The trouble with ticks

Tick-borne diseases are among the most difficult to diagnose and challenging to treat. Eliminating ticks from your horse’s environment is practically impossible–but you can take steps to safeguard his health.

Finding something positive to say about ticks isn’t easy. These tiny arachnids may serve as a food source for some birds, reptiles and other small creatures. They can also be said to be a highly successful life form that has survived a long time: Two species of hard-bodied ticks (Ixodidae) have been found preserved in amber dating to the Cretaceous Period, about 100 million years ago—likely, they were feeding on dinosaurs long before large mammals even existed. Currently, about 900 species of ticks live worldwide, and about 90 in the United States.

But, scientists tend to agree, the primary role ticks play in the eco-system is to act as host to a range of disease-causing organisms, many of which rely on the ticks to pass them around. That’s right—these blood-sucking parasites that once dined on Tyrannosaurus rex exist mainly to spread diseases to large animals. Biologists suggest that this helps to cull weaker individuals and leave more resources behind for the stronger ones.

In the modern world, of course, our goal is to protect our animals, and ourselves, against the many diseases ticks can carry. And this challenge can be especially difficult with our horses, who live in the grassy pastures and shrubby areas where ticks thrive. Eliminating ticks from your horse’s environment is practically impossible—but you can take steps to safeguard his health.

Lyme disease, equine piroplasmosis and anaplasmosis are three of the better known equine diseases spread largely by ticks, but there are others as well. Here’s what you need to know.

Equine piroplasmosis (babesiosis): Cause for constant vigilance

Equine piroplasmosis (EP), also called babesiosis, is found around the world. It is currently not endemic to the United States, Canada, Australia, Japan, England, Ireland or Iceland, and these countries have instituted measures to keep the disease out. Horses imported into the United States from endemic areas must undergo quarantine and testing to ensure that they are not carrying antibodies to either of the two protozoa that cause EP: Theileria equi and Babesia caballi. Nevertheless, several outbreaks have occurred in recent years.

Today, if a horse in the United States were diagnosed with EP, there would be two priorities—clearing the organism from his system through treatment and preventing the disease from getting established in this country.

• The ticks: The primary vectors of T. equi are Amblyomma mixtum and Rhipicephalus microplus, Central and South American tick species with a range extending into southern Texas. Ticks pick up the parasites by consuming the blood of infected horses, then pass them along when they take their next meal. Once established in an equine population, the protozoa can also be transmitted by more than a dozen other tick species, including Dermacentor variabilis, the American dog tick, which is common east of the Rocky Mountains and in California. The protozoa can also spread by blood-to-blood contact, which can occur through reuse of hypodermic needles and other improper hygiene. Dermacentor nitens is the vector of B. caballi, and this parasite may also infect a tick’s ovaries, which means that the entire next generation of larvae and nymphs will be able to pass on the parasites. A single female tick can lay 2,000 to 3,000 eggs.

• The disease: The protozoa that cause EP damage red blood cells, reducing their ability to carry oxygen, which leads to anemia. Signs develop within one to three or four weeks after exposure, depending on the species of protozoan. Horses with mild EP may become weak, lethargic and lose weight. In acute cases, signs include fever, jaundice, roughened coats, colic, a swollen abdomen, edema (swelling) in the legs and red urine. Some infected horses may show no signs of illness. Unless the parasites are eliminated through treatment, a horse may carry the parasites for the rest of his life, remaining a potential source of infection for others.

Diagnosis:A competitive enzyme-linked immunosorbent assay (cELISA) is the current test used by the U.S. Department of Agriculture–Animal and Plant Health Inspection Service (USDA–APHIS) in Ames, Iowa, to screen horses for piroplasmosis. Separate tests are available for T. equi and B. caballi-. Depending on the suspected stage of the disease and the reason for testing, other tests that may be used include the complement fixation test (CFT), which identifies antibodies in a horse’s blood serum; the immunofluorescence assay, which uses fluorescent substances to detect antibodies bound to specific antigens; and polymerase chain reaction (PCR), which uses DNA amplification to detect the presence of organisms.

• Treatment:EP is a reportable disease, meaning that veterinarians who diagnose a case must notify state and/or federal authorities. Infected horses can be enrolled in a treatment program to receive imidocarb dipropionate, an antiprotozoal medication originally used to treat babesiosis in dogs that can eliminate the parasites from the horse’s system. If treatment fails to clear the organisms, the horse may be euthanatized, put into permanent quarantine or be exported to an endemic area.

