Post by Calaminty Jane on Feb 27, 2012 10:35:46 GMT -5
Ticked Off!
From Amesbury Animal Hospital
Understanding Equine Lyme Disease
Just to warn you ahead of time, this article will not be like my other columns. It will not explain Lyme disease away in any definitive language. You may read this article and end up being more confused then when you started! Unfortunately, I don't have all the answers about equine Lyme disease - no one does. Lyme disease in horses is not completely understood yet. Ask four vets about Lyme disease and you may end up with five different answers. But we still trek along gamely with the information we do have and work to practice the best medicine available for our equine patients. And thus forewarned, read on!
This is an adult deer tick.First identified in Lyme CT (and hence its name), Lyme disease is caused by a bacterial spirochete called Borrelia burgdorferi. It is spread primarily by the tiny deer tick, although it has been identified in several other species including the common dog tick. A minimum of 24 hours of attachment is required to transmit the spirochete bacteria, during which the tick continues feeding. Now, it appears that the bacteria may stay in the skin at the site of the bite before spreading and causing clinical signs. This may account for the delay in signs after an infected tick bite in some animals.
Once the bacteria does spread, it can produce a variety of clinical signs which are most commonly mild fevers, stiffness and lameness in more than one limb, muscle tenderness or behavioral changes. Some believe it also can cause uveitis (moon blindness), dermatitis or neurological disease. Many horses display little to no signs, and it is estimated that 50% of our horses have been infected at one point in their lives. To further complicate things, many horses are co-infected with another tick-borne disease called Anaplasmosis. Anaplasmosis or Granulocytic Ehrlichiosis is spread by the same types of ticks as Lyme disease and it is characterized by fever, lethargy or limb edema. As these signs are very similar to those associated with Lyme disease, it can be easy to confuse the two diseases and makes definitive diagnosis more difficult. However, treatment for both diseases is essentially the same.
There is a controversy over the clinical signs and consequences of Lyme disease in our horses. It stems from an inability of researchers to demonstrate reproducible cause and effect relationship between Lyme bacterial infection and specific clinical signs. Attempts to artificially infect ponies have only been found to produce very mild disease. Pathology was present when tissue was biopsied in this study, but the minimal clinical signs seen compared to what horse owners and veterinarians see in the field leave some questions regarding the reliability of signs caused by the Lyme bacteria.
To summarize so far; it is estimated that nearly half of our horses have been or are infected with Lyme bacteria. They may or may not have any clinical signs. If they do, these signs could also be attributed to a co-infection with Anaplasma or to a variety of other diseases that cause fever, lameness or muscle soreness. Still with me so far? Good!
Now, if one suspects Lyme disease or if an owner is just curious about their horse's status, there are several diagnostic tests to determine if a horse has been exposed. All of these tests measure exposure to the Lyme bacteria and NOT the level of organism in a horse's system. Traditionally, an ELISA and/or Western Blot have been the test of choice, requiring just a simple blood sample. The ELISA provides a quantitative number or antibody level, which makes it useful for comparing previous and future titers. Sometimes, the ELISA comes back as "equivocal" or a possible false positive and a Western Blot (WB) must be performed to further confirm the results. A Western Blot provides a more qualitative result - high, moderate or low antibody level. Testing for Anaplasma requires a separate test altogether, usually an IFA test.
Recently, there has been some research into using the SNAP 4Dx test developed by Idexx laboratories. This test was developed for testing dogs for heartworm, Lyme, Anaplasma and Ehrlicia infections (four diseases, hence the 4Dx). It can easily be run in-house and only takes 8-10 minutes. In a 2008 study, researchers found that the 4Dx could successfully be used in horses to detect antibodies to the Lyme and Anaplasma bacteria. If this holds true, it would be a valuable diagnostic tool because it provides rapid results in suspected cases and tests for both diseases at the same time. However, a single study with a relatively small sample group may not be proof enough to give us reason to rely solely on a single test to diagnose our horse patients. We would prefer more research be done in this area before we decide to use the 4Dx in all cases. This test also only provides a simple negative or positive result if you horse has been exposed. Since it does not result in an actual number as with the ELISA, the 4Dx does not allow for comparing results of future tests. If your horse has been exposed to Lyme disease (like 50% of horses out there), a positive 4Dx may simply represent exposure versus true disease. If your horse is suspected to be infected with Lyme or Anaplasma, we will discuss the best testing options with you on a case-by-case basis.
