Leptospirosis

• In New Zealand the main risk factor is animal exposure e.g. farming, meat processing, but contact with soil or water contaminated by animals is an additional source of exposure. Seen in travelers, particularly to the tropics, who have had water exposure.
• Typically causes a non-specific febrile illness, but conjunctival redness is a more specific sign. Muscle tenderness may also be a feature. Can involve a number of organ systems. Illness ranges from very mild to severe.

The organism infects the kidneys of various animals and is shed in the urine – this is how it contaminates the environment and human exposure occurs.

• Can be difficult to diagnose, as often presents non-specifically. Consider leptospirosis in those with possible exposures, especially if accompanied by conjunctival redness.
• Consider the diagnosis in patients with febrile multisystem illnesses e.g. lung infiltrates, kidney, liver derangement.
• Mimics to consider: Primary HIV infection, syphilis, viral hepatitis (if significant LFT derangement), bacteraemia.

IgM serology is the recommended initial screening test for most patients, however it may be negative within the first week of symptoms.

• Reasonable sensitivity – a negative test in a person with symptoms >7 days makes the diagnosis less likely
• Moderate specificity – a positive test does not necessarily confirm infection, as other febrile illnesses can cause cross-reactions

For this reason, positive tests are sent to a reference laboratory for microscopic agglutination testing (MAT), which is more specific.

• To improve diagnostic accuracy, a convalescent sample collected 14-21 days after the first is recommended.
• A significant rise on MAT between the first (acute) and convalescent samples confirms leptospirosis.

PCR is also used to diagnose leptospirosis. It can be used on blood samples within the first 7 days of illness. After this time, the organisms move to the kidneys, so a urine is required.

• In the first 7 days of illness: on blood and urine
• After 7 days of illness: on urine (the organisms leave the blood and move to the kidneys after this time).
• Moderate sensitivity – particularly in the urine, shedding of organisms is intermittent, so a negative test does not exclude infection
• Excellent specificity  – a positive test confirms the diagnosis.

We recommend IgM serology as the initial test for most patients. In general PCR is reserved for hospitalised patients. In instances of possible occupational exposure (where WorkSafe NZ may investigate and an ACC claim may be made) PCR is sometimes used, due to its excellent specificity.