Considerations:EP was first identified in the United States in 1961, when a Florida horse’s illness was linked to B. caballi infection. Although the source of the infection was never pinned down, it was believed to have originated in horses imported from Cuba in 1959. More than 100 more cases were subsequently diagnosed, mostly in Florida and other southern states. In 1964, another Florida horse was found to be infected with T. equi, and in 1969, two more cases were diagnosed in Florida, as was one in New Jersey, in a horse imported from Europe.

A program to eradicate B. caballi from Florida was established in 1962. A joint effort of the USDA and the state, the program involved quarantine and treatment of infected animals, spraying to kill ticks and transport controls to prevent spread of this disease.

Beginning in 1968, all imported horses were screened for EP using the CFT. In 2005, a more sensitive cELISA test, which better identifies chronic EP cases, became the official import test. “The United States was able to maintain an EP-free status by checking all horses coming in, using serology,” says Massaro Ueti, DVM, PhD, a research veterinary medical officer at the USDA Animal Disease Research Unit in Pullman, Washington.

The United States was declared free of EP in 1988, but in late 2008 and early 2009 two outbreaks occurred in Florida and Missouri, respectively. Both were connected with horses imported from Mexico to take part in unsanctioned Quarter Horse races. Researchers found no evidence of the parasites in local tick populations. Instead, “the disease agent was found to have been transmitted through less-than-optimal hygiene practices, including reuse of the same needles on multiple horses, and blood doping among horses involved in unsanctioned racing. Regulatory actions resolved both outbreaks,” says Angela Pelzel-McCluskey, DVM, an epidemiologist with the USDA–APHIS in Fort Collins, Colorado.

A larger outbreak in October 2009 contributed to an EP treatment breakthrough. In this case, 292 horses living at a large ranch in South Texas tested positive for T. equi, which was also discovered in two species of local ticks found on the horses. The parasite most likely came in with horses imported from South America.

With the cooperation of state and federal researchers and veterinarians as well as the ranch owners, 25 of the naturally infected horses were chosen for a research study to test whether the drug imidocarb dipropionate could clear the EP infection in horses. “The Texas situation was very interesting for us because we worked very closely with the ranch to try to eliminate the parasite by the treatment that Dr. Don Knowles [research leader for the USDA Animal Disease Research Unit] developed here in cooperation with Washington State University,” says Ueti.

“There were a few animals that did not clear the parasite with one treatment series, and we had to do a second one for them,” Ueti adds. “There seems to be some difference in the strain, regarding resistance or susceptibility to this drug.” However, he says, “Today we can safely say the treatment was very successful and the ranch is now free of this parasite.”

At this point, the United States is once again considered essentially free of EP. “In 2014 nationwide we identified only 31 cases, out of 27,800 horses tested,” says Pelzel-McCluskey. “These positive cases were all in the Quarter Horse racing population [spread by using the same needles on multiple horses] and previous imports, so there are no new categories of horses infected. The Texas Animal Health Commission created a new rule this year regarding their test requirement expanding it to include horses participating in any race, including unsanctioned races at fairs or match races.”

With continued vigilance, it is hoped, EP will remain only a minor concern to U.S. horse owners. “If we don’t have the parasite in the United States, it doesn’t matter whether we have a tick vector here,” says Ueti. “But if we introduce infected horses into the United States and have the vector, this could be catastrophic for the horse industry. This disease could easily spread if it is introduced again.”

Lyme disease: An ever-present threat

Named for Lyme, Connecticut, where it was first identified in 1975, Lyme disease cases are most common in New England and Mid-Atlantic regions as well as the upper Midwest, but they can appear in other states as well. “It’s also been found in Minnesota, Wisconsin, northern California and occasionally in the West Central states,” says David Trachtenberg, DVM, of Trachtenberg Veterinary Associates in Penfield, New York. According to the CDC, the number of cases diagnosed in the United States has been increasing over the past dec-ade, and the area of concentration has been growing, up into Maine and Ontario, out into western Pennsylvania and New York as well as down into Virginia.

The spirochete bacterium responsible for Lyme disease, Borrelia burgdorferi, causes illness in a number of animals, including people and dogs, as well as horses. “Various Borrelia species of spirochete occur around the world, and although there are some genetic variations of this bacterium, they cause much the same disease in whatever host they infect,” explains Stephen Barthold, DVM, PhD, professor emeritus of the University of California–Davis.

• The ticks:Ixodes scapularis, also called the deer tick or the bear tick, in the Northeast/East Coast and north central/midwestern regions of the country; Ixodes pacificus, called the Western black-legged tick, on the West Coast. The ticks undergo three stages in their life cycle—larvae, nymphs and adults—and they must take a blood meal at each stage to pass to the next and then to lay eggs.

