Modeling the Spread of Vector-Borne Diseases on Bipartite Networks

BACKGROUND: Vector-borne diseases for which transmission occurs exclusively between vectors and hosts can be modeled as spreading on a bipartite network. METHODOLOGY/PRINCIPAL FINDINGS: In such models the spreading of the disease strongly depends on the degree distribution of the two classes of nodes. It is sufficient for one of the classes to have a scale-free degree distribution with a slow enough decay for the network to have asymptotically vanishing epidemic threshold. Data on the distribution of Ixodes ricinus ticks on mice and lizards from two independent studies are well described by a scale-free distribution compatible with an asymptotically vanishing epidemic threshold. The commonly used negative binomial, instead, cannot describe the right tail of the empirical distribution. CONCLUSIONS/SIGNIFICANCE: The extreme aggregation of vectors on hosts, described by the power-law decay of the degree distribution, makes the epidemic threshold decrease with the size of the network and vanish asymptotically

Avian Host-Selection by Culex pipiens in Experimental Trials

Evidence from field studies suggests that Culex pipiens, the primary mosquito vector of West Nile virus (WNV) in the northeastern and north central United States, feeds preferentially on American robins (Turdus migratorius). To determine the contribution of innate preferences to observed preference patterns in the field, we conducted host preference trials with a known number of adult female C. pipiens in outdoor cages comparing the relative attractiveness of American robins with two common sympatric bird species, European starling, Sternus vulgaris and house sparrow, Passer domesticus. Host seeking C. pipiens were three times more likely to enter robin-baited traps when with the alternate host was a European starling (n = 4 trials; OR = 3.06; CI [1.42–6.46]) and almost twice more likely when the alternative was a house sparrow (n = 8 trials; OR = 1.80; CI = [1.22–2.90]). There was no difference in the probability of trap entry when two robins were offered (n = 8 trials). Logistic regression analysis determined that the age, sex and weight of the birds, the date of the trial, starting-time, temperature, humidity, wind-speed and age of the mosquitoes had no effect on the probability of a choosing a robin over an alternate bird. Findings indicate that preferential feeding by C. pipiens mosquitoes on certain avian hosts is likely to be inherent, and we discuss the implications innate host preferences may have on enzootic WNV transmission

Associations of passerine birds, rabbits, and ticks with <it>Borrelia miyamotoi and Borrelia andersonii</it> in Michigan, U.S.A.

<p>Abstract</p> <p>Background</p> <p>Wild birds contribute to maintenance and dissemination of vectors and microbes, including those that impact human, domestic animal, and wildlife health. Here we elucidate roles of wild passerine birds, eastern cottontail rabbits (<it>Sylvilagus floridanus</it>), and <it>Ixodes dentatus</it> ticks in enzootic cycles of two spirochetes, <it>Borrelia miyamotoi</it> and <it>B. andersonii</it> in a region of Michigan where the zoonotic pathogen <it>B. burgdorferi</it> co-circulates.</p> <p>Methods</p> <p>Over a four-year period, wild birds (n = 19,631) and rabbits (n = 20) were inspected for tick presence and ear tissue was obtained from rabbits. Samples were tested for <it>Borrelia</it> spirochetes using nested PCR of the 16S-23S rRNA intergenic spacer region (IGS) and bidirectional DNA sequencing. Natural xenodiagnosis was used to implicate wildlife reservoirs.</p> <p>Results</p> <p><it>Ixodes dentatus,</it> a tick that specializes on birds and rabbits and rarely bites humans, was the most common tick found, comprising 86.5% of the 12,432 ticks collected in the study. The relapsing fever group spirochete <it>B. miyamotoi</it> was documented for the first time in ticks removed from wild birds (0.7% minimum infection prevalence; MIP, in <it>I. dentatus</it>), and included two IGS strains. The majority of <it>B. miyamotoi</it>-positive ticks were removed from Northern Cardinals (<it>Cardinalis cardinalis</it>). <it>Borrelia andersonii</it> infected ticks removed from birds (1.6% MIP), ticks removed from rabbits (5.3% MIP), and rabbit ear biopsies (5%) comprised twelve novel IGS strains. Six species of wild birds were implicated as reservoirs for <it>B. andersonii.</it> Frequency of <it>I. dentatus</it> larval and nymphal co-feeding on birds was ten times greater than expected by chance. The relatively well-studied ecology of <it>I. scapularis</it> and the Lyme disease pathogen provides a context for understanding how the phenology of bird ticks may impact <it>B. miyamotoi</it> and <it>B. andersonii</it> prevalence and host associations.</p> <p>Conclusions</p> <p>Given the current invasion of <it>I. scapularis</it>, a human biting species that serves as a bridge vector for <it>Borrelia</it> spirochetes, human exposure to <it>B. miyamotoi</it> and <it>B. andersonii</it> in this region may increase. The presence of these spirochetes underscores the ecological complexity within which <it>Borrelia</it> organisms are maintained and the need for diagnostic tests to differentiate among these organisms.</p