A New Reason To Hate Ticks On Horses: Equine Anaplasmosis

Equine anaplasmosis (also known as Equine Granulocytic Erlichiosis) is a tick-borne disease caused by the bacterium Anaplasma phagocytophila (formerly known as Erlichia equi), also the cause of human granulocytic erlichiosis (HGE). It was first described in horses in California in the 1960’s, but has since been reported in multiple states of the USA and in Europe. Several cases have been diagnosed in the Shenandoah Valley so far this fall.

Anaplasma phagocytophila is a bacterium that lives within blood cells and is carried by deer ticks, the same ticks that carry Lyme disease. Possible natural reservoirs of the disease include deer, rodents and birds. Horses seem to only suffer transient infection, so that they do not harbor latent disease. The disease is not contagious from one horse to another, but multiple horses on a property may be infected, depending on the local tick population.

Horses most commonly get sick with anaplasmosis in the fall to late winter. They commonly show signs of fever, loss of appetite, reluctance to move (people with the disease complain of headache and muscle pain), leg swelling, petechiation (red spots on the gums and whites of the eyes), jaundice, and lethargy. Occasionally, horses may show neurologic signs, or have trouble rising. The clinical signs are in part due to the fact that the bacteria invades some white blood cells (neutrophils and eosinophils), leading to the destruction of red blood cells, white blood cells, and platelets. Bloodwork shows low platelets, which are important for clotting the blood, and anemia.

Diagnosis can be made in several ways. The most definitive way to diagnose the disease is to look at the blood cells under a microscope to look for the presence of the bacteria inside the neutrophils. In some infections, though, only a small number of neutrophils may be affected, and therefore they may not be detected this way. Because horses mount a strong immune response to the bacteria, titer levels can be tested to confirm recent exposure.

The treatment for anaplasmosis is the administration of tetracycline, usually oxytetracycline, minocycline, or doxycycline. Some horses with mild disease may recover without treatment. Most horses recover fully, although some may relapse in 3-4 weeks. Fatality from the disease is rare. There is currently no vaccine available, so prevention is limited to tick control.

Strangles in Horses: Causes, Symptoms & Treatment

copyright Dr. Josie Traub-Dargatz, CSU

copyright Dr. Josie Traub-Dargatz, CSU

The disease known as strangles in the horse is caused by the bacteria Streptococcus equi var. equi (S. equi). Horses with strangles usually have a high fever, followed by nasal discharge and enlargement and abscessation of the lymph nodes under the jaw and in the throatlatch region. Affected horses often have a loss of appetite and may stand with their head and neck extended. Occasionally some horses develop abscessation of lymph nodes in the chest and abdomen known as “bastard strangles”.

S. equi is spread through contact with fluid draining from the abscess or nasal

secretions. The incubation period from infection to clinical signs is 3-14 days. Horses infected with Strangles do not shed the bacterium from the nasal secretions for 1-2 days after the onset of fever. If temperatures are regularly taken, new cases can be quarantined before they become infectious, limiting spread through a barn. The organism can be carried on the boots, clothes and equipment of those who have contact with an infectious animal, necessitating strict biosecurity measures to prevent spread. Most horses will shed the organism for 2-3 weeks post infection. A veterinarian can perform nasopharyngeal washes one week apart to ensure a horse is free of infection before returning him to the barn. Some horses may remain persistently infected due to bacteria harbored in the guttural pouches. These horses can shed the organism intermittently and are known as carriers. They may be the source of infection when introduced into a herd of susceptible horses. Carriers can be detected by performing nasopharyngeal washes. S. equi may persist in the environment for a few months.

Treatment of Strangles depends on the situation and severity of the disease. If antibiotics are begun as soon as the horse shows fever, lymph node abscessation may be prevented. Once a horse has lymph node involvement, antibiotics are generally contraindicated. Antibiotics at this stage may prolong the time to abscessation and resolution of the signs. Instead therapy should be directed at encouraging maturation and drainage of the abscess. Some horses develop complications that necessitate additional therapies.

