1.2 Transmission

Portals of entry include cuts and abrasions or mucous membranes such as the conjunctival, oral, or genital surfaces. Exposure may occur through either direct contact with an infected animal or through indirect contact via soil or water contaminated with urine from an infected animal. Individuals with occupations at risk for direct contact with potentially infected animals include veterinarians, abattoir workers, farm workers (particularly in dairy milking situations), hunters and trappers, animal shelter workers, scientists, and technologists handling animals in laboratories or during fieldwork. The magnitude of the risk depends on the local prevalence of leptospiral carriage and the degree and frequency of exposure. Most of these infections are preventable by the use of appropriate personal protective equipment such as rubber boots, gloves, and protective eyewear. Since many of these infections are covered by occupational health and safety regulations, local risk assessments and training are essential (Steneroden et al. 2011).

Indirect contact with water or soil contaminated with leptospires is much more common, and can be associated with occupational, recreational, or avocational activities. In addition to the risks associated with outdoor work listed above, sewer work, military exercises, and farming in high rainfall tropical regions are recognized; the latter is by far the most important numerically. Agricultural workers at risk for leptospirosis include rice field workers, taro farmers, banana farmers, and sugar cane and pineapple field harvesters (Levett 2001). These occupations involve activities likely to result in exposure of cuts and abrasions to soil and water contaminated with the urine of rodents and other animals attracted to food sources. For example, banana workers accounted for two-thirds of the reported leptospirosis cases in a tropical region of Queensland, Australia (Smythe et al. 1997).

Recreational exposures include all freshwater water sports including caving (Self et al. 1987), canoeing (Waitkins 1986), kayaking (Jevon et al. 1986; Shaw 1992), rafting (Wilkins et al. 1988), and triathlons (Morgan et al. 2002; Sejvar et al. 2003). The importance of this type of exposure has increased over the past 20 years as the popularity of adventure sports and races has increased, and also because the relative cost of travel to exotic destinations has decreased (Lau et al. 2010a). Competitive events create the potential for large outbreaks; 80 and 98 leptospirosis cases occurred as part of the 2000 Eco-challenge competition (Fig. 2a) (Sejvar et al. 2003) and 1998 Springfield triathlon (Morgan et al. 2002), respectively. Participants in international events may become ill after having returned home, often to multiple destination countries, which complicates recognition and investigation of outbreaks. In many series, the incidence of leptospirosis is much higher in males than females (Everard et al. 1992; Guerra-Silveira and Abad-Franch 2013; Katz et al. 2011). However, it seems likely that gender differences in leptospirosis incidence are due entirely to exposure-related bias, as reports of leptospirosis outbreaks related to athletic events where males and females have similar levels of exposure have found no significant effects of gender on development of illness (Morgan et al. 2002; Sejvar et al. 2003).

Fig. 2

Epidemiologic settings for leptospirosis. a A high proportion of contestants in the 2000 Eco-Challenge multisport race held in Malaysian Borneo developed leptospirosis. Of 189 participants contacted by the Centers for Disease Control, 80 (42 %) met the case definition for leptospirosis. Risk factors included exposure for extended periods of time to the rain-swollen Segama river (photograph credit Reed Hoffmann). b This rural village in Laos is a typical epidemiologic setting for leptospirosis. Residents of tropical regions of the world with high levels of rainfall are at increased risk of leptospirosis, particularly when standing water is contaminated by urine from wild or domesticated animals, which may serve as reservoir hosts for pathogenic Leptospira species (photograph credit Ben Adler)

Avocational exposures are by far the most important exposures, affecting millions of people living in tropical regions. As illustrated in Fig. 2b, lack of adequate sanitation and poor housing combine to exacerbate the risk of exposure to leptospires in both rural and urban slum communities (Bharti et al. 2003; Felzemburgh et al. 2014; Hotez et al. 2008; Reis et al. 2008). The role of poor housing is also suggested by the study of Maciel et al. (2008), which showed a greatly increased risk (odds ratio 5.29) of leptospirosis exposure among individuals who live in the same household as a leptospirosis patient. These factors are most likely surrogates for rat exposure, as proximity to uncollected trash and sighting of rats increased the risk of leptospirosis among residents of urban slums (Reis et al. 2008). The recognition of large outbreaks following excess rainfall events (Ahern et al. 2005; Dechet et al. 2012; Ko et al. 1999; Zaki and Shieh 1996) led to the labeling of leptospirosis as an emerging infectious disease two decades ago (Levett 1999). More recently, the interaction of urbanization and climate change has been identified as a significant risk for both increased incidence and increasing frequency of outbreaks of leptospirosis (Lau et al. 2010b). The need for interdisciplinary research to understand the effects of anthropogenic change and its effect on the epidemiology of leptospirosis has been proposed (Vinetz et al. 2005).

1.3 Global Burden of Disease

Early studies of leptospirosis incidence concentrated on occupational disease, primarily in developed countries related to leptospirosis in livestock animals (Alston and Broom 1958; Faine et al. 1999). As the importance of the disease in tropical countries became better recognized, guidelines were developed for the diagnosis and control of leptospirosis (Faine 1982). As diagnostic methods became more widely available, numerous epidemiologic studies were reported from many countries. An initial attempt to gather global data on the incidence of leptospirosis was published over 15 years ago (WHO 1999). Based on global data collected by International Leptospirosis Society surveys, the incidence was estimated to be 350,000–500,000 severe leptospirosis cases annually (Ahmed et al. 2012). Despite these efforts, the global burden of leptospirosis was felt to be largely underestimated for a number of reasons, including the fact that the vast majority of countries either lack a notification system or notification is not mandatory (Ahmed et al. 2012). To address these shortcomings, the WHO established the Leptospirosis Burden Epidemiology Reference Group (LERG) (Abela-Ridder et al. 2010). The LERG met for the first time in 2009 and for a second time in 2010. The specific objectives of the second LERG meeting were: (1) To review and appraise the revised systematic review of mortality, morbidity, and disability from human leptospirosis; (2) To review a draft disease transmission model for leptospirosis and provide technical input for the further development and refinement of the model; (3) To assemble preliminary estimates of the disease burden; (4) To identify gaps in knowledge and research; and (5) To advise WHO on the next steps for estimation of the burden of human leptospirosis and the implications for policy. The resulting LERG report included a systematic literature review that estimated the overall global annual incidence of endemic and epidemic human leptospirosis at 5 and 14 cases per 100,000 population, respectively (WHO 2011). Endemic human leptospirosis rates varied by region from 0.5/100,000 population in Europe to 95/100,000 population in Africa.