Lyme Disease in Horses
Lyme disease might not be the most prevalent equine disease you face with your horses, but it does exist, and it can seriously impact a horse's health. And that's not all. Although documented cases of Lyme disease in horses are relatively rare, there are indications that the disease is on the increase.
Lameness, arthritis, neurologic disease, uveitis (moon blindness), and dermatitis have been the most commonly described signs of Lyme disease in horses, although, as we shall learn, both symptoms and test results often are confusing.
In humans, Lyme disease is definitely on the increase.
Lyme disease was first recognized as a distinct clinical illness in 1975, when 51 residents from Old Lyme, Lyme, and East Haddam, Conn., were diagnosed as having a unique form of oligoarticular arthritis.
Since its description in 1975, reports from the University of Rhode Island's Tick Research Laboratory have noted that Lyme disease has emerged as a significant threat to the public's health, particularly in the northeastern United States.
(The Tick Research Laboratory is a non-profit adjunct of the Department of Biological Sciences, University of Rhode Island. It is devoted to the study of various aspects of tick-borne diseases, especially Lyme disease, including epidemiology, prevention, and control strategies. Since 1983, research on ticks and other ectoparasites of both mammals and birds has been in progress in this laboratory, and the results have been reported in scientific literature and at professional meetings. This work has been supported by various governmental and private agencies. The establishment of this laboratory was the result of a growing demand from the private sector for identification and consultation services. Much of the information that follows concerning ticks and how they can spread disease is based on information from the Rhode Island laboratory.)
Statistics bear out the laboratory's assertion that Lyme disease is a growing threat to public health. Nationally, Lyme disease increased from 523 reported cases in 1982, to 4,507 reported cases in 1988; 8,552 cases in 1989; and 13,043 cases in 1994.
The Centers for Disease Control reported that Lyme disease accounted for 81% of all reported cases of arthropod-transmitted diseases in the United States between 1986 and 1991.
Once the clinical illness had been described in 1975, scientists launched research aimed at finding the causative agent. They struck pay dirt in 1982, when a treponema-like spirochete was isolated from the midgut of a blacklegged tick, also known as a deer tick. (A spirochete is an order of slender, spirally undulating bacteria. A treponema is any of a genus of spirochetes that parasitize man and other warm-blooded animals.) The scientific name for the blacklegged or deer tick is Ixodes scapularis.
Isolating the spirochete strongly suggested that this organism was involved in the
etiology of Lyme disease.
Research progressed rapidly in the wake of that accomplishment. Shortly thereafter, spirochetes were isolated from the blood of Lyme disease patients and from adult blacklegged ticks. The following year, the blacklegged tick spirochete was recognized as a new species and named Borrelia burgdorferi. All evidence indicated that the blacklegged tick was the most important vector of this particular spirochete.
Time to take a look at this vector, which is the prime carrier of Lyme disease.
The Life Cycle
The genus Ixodes contains almost 250 species worldwide, with at least 35 species reported in North America. Pathogens transmitted by these ticks include viruses, bacteria, rickettsia, protozoans, and
In Europe, the sheep tick (Ixodes ricinus) has been implicated in the transmission of at least eight disease-causing organisms. A tick in Eurasia--Ixodes persulcatus--transmits four infectious agents. In the western and southern United States, both the western blacklegged tick (Ixodes pacificus) and the blacklegged tick (Ixodes scapularis) have been found to transmit the Lyme disease
Like all ticks in the genus Ixodes, the blacklegged tick is a three-host vector. This means that each feeding stage requires one vertebrate blood meal for its development. The three stages are larva, nymph, and adult.
The blacklegged tick is commonly found in the northeast and midwest. The two states where it is most numerous in the midwest are Minnesota and Wisconsin. In the northeast and midwest, blacklegged ticks usually mate in the fall or early spring.
Typically, the blacklegged tick takes about two years to complete one life cycle. As mentioned, the adults appear to exhibit two breeding periods. Adults resulting from spring nymphs first emerge in the early fall and undergo a fall breeding period.
While on the host animal, primarily white-tailed deer, the female tick gorges itself on blood. The male mates repeatedly with several females. The fully engorged (full of blood) female drops off her host and lives in leaf litter or other protected areas until she lays her eggs. The eggs are minute, and the female can lay up to 3,000 of them at one time. After laying her eggs, the female dies.
The second breeding season results from unengorged adults which have overwintered and by adults which have developed from overwintering nymphs. The occurrence of two breeding seasons results in both spring and summer distribution of larvae.
In either case, the eggs hatch into larvae which are extremely small--about the size of a grain of sand. They must have high humidity in order to survive. The larvae are rarely infected, but they must find a host on which to feed, and it is at this phase that infection can begin.
The first larval activity is seen in May and results from females that successfully mated and deposited their eggs the previous fall. The second, and much larger, larval activity peak is seen in August. This activity peak results from the more successful spring breeding season.
The larval stage is the only six-legged stage in the life cycle. The larvae usually feed on small mammals for three to five days before dropping from the host to metamorphose over a period of time to the nymphal stage in the leaf litter.