Confirming a true infection with Lyme disease is important because treatment can be very intensive, both in terms of time and money. It is generally agreed that the gold standard for treatment is daily intravenous tetracycline for 2-3 weeks. Because this requires an IV catheter and monitoring of blood values, it is strongly recommended that a horse be hospitalized for the duration of treatment. We usually send clients to one of the nearby referral hospitals for this treatment option. The most common treatment option is oral doxycycline twice daily for a period of 1-3 months. At the moment, there does not seem to be a consensus on duration of treatment with oral doxycycline, but we generally recommend 2-3 months depending on a horse's clinical improvement on medication. This can be an expensive treatment option, depending on the size of your horse. Giving oral medication to a horse can also be a difficult and messy process! A third option is ceftiofur (or Naxcel) given twice daily in the muscle, but we have found that few clients opt for potentially painful intramuscular injections with their horse. Ceftiofur is also considered not as good as IV tetracycline for treating Lyme disease.
This is the Lyme bacteria magnified. We generally consider treatment successful if a horse's clinical signs improve. However, doxycycline is also an anti-inflammatory agent. Horses with mild inflammation causing the clinical signs associated with Lyme disease may simply improve because of the anti-inflammatory nature of doxycycline and not due to treatment of actual Lyme disease. There is controversy over horses with "chronic" Lyme disease who improve with repeated doxycycline treatment that simply addresses an undiagnosed inflammatory condition. In many cases, re-testing for Lyme disease with an ELISA and/or WB is recommended at some interval to determine if a titer has decreased with treatment. Unfortunately, some horses maintain a high titer for months or years, regardless of treatment. Horses may also be re-infected at any time, leading to another increase in titer numbers. Re-testing for Lyme disease can obviously be very frustrating for both veterinarian and client in these cases.
How do we know which horses to treat if Lyme disease can cause some rather vague clinical signs, nearly 50% of horses have been exposed, and testing can often be confusing or misleading? Well, it depends. It can depend on the clinical signs we see, the results of testing, or even a client's preference in some cases. Honestly, how many stiff or "off" horses are out there? How many do you think are positive for Lyme disease? Should we treat every positive horse? If so, with what and for how long? If a horse improves on doxycycline but then "regresses" once off, do you continue treatment for longer? Or do you consider the anti-inflammatory properties of doxycycline as playing a role? At the moment, there are no right or wrong answers to these questions. We just don't know enough yet about Lyme disease in our horses. Generally, we tend to treat a horse if they show obvious clinical signs AND test positive. If clinical improvement is seen initially on medication, we will recommend continuing treatment for 2-3 months. If no improvement is seen, other differentials are considered. We approach every Lyme-suspect horse on a case-by-case basis and work out a diagnostic and treatment plan best suited for each horse.
Many clients ask about ways to prevent Lyme disease. At the moment, prevention is the best way to protect your horse. Perform a "tick check" daily if able. There are some insect repellants that do help decrease the numbers of ticks on a horse. Avoid riding through high grasses or brush during peak tick seasons. There is currently no Lyme vaccine for horses. Recently, some veterinarians have used the Lyme vaccine developed for dogs on horses. Theoretically, this vaccine was developed in a way that should work the same way in horses as it does in dogs. However, it is NOT approved for horses and there are no studies determining its safety in horses. We do not use this vaccine in our horse patients. Our practice does not believe the benefits outweigh the potential risks associated with using off-label vaccines in this way.
There you have it. You know everything there is to know about Lyme disease in horses. Or you know everything there isn't to know! As I mentioned before, we are still learning about this often exasperating and persistent disease. Hopefully, one day we will be able to better prevent, diagnose and treat Lyme disease in our horses. Until then, stay tuned!