Seventy-five percent of horses that have lymph node involvement and recover have a solid immunity that lasts 5 years or longer. Antibody titers can determine the strength of immunity. Vaccination should be administered on a case-by-case basis by a veterinarian depending on previous exposure and likelihood of future exposure. Horses that have had previous exposure to S. equi may develop immune mediated complications following vaccination.

Most horses that suffer from Strangles will recover without complications on their own. But to prevent the disease, all new horses or horses returning from an event where horses are comingled should be quarantined with temperatures taken daily or twice daily. New horses should be isolated for 3 weeks. Ideally, no new horses should be introduced to a farm without 3 negative nasopharyngeal washes.

Many thanks to Dr. Josie Traub-Dargatz from Colorado State University for allowing us to use her pictures.


What You Need To Know About Equine Herpes Virus Myeloencephalopathy Outbreaks

Over the last several years there have been numerous outbreaks of Equine Herpes Virus (EHM) myeloencephalopathy infections in horses. Many of the outbreaks originated at shows or events where numerous horses were stabled together for a few days.

The cause of the outbreaks is Equine Herpes Virus-1 (EHV-1). Usually the virus causes respiratory signs including fever, poor appetite, coughing, nasal discharge and lethargy. It can cause abortion and neonatal death. In rare circumstances the virus may produce neurologic signs known as myeloencephalopathy. Loss of bladder and anal tone, mild incoordination, stumbling, hindlimb paralysis, and recumbency are signs of equine myeloencephalopathy.

Transmission from one horse to another comes from direct contact (nose to nose) or by contact with infected discharges carried on boots, buckets, blankets, trailers, hands, aborted fetus etc. The incubation period (time from infection to exhibiting clinical signs) is 2-14 days. Most horses have been infected with the respiratory form before the age of 2 years. Previous infection does not protect the horse from reinfection later in life. Some horses have a carrier or latency state in which they test positive but have no clinical signs. Stress may cause a recrudescence of the disease or result in shedding of the virus.

The virus can be treated with anti-viral drugs, which will lessen the severity of clinical signs if caught in the very early course of the disease. However, treatment is usually supportive (bladder drainage, anti-inflammatory drugs, sling) until the horse recovers on it own. If the horse becomes recumbent, the prognosis is poor.

Although vaccination for EHV-1 does give some protection against the respiratory form, it does not protect against the neurologic form. Following some simple management practices while at home and away can help protect your horse from infection. New horses on the farm, or horses that are returning to the farm after co-mingling with other horses should be kept away from horses on the farm for 21 days.

While at a horse show and when returning take your horse’s temperature every day. Contact a veterinarian the first day your horse has a fever. If EHV-1 is suspected, a nasopharyngeal swab can be obtained and submitted for tests to determine of EHV-1 is to blame. Horses exposed to EHM will be quarantined for 21 days following clinical signs to prevent spread of the disease.

Equine Lyme Disease

Over one hundred horse owners/ enthusiasts gathered at the Virginia Horse Center on March 19 for dinner and an educational seminar sponsored by Blue Ridge Equine Clinic, Augusta Cooperative Farm Bureau and the Virginia Horse Center. We were delighted to see old friends, clients and to get acquainted with new friends from our horse community.

Dr. Abby Sage, VMD from Blue Ridge Equine Clinic, gave an excellentCE mtg Dr Sage presentation on Equine Lyme Disease: What We Know and What We Still Don’t Know. Although there are been a few studies on the disease, the frustrating reality is that there is still much that has not been proven scientifically about Lyme Disease in horses, which has reached epidemic levels in our region since 2009.

