06/18/2025
The Expanding Threat of Tick-Borne Diseases in Horses
Lyme is the tick-borne disease most people think of first, but it isn’t the only one. Anaplasmosis is another disease of clinical significance in horses, and other diseases might be on the rise (Have you heard of ehrlichiosis in horses?). In this story we’ll look at what ticks are troublesome to horses, when and where they can be found, then discuss the clinical signs, diagnosis, and treatment of anaplasmosis, the “other” tick-borne disease that always seems to take a back seat to Lyme.
Which Ticks are Found on Horses?
In the U.S., many hard ticks infest horses, including the American dog tick (Dermacentor variabilis), the winter tick (Dermacentor albipictus), the lone star tick (Amblyomma Americanum), the Gulf Coast tick (A. maculatum), and the black-legged tick (Ixodes scapularis).
“Of particular interest to horse owners and veterinarians is the ability of these ticks to transmit disease agents,” says Kathryn Duncan, DVM, PhD, Dipl. ACVM, adjunct assistant professor at Oklahoma State University, in Stillwater, who has studied the subject with colleague Kellee Sundstrom, MS, senior research specialist. “Occasionally, a foreign animal disease, such as equine piroplasmosis, is diagnosed in the U.S. Ticks can be implicated as the cause of transmission, but the tick-borne disease agents most often transmitted to horses are through a bite from Ixodes species.”
Where Ticks are Found
Geographically Traditionally, I. scapularis are found in the eastern parts of the United States, whereas I. pacificusare found along the West Coast, including California, Washington, and Oregon, with a few reports coming from Nevada and Utah. With continued biogeographical changes, however, the “natural” ranges of ticks are evolving, experts say, and many ticks are found well outside what has been considered their traditional geographical areas.
Sonenshine et al. (2018) reported that the main reason for expansion of ticks is climate change, notably the rapidly rising global temperatures. That said, host availability and specificity, habitat suitability, relative humidity tolerance, the extent and duration of freezing temperatures, and human impact (habitat modification) all contribute. As an example of expanding tick ranges, Sonenshine et al. noted that I. scapularis densities have also intensified in many areas, and they appear to be advancing into parts of southern Canada at a rate of about 46 km (28.5 miles)/year.
Anatomically (on the Horse) In a study on horses in northeastern Oklahoma, Sundstrom et al. (2021) looked at the types of ticks infesting horses and the preferred site of attachment. Client-owned horses residing on eight farms were examined twice monthly for one year. Researchers collected ticks from those horses and recorded the anatomic location of attachment. They included 88 horses in the study, and 84.1% of horses were infested with ticks at least once during the study period.
The median number of ticks per horse was three. Researchers identified five different tick species, the most common being A. Americanum—the lone star tick (78.2%)—followed by I. scapularis, which was much less prevalent (18.2%). They found the lone star tick most often attached in the inguinal region (groin), and they found I. scapularis primarily on the chest and armpits (axilla) of the horses.
What Times of Year In that same study Sundstrom et al. also noted ticks were discovered on horses year-round in northeastern Oklahoma with the largest number seen in May. I. scapularispredominated October through February and was still common in March.
“The belief that ticks are only out in the summer is a myth that many tick researchers and veterinarians have been trying to bust for a while now,” explains Duncan. “Tick numbers may decline in the winter, but the extent to which this happens is dependent on the geographic region.”
In fact, she says that Ixodes spp and D. albipictus prefer cooler months.
“Ticks will ‘overwinter’ in leaf litter, under snow cover, and in reservoir host habitats but reemerge when there are ample hosts available and slightly warmer days,” Duncan explains. “In order to reemerge in the spring, some stages are required to survive the winters. With milder winters and premature emergence of warm weather, we can expect ticks to be noted earlier than we’ve seen in the past. This is likely a trend that will remain based on continued tick habitat changes.”
Similar to Lyme disease, A. phagocytophilum is transmitted to horses via the bite of an infected black-legged tick—I. scapularis, I. pacificus, and other Ixodes spp. After the bite, the tick injects the bacterium into the horse’s skin. Migrating granulocytes respond to the local inflammation and engulf the bacteria, attempting to kill them.