The normal host for the larvae is the white-footed mouse. If the white-foot mouse is infected with the bacterium (Borrelia burgdorferi)--and many of them are--the larvae are frequently infected. Estimates are that 25% of the larvae are infected.
The white-footed mouse is not the only host. Larvae and nymphs are known to infest 31 mammalian species and 49 avian species, and the adults have been found on at least 13 species of medium to large-sized mammals.
The nymphal stage follows the larval. The eight-legged nymphs are most active in the months of May, June, and July. Larvae that have been infected will transmit the infection on to the nymph stage.
Nymphs are very small; about the size of a poppy seed. In the late spring and early summer months, nymphs seek a host. Normally, this will be a mouse or some other small to medium-sized mammal. They will attach themselves to the host and feed. Because of their small size, nymphs can easily go undetected.
If the host is infected, the nymph has a second opportunity to acquire infection with Lyme disease bacteria. Once they are engorged with blood, the nymphs drop into the leaf litter and turn into sexually mature blacklegged ticks.
The adult ticks are active from fall to spring at times when the temperature is above 40ï¿½ F. The adult feeds on large mammals like horses, dogs, and deer--and on man. After mating, the engorged adult female drops off and the cycle begins anew.
As is the case in the passage from larval to nymph stage, the infectious bacteria can be passed from nymph to adult tick, with the adult thus becoming the third and final carrier of Lyme disease in a single life cycle.
The Lyme disease spirochete is transmitted from the nymph or tick to the host via salivation and/or regurgitation. Although the infectious bacteria is found primarily in the lumen of the tick's digestive tract, the spirochete also disseminates through the tick's hemolymph and enters the salivary glands. The spirochete is then most likely transmitted through the saliva as ticks salivate excessively during feeding. It appears that a minimum of 24 hours of tick attachment is required for spirochete transmission.
Although the adult ticks are the ones we generally find most frequently on our own bodies or firmly attached to our horses, it is at the nymphal stage that Lyme disease transmission is the greatest.
Lyme disease transmission is highest during May and June, which coincides with peak nymphal stages. Because the nymphs are so small, they often go unnoticed, whereas the large adults frequently are spotted and removed before they can transmit the infectious bacteria.
Although research has proven adult blacklegged ticks transmit Lyme disease to humans and warmblooded mammals, the disease transmission decreases decidedly during the adult season. This, despite the fact that a larger segment of the population is infected at the adult stage compared to the nymphal stage. Estimates are that up to 50% of the adult population carries the infectious bacteria.
In addition to the fact that the adults are larger and more apt to be spotted and removed, there is also the matter of the weather. The adults are at their peak during a cooler part of the year when people are less active outdoors and when active, they frequently wear long pants and long-sleeved shirts. This means that fewer adult ticks would be able to attach and begin feeding and, thus, transmit Lyme disease.
Diagnosing Lyme Disease
Unfortunately, Lyme disease is often difficult to diagnose in both humans and animals because its symptoms and signs mimic those of many other diseases. The fever, muscle aches, and fatigue of Lyme disease easily can be mistaken for viral infections, such as influenza, infectious mononucleosis, or chronic fatigue syndrome. Joint pain can be mistaken for other types of arthritis, such as rheumatoid arthritis, and neurologic signs can mimic those caused by other conditions, such as multiple sclerosis.
One of the first signs of infection in humans comes with erythema migrans (EM), which is a red, circular patch that usually appears between three days to one month after the bite of an infected tick. The initial red patch will be at the site of the tick bite. The patch then expands, quite often to cover a large area.
Sometimes, say researchers at the Rhode Island laboratory, many patches appear, varying in shape, depending on their location. Common sites are the thigh, groin, trunk, and the armpits. The center of the rash might clear as it enlarges, giving it a bulls-eye appearance. The rash might be warm, but it usually is not painful.
Nothing is easy in diagnosing Lyme disease. For example, not all rashes that occur at the site of a tick bite are due to Lyme disease. The rash could be an allergic reaction to tick saliva. However, this type of rash usually occurs within from a few hours to a few days following a tick bite, but normally does not expand. Generally, these rashes will disappear within a few days.
Despite the confusion, Rhode Island experts say that the early stages of Lyme disease in humans are usually marked by one or more of the following symptoms: fatigue, chills and fever, headache, muscle and joint pain, swollen lymph nodes, and the characteristic skin rash described above.
Some symptoms and signs of Lyme disease might not appear until weeks, months, or years after a tick bite:
- Arthritis is most likely to appear as brief bouts of pain and swelling, usually in one or more large joints, especially the knees.
- Nervous system abnormalities can include numbness, pain, Bell's palsy (facial paralysis, which usually occurs on one side), meningitis (fever, stiff neck, and severe headache).
Proper diagnosis of Lyme disease depends on several factors for both humans and horses. One of the best ways to determine if you or your horse is a candidate for Lyme disease is to save the tick that has done the biting and have it tested for presence of the infectious bacteria.