The disease is caused by a bacteria called Borrelia burgdorferi. It is carried by the tiny deer or black legged tick and can be transmitted throughout the year. It is difficult to be certain what the clinical signs of the disease are in the horse. When ponies were experimentally infected with the organism, none of them developed clinical signs even though the organism was recovered from the ponies at necropsy. There have only been six documented cases of Lyme Disease in the horse that have been reported in the scientific literature. However clinical signs that have been attributed to the disease are: stiff, swollen joints, anterior uveitis (moonblindness), neurologic signs, low grade fever, sensitivity to touch, lameness, weight loss, tremors, neck pain, lethargy, laminitis and pseudolymphoma. Unfortunately, these signs may be caused by many other diseases that are commonly seen. Those include anaplasma, EPM, arthritis, typing up, PSSM and many more.

Lyme test results are based on an antibody response. If the horse is infected with the organism, it will develop an antibody response. However, 40-50% of all horses in the mid-atlantic region test positive (75% in New England, where Lyme Disease was first reported) even though they have no clinical signs nor will they develop clinical signs in the future. They have been exposed, develop an antibody response but do not get sick. So a positive test does not mean that the clinical signs the horse is experiencing are definitely caused by the Borrelia organism.

So how do we know if a horse is infected with Lyme Disease? For lack of a better method, veterinarians have adopted the rules for humans the CDC uses:

* the horse must be in an endemic area

* the horse must have compatible clinical signs

* all other potential diseases must be ruled out

* the horse must have a positive test

* if the horse dies, it must have compatible pathology

Results from experiments on treating Lyme Disease have not been definitive. An experiment performed at Cornell University College of Veterinary Medicine divided 16 ponies into 4 groups of four. Horses which were infected with Lyme Disease and treated for 28 days four different ways: Doxycycline (oral form of tetracycline), ceftiofur (antibiotic in Naxcel or Excede), IV Tetracycline or with no medication (control group). The only group that showed a negative antibody test after 28 days of treatment was the IV tetracycline. However another experiment in naturally infected horses did not show a similar effect with IV tetracycline. It is not clear why these horses did not have a negative test after treatment. It may be that they became reinfected or they remain chronically infected.

Considering the results from the first treatment experiment, one may wonder why not just treat all suspected cases with IV Tetracycline? Several reasons: it’s expensive, as it requires a daily visit from the veterinarian for 30 days, horses can collapse if it is giving too quickly, it’s very irritating if it gets outside the vein, it can cause GI upset and renal failure. As you can see, it’s not a treatment decision to be made lightly in a suspected case of Lyme Disease. If you or your veterinarian suspects Lyme’s Disease a discussion about the test and treatment options should follow. Today veterinarians use Monocycline (oral tetracycline) because it is given orally and rises to therapeutic levels in the blood stream of the horse. But there is no clinical evidence confirming if it actually works or how long it should be given.

As you can see, it’s frustrating for all of us that there is so much that is still unknown about Lyme Disease: its prevalence, definitive clinical signs, how to diagnose it and best treatment method. Until more research dollars are invested in further research, the only things we know for sure are the organism that causes it and that tetracycline has some effect on it.


Why Do Horses Tie Up?

There are many names for “it”: azoturia, set fast, paralytic myoglobinuria, and chronic exertional rhabdomyolysis (ER), but most of us know “it” as tying up. It is a common muscle problem in horses with multiple causes. The most common syndromes include Polysaccharide Storage Myopathy (PSSM), Recurrent Exercise Rhabdomyolysis (RER), vitamin/mineral deficiency, electrolyte imbalance, exhaustive exercise, especially on hot, humid days.  Some horses can have chronic episodes starting at a young age when exercised lightly. Depending on the cause of tying up, keeping your horse well hydrated and fit for exercise can help reduce tying up episodes.

PSSM is a common cause of chronic tying up in Quarter Horses, Paints, snow pictureAppaloosas, Warmbloods, and Draft Breeds, although other breeds may be affected. In PSSM, horses have a genetic defect in the muscle cells which results in excessive storage of sugar known as glycogen. Most horse owners have reported that horses with PSSM have exercise intolerance. A genetic test is available for some breeds of horses that require only a hair sample or blood test. However, other breeds may need to have a muscle biopsy performed to diagnose the disorder. The diet for these horses should consist of low starch/low sugar feeds with a high fat percent.. Any change in diet should be done gradually. These horses should also have daily exercise to prevent muscle stiffness and liberal access to turnout.