“But A. phagocytophilum is now exactly where it wants to be, inside the cell, where it is able to avoid natural immune defenses,” says Duncan. “Disease symptoms are caused by natural immune chemicals like interferon trying to fight off the infected cells. It takes about one week before the horse begins showing signs of disease.”
Clinical Signs of Anaplasmosis
Classic clinical signs of anaplasmosis can be as mild as the horse having a fever, depression, subtle limb edema (fluid swelling), and ataxia (incoordination).
“Many horses with anaplasmosis probably don’t show any clinical signs at all,” says Janet Foley, DVM, PhD, professor in the Department of Medicine and Epidemiology at the University of California, Davis (UC Davis), School of Veterinary Medicine.
More severe signs, which typically develop in older, geriatric horses, include loss of appetite, reluctance to move, icterus (jaundice), and petechiae, which are small pinpoint bruises on the skin.
When present, the fever is quite high, usually 103-104 F, but can reach 107-108 F and lasts for about five days.
That said, tick-borne diseases are true masters of disguise, and researchers have described various other clinical presentations. John Madigan, DVM, MS, Dipl. ACVIM, a professor in the Department of Medicine and Epidemiology at the UC Davis School of Veterinary Medicine, recently published an article describing one horse with dysphagia (difficulty swallowing) and another with respiratory distress attributable to anaplasmosis (2021).
Veterinary Diagnosis
Horses with potential exposure to ticks—essentially, those in every state in the U.S.—presenting with the above-described clinical signs, particularly a high fever, should be tested for anaplasmosis.
“A horse presenting with a high-ish fever that resides in a place that overlaps
geographically with I. pacificus would definitely make you want to test for anaplasmosis,” says Foley.
Testing includes the following steps:
* A complete blood cell count. Typical findings include either a decrease in white blood cell counts or a decrease in all three cell lines: white blood cells, red blood cells, and platelets. This latter presentation is called pancytopenia.
* Microscopic analysis of the blood cells. The white blood cells might show “cytoplasmic inclusion bodies,” which are the rickettsial organisms living inside the granulocytes.
* A polymerase chain reaction (PCR) test, a molecular method for detecting DNA of the bacterium in a horse’s blood sample.
* The IDEXX SNAP 4Dx that detects antibodies to anaplasmosis. It provides a “positive” or “negative” indication for antibodies, but it does not confirm an active infection.
* An immunofluorescence antibody test (IFAT) that identifies antibodies against A. phagocytophilum. Veterinarians must interpret this result with caution, however, because many horses living in areas where hard ticks are endemic might have those antibodies.
* Acute and convalescent titers. This is an IFAT performed when the horse is first sick to measure the antibody levels, then again two to four weeks later. The antibody levels increase about fourfold in horses that have been actively infected.
Anaplasmosis must be differentiated from other diseases. Because horses can exhibit ataxia, the top differentials include viral encephalitides such as equine herpesvirus myeloencephalopathy (EHM), West Nile virus, and the Eastern and Western encephalitis viruses. Other conditions to consider are liver disease (because of the jaundice); purpura hemorrhagica, which is a secondary complication to strangles(Streptococcus equi bacterial) infection; viral arteritis; and even the reportable and highly fatal disease equine infectious anemia (EIA).
Treating Anaplasmosis
Like Lyme disease, anaplasmosis is easily treatable. Veterinarians’ antibiotic of choice is oxytetracycline.
“There are different treatment protocols out there, but a standard full course of antibiotics is appropriate, 10 days,” says Foley.
For more severely affected horses, practitioners can administer corticosteroids in addition to intravenous fluids and nonsteroidal anti-inflammatories.
Unlike many other diseases that fail to spare the feet, few horses with anaplasmosis ever develop laminitis.
How Commonly Does Anaplasmosis Strike?
“In Northern California, anaplasmosis is actually pretty common,” says Foley.
Duncan agrees, adding, “These infections are probably more common than we realize. But considering most horses are never tested for tick-borne diseases, we can’t know for sure.”
