Berry Wilkinson asked:

I was wondering if you can titre test for leptospirosis? Or is it only useful when you are testing sick dogs? Thanks.

Answer:

Hi Berry, thanks for your question about testing for Leptospirosis. To answer it, I’ll briefly discuss Leptospirosis as a disease, then talk about the different diagnostic techniques available. Finally, I’ll discuss vaccination and the implications for diagnosis.

What is Leptospirosis?

Leptospirosis (“Lepto”) is a disease caused by bacteria of the genus Leptospira. There are more than 300 strains (technically called serovars) of the bacteria. In the UK, Leptospira icterohaemorrhagiae and L. canicola used to be the most common, but since widespread vaccination against these has started, it is now thought that L. interrogans and L. kirschneri may be more important.

The disease is transmitted by body fluids of infected animals, including rats. The symptoms of Leptospirosis in dogs include:

  • Fever and sore muscles.
  • Loss of appetite, vomiting, diarrhoea and dehydration.
  • It may cause kidney or liver failure
  • Sometimes the only symptom is sudden death.
  • Infected dogs may shed the bacteria in their urine for months or years without showing any clinical signs.
  • Leptospirosis is highly zoonotic – i.e. it is a high risk pathogen for infecting humans.

How is Leptospirosis diagnosed?

There are four methods to test for Leptospira in clinical samples, of which two are clinically useful. They are:

  • Darkfield microscopy – looking for the bacteria themselves. This is very siple, but is notoriously unreliable, unfortunately!
  • Bacterial culture – attempting to grow the bacteria; however, in many cases the bacteria are very hard to culture, so even in confirmed infections, this test may come back negative.
  • Serology – looking for antibodies produced by the immune system in response to the presence of the bacteria. However, vaccination will often lead to a positive response, and low-positive titres (levels of antibody) may persist for a prolonged period. In addition, the levels of antibodies often won’t be significant in the first week of infection.
  • PCR – testing blood (early infection) or urine (later stages of infection or carrier status) for genetic material from the Leptospira bacteria; this is a very sensitive and specific test. However, a negative PCR result doesn’t rule out carrier status because the bacteria are only shed intermittently in the urine, and will not be present in the bloodstream; and it can also appear negative in some milder infections.

So how is serology interpreted?

  • The normal screening test for Lepto is an antibody test (“ELISA testing“) that gives a simple positive or negative result.
    • If this is negative, then in general either:
      • The dog doesn’t have Lepto, or
      • The dog has only been infected in the last week or so.
    • If the result is positive, then:
      • The dog has Lepto, or
      • The dog has had Lepto in the past, or
      • The dog has been vaccinated and still has high levels of circulating antibody.
  • If the ELISA-test is positive; or if the symptoms are suspicious but PCR (genetic) testing is negative, the next phase is to use a different type of antibody testing (“MAT serology“) to determine the level of antibodies in the blood (the titre).
    • On a single test:
      • Low titres are most likely to represent vaccination or past infection.
      • Moderate titres may indicate vaccination or infection.
      • High titres usually represent acute infection.
    • However, it is far more useful to carry out paired serology – 2 tests 7-10 days apart:
      • In a genuine infection, the titre would normally be expected to rise by at least four-fold.
      • In chronic infection, or asymptomatic shedding, diagnosis can be really difficult, but a persistent moderate titre that doesn’t decay over time is highly suggestive of chronic infection; however, demonstration of the organism’s genetic material by PCR in repeated urine samples is often more practical.

What about vaccination?              

There are a number of different Leptospirosis vaccines available; most of them cover 2 strains (“bivalent vaccines”), although some now cover 4 (“quadrivalent vaccines”). They are aimed at covering the most common types that cause disease, and there is relatively little cross-protection between strains (so immunity to one strain or serovar won’t usually protect against another).

The vaccine doesn’t necessarily prevent infection, but it should reduce the risk of infection, and it does reduce the severity of clinical disease and shedding (for whichever strains or serovars are covered by that vaccine).

There are some commercial tests that claim to determine whether a dog requires vaccination against Leptospira by testing circulating antibodies. This may work in some cases, but it is very limited. There are a number of problems with this approach:

  • The serological titre (level of antibodies in the blood) can only tell you how much antibody there is in the bloodstream at the specific time the test is done – it cannot tell you whether the levels will remain high for the following 12 months.
  • The link between antibody levels and protection isn’t consistent – some dogs appear to utilise other parts of the immune system (cell mediated immunity) and are protected against Lepto even in the absence of significant circulating antibody titres.
  • After vaccination, titres normally drop off over 4-5 months, but protection lasts for 12 months.

As a result, it is wisest to maintain annual vaccination against Leptospirosis, to reduce the risk of infection to your dog and to you.

I hope that helps!

David Harris BVSc MRCVS