Larvae are not born carrying B. burgdorferi but will pick up the organism if they feed on an infected carrier, often mice, squirrels or other small animals. Once infected, the nymphs and adults can pass the bacterium on to their next host. Nymphs may feed on a variety of large or small animals; adult ticks feed primarily on white-tailed deer, but they can also bite horses and other large animals.

Many smaller animals, especially the white-footed mouse but also raccoons, foxes, opossums and feral cats, are reservoirs for B. burgdorferi—once infected, these animals harbor the bacterium and remain a continuous source of infection for new ticks. “When the spirochetes get into a mouse, they basically swim through the skin and infect the entire body surface of the mouse because it’s so small. Then the immune response kicks in and drives the spirochetes into hiding, where they wait for a tick to come along—and infect the tick when the tick bites the mouse,” says Barthold.

An infected tick needs to feed on its host for 36 to 48 hours before it passes along B. burgdorferi. Both adults and nymphs can spread Lyme disease, but the mature ticks are more likely to be noticed and removed before infection occurs.

• The disease:When a horse is bitten by an infected tick, B. burgdorferi travels into the skin. This incites a localized inflammatory reaction—the characteristic “bull’s-eye rash” at the site of the bite—which in horses is often missed under the hair coat. In some cases, the bacteria then enter the bloodstream and are carried throughout the horse’s body to cause localized infection in various tissues—including joints, muscles or organs.

Lyme disease is considered acute if it is diagnosed within the first weeks after exposure and chronic if the infection persists for months. Signs can be vague and varied, depending on the tissues affected. They include generalized stiffness, lameness that shifts from limb to limb, swollen joints, weight loss, hyperesthesia (sensitive skin), lethargy, behavioral changes or rarely uveitis (inflammation of the eye). The horse may just generally not feel good or develop a surly attitude.

Still, B. burgdorferi infection does not always mean illness. In fact as many as 45 percent of horses in endemic areas may test positive for exposure to the organism without showing signs of disease. “The problem with dogs and horses, from a veterinary standpoint, is that many of them are becoming infected but never develop clinical signs of Lyme disease,” says Barthold. “This is one of the mysteries of Lyme disease; it is so variable in presentation—-in humans as well as dogs and horses. There are people who test positive for Lyme disease who never had any symptoms either. Perhaps they have a stronger immune system or encountered weaker bacteria; we don’t know.”

• Diagnosis:Lyme disease is difficult to diagnose because exposure to the bacterium alone is not proof of illness, and the outward signs mimic so many other problems. For that reason, other illnesses and injuries are generally ruled out before a Lyme diagnosis is made.

ELISA and Western blot tests, which detect antibodies to B. burgdorferi in a horse’s blood, have long been used to screen horses for Lyme disease, but they have limitations. For one, they offer nothing more than a yes/no as to whether a horse has been exposed to the bacterium.

The Lyme multiplex assay, offered by Cornell University, is the most advanced test for Lyme currently available. This assay identifies three distinct outer-surface antigen proteins (Osp A, Osp C, Osp F), each of which fluctuates in intensity as the disease progresses. “If there is a high Osp A, this is typically considered an antibody response due to vaccination because this is the antigen in the dog vaccine [also being used in horses], versus Osp C which would mean early natural infection, versus Osp F, which would mean chronic natural infection,” explains Trachtenberg, adding that the test’s interpretation is still undergoing refinement. “Since the test’s introduction, we’ve found that we have to tweak this interpretation a little.”

One of the more valuable uses of the multiplex assay is to distinguish between acute and chronic infections. “Osp C tends to rise early in the infection and then drops later, and it has been considered the most valuable one to assess,” says Trachtenberg. “If I treat a horse with a high Osp C, his chances of a full recovery are greater than when he is treated in the later stages of the disease. If the test shows a high Osp F and a low Osp C, this likely means that the horse has had the disease longer, and it could be tougher to treat,” he adds. “Additionally, with high Osp F/low Osp C horses, I need to be more convinced the signs are related to Lyme and, if in doubt, am less inclined to treat for it.”

Because diagnostic tests still leave some room for doubt, “diagnosis through treatment” is sometimes the best option for an ill horse who has high antibody titers for the Lyme organisms: If he improves with the standard Lyme therapy (antibiotics, often tetracycline), then Lyme disease is a likely diagnosis. But this approach has its limitations. “Tetracyclines work as anti-inflammatories as well as antibiotics, so they will make any kind of arthritic condition improve,” says Trachtenberg. “Thus a good response while on tetracyclines doesn’t necessarily mean that it’s Lyme disease. If the horse goes off the tetracycline and continues to do better for a period of time, this would be more conclusive—in my mind—than the temporary response to treatment in a case of arthritis. So we can use response to treatment as an indicator of recovery from Lyme, in helping to establish the diagnosis.”

