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October 6, 2011

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Lyme Disease in horses – Symptoms and Diagnosis

by DePaolo Equine Concepts

Lyme Disease was first identified in the 1970s in Lyme, Connecticut. The Northeastern and North Central United States as well as some Southern states are the most common locales where Lyme disease is present.  Lyme disease is carried by deer ticks and Western black-legged ticks which are most prevalent on the eastern seaboard.  It is estimated that roughly 50% of the black-legged tick population carries the bacteria, Borrelia burgdorferi which causes Lyme disease. Ticks must attach and feed for a minimum of 24 hours to transmit Borrelia burgdorferi. The ticks pick up the bacteria during their larval and nymph stages while feeding on infected mice.  Areas with cold-winter weather see adult ticks emerge in the fall and spring after a winter dormancy in brush and leaf litter.  Adult horses are most likely to be affected as hosts for the ticks.[1]

Symptoms

  • Low energy
  • Stiffness and muscle pain
  • Soreness in the large working joints
  • Edema or eye inflammation
  • Irritability and sensitivity to touch
  • Behavioral or attitude changes
  • Fever
  • Laminitis
  • Gait abnormalities

Diagnosis of Lyme disease can be difficult largely because it can mimic so many other ailments. To complicate matters, many horses in the early stages of infection do not exhibit enough antibodies to make testing very accurate.  Horses in active use are more likely to be correctly diagnosed due to the affect the infection will have on their performance. Less than 10% of the horse population that is infected with Borrelia burgdorferi will display symptoms[2] while 75% of the equine population living in tick-infested areas will test positive for antibodies to Borrelia burgorferi[3].  Persistent symptoms such as chronic weight loss, stiff swollen joints and neurologic problems that do not respond to other treatment protocols are likely the body’s reaction to immune inflammation caused by the organism.

Diagnostic Tests

ELISA (enzyme-linked immunosorbent assay)test:  This test uses the horse’s blood mixed with antigen bound to an enzyme. Antibodies that are being tested for will bind to the test.  Levels above 200-300 units indicate infection.  Lower levels do not mean that there is no infection but instead suggest that the horse has been exposed in the past. 

Drawback: This test is not accurate in early-stage infection and can obtain false-positives due to related enzymes in the blood.  ELISA tests are typically confirmed using a Western Blot test.

Western Blot test:  This test is more sensitive than the ELISA method because it seeks to locate antibodies specific to certain proteins of the bacteria.  The antigen/antibody reactions are separated into bands when the test is read.  Three or more bands indicate infection.

Snap 3DX test: New to the ELISA test is the C6 antigen test.  This test is derived from a human test.  Researchers identified the C6 antigen in all Lyme organisms and noticed that it is consistently present at all life stages of the parasite.  This test suffers far less false-positives in the early stages of infection and nearly none in later stages.  A further advantage to this test is that it can be performed either on site or in the lab.

There are a few less common testing methods offered by IgenX Laboratory in California. The Lyme Dot-Blot assay uses urine to test for comparative antigens and antibodies but is prone to false positives due to cross-reactions.  The Reverse Western Blot test tests for antigens and separates more specifically for Lyme antibodies.  The Multiplex PCR test is a very sensitive test run on tissues, blood, urine, spinal fluid or joint fluid.  It looks for DNA level evidence of organisms or pieces of them.[4]

Often a combination of blood tests, physical exam and the likelihood of exposure must be combined when considering a diagnosis of Lyme disease.  Next week we will discuss Lyme Disease in horses – Treatment and Prevention.

[1] Elaine Pascoe.Ticked Off. Practical Horseman, sept.2010

[2] Practical Horsekeeping.com. Equine Lyme: Diagnosis, Treatment and Prevention. 1/25/2011

[3] MyHorse.com. Lyme Disease in Horses requires Health Treatment.1/25/2011

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