Nonetheless, tick-borne diseases are frequently considered as causes for disease (i.e., fever, lethargy, lameness, ataxia) in horses where ticks are endemic. But a Pennsylvania survey shows that infection with A. phagocytophilum and B. burgdorferi is not all that common even though ticks are commonly found in the state.
In that survey Thompson et al. analyzed blood samples collected from 271 horses whose veterinarians suspected tick-borne disease. The researchers used PCR to detect DNA from both bacteria and used the IDEXX SNAP 4Dx and an IFAT to detect antibodies.
Key findings were:
* Researchers identified DNA for A. phagocytophilium in 7% of the horses. They did not find B. burgdorferi DNA in any horse;
* They detected antibodies to A. phagocytophilium and B. burgdorferi in 28.4% and 67.9% of horses, respectively;
* With IFAT they discovered antibody titers of 1:50 or more in 41.3% and 60.5% of horses for A. phagocytophilium and B. burgdorferi, respectively.
The high prevalence of antibodies to both bacteria in this survey shows that horses in Pennsylvania, where ticks are common, could have prolonged, repeated exposure to these pathogens
“Again, though, simply identifying antibodies does not confirm that either of these bacteria are causing the disease. The presence of antibodies simply proves exposure to the bacteria,” says Foley.
The lack of B. burgdorferi DNA found in that study wasn’t surprising because the bacteria do not persist in the bloodstream for very long.
“For Lyme disease the bacteria can be sequestered in the joint, as well as any connective tissue, including kidney capsule, etc. For anaplasmosis, though, we do expect circulating blood to be PCR-positive during active infection,” says Foley.
Identifying A. phagocytophilium DNA together with the lack of circulating antibodies in the bloodstream (the body hasn’t had time to produce them yet) and the presence of high fever or other clinical signs consistent with disease supports a diagnosis of anaplasmosis.
Emerging Tick-Borne Diseases
While Lyme, anaplasmosis, and piroplasmosis are the only three tick-borne diseases discussed in the U.S. for equids, surveys from other regions suggest Rickettsia spp and Ehrlichia spp might be clinically relevant in horses.
Therefore, Duncan et al. (2022) wanted to better understand the transmission risk of these tick-borne rickettsial disease agents. In their study they tested ticks from horses residing in Oklahoma using PCR.
Additionally, they analyzed blood samples from horses both infested and not infested with ticks for antibodies to these bacteria.
They reported that illnesses caused by Rickettsia spp and Ehrlichia spp could be emerging tick-borne diseases in horses.
“We found that one-quarter of the ticks recovered from tick-infested horses contained DNA of Rickettsia spp, and R. amblyommatis was the most common species identified. This is part of the spotted fever group of bacteria, is transmitted by the lone star tick, and is currently considered nonpathogenic,” relays Duncan. “Ehrlichia spp were found far less commonly in ticks recovered from horses.”
In contrast, researchers did not find circulating antibodies for R. rickettsii in any of the study horses, but almost 30% of horses had antibodies for Ehrlichiaspp.
“These results suggest there is a need to further explore the role of Rickettsia spp and Ehrlichia spp in equine health,” states Duncan.
The fact other diseases might also be transmitted by ticks isn’t surprising, considering ticks are known to transmit several different disease agents to humans in addition to Borrelia and Anaplasma. Examples include the agents of Rocky Mountain spotted fever, ehrlichiosis, tularemia, babesiosis, Colorado tick fever, and Powassan virus disease, among others.
Take-Home Message
Some of the main takeaways from the current research are:
1. Many tick types can infest horses, but few are currently known to actually transmit disease to horses;
2. Tick ranges are expanding, so ticks are found in geographical ranges and at times of year they haven’t previously been found;
3. Anaplasmosis can occur commonly in certain geographic areas and should be considered as important a tick-borne disease as Lyme disease.
“If residing in a region with a heavy tick burden, be aware of practices to prevent tick-borne diseases including using regular tick control, screening annually with an antibody test, and performing thorough tick checks with proper removal,” advises Duncan.
The horses.com