• Treatment:Lyme disease can be treated with antibiotics, typically intravenous oxytetracycline and/or oral doxycycline, although your veterinarian may make different choices. One newer drug is minocycline, which, Trachtenberg says, “has a higher penetration into the nervous system. It is the treatment of choice for neuroborreliosis [the rare neurological impairment that occurs when the Lyme organism attacks the brain], and there has also been some research that shows that it gets into joints better. Minocycline is one of the first drugs I use now, although it varies a lot from practice to practice.”

• Considerations:Currently, no vaccine for Lyme disease is labeled for use in horses; however, many owners opt to administer the canine formula to their horses. “If there is ever a licensed horse vaccine, it will probably be similar to the dog vaccine,” Barthold says. “The difference is that it would have to be tested for safety and efficacy in horses. It’s not a matter of research—the research has already been done—as much as convincing a drug company to do the necessary clinical safety trials in horses, which are expensive.”

Talk to your veterinarian if you believe a canine Lyme vaccine might be a good idea for your horse. If your veterinarian agrees that vaccination is appropriate for your horse, he will choose the appropriate formula. “One of [the canine vaccines] does include the Osp C antigen,” says Trachtenberg. “It is generally recommended to not use that one because it could get really confusing when the horse is being tested for Lyme. The preliminary work was done with an Osp A vaccine, so that’s generally what is used for horses.”

Using any product off-label carries both pros and cons, but so far, the canine vaccine has been promising. “The longer this vaccine is used extra-label for horses, the more accepted it will be, as long as we do not hear of many serious vaccine reactions,” says Trachtenberg. That said, extra-label use of vaccines carries risk, and before it is administered a serious talk needs to take place between you and your veterinarian. “Hopefully there will eventually be an approved vaccine for horses, but at this point the dog vaccine shows promise for use in prevention of Lyme disease,” he adds.

Equine granulocytic anaplasmosis: The other threat from deer ticks

The same ticks that transmit the Lyme disease organism can also carry Anaplasma phagocytophilum (formerly Ehrlichia equi), the bacterium responsible for anaplasmosis (previously known as ehrlichiosis). It’s possible for a horse to have both at once. Fortunately, anaplasmosis is rarely fatal and is easy to treat.

• The ticks:Ixodes scapularis, also called the deer tick or the bear tick, in the Northeast/East Coast and north central/Midwest regions of the country; Ixodes pacificus, called the western black-legged tick, on the West Coast. Also as with Lyme, the juvenile ticks pick up the bacteria from rodents, primarily mice, and then the adults pass the pathogens on to their next hosts.

• The disease:A. phagocytophilum infects white blood cells, “primarily neutrophils and eosinophils,” says Ueti. Signs include high fever (up to 104 degrees Fahrenheit) as well as depression, lethargy, poor appetite, jaundice, ataxia (incoordination) and edema (soft swellings) in the legs.

• Diagnosis:A. phagocytophilum may be visible when blood smears are viewed under a microscope, or theorganism may be detected via polymerase chain reaction (PCR), which identifies the presence of its DNA in a horse’s blood sample. Laboratory tests may reveal a low white blood cell (WBC) count.

• Treatment:Antibiotics—primarily oxytetracycline and tetracycline—can eliminate an A. phagocytophilum infection. The corticosteroid dexamethasone can help to alleviate more severe signs. Horses remain immune once the infection clears. There is no vaccine against A. phagocytophilum.

• Considerations:“If we find Ixodes ticks on the horse, there are two possibilities for disease,” says Ueti: Lyme disease and anaplasmosis. When a horse contracts both infections at the same time, the combination of signs can make diagnosis difficult. Fever, however, is one key difference. “Fever is much more associated with anaplasmosis,” says Trachtenberg. “A significant number of horses can be co-infected with both anaplasma and Lyme disease.”

Another primary sign of anaplas-mosis, edema of the legs, can also accompany other illnesses, such as purpura hemorrhagica, protein loss and liver disease.

“These can all look similar,” says Trachtenberg. “Veterinarians consider these on the list of possibilities whenever we see limb edema and fever. Fortunately, anaplasmosis and Lyme disease can be effectively diagnosed, and the treatment for both of them is tetracyclines. This is convenient because you can effectively treat a horse with concurrent infections.”

The disease threats from ticks may be ever-present, but advancing research is continuing to make strides to help keep the risks under control. With continued vigilance to keep ticks off of your horse, you can go a long way toward keeping him healthy.

This article first appeared in EQUUS issue #456, September 2015. 




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