During the 2024 National Alliance of Equine Practitioners (NAEP) Saratoga Equine Practitioners Convention, John Madigan, DVM, MS, Dipl. ACVIM, ACAW, Professor Emeritus, Department of Medicine and Epidemiology at the University of California’s Davis School of Veterinary Medicine, recalled and regaled how veterinarians unearthed the cause and treatment of anaplasmosis in horses.
“Between 1958 and 1969, only six horses were identified at UC Davis as having inclusions in their neutrophils and eosinophils together with a combination of anemia, thrombocytopenia, leukopenia, fever, ataxia, limb edema, and depression,” relayed Madigan. “If you didn’t do anything but provide supportive care, the horse recovered.”
Those inclusion bodies resembled rickettsial (ehrlichial) disease in sheep seen in the United Kingdom in the 1950s, and so the disease was called the “California ehrlichia agent.”
Madigan said he learned about the disease from brief comments—about six seconds worth—during a lecture in school.
Early in his career, Madigan recalled seeing a 3-year-old horse that was icteric, depressed, and had limb edema, fever, and … ticks.
“I ran a CBC, buying myself time because I didn’t have a clue what was going on,” he explained. “There was a low white count and no thrombocytes. Under oil I saw inclusion bodies and therefore boldly declared this a clear-cut case of Ehrlichia equi.”
He treated the condition with oxytetracycline designed for turkeys, and the horse recovered.
He then started seeing more of these cases and found them rewarding because the horses turned around in about 12 to 24 hours.
An Explosion of Disease
At the American Association of Equine Practitioners conference in 1981, Madigan presented 41 cases of ehrlichiosis.
Veterinarians in other states started reporting cases, and in 1994 the first known human case of inclusion bodies occurred in a woman who had been hiking and got a tick. Unlike the horses, she went downhill in the hospital rather than recovering quickly.
Madigan was hired at UC Davis in 1983. He collaborated with colleagues from Johns Hopkins University School of Medicine to gain more information about the life cycle of what they still called ehrlichiosis.
One roadblock was that Madigan and his team could not get the agent in the inclusions to grow in the lab until the late 1990s. In 1996 they reported the first cultivation of the agent in a cell culture in the New England Journal of Medicine, which then allowed further study and characterization of the agent.
The number of human cases reported skyrocketed from 2000 to 2019; those patients were diagnosed with human granulocytic ehrlichia.
“Many more veterinarians started diagnosing the disease in New England, then other states where the vector is found,” said Madigan.
A Tricky Presentation
Madigan then quizzed conference attendees on the signs they thought were associated with anaplasmosis in horses (including fever, depression, mild limb swelling, and lack of coordination), and all but laminitis were correct.
“Some horses are pretty ataxic when the primary presentation is vasculitis, and we can get some really funny signs depending on where the vasculitis occurs,” he said.
Madigan conducted research studies evaluating dexamethasone’s effects on the infection and found it minimized clinical signs.
“This really diminished their clinical signs, even without tetracycline,” he explained. “And the persistent low-grade fever lasts one week longer without treating with dexamethasone. It’s pretty safe in this infection.”
He encouraged practitioners to consider administering a dose of dexamethasone in cases of severe ataxia or other cardiac signs, such as arrythmias, from the vasculitis that can affect any organ.
“There are published reports of horses that develop upper respiratory signs and have even had to perform tracheostomies on them. There are some tricky presentations with anaplasmosis,” he added.
Treating (and Retreating?) Anaplasmosis in Horses
The standard intravenous dose of tetracycline is 6.6 mg/kg once daily for seven days.
“Be sure to give it slowly or you’ll knock the horse down,” said Madigan. “But they’ll warn you with a deep breath first at least.”
Doxycycline and minocycline are oral treatment options, neither of which tend to cause diarrhea like other antibiotics often do.
“Doxycycline also has immunomodulatory properties, suppressing proinflammatory cytokines,” said Madigan.
Unfortunately, relapses can occur with shorter therapy periods when disease is detected in its early stages. Madigan assured his audience that this is not antibiotic resistance. Further, he reminded attendees it’s often self-limiting in 10 to 14 days.
“There are no chronic infections,” he said.
Additional Comments
While thrombocytopenia associated with anaplasmosis can be severe, horses do not bleed.
“It is possible that the thrombocytes are being sequestered somewhere, possibly lining the blood vessels,” he said.
Madigan noted that affected horses likely have a seven- to eight-year span of immunity. However, attendees reported seeing disease relapse in only a few months. Madigan suggested they treated too early in the course of disease (i.e., they have very attentive owners who notice the disease earlier) or they did not treat long enough.
“When horses are treated early, they may not have had enough immune system exposure to develop full immunity,” he relayed.
For diagnosing anaplasmosis in the field, some attendees reported using a canine anaplasmosis SNAP test. If the horse is “off” and the test is positive, they’ll treat. Some attendees also reported using the test in asymptomatic horses and treating if the test was positive. In Madigan’s opinion, this is not a reliable test for the disease.
Finally, he reminded practitioners, “Most horses survive without any treatment at all.”
Related Reading
- The Future of Equine Infectious Diseases in a Changing Climate
- Causes of Immune-Mediated Myopathies in Horses
- Disease Du Jour: Lyme Disease in Horses
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