As mentioned earlier, the chances of being bitten by an infected nymph or adult tick varies with geography, which can also mean a difference in hosts. In the southern and Pacific coastal states, for example, the host of the larvae and nymphs is frequently a lizard, which is a cold-blood reptile. Some lizards are susceptible to infection with the Lyme disease spirochete and others, such as the western fence lizard, are not.
This means that the actual infection rate varies in different parts of the country. Where the infection rate of adult ticks in the Northeast and Midwest may be 50% or more, the infection rate in nymphs or adults in most areas of the South and West are only 1-2%.
And, it stands to reason, if Lyme disease is not a serious threat to humans because of a low population of infected ticks, it also will not represent a serious threat to the equine population. If, however, the reverse is true, the threat to horses rises in proportion. Human cases parallel animal cases geographically.
The chances of you or your horse becoming infected with Lyme disease in the Northeast and certain parts of the Midwest may be 25 times greater than it would be in other parts of the country.
It should be pointed out that although the Northeast and Midwest seem to be the areas where Lyme disease is most prevalent, they are not the only areas. The disease has been found in 44 states, Canada, Europe, and Australia.
Blacklegged ticks are most abundant in rural locales, so if one lives in the Northeast or Midwest where there are vast acreages of woodlands, it is virtually impossible to avoid infested areas. Ticks are also found in lawns, gardens, and on bushes near residential housing.
To this point, we have discussed Lyme disease from the viewpoint of its origin and transmission. Time to turn attention to just what happens to horses which contract it.
At the risk of being redundant, we must again point out that a definitive diagnosis of the disease in horses is difficult. As mentioned earlier, there are the clinical signs of lameness, arthritis, neurologic disease, eye disease, and dermatitis.
Perhaps the most frequently reported clinical sign in this grouping is arthritis. This is where the deep diagnostic water begins. Lameness and joint swellings occur frequently in horses, and they can be from a variety of causes. Thus, the veterinarian first would eliminate these potential causes before turning attention to Lyme disease.
Arthritis that results from Lyme disease can involve one or several joints. The arthritis is often episodic, but can become chronic. Researchers say that joints of the forelimb seem to be affected most frequently and are often painful, warm, and swollen.
A variety of other maladies have been associated with Lyme disease to further confuse the issue. Noah D. Cohen, VMD, PhD, Diplomate ACVIM, of Texas A&M University, reported that encephalitis (inflammation of the brain) has been associated with Borrelia burgdorferi infection in a horse.
The horse, he said, had a head tilt, paralysis of the tail, difficulty swallowing, glazed eyes, profuse sweating, and wandered aimlessly.
In another case, a pony was reported to have suffered eye problems after being infected with Lyme disease. The eye disease was similar in appearance to periodic recurrent uveitis (moon blindness).
On the bright side for equines, heart, liver, and kidney diseases that have been reported in humans as a result of Lyme disease have not been seen in horses.
Diagnosis of Lyme disease in horses can be confirmed by isolation of the infectious bacteria. However, this is not easily done. An alternative approach involves a blood test, but this has serious limitations.
The blood test is based on detecting antibodies to the infectious bacteria. Unfortunately false negative and false positive results can occur if a horse is tested during the first several weeks of infection before antibodies have developed.
Other tests have been developed based on molecular genetic technique, but they are not widely available.
Thus, once again the veterinarian is called upon to play the role of detective. Is the horse in a tick-infested area where Lyme disease has been reported? Does it show all the classic symptoms, with other diseases with the same symptoms being ruled out? The investigative list goes on.
If the diagnosis is Lyme disease, the treatment protocol normally will include antibiotics. Tetracyclines and ampicillin have been used successfully.
However, if the disease is in an advanced stage, prolonged treatment might be required.
For horse owners in endemic areas, it's a case of an ounce of prevention being worth a pound of cure. Because ticks do not transmit the spirochete immediately after attachment, the most effective preventive measure is to carefully examine horses on a daily basis and remove nymphs and ticks immediately. Nymphs can be difficult to locate because of their size.
Ticks can be found about anywhere on a horse's body, but one should be certain to examine the area along the neck at the base of the mane and around the rectal area.
The experts say there is a right way and a wrong way to remove ticks from oneself and from horses. The wrong way is to grasp it between thumb and finger and pull it free. By squeezing it in that manner, one can force material from the tick into the skin.
The correct way is to grasp the tick with fine tweezers as close to the skin surface as possible and then pull straight up with a slow, steady force. If one wants the tick examined by a laboratory, it should be placed in a sealed container. The Tick Research Laboratory in Rhode Island is one facility that can identify the infectious bacteria that causes Lyme disease, along with certain health departments and other laboratories.
Once the tick has been removed, an antiseptic, such as alcohol or an antibiotic ointment, should be applied to the site of the bite.
At present there is no vaccine for horses against Lyme disease, although a vaccine has been developed for dogs.
Until something is available for equines, it behooves the responsible horse owner to keep horses clear of tick-infested areas if possible, and to carefully examine horses on at least a daily basis for the blood-sucking parasites.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: University Equine Hospitals