From Amesbury Animal Hospital
Understanding Equine Lyme Disease
Just to warn you ahead of time, this article will not be like my other columns. It will not explain Lyme disease away in any definitive language. You may read this article and end up being more confused then when you started! Unfortunately, I don't have all the answers about equine Lyme disease - no one does. Lyme disease in horses is not completely understood yet. Ask four vets about Lyme disease and you may end up with five different answers. But we still trek along gamely with the information we do have and work to practice the best medicine available for our equine patients. And thus forewarned, read on!
This is an adult deer tick.First identified in Lyme CT (and hence its name), Lyme disease is caused by a bacterial spirochete called Borrelia burgdorferi. It is spread primarily by the tiny deer tick, although it has been identified in several other species including the common dog tick. A minimum of 24 hours of attachment is required to transmit the spirochete bacteria, during which the tick continues feeding. Now, it appears that the bacteria may stay in the skin at the site of the bite before spreading and causing clinical signs. This may account for the delay in signs after an infected tick bite in some animals.
Once the bacteria does spread, it can produce a variety of clinical signs which are most commonly mild fevers, stiffness and lameness in more than one limb, muscle tenderness or behavioral changes. Some believe it also can cause uveitis (moon blindness), dermatitis or neurological disease. Many horses display little to no signs, and it is estimated that 50% of our horses have been infected at one point in their lives. To further complicate things, many horses are co-infected with another tick-borne disease called Anaplasmosis. Anaplasmosis or Granulocytic Ehrlichiosis is spread by the same types of ticks as Lyme disease and it is characterized by fever, lethargy or limb edema. As these signs are very similar to those associated with Lyme disease, it can be easy to confuse the two diseases and makes definitive diagnosis more difficult. However, treatment for both diseases is essentially the same.
There is a controversy over the clinical signs and consequences of Lyme disease in our horses. It stems from an inability of researchers to demonstrate reproducible cause and effect relationship between Lyme bacterial infection and specific clinical signs. Attempts to artificially infect ponies have only been found to produce very mild disease. Pathology was present when tissue was biopsied in this study, but the minimal clinical signs seen compared to what horse owners and veterinarians see in the field leave some questions regarding the reliability of signs caused by the Lyme bacteria.
To summarize so far; it is estimated that nearly half of our horses have been or are infected with Lyme bacteria. They may or may not have any clinical signs. If they do, these signs could also be attributed to a co-infection with Anaplasma or to a variety of other diseases that cause fever, lameness or muscle soreness. Still with me so far? Good!
Now, if one suspects Lyme disease or if an owner is just curious about their horse's status, there are several diagnostic tests to determine if a horse has been exposed. All of these tests measure exposure to the Lyme bacteria and NOT the level of organism in a horse's system. Traditionally, an ELISA and/or Western Blot have been the test of choice, requiring just a simple blood sample. The ELISA provides a quantitative number or antibody level, which makes it useful for comparing previous and future titers. Sometimes, the ELISA comes back as "equivocal" or a possible false positive and a Western Blot (WB) must be performed to further confirm the results. A Western Blot provides a more qualitative result - high, moderate or low antibody level. Testing for Anaplasma requires a separate test altogether, usually an IFA test.
Recently, there has been some research into using the SNAP 4Dx test developed by Idexx laboratories. This test was developed for testing dogs for heartworm, Lyme, Anaplasma and Ehrlicia infections (four diseases, hence the 4Dx). It can easily be run in-house and only takes 8-10 minutes. In a 2008 study, researchers found that the 4Dx could successfully be used in horses to detect antibodies to the Lyme and Anaplasma bacteria. If this holds true, it would be a valuable diagnostic tool because it provides rapid results in suspected cases and tests for both diseases at the same time. However, a single study with a relatively small sample group may not be proof enough to give us reason to rely solely on a single test to diagnose our horse patients. We would prefer more research be done in this area before we decide to use the 4Dx in all cases. This test also only provides a simple negative or positive result if you horse has been exposed. Since it does not result in an actual number as with the ELISA, the 4Dx does not allow for comparing results of future tests. If your horse has been exposed to Lyme disease (like 50% of horses out there), a positive 4Dx may simply represent exposure versus true disease. If your horse is suspected to be infected with Lyme or Anaplasma, we will discuss the best testing options with you on a case-by-case basis.