Recurrent Exercise Rhabdomyolysis (RER) is another cause for chronic tying up and occurs when there is an abnormality in the way muscle cells regulate intracellular calcium. This type of tying up is not due to the calcium in the horse’s diet, but is also believed to be a genetic defect in the muscle cell. RER episodes occur with exercise or excitement, so keeping the horse calm is one part of prevention. Getting the horse conditioned to the stimuli, providing controlled daily exercise, and the least amount of time in the stall possible are the best ways to prevent RER.  A proper diet for these horses can be difficult because most are in training; they need enough calories to maintain that level without making them high strung.  Fat is a good energy source without making them “hot”.  The most common breeds with RER are Arabians, Standardbreds, and Thoroughbreds.

Tying up episodes can range from mild to severe. A horse can present with a tucked-up abdomen, excessive sweating, muscle stiffness, reluctance to move, muscle twitching in the flank, a camped-out position, back muscle pain and/or a shortened stride. Sometimes this can happen very quickly or, in the cases with RER, some Standardbreds have been known to tie up 15 minutes after exercise. If your horse presents with these symptoms, call your veterinarian and do not move the horse but keep them standing.  Offer the horse water and prevent them from getting chilled with a blanket.  In cases of significant muscle damage, myoglobin (the protein which carries oxygen in the muscle cell) may be released, resulting in red or dark colored urine. Fluids, under the direction of your veterinarian, may be required to prevent kidney damage.

A horse who chronically ties up should be examined by a veterinarian to rule out specific diseases that could be the cause.  There is still much to learn about the causes of tying up, but research continues to offer new strategies to manage this difficult condition.

Equine Enteric Coronavirus – A Newly Emerging Disease Of Horses

Cornell University’s School of Veterinary Medicine has provided current information on Equine Enteric Coronavirus, a newly emerging disease most often found in the Northeast during colder seasons.  Good information to keep in mind as you monitor your horse’s health this winter.

As always, please feel free to call us with any questions or concerns about your horse’s health:  (434)973-7947 Earlysville Clinic or (540)460-5702 Valley Division.

Would You Recognize Botulism If You Saw It?

Most of us have probably heard that your horse should have a botulism vaccine if you feed round bales, but if you’re like me, you may have never actually seen or known a horse with the disease.  Just recently, we did have a patient with a confirmed diagnosis of botulism and we hope it will be helpful to detail the symptoms.  Of course, the best scenario is to avoid botulism in your horse, which is easily done by keeping your horse vaccinated against it.

We received a call from a client whose horse was just not himself:  lethargic, depressed, not cleaning up all of his feed, slight fever of 101 degrees, slightly loose stool with an occasional trip/mistep.  The horse had been ridden the day before and seemed fine.  The owner knew the horse well and was convinced something was just not right.  Bute had not seemed to make the horse more comfortable.  He was up to date on his Rabies vaccine only, lived with four other horses who remained well and had access to a round bale in his pasture.

When Dr. Hecking saw the horse, he displayed progressive depression and exhibited weakness, stumbling, and incoordination.  She gave the horse electrolyte paste, Banamine and pulled bloodwork, but was concerned enough about possible neurologic conditions, EHV or enchepalitis that she referred the horse to an equine hospital for evaluation.

Upon presentation at the hospital, temperature/pulse/respiration were normal.   A neurologic exam confirmed that he was very dull, had little to no sensation to the face, eyelid tone was reduced and he would not pull his tongue back into his mouth when it was pulled out.  Likewise, his tail tone was reduced and he had decreased resistance to tail pulls.  His bloodwork showed mild electrolyte abnormalities and increased fibrinogen consistent with inflammation and anorexia.

Based on his access to a round bale and his muscle weakness, a presumptive diagnosis of botulism was made.  Botulism is caused by a toxin produced by the bacteria Clostridium botulism, which can result in stimulated muscle contraction.  This can cause horses to develop a flaccid paralysis, usually starting in the facial muscles, moving to limb muscles and in severe cases, paralysis of the diaphragm.

