Abstract
In New Zealand (NZ), leptospirosis is a mostly occupational zoonosis, with >66% of the recently notified cases being farm or abattoir workers. Livestock species independently maintain Leptospira borgpetersenii serovar Hardjo and L. interrogans serovar Pomona, and both are included in livestock vaccines. The increasing importance in human cases of Ballum, a serovar associated with wildlife, suggests that wildlife may be an overlooked source of infection. Livestock could also act as bridge hosts for humans. Drawing from disease ecology frameworks, we chose five barriers to include in this review based on the hypothesis that cattle act as bridge hosts for Ballum. Using a narrative methodology, we collated published studies pertaining to (a) the distribution and abundance of potential wild maintenance hosts of Ballum, (b) the infection dynamics (prevalence and pathogenesis) in those same hosts, (c) Ballum shedding and survival in the environment, (d) the exposure and competency of cattle as a potential bridge host, and (e) exposure for humans as a target host of Ballum. Mice (Mus musculus), rats (Rattus rattus, R. norvegicus) and hedgehogs (Erinaceus europaeus) were suspected as maintenance hosts of Ballum in NZ in studies conducted in the 1970s–1980s. These introduced species are distributed throughout NZ, and are present on pastures. The role of other wildlife in Ballum (and more broadly Leptospira) transmission remains poorly defined, and has not been thoroughly investigated in NZ. The experimental and natural Ballum infection of cattle suggest a low pathogenicity and the possibility of shedding. The seroprevalence in cattle appears higher in recent serosurveys (3 to 14%) compared with studies from the 1970s (0 to 3%). This review identifies gaps in the knowledge of Ballum, and highlights cattle as a potential spillover host. Further studies are required to ascertain the role that wild and domestic species may play in the eco-epidemiology of Ballum in order to understand its survival in the environment, and to inform control strategies.
Highlights
Introduction distributed under the terms andLeptospira are conjectured to be saprophytic soil bacteria that evolved into pathogenic strains by adaptation to mammalian hosts, and genomic tools are beginning to unravel the diversity of the species in this genus [1,2]
As meat workers had a lower incidence than farmers for Ballum [20], and as dairy farmers are in more frequent contact with livestock species compared to other farmers, we limited the notion of bridge host to this production system
Leptospirosis due to L borgpetersenii sv Ballum is emerging as an important problem in humans in New Zealand (NZ) [19,20,173], but has not been thoroughly studied, and has even been overlooked in NZ in the past decades
Summary
The online databases used were Scopus, Web of Science, and SciQuest. Google Scholar, the Massey University library database (Discover) and NZResearch.org.nz were searched for additional grey literature on Leptospira in NZ, as well as the paper archives of the Leptospirosis Reference Centre in Amsterdam (spanning 1915–1990). S1), informing a list of maintenance hosts to include in the rest of this review (barriers a and b; see below): the house mouse (Mus musculus), ship rat (Rattus rattus), brown rat (R. norvegicus) and hedgehog. Drawing from two disease ecology frameworks conceptualising the barriers to be crossed for pathogen spillover [22] and the notion of bridge host [23], we chose five barriers (a–e) to include in the review based on the hypothesis that cattle act as bridge hosts (Figure 1):. (e) the infection dynamics of Ballum in maintenance hosts, including its prevalence and pathogenesis; Ballum shedding and survival in the environment; the exposure and competency of cattle as a potential bridge species; and the exposure (risk factors) for humans (target hosts). As meat workers had a lower incidence than farmers for Ballum [20], and as dairy farmers are in more frequent contact with livestock species compared to other farmers, we limited the notion of bridge host to this production system
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