Lyme disease in dogs is not easy to diagnose. Most infected dogs are asymptomatic, which means that they have been exposed to and infected by Borrelia burgdorferi but show no outward signs of illness. In addition, dogs with this disease often have other tick-borne ailments, such as ehrlichiosis or babesiosis, which can mimic the signs of Lyme disease. The initial diagnostic focus usually is to rule out other possible causes of lameness and appetite loss in a dog with a fever of unknown origin. In most cases, the veterinarian will make the diagnosis based on the dog’s history, physical examination results, observable symptoms, laboratory test results and ruling out other causes of the dog’s clinical signs.
The results of routine urine and blood tests are almost always unremarkable in dogs with Lyme disease. A history of recent exposure to ticks, or to heavily vegetated areas known to harbor ticks that carry Borrelia burgdorferi, is important to the diagnosis. A dog’s prompt response to antibiotic treatment can also help to confirm that it is suffering from Lyme disease. Radiographs (X-rays) of painful or swollen joints can be used to rule out trauma and some other potential causes of lameness. X-rays of the joints of dogs with clinical Lyme disease typically show accumulated fluid without any fractures or obvious degenerative joint changes.
Specialized testing for Lyme disease is available. The presence of antibodies to the infective bacteria can be detected in blood samples using several different advanced tests. Antibodies usually can be detected several weeks after a dog has been exposed to the bacteria. A positive result on blood tests means that the dog has been exposed to, and has mounted an immune response against, the “bugs” that cause Lyme disease. If the dog has been vaccinated against Lyme disease, the results of blood tests can become more difficult to interpret. A fairly recent test is available that is both convenient and effective in differentiating between a dog’s antibody response to natural infection and one caused by a vaccine.
For a number of reasons, blood tests alone usually are not enough to definitively diagnose Lyme disease. Samples of joint fluid taken from dogs with Lyme disease often contain very high numbers of white blood cells. This is a sign of inflammation. The procedure used to get samples of joint fluid is called “arthrocentesis.” Unfortunately, evaluation of joint fluid usually does not provide information about the cause of the elevated white blood cell count, unless the bacteria can be identified microscopically.
Although the organisms that cause Lyme disease are somewhat difficult to grow in a laboratory, culture is still the preferred and most reliable way to diagnose Lyme disease. This procedure is usually performed at an outside laboratory on skin samples taken from the infected dog. The laboratory personnel will attempt to grow colonies of Borrelia burgdorferi from those skin samples. If these organisms grow from the skin samples, the culture is considered to be positive, and the diagnosis of Lyme disease is considered to be definitive.
Only about 5% of dogs bitten by infected deer ticks go on to develop observable signs of Lyme disease. As a result, this disease is rarely diagnosed in companion dogs. When a dog is diagnosed with Lyme disease, its owner should be very conscious of the chances that she, too, may be infected. The source of infection for people and for dogs is the same: infected ticks. The fact that a dog develops clinical disease means that it has been to some place where infected ticks are present. Presumably, the owner has also been to that place and may also have been exposed to infected ticks. Ticks that have not attached to a dog can be transferred to people, although this is unlikely because deer ticks usually latch on very quickly to their warm-blooded hosts.