Collecting data on leptospirosis prevalence is often difficult. In some countries the infection is nationally notifiable, and so case data is available to researchers. In others it is not, and whilst it is possible to search individual local records the accuracy of the resulting data is very poor. In areas where the infection is endemic data is impossible to collect and only rough statistical figures are used. What is clear is that infection rates are not falling, and in some countries there are signs of an increase over the last few years.
In the ICD10 disease classification system, leptospirosis is code A27.
The table on the right shows nationally reported cases of human leptospirosis. The data is based only on reported and verified cases, and so is estimated to be between 5 and 20 times below the true figures. Leptospirosis ceased to be a nationally notifiable disease in the USA in 1995, hence no recent data is available. Figures for France show the total territorial figures (including overseas territories) then the continental figure in brackets. As is obvious, the figures from France are far higher than for the USA, and given the much larger population we would expect the US figures to be many orders of magnitude higher. The French data is based on confirmed serology and so is accurate, therefore our conclusion has to be that the US reporting and testing system failed.
Note that data for Australia has a margin of error of 10 cases as the data is based on semi-voluntary submission of reports, so historical figures are revised as new data is submitted.
The USA data shown here was supplied by the Centers for Disease Control & Prevention NNDSS Service. Data for France from 1996 to 2004 was supplied by the Institut Pasteur and Australian data is from the QHPSS.
The International Leptospirosis Society is developing a global database of human infection, however this is not yet available to the public and is very much a work in progress. Details can be obtained from the ILS database website at LeptoNet.
The incidence of leptospirosis is entirely dependent on the geographical location – weather conditions, local vegetation, presence of rats, human lifestyles and population density all influence infection. Globally there are three classes of location:-
L2 – Endemic
Locations where the infection is commonplace, caused by high rainfall, close human contact with livestock or wild animals, poor sanitation or workplace exposure (rice farming, etc). Example countries in class L2 are India, Central America and certain areas of the Pacific Rim.
L1 – General
Locations where the infection is at the international average of 0.05 cases per 100,000, and infection is usually the result of accidental exposure through wild rats, livestock or direct contact with water through leisure or occupation. Example countries in class L1 are North America, Europe, Australia and the former Soviet territories.
L0 – Clear
Locations where the infection is technically impossible to sustain in the environment, through extremes of climate or rodent absence. Example countries in class L0 are the Arctic and Antarctic zones, desert climates such as Saharan Africa, or isolated island communities with no rat population.