In adult horses, intoxication usually comes from one or two sources:  contaminated hay or from a toxin produced within a wound infected with the bacteria.  Though this patient had a wound on his left hock, it was reported to be old and was not painful or hot on palpation, so the most likely source of infection was the round bale.

The patient received Botulism antitoxin plasma, IV fluids with electrolytes, DMSO and Banamine.  Within 24 hours, he appeared much brighter and his facial sensation was almost normal, with tongue, eyelid and tail tone significantly improved.  Cerebrospinal fluid was normal, making the botulism intoxication more likely than neurologic diseases like herpes, EEE, WEE or WNV.  A test for EPM was done, although his marked, quick response to treatment made EPM less likely.

Only three days after he was initially seen by Dr. Hecking, our patient was bright, alert, and responsive with a good appetite.  He is expected to make a full recovery, in large part due to his owner’s quick call to us the first day he noticed a departure from normal behavior.

To protect your horse against botulism, we do recommend the vaccine for all equines having access to round bales.  Initially, a series of three vaccines (three to four weeks apart) are necessary for full protection, with only an annual booster required thereafter.  Please call us (434)973-7947 or (540)460-5702 if we may be of help with your vaccine questions.

Overview Of PPID (Equine Cushing’s Disease)

Thank you to Boehringer-Ingelheim for this Update on PPID (Equine Cushing’s Disease) for Horse Owners

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Pituitary Pars Intermedia Dysfunction (PPID), also known as Equine Cushing’s Disease, is the most common endocrine disorder in horses. An estimated 21% of horses and ponies over the age of 15 are affected by this disease.1 PPID is a chronic progressive disease. PPID causes multiple health problems for the horse, including laminitis and recurrent infections.


The pituitary gland is located at the base of the brain. As PPID horses age, dopamine-secreting neurons that control the pituitary degenerate at a faster rate. The exact cause of this degeneration is poorly understood, but is thought to result from oxidative damage, similar to human Parkinsons disease.2 When degeneration of neurons occurs, the pituitary becomes abnormally active, increasing secretion of hormones which result in the individual clinical signs of PPID (below.)


Most horses affected with PPID are between 15-21 years of age, although horses as young as 4 have been reported. Ponies and Morgans are overrepresented.3  Horses with a history of Equine Metabolic Syndrome (EMS) are thought to be at higher risk.3 PPID-affected horses show a wide variety of nonspecific clinical signs which worsen over time.

Early signs of PPID may include:

  • Abnormal haircoat (delayed shedding compared to herdmates, dull, coarse, thicker haircoat, change in hair color, presence of longer, lighter hairs along the jaw, neck, elbow, lower legs)
  • A shift in metabolism from an “easy keeper” to more lean body condition
  • Regional fat deposits, i.e. “cresty neck”
  • Poor performance, attitude/behavioral changes, i.e. “dullness, lack of energy”
  • Subfertility
  • Unexplained laminitis episodes,“foot soreness”

Advanced signs of PPID may include:

  • Generalized long, curly, dull haircoat that fails to shed despite change in season
  • Muscle wasting with regional fat deposits,  “pot-bellied appearance”
  • Increased thirst and urination
  • Recurrent infections, i.e. white line disease, hoof abscesses, sinusitis
  • Inappropriate sweating (not sweating or excessive sweating)
  • Chronic laminitis
  • Neurologic deficits/blindness

