Infectious diseases (Proceedings)


Equine infectious diseases continue to emerge and re-emerge, infecting horses across the US and beyond. For the purpose of this discussion, we will discuss equine piroplasmosis (EP), methicillin-resistant Staphylococcus aureus, and Lyme Disease in the horse.

Equine infectious diseases continue to emerge and re-emerge, infecting horses across the US and beyond. For the purpose of this discussion, we will discuss equine piroplasmosis (EP), methicillin-resistant Staphylococcus aureus, and Lyme Disease in the horse. If time permits, Leptospirosis will also be covered. The clinical syndromes, diagnostic techniques, and therapy will be included.

Equine Piroplasmosis

As of September 2010, almost 500 horses in the United States had tested positive for equine piroplasmosis (EP). Initial estimates from the USDA indicate approximately 100,000 U.S. horses have been tested for EP from November 2009-present. And excluding the several hundred cases from the Texas ranch outbreak, an additional 163 horses have tested positive throughout the US. Although this disease can cause severe hemolytic anemia and vasculitis necessitating euthanasia, it can also be silent and cause no apparent signs at all. Piroplasmosis is a reportable disease, and its emergence is affecting equine transportation across state lines and to other countries.

Piroplasmosis is caused by the protozoan parasites Babesia caballi and Theileria equi (formerly called Babesia equi). It also can affect donkeys, mules, and zebras; but clinical disease in those equids is rare. The disease is transmitted by ticks and other biting insects; however, shared needles and/or blood contamination has been implicated in several disease outbreaks. Once horses are infected with T. equi, carrier status may be lifelong. Carrier horses are also capable of transmitting the disease to ticks—vectors that can transmit it to other horses. The disease is considered endemic in Africa, Central and South America, Asia, the Middle East, the Caribbean, and the Mediterranean. The U.S. has not been considered an endemic region. When infection occurs, T. equi tends to be the most common agent, rather than B. caballi. However, infection with both parasites can occur simultaneously.

Once horses become infected with the parasite, it usually takes between 5 and 30 days for any signs of the disease to appear. As previously stated, infected horses may not have any signs of EP at all. Generally, affected horses display nonspecific signs that can look similar to other diseases. Fever, depression, anorexia, pale or icteric mucous membranes, and edema of the limbs or along the ventral abdomen have been commonly reported. Reddish-brown or discolored urine may also be observed. Laboratory abnormalities typically include anemia and thrombocytopenia.

Several laboratory tests are available for diagnosis of EP. Occasionally, the parasite can be seen on microscopic examination of a blood smear. The U.S. Department of Agriculture (USDA) standard test is the cELISA (competitive enzyme-linked immunosorbent assay). Specific laboratories (the National Veterinary Services Laboratories, Texas Veterinary Diagnostic Services Laboratories, Florida's State Diagnostic Laboratory) have been identified to run the tests and report the results. The Bronson Animal Disease Diagnostic Laboratory (BADDL, formerly Kissimmee Animal Disease Diagnostic Laboratory) in Florida was approved by the USDA for equine piroplasmosis testing. BADDL can test blood samples for interstate and intrastate purposes, but the National Veterinary Services Laboratories is still testing all international transport samples.

Horses that test positive for equine piroplasmosis MUST be quarantined. Local veterinarians can work with state and federal veterinarians to ensure that manageable quarantine guidelines are being followed and are in place. Although there are several drugs (imidocarb, etc) that have been identified for treatment of piroplasmosis, the organisms can be refractory to treatment, and the carrier state is difficult to clear. Euthanasia for positive horses is not necessarily required, nor is it being recommended in every case by the USDA, especially since so many positive horses are asymptomatic. State and USDA veterinarians are working in conjunction with local veterinarians and owners to determine the best recommendations for each positive horse. Some owners elect to transport positive horses out of the country—to countries that have endemic piroplasmosis—but that is not a palatable option for most. In addition to quarantine, there is a treatment research program available for positive horses. This program is in conjunction with Washington State University and Dr. Don Knowles. Owners and their local veterinarians work with the USDA and Dr. Knowles to determine if they have a horse that is eligible for enrollment.

