Understanding Lyme disease in horses

Passed on by the sheep tick, Lyme disease is a rare illness that is difficult to diagnose as it mimics other complaints.

In 1975, a mystery illness in adults and children in Lyme, Connecticut, USA, occurred, which was subsequently recognised as Lyme disease. In 1985, the disease was first identified in horses in the USA, and the first definite case in a British horse was diagnosed in Sussex in 1993.

Lyme disease is a caused by a specialised bacteria, known as borrelia burgdorferi, and is transmitted to horses by sheep ticks. As ticks need thick vegetation in which to live, the disease is common in woodland areas.

How to recognise Lyme disease in horses

In the horse, Lyme disease can cause a variety of illnesses and mimic numerous different diseases, which makes it difficult to diagnose just by an examination.

The signs include:

• The affected horse is often depressed and has a temperature.
• The horse will go off its food and can rapidly lose weight.
• Arthritis is frequently found, and horses are often lame on more than one limb, with swellings in numerous joints.
• The whole of one or more legs will often be extremely swollen.
• The horse will occasionally show signs of eye pain.
• Sporadically, the disease can affect the horse’s brain, causing behavioural changes, paralysis and other nervous disorders.
• The disease is rarely fatal, except occasionally in foals, but it can cause long-term effects in horses with arthritis or brain problems.

How to diagnose Lyme disease in horses

A diagnostic aid is to analyse a blood sample for specific antibodies (special blood markers) to the bacteria which cause Lyme disease. Research carried out at the University of Liverpool has shown that many normal horses from certain areas in the UK have these antibodies. Infection appears to be common in East Anglia, around Thetford Forest, and along the south coast, near the New Forest.

In these parts of Britain, it appears that many horses have been infected with Lyme disease, but only a few appear to show any illness despite the infection. In other parts of the world, such as the USA or Scandinavia, infection seems to cause disease more commonly in the horse. This is probably due to different strains of the same bacteria being present.

A more sensitive and accurate test has been developed, known as PCR (polymerase chain reaction). Once samples of either blood, urine or joint fluid have been taken from the horse, this test can recognise the specific bacteria which causes Lyme disease. While the test can definitely say whether the bacteria which causes the disease is present in the horse, it cannot tell us whether those bacteria are causing problems.

Since most infections with this bacteria do not cause any illness in the horse, it is still uncertain how useful the test will be in diagnosing and managing Lyme disease.

As Lyme disease can mimic numerous other diseases, and because we do not have a perfect test, diagnosis is extremely difficult. Often, diagnosis is made on the basis of the clinical signs in the sick horse, combined with a history of its exposure to ticks. The likelihood that such a horse is suffering from Lyme disease is increased by the positive results of blood tests.

Treatment of Lyme disease in horses

Fortunately for such an unpleasant disease, affected horses can be treated easily with antibiotics. The bacteria which causes Lyme disease is sensitive to several of the antibiotics which vets generally prescribe for the horse. The only problem is that horses may have to remain on the drugs for more than a month. Sometimes, horses which have the arthritis form of the disease are so sore they also have to receive a painkiller. Bute is often used while the horse slowly responds to the antibiotics.

Lyme disease in the horse is a rare illness, with only a few horses each year falling sick with it in the UK. Despite this, we see from blood samples that although many horses get the infection, the vast majority never show any illness. As a result, there is a danger that vague signs of other diseases may be incorrectly called Lyme disease. The presence of antibodies in a blood sample can only confirm a clinical picture. If the picture does not fit, the antibodies may be a red herring.

Avoiding infection

Unfortunately, there is no effective vaccine for Lyme disease, and the best way to avoid infection is to make sure your horse is not exposed to ticks. In certain areas of the country this can be virtually impossible, although careful grooming can identify the ticks, which can be removed, and decrease the risk of infection.