Signs of PPID may be erroneously attributed to the normal ageing process. Subtle alterations in length, color, texture, or thickness of hair may indicate early PPID. Owners should also record when their ageing horse sheds its winter haircoat and compare this time with herdmates to detect delayed shedding. Chronic, recurrent infections (white line disease, hoof abscesses, sinus infections, skin infections, etc.) that fail to respond to treatment may occur in PPID due to immunosuppression. Another indicator of PPID may be subtle foot soreness. Foot soreness may be difficult to recognize unless the horse is examined on a hard surface. Subtle foot soreness without an inciting cause can indicate the presence of chronic laminitis, which can become debilitating. The author has also encountered several reports of confirming PPID in horses with a history of excessive sweating, or alternatively, failure to sweat. The presence of a generalized long, curly, dull haircoat that fails to shed is easily recognized and considered pathgnomonic for PPID. However, the presence of this “wooly mammoth” haircoat indicates advanced disease. Ideally, detection and treatment of PPID should begin long before this classic sign is observed. The diagnosis of PPID may be missed altogether due to focus on the primary problem (i.e. infection, failure to sweat) and lack of other overt signs.


High blood insulin with insulin resistance (IR) now collectively referred to as insulin dysregulation,3 is a common finding in approximately 30% of PPID horses.Insulin works within the body to mediate absorption of glucose into tissues such as liver, adipose, and skeletal muscle. The concern with insulin dysregulation is that certain tissues can be deprived of proper nutrients. Tissues within the hoof are extremely sensitive to changes in nutrient supply, and horses with a history of insulin dysregulation usually suffer from laminitis. Insulin dysregulation occurs in a minority of horses with PPID, but is a defining component of Equine Metabolic Syndrome (EMS.)


Diagnosis of PPID can be challenging. As of this writing (July 2013), a diagnosis of PPID emphasizes one screening Tier 1 blood test, resting ACTH, or another dynamic Tier 1 blood test, TRH Stimulation measuring ACTH. TRH measuring ACTH may be used when ACTH is inconclusive or to confirm a positive or negative result. Discuss these tests with your veterinarian to initiate the best diagnostic plan.

Detection of early PPID remains difficult, although this is the time period in which medical intervention is most satisfying. Unlike screening ACTH, TRH w/ACTH is showing potential for detection of earlier PPID.3 However, in horses with negative or “grey area” initial test results, tests should be repeated in 6 months or another Tier 1 test used.  If test results remain negative in light of clinical signs, a 6-month treatment “trial” with pergolide (Prascend®) can be considered. PPID tests are affected by season, and should always be interpreted alongside clinical signs. In advanced disease, the presence of the classic “wooly mammoth” haircoat remains the most sensitive indicator of an abnormally functioning pituitary, although with advanced disease, medical treatment is palliative.

It is also recommended that when screening for PPID, insulin status should also be evaluated for assessment of laminitis risk. Insulin, leptin, and triglcyerides may also be measured as part of a comprehensive “panel” in horses with a history of obesity. As the relationship between EMS and PPID is emerging, a complete diagnostic evaluation should include testing parameters for both disorders.


PPID is a chronic, lifelong condition for which there is no cure. Treatment of PPID focuses on administration of pergolide and attention to other significant issues, such as laminitis, dental disease, and maintenance of proper diet. Pergolide is the gold standard for treatment of PPID. Pergolide acts to restore dopamine to the pituitary, controlling its activity, and decreasing the production of detrimental hormones. Until 2011, only compounded pergolide was available. Compounded pergolide products have demonstrated rapid declines in potency and stability over time.4  In one recent study on compounded pergolide, a high degree of variation was found “between two containers of same product ordered from same pharmacy on the same date.”4 In 2011, FDA-approved pergolide became available as Prascend® Prascend(www.prascend.com.) Prascend® is now the treatment of choice for PPID.5 Owners should expect significant improvement in their horse’s clinical signs and test results when properly controlled with pergolide. However, similar to human Parkinsons, it should be expected that PPID will progress over time. Some horses with advanced PPID can be managed with higher doses of pergolide, with the addition of cyproheptadine.  Pergolide, however, has not been conclusively shown to improve insulin parameters, therefore management of laminitis due to insulin dysregulation should be primarily addressed with diet, exercise, +/- medical therapies. Two recent studies do demonstrate a beneficial effect of pergolide on certain insulin parameters and body weight, although this remains a subject of ongoing investigation.6,7 Although PPID is managed medically, body clipping, farrier care, regular deworming, and routine dentistry are of equal importance. Additional water should be provided if the horse drinks and urinates excessively.