The outbreak of equine piroplasmosis in Florida in 2008 identified 20 positive horses. Twenty-five quarantines were placed in Manatee, Polk, DeSoto, Lake, and Dade counties; seven premises had positive horses. The last premise in Florida was released from quarantine in February of 2009. Since then, additional positive cases were reported in September of 2010. An active investigation is still underway. Fortunately, tick surveillance in Florida thus far has not revealed evidence of natural (tick) transmission. Blood contamination from shared needles was implicated in the outbreak. Unlike Florida, tick transmission was identified in the Texas outbreak. The USDA and state veterinarians are involved in an ongoing investigation in that state.

Fortunately, it does not appear that tick transmission has been significantly involved in EP transmission outside of the affected premises in Texas. However, people can spread this disease from horse to horse, and we can prevent that mode of transmission. All dental, surgical, and tattoo equipment must be thoroughly disinfected between horses. Horses have contracted the disease though the use of shared needles and/or syringes, as well as from blood transfusions. A new sterile needle and syringe should be used for each injection, whether into a muscle or a vein. Additionally, a previously used needle should never be inserted into a drug or vaccine multidose vial—and owners/trainers should be reminded of these infection control measures. Work with your veterinarian to ensure that all equipment is thoroughly cleaned and disinfected between horses. EP is still a very uncommon disease in the U.S., but it is critical to be vigilant and follow preventative measures.


The prevalence of Methicillin-resistant Staphylococcus aureus (MRSA) in routine infections in people and hospital outbreaks has initiated world-wide concern. Staphylococcus aureus is a common bacterium that colonizes the skin and has been found to cause disease in many species. Staphylococcus aureus began developing resistance to antibiotics almost as soon as they were introduced, beginning with penicillin, and progressing to methicillin resistance. MRSA is typically resistant to all beta-lactam antibiotics (penicillin and cephalosporin families) and often many other antibiotics as well. This antimicrobial resistance can make MRSA infections a challenge to treat. The percentage of community-associated infection (milder, outpatient type illness like skin infections/abscesses) from MRSA had risen from 35.1% in 2003 to 50.0% in 2006 in Florida.

Methicillin-resistant Staph aureus infections in the horse have manifested as wound and surgical site infections, cellulitis (soft tissue infections, typically of the limb), catheter-site infections, pneumonia, septic arthritis, and skin infections, among others. Historically, equine MRSA infections were uncommonly reported, and began to increase in prevalence in the late 1990s. More recent studies have demonstrated that MRSA is an important emerging pathogen in horses and can be zoonotic. It is also possible for humans to transmit the bacteria to horses as well. Approximately 25% of healthy children and adults can carry the Staphylococcus aureus bacteria in their nose or on the skin. For most people and horses, carrying the bacteria in the nose or on the skin causes no ill-effects. Certain circumstances such as a wound or an illness requiring hospitalization can result in active infection.

Studies done in horses have found that approximately 0-5% of horses carry the MRSA bacteria in their nasal passages, which is the most common site for colonization (Weese et al, 2005). Horses can also have the bacteria on their skin or in their intestinal tract. Although generally very few horses carry the bacteria, some farms with a history of MRSA infections in horses have demonstrated carrier rates of 0-45% (Weese et al, 2005). Horses that carry the bacteria in their nasal passages may not ever develop a clinical MRSA infection. However, these horses may transmit MRSA to other horses or people, and will sometimes develop active infections under certain conditions. People who work with horses seem to have a higher carrier rate of MRSA. Studies of equine veterinarians have reported colonization rates ranging from 10-14%, with predominantly the equine strain of the bacterium. This provides further evidence that carrier horses can transmit MRSA to humans.

