Testing for Leptospirosis in Dogs
What is leptospirosis?
Leptospirosis is an infectious disease caused by a type of bacteria called Leptospira. The disease causes severe damage to the kidney and liver and may be fatal in severe cases. Bacteria are passed in the urine of infected animals and can survive in the environment for long periods in warm, stagnant water or moist soil. Wild animals can spread the infection to dogs, including skunks, raccoons, opossums, rats, wolves, and deer. See the handout “Leptospirosis in Dogs” for more information about this disease.
Can leptospirosis be diagnosed with routine blood tests?
No. Routine blood tests cannot diagnose leptospirosis definitively, but they can provide valuable clues and should be considered the starting point of any investigation. If routine testing suggests a dog has leptospirosis, then additional definitive testing will likely be recommended to confirm the diagnosis.
Routine tests include complete blood count (CBC), serum biochemistry, and urinalysis. (See handouts “Complete Blood Count”, “Serum Biochemistry”, and “Urinalysis” for further information on these tests.) The most common abnormalities found on routine testing in dogs with leptospirosis include the following:
- CBC: increased numbers of white blood cells (indicating infection and tissue damage), decreased numbers of platelets (indicating infection and severe disease), and sometimes, decreased numbers of red blood cells (due to bleeding).
- Serum biochemistry: high liver and/or kidney values (indicating damage to the liver and/or kidney) and abnormal values for sodium, chloride, phosphorous, and potassium (indicating damage to the kidney and unbalanced metabolism, indicating severe illness).
- Urinalysis: dilute urine, the presence of protein, and evidence of inflammation (all signal kidney damage).
What definitive tests are available to diagnose leptospirosis?
There are several tests for diagnosing leptospirosis, but the two most common ones are the DNA-PCR test and the microscopic agglutination test (MAT). Infection can be diagnosed with either test, but each has weaknesses, and in some situations, both tests may be needed to reach a diagnosis.
What is the DNA-PCR test for leptospirosis?
The DNA-PCR test is a rapid test that detects the DNA of Leptospira in whole blood or urine. Urine is often the preferred sample because of the large numbers of bacteria that are usually present. The test is faster and often less expensive than the MAT.
Does the DNA-PCR test always work?
The DNA-PCR is an excellent test, but it has limitations. Most importantly, the DNA-PCR test must be done before the dog is given antibiotics. Leptospira are easily killed by antibiotics, and even small doses can make it difficult to detect infection using DNA-PCR. The test is most useful in the early stages of moderate to severe disease when large numbers of bacteria are present. Dogs that have been sick for a long time or have only mild signs of illness may be difficult to diagnose by DNA-PCR due to the small numbers of bacteria present.
In some cases, dogs that are truly infected may give a negative result on the DNA-PCR test (false negative). This is especially common when antibiotics are given before the test is done. It is important to remember that a negative result does not rule out infection. If leptospirosis is suspected and the DNA-PCR test is negative, further testing must be done to confirm the infection. This usually means doing the microscopic agglutination test (MAT).
In rare cases, the DNA-PCR may be positive even when true infection is not present (false positive). It usually occurs in dogs that have been in contact with another type of Leptospira that does not cause disease. These dogs are not ill and usually have unremarkable routine test results. For this reason, routine testing should always be completed before testing for leptospirosis by DNA-PCR.
What is the MAT test for leptospirosis?
The MAT or microscopic agglutination test detects the presence of antibodies against Leptospira in a dog's blood. If the level of antibodies (called a titer) is high enough or can be shown to rise over time, then infection is confirmed.
Does the MAT always work?
The MAT is an excellent test, but it also has limitations. It is a slower test than the DNA-PCR test, and it may take several days to get results from the laboratory. Sometimes a follow-up test (called a convalescent sample) is needed to confirm infection, which further delays the diagnosis. Also, test results can be inconclusive if the patient has been previously vaccinated for leptospirosis or if antibiotics were given early in the course of the disease before the immune system had time to start producing antibodies.
Several in-clinic tests can be run that act similarly to MAT. They have similar limitations but can offer more rapid diagnosis in certain situations.
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