The determination of whether an ageing horse is suffering from PPID is imperative to overall care. With awareness of subtle clinical signs, owners and veterinarians can work together as a team to identify this progressive disease. And by intervening earlier with treatment and control strategies, we can provide our horses with the optimal chance at maintaining healthier, more productive lives.


Marian G. Little, DVM, Field Equine Professional Services, Boehringer Ingelheim, Paris, KY

Inquiries via email: marian.little@boehringer-ingelheim.com


1McGowan TW, Pinchbeck GP, McGowan CM. Prevalence, risk factors and clinical signs predictive for equine pituitary pars intermedia dysfunction in aged horses. Equine Vet J 2013;45:74-79.

2McFarlane D, Cribb AE. Systemic and pituitary pars intermedia antioxidant capacity associated with pars intermedia oxidative stress and dysfunction in horses. Am J Vet Res 2005;66:2065-2072.

3Frank N. Pituitary Pars Intermedia Dysfunction. Current Therapy 2013. Pending publication.

4Stanley SD,Knych HD. DVM, Ph.D. Comparison of Pharmaceutical Equivalence for Compounded Preparations of Pergolide Mesylate. AAEP Proceedings 2012; 56: 274-276.

5PRASCEND® (pergolide mesylate) [Freedom of Information Summary]. St. Joseph, MO: Boehringer Ingelheim Vetmedica, Inc.; 2011.

6Durham A, Campbell J. Changes in Resting Serum Insulin Concentrations Following Pergolide Treatment in Horses with PPID. Equine Endocrinology SIG. ACVIM, Seattle, WA. 2013.

7McFarlane D, Banse HE. Response of Serum Insulin, Oral Sugar Testing and Body Weight in Horses Treated with Prascend.® Equine Endocrinology SIG. ACVIM, Seattle, WA, 2013.

Potomac Horse Fever Information


In response to the recent cases of Potomac Horse Fever in Virginia the veterinarians at Blue Ridge Equine Clinic would like to offer the following advice. If your horse has been vaccinated for PHF after May 1st of this year, boostering at this time is not likely to offer any significant benefit. If your horse was vaccinated prior to May first then there may be some benefit to revaccinating your horse, we recommend calling and consulting with your vet about your individual circumstances. The PHF vaccine offers varying degrees of protection depending on who you ask, but we feel that it seems to lessen the severity of the disease in many, but not all cases. At this time we are not aware of any confirmed cases in Albemarle county, but there are cases reported in the Culpepper and Richmond areas and suspect cases elsewhere in central Virginia.




Potomac Horse Fever – Can You Limit Your Horse’s Exposure?

Potomac Horse Fever or PHF is a rickettsial disease caused by the organism Neorickettsia risticii.  There have been about 10 confirmed cases of the disease between Richmond and the Shenandoah Valley this spring.

The disease is often (but not always) seen in horses that live near water because part of the life cycle of the fluke organism that is the vector or carrier for N. risticii goes through freshwater snails.  It is believed that larval stages of several varieties of flies ingest the fluke, and that the horse accidentally ingests the infected adult flies.

The main clinical signs include fever, lethargy, diarrhea, and laminitis.  There is a  test for PHF however it takes several days to get the results so your veterinarian may choose to initiate treatment before confirmation of the disease.

There is also a vaccine for the PHF, however it does not completely prevent the disease from occurring in the horse, but it may lessen the severity of clinical signs.  Vaccination may occur in both the spring and fall.

Treatment for PHF includes IV oxytetracycline (an antibiotic), in addition to supportive care for other symptoms.  Recommendations to help prevent the disease include maintaining barriers along bodies of water to keep aquatic insects near their place of origin, turning off outside lights around the barn to prevent insects from being attracted to the area, and cleaning water buckets and feed areas often in addition to keeping feed covered to reduce the chances of the horse ingesting infected insects.