A modest model explains the distribution and abundance of Borrelia burgdorferi strains

Abstract

The distribution and abundance of Borrelia burgdorferi, including human Lyme disease strains, is a function of its interactions with vertebrate species. We present a mathematical model describing important ecologic interactions affecting the distribution and abundance of B. burgdorferi strains, marked by the allele at the outer surface protein C locus, in Ixodes scapularis ticks, the principal vector. The frequency of each strain in ticks can be explained by the vertebrate species composition, the density of each vertebrate species, the number of ticks that feed on individuals of each species, and the rate at which those ticks acquire different strains. The model results are consistent with empirical data collected in a major Lyme disease focus in New England. An applicable extension of these results would be to predict the proportion of ticks carrying human infectious strains of B. burgdorferi from disease host densities and thus predict the local risk of contracting Lyme disease.

Figure 1

Distribution (%) of outer surface protein c major groups (oMGs) in host-seeking nymphs caught at the Institute for Ecosystem Studies in Dutchess County, New York in 2002. 2002.

Figure 2

Variance of the estimates of P_species plotted against G, the measure of the fit of the simulated oMG frequencies to the observed data. The variance for all parameters remain relatively constant when G is less than 0.025. When values of G greater than 0.025 are included, the variance in the estimates of P_species increases sharply for all species. The variance continued to increase for all P_species through G = 0.3, the largest value retained during the simulation. m = mouse; c = chipmunk; b = shrew; s = squirrel; x = category X.

Figure 3

Box-and-whisker plot of the measured transmission probabilities probabilities as well as those estimated by the simulation (category X). The box represents the median (line) and upper/lower quartiles and the whiskers include all values (the small circle is an outlier). The transmission probabilities for category X are quantitatively similar to those of short-tailed shrews and are much greater than those of squirrels, suggesting that category X does not represent species with low transmission probabilities.

Figure 4

Percentage of nymphs predicted to carry a human infectious strain of Borrelia burgdorferi in areas with different vertebrate community structures. Pl = Peromyscus leucopus; T ; Ts = Tamias striatus; B ; Bb = Blarina brevicauda; S ; Sc = Sciurus carolinensis.