Humans can become infected with the spirochete Leptospira if they have wounds that contact contaminated soil or water. Leptospira live in the kidneys of reservoir animals and are released in urine into the environment. The majority of leptospirosis cases occurs in tropical locales, where the warm, moist conditions promote the survival of Leptospira in the environment. Flooding following heavy rainfall can cause large epidemics of leptospirosis in these regions of the world. A research article published in the March 15th issue of Clinical Infectious Diseases by Desai and colleagues describes an unexpected outbreak of leptospirosis among strawberry harvesters in Germany, a country not known for tropical weather. The July 2007 outbreak, while small by the standards of those that occur in tropical countries, was Germany's largest since the 1960s. The authors searched for factors that may have caused the epidemic.
The outbreak occurred among laborers who worked on a strawberry farm near Düren. The strawberry harvesters were seasonal workers from Romania, Slovakia, and Poland. Among the 153 who were employed on the farm during the outbreak, 24 were stricken with leptospirosis, and 13 were hospitalized. Fortunately, no one died. Most who became sick had antibodies against the Leptospira serogroup Grippotyphosa in their blood.
In September 2007, the authors conducted a retrospective cohort study to identify risk factors for acquiring leptospirosis. The strawberry harvesters were questioned about possible sources of exposure to Leptospira before the outbreak, including the presence of wounds, contact with rodents, and consumption of unwashed strawberries. Logistic regression of the data revealed two statistically significant risk factors. First, the odds of the harvesters acquiring leptospirosis increased with the number of days worked with unprotected hand lesions (OR, 1.1; 95% CI, 1.04-1.1). Leptospira probably entered the lesions as the workers reached into the water-logged soil. "Accidental contact with rodents" was listed as the other significant risk factor (OR, 4.8; 95% CI, 1.5-15.9), although the nature of the contact was not described by the authors. Of course recall bias may have skewed the results of the retrospective study. However, there is nothing unusual about these risk factors; contact of skin wounds with moist soil laden with Leptospira and exposure to infected rodents are typical means of acquiring leptospirosis.
The common vole was the main suspect in contaminating the strawberry field with Leptospira. More than 10 mouse holes per square meter were found in the field and surrounding areas when the site was examined in September 2007. Kidneys of voles captured in and near the field were infested with the spirochete. Leptospira was successfully cultured from the kidneys and were identified as members of the serogroup Grippotyphosa, the same serogroup that the antibodies in the patients' blood were targeting.
The strawberry field outbreak is reminiscent of the large epidemics of leptospirosis that sickened thousands of German agriculture workers from the 1920s through the 1960s, with the Leptospira serogroup Grippotyphosa carried by voles and hamsters responsible for many cases. In contrast, most German cases of leptospirosis of late are related to activities outside of work. Recreational activities involving water and exposure at home to infected pets and contaminated soil while gardening are common risk factors. Icterohaemorrhagiae, with rats as the reservoir host, is now the most prevalent serogroup in Germany. So why did a 1960s-style leptospirosis outbreak occur in 2007? The authors propose that the unusual weather in the months preceding the outbreak was a critical factor.
Europe experienced its warmest autumn in 2006 since recordings began, and the warm weather continued through the winter into 2007. In Germany, heavy rainfall accompanied the high temperatures in the months preceding the outbreak. May and June 2007 were Germany's wettest months since 1901. The tropical conditions may have augmented the amount of food available to the voles. In addition, the vole population, which naturally fluctuates every 3-5 years, may have already been at the peak of its cycle in 2007. A population explosion of infected voles may have increased the density of Leptospira residing in the strawberry field. Because the wet, warm conditions were favorable to survival of Leptospira, it is easy to imagine how an outbreak would have occurred among workers who spent days reaching into the contaminated soil with unprotected hands.
In conclusion, the authors present leptospirosis as a potential model for an infectious disease affected by global warming. Only time will tell whether the leptospirosis outbreak caused by the confluence of warm weather and heavy rainfall in a normally temperate region was a statistical anomaly or a sign of things to come.
Desai, S., van Treeck, U., Lierz, M., Espelage, W., Zota, L., Sarbu, A., Czerwinski, M., Sadkowska‐Todys, M., Avdicová, M., Reetz, J., Luge, E., Guerra, B., Nöckler, K., & Jansen, A. (2009). Resurgence of Field Fever in a Temperate Country: An Epidemic of Leptospirosis among Seasonal Strawberry Harvesters in Germany in 2007 Clinical Infectious Diseases, 48 (6), 691-697 DOI: 10.1086/597036
Jansen, A., Schöneberg, I., Frank, C., Alpers, K., Schneider, T., and Stark, K. (2005). Leptospirosis in Germany, 1962-2003. Emerging Infectious Diseases 11(7):1048-1054.
Pappas, G., Papadimitriou, P., Siozopoulou, V., Christou, L., Akritidis, N. (2008). The globalization of leptospirosis: worldwide incidence trends. International Journal of Infectious Diseases 12(4):351-357.