% preventative. Additional prevention strategies should be targeted at reducing insect populations around barns (turning off lights at night, preventing standing water).

A strong vaccination protocol paired with knowledge of the clinical signs for vector-borne diseases is essential for disease prevention and early intervention for non-preventable illnesses. If you have any questions regarding vaccination protocols, please do not hesitate to contact our office and speak with one of our doctors.

*When administered by a veterinarian, vaccination against these diseases is backed by an immunization support guarantee. Our practice always uses reputable vaccines that offer this support so that in the unlikely event your horse would become infected with a disease for which they received a covered vaccine, you can receive financial support to get your horse the help they need.*

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