Confirming a true infection with Lyme disease is important because treatment can be very intensive, both in terms of time and money. It is generally agreed that the gold standard for treatment is daily intravenous tetracycline for 2-3 weeks. Because this requires an IV catheter and monitoring of blood values, it is strongly recommended that a horse be hospitalized for the duration of treatment. We usually send clients to one of the nearby referral hospitals for this treatment option. The most common treatment option is oral doxycycline twice daily for a period of 1-3 months. At the moment, there does not seem to be a consensus on duration of treatment with oral doxycycline, but we generally recommend 2-3 months depending on a horse's clinical improvement on medication. This can be an expensive treatment option, depending on the size of your horse. Giving oral medication to a horse can also be a difficult and messy process! A third option is ceftiofur (or Naxcel) given twice daily in the muscle, but we have found that few clients opt for potentially painful intramuscular injections with their horse. Ceftiofur is also considered not as good as IV tetracycline for treating Lyme disease.
This is the Lyme bacteria magnified. We generally consider treatment successful if a horse's clinical signs improve. However, doxycycline is also an anti-inflammatory agent. Horses with mild inflammation causing the clinical signs associated with Lyme disease may simply improve because of the anti-inflammatory nature of doxycycline and not due to treatment of actual Lyme disease. There is controversy over horses with "chronic" Lyme disease who improve with repeated doxycycline treatment that simply addresses an undiagnosed inflammatory condition. In many cases, re-testing for Lyme disease with an ELISA and/or WB is recommended at some interval to determine if a titer has decreased with treatment. Unfortunately, some horses maintain a high titer for months or years, regardless of treatment. Horses may also be re-infected at any time, leading to another increase in titer numbers. Re-testing for Lyme disease can obviously be very frustrating for both veterinarian and client in these cases.
How do we know which horses to treat if Lyme disease can cause some rather vague clinical signs, nearly 50% of horses have been exposed, and testing can often be confusing or misleading? Well, it depends. It can depend on the clinical signs we see, the results of testing, or even a client's preference in some cases. Honestly, how many stiff or "off" horses are out there? How many do you think are positive for Lyme disease? Should we treat every positive horse? If so, with what and for how long? If a horse improves on doxycycline but then "regresses" once off, do you continue treatment for longer? Or do you consider the anti-inflammatory properties of doxycycline as playing a role? At the moment, there are no right or wrong answers to these questions. We just don't know enough yet about Lyme disease in our horses. Generally, we tend to treat a horse if they show obvious clinical signs AND test positive. If clinical improvement is seen initially on medication, we will recommend continuing treatment for 2-3 months. If no improvement is seen, other differentials are considered. We approach every Lyme-suspect horse on a case-by-case basis and work out a diagnostic and treatment plan best suited for each horse.
Many clients ask about ways to prevent Lyme disease. At the moment, prevention is the best way to protect your horse. Perform a "tick check" daily if able. There are some insect repellants that do help decrease the numbers of ticks on a horse. Avoid riding through high grasses or brush during peak tick seasons. There is currently no Lyme vaccine for horses. Recently, some veterinarians have used the Lyme vaccine developed for dogs on horses. Theoretically, this vaccine was developed in a way that should work the same way in horses as it does in dogs. However, it is NOT approved for horses and there are no studies determining its safety in horses. We do not use this vaccine in our horse patients. Our practice does not believe the benefits outweigh the potential risks associated with using off-label vaccines in this way.
There you have it. You know everything there is to know about Lyme disease in horses. Or you know everything there isn't to know! As I mentioned before, we are still learning about this often exasperating and persistent disease. Hopefully, one day we will be able to better prevent, diagnose and treat Lyme disease in our horses. Until then, stay tuned!