Manlove Kezia R, Cassirer E Frances, Cross Paul C, Plowright Raina K, Hudson Peter J
Center for Infectious Disease Dynamics, Pennsylvania State University, University Park, PA 16802, USA
Idaho Department of Fish and Game, Lewiston, ID 83501, USA.
Proc Biol Sci. 2014 Dec 22;281(1797). doi: 10.1098/rspb.2014.2331.
Group living facilitates pathogen transmission among social hosts, yet temporally stable host social organizations can actually limit transmission of some pathogens. When there are few between-subpopulation contacts for the duration of a disease event, transmission becomes localized to subpopulations. The number of per capita infectious contacts approaches the subpopulation size as pathogen infectiousness increases. Here, we illustrate that this is the case during epidemics of highly infectious pneumonia in bighorn lambs (Ovis canadensis). We classified individually marked bighorn ewes into disjoint seasonal subpopulations, and decomposed the variance in lamb survival to weaning into components associated with individual ewes, subpopulations, populations and years. During epidemics, lamb survival varied substantially more between ewe-subpopulations than across populations or years, suggesting localized pathogen transmission. This pattern of lamb survival was not observed during years when disease was absent. Additionally, group sizes in ewe-subpopulations were independent of population size, but the number of ewe-subpopulations increased with population size. Consequently, although one might reasonably assume that force of infection for this highly communicable disease scales with population size, in fact, host social behaviour modulates transmission such that disease is frequency-dependent within populations, and some groups remain protected during epidemic events.
群居生活促进了病原体在群居宿主间的传播，然而，长期稳定的宿主社会组织实际上可以限制某些病原体的传播。当在疾病流行期间亚种群间的接触很少时，传播就局限于亚种群内。随着病原体传染性的增加，人均感染接触数接近亚种群规模。在此，我们证明在大角羊（加拿大盘羊）的高传染性肺炎流行期间就是这种情况。我们将单独标记的大角羊母羊分类到不相交的季节性亚种群中，并将羔羊存活至断奶的方差分解为与个体母羊、亚种群、种群和年份相关的成分。在疫情期间，羔羊存活率在母羊亚种群之间的差异远大于在种群之间或年份之间的差异，这表明病原体传播是局部性的。在无疾病的年份未观察到这种羔羊存活模式。此外，母羊亚种群中的群体规模与种群规模无关，但母羊亚种群的数量随种群规模增加。因此，尽管人们可能合理地认为这种高传染性疾病的感染强度与种群规模成比例，但实际上，宿主的社会行为调节了传播，使得疾病在种群内是频率依赖的，并且在疫情期间一些群体仍受到保护。
