Costs and benefits of group living with disease: a case study of pneumonia in bighorn lambs (Ovis canadensis).
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.
Center for Infectious Disease Dynamics, Pennsylvania State University, University Park, PA 16802, USA krm17@psu.edu.
Kezia R
http://orcid.org/0000-0002-7200-5236
KR
Manlove
Idaho Department of Fish and Game, Lewiston, ID 83501, USA.
E Frances
EF
Cassirer
US Geological Survey, Northern Rocky Mountain Science Center, Bozeman, MT 59715, USA.
Paul C
PC
Cross
Center for Infectious Disease Dynamics, Pennsylvania State University, University Park, PA 16802, USA Department of Microbiology and Immunology, Montana State University, Bozeman, MT 59717, USA.
Raina K
RK
Plowright
Department of Biology and Huck Institute for Life Sciences, Pennsylvania State University, University Park, PA 16802, USA.
Peter J
PJ
Hudson
D000818:Animals; D001522:Behavior, Animal; D003362:Cost-Benefit Analysis; D004196:Disease Outbreaks / Q000662:veterinary; D005260:Female; D011014:Pneumonia / Q000453:epidemiology / Q000401:mortality / Q000662:veterinary*; D011156:Population Density; D012621:Seasons; D012756:Sheep; D012757:Sheep Diseases / Q000382:microbiology / Q000401:mortality / Q000635:transmission*; D034581:Sheep, Bighorn / Q000382:microbiology / Q000502:physiology; D012919:Social Behavior
bighorn sheep; epidemic size; social contact network; social immunity; transmission process; variance decomposition