Clustering of Mycobacterium avium subsp. paratuberculosis in rabbits and the environment: how hot is a hot spot?

Abstract

Clustering of pathogens in the environment leads to hot spots of diseases at local, regional, national, and international levels. Scotland contains regional hot spots of Johne's disease (caused by Mycobacterium avium subsp. paratuberculosis) in rabbits, and there is increasing evidence of a link between paratuberculosis infections in rabbits and cattle. The spatial and temporal dynamics of paratuberculosis in rabbits within a hot spot region were studied with the overall aim of determining environmental patterns of infection and thus the risk of interspecies transmission to livestock. The specific aims were to determine if prevalence of paratuberculosis in rabbits varies temporally between seasons and whether the heterogeneous spatial environmental distribution of M. avium subsp. paratuberculosis on a large scale (i.e., regional hot spots) is replicated at finer resolutions within a hot spot. The overall prevalence of M. avium subsp. paratuberculosis in rabbits was 39.7%; the temporal distribution of infection in rabbits followed a cyclical pattern, with a peak in spring of 55.4% and a low in summer of 19.4%. Spatially, M. avium subsp. paratuberculosis-infected rabbits and, thus, the risk of interspecies transmission were highly clustered in the environment. However, this is mostly due to the clustered distribution of rabbits. The patterns of M. avium subsp. paratuberculosis infection in rabbits are discussed in relation to the host's socioecology and risk to livestock.

FIG. 1.

Distribution of all rabbits sampled, divided into the five local populations.

FIG. 2.

K-function analysis of the distribution of M. avium subsp. paratuberculosis-infected rabbits across the site (dotted lines, 95% confidence envelope; solid line, CSR; boldface line, M. avium subsp. paratuberculosis distribution). Above CSR indicates that the M. avium subsp. paratuberculosis infection is clustered; below indicates that it is regularly distributed.

FIG. 3.

K-function analysis of the distribution of all rabbits across the site (dotted lines, 95% confidence envelope; solid line, CSR; boldface line, rabbit distribution). Above CSR indicates that the rabbits are clustered; below indicates that they are regularly distributed.

FIG. 4.

K-function analysis of the difference in the distributions of infected rabbits [K(1)] and noninfected rabbits [K(0)] across the site (dotted lines, confidence envelopes; solid line, CSR; boldface line, difference between K-functions of infected and noninfected rabbits). Above CSR indicates that the M. avium subsp. paratuberculosis-infected rabbits are clustered; below indicates that they are regularly distributed within the distribution of all rabbits.

FIG. 5.

K-function analysis of the distribution of warrens across the site (dotted lines, 95% confidence envelope; solid line, CSR; boldface line, warren distribution). Above CSR indicates that the warrens are clustered; below indicates that they are regularly distributed.

FIG. 6.

Representations of a regular distribution (a) and a clustered distribution (b) of infected individuals in a heterogeneously distributed host population (•, infected; ○, noninfected).