Clinical infection with MRSA is certainly concerning. Fortunately, a multicenter study reported that 83% of horses with MRSA infections survived (Anderson et al, 2009). Many horses with clinical infections such as pneumonia or wound infections required prolonged hospital stays and needed additional surgeries. However, acceptable antibiotic options exist in most cases, especially when infection is identified early. In this author's experience, catheter site infections with MRSA, while uncommon, result in the highest mortality rates if the infection spreads through the bloodstream and into the lungs or other sites. Early detection and treatment is certainly critical for the best outcome for the horse. Currently, there is no evidence that horses that carry MRSA need to be treated with antibiotics. Farms with documented MRSA prevalence have successfully eradicated MRSA with good hygiene and infection control practices.

Lyme Disease

Lyme disease is a tick-borne infectious disease that occurs in parts of North America, Europe, and Asia caused by the spirochete Borrelia burgdorferi. In humans, Lyme disease has been associated with a variety of neurologic, rheumatologic, and psychiatric manifestations. Infection in most horses appears to be subclinical. Clinical signs reported in the horse include low grade fever, stiffness and lameness in multiple limbs, muscle tenderness, hyperaesthesia, swollen joints, lethargy, and behavioral changes. Neurologic dysfunction and panuveitis have been reported in one pony and one horse. However, cause and effect of clinical signs and infection with Borrelia in horses has been difficult to document (Divers 2007).

Diagnosis of Lyme disease is based on the horse being housed in an endemic area, compatible clinical signs, ruling out other causes of clinical signs, and a high titer using the kinetic enzyme-linked immunosorbent assay (KELA) (generally > 300 KELA units). Other serologic tests or positive western blot (WB) results for anti-B. burgdorferi antibodies have also been utilized for diagnosis. There is a 99% probability that the WB will be positive if the ELISA titer is >300 (Divers 2007). Time from infection to seroconversion is reported to be 3-10 weeks. A limitation of serologic testing is that it does not distinguish between active infection and previous exposure. Therefore, PCR testing of B. burgdorferi DNA in a synovial membrane of a painful joint may be utilized and is strongly indicative of infection.

Common disorders that may have similar signs should always be ruled out to establish a diagnosis. These include osteoarthritis of the hock, OCD, polysaccharide storage myopathy, other chronic myopathies, polysynovitis, thoracic spinous process osteoarthritis, and EPM. Thorough lameness and neurologic exams should be performed when examining these cases. Additional diagnostic tests may include radiography, scintigraphy, muscle biopsy, CSF collection, and laboratory muscle enzyme evaluation.

The most common antimicrobials for treating Lyme disease in the horse are oxytetracycline (6.6-10 mg/kg, IV every 24h) and doxycycline (10 mg/kg PO every 12h). Horses with the more typical clinical signs of chronic stiffness, lameness, and hyperaesthesia are typically treated with a one month course of oral doxycycline. Owners must be vigilant in watching for evidence of soft stool or development of diarrhea. Improvement in some cases may be related to nonspecific anti-inflammatory response due to doxycycline inhibiting metalloproteinase activity. Oxytetracyline should not be utilized if dehydration is present or in horses with pre-existing renal dysfunction. Acute renal failure may occur after prolonged use in dehydrated horses. In addition to antimicrobials, other supportive therapies may include NSAIDS, chondroprotective agents, and acupuncture.

Prevention of Lyme disease in endemic areas would require preventing tick exposure or prolonged attachment. There is no commercially approved equine vaccine at present. Efficacy of administration of the commercial canine vaccination is currently unknown.

Further References:

Anderson et al. Retrospective multicenter study of MRSA infections in 115 horses. Equine Vet J; 2009, Nov; 41(4): 401-405.

Weese et al. Community-associated MRSA in horses and humans who work with horses. JAVMA, 2005; Feb 15; 226(4): 580-583.

Divers TJ, Mair TS, and Chang YF. Lyme Disease in Horses. Infectious Disease of the Horse, Ed TS Mair and RE Hutchison, 2009, EVJ Ltd: p 286-292.

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