Table 2.
Description of predictor and response variables used in the GAM analyses.
| term | type | description |
|---|---|---|
| response variables | ||
| human case fatality rate | numeric | the proportion of human cases for a given virus that are fatal |
| human transmissibility | categorical ranking (1–4) | a virus’s capacity for human-to-human transmission see the electronic supplementary material, Methods for a description of the ranking system |
| zoonotic potential | binary (0,1) | the probability that a virus has the capacity to infect humans (i.e. is or is not zoonotic) |
| host predictors | ||
| length of gestation period | numeric | length of pregnancy (in days) |
| litter size | numeric | average number of offspring produced at one time |
| maximum lifespan | numeric | maximum recorded longevity (in months) |
| body mass | numeric | average body mass (in grams) |
| phylogenetic distance from humans | numeric | distance from humans on a cytochrome b phylogenetic tree |
| taxonomic order | categorical | taxonomic classification |
| number of disease-related citations | numeric | number of PubMed citations relevant to zoonotic diseases; to control for any potential publication bias |
| viral predictors | ||
| whether or not the virus is enveloped | binary | does the virus have an external envelope? |
| whether or not the virus replicates in the cytoplasm | binary | does the virus replicate in the cytoplasm? |
| average genome length | numeric | metric for genome size |
| DNA or RNA | binary | is the virus DNA or RNA? |
| genome composition | categorical | viral genome classification |
| maximum host phylogenetic distance from humans | numeric | phylogenetic distance from humans of the most distantly related known host for a given virus |
| maximum host phylogenetic breadth | numeric | maximum phylogenetic distance between the two most distantly related known hosts for a given virus |
| number of citations | numeric | number of relevant PubMed citations; control for research effort bias |
| categories for virus–mammal associations | ||
| reservoir status | binary (1,2) | host species role in the maintenance of zoonotic transmission. Species that maintain viruses endemically are reservoir hosts (reservoir status = 1). Species that harbour the virus but are not implicated in zoonotic maintenance are secondary hosts (reservoir status = 2) |
| spillover capacity | binary (1,0) | host species' role in the spillover of zoonoses to humans. Spillover hosts are (1) defined as species that are a source of human infection. Non-spillover hosts are (0) defined as species that have no record of transmission to humans |
| spillover type | binary (1,2) | chain of spillover transmission for a given virus–host system. Spillover to humans from a reservoir host is ‘primary spillover’ (reservoir status = 1, spillover capacity = 1, spillover type = 1). Spillover to humans from a secondary host (bridge host) is ‘secondary spillover’ (reservoir status = 2, spillover capacity = 1, spillover type = 2) |