Epidemics:guineapigs:data

The underlying guinea pig experiments were conducted using eight animals per trial: four infected and four healthy but susceptible to infection. Individual pigs were placed together in a chamber and arranged pairwise on four shelves such that each shelf contained one healthy and one infected individual. Thus there were two vertical columns - one consisting of the healthy and the other, of the infected guinea pigs. The air flowed horizontally from the infected towards the healthy animals to maximize the infection rate. In order to control infectivity, the virus concentration in nasal wash of each guinea pig was measured every two days. The goal was to discover how many susceptible animals would become infected, and after how many days, under controlled air temperature and humidity conditions.

The experiments were performed at five different relative humidity values (20%, 35%, 50%, 65%, and 80%) and three different temperatures (30$^\mathrm{o}$C, 20$^\mathrm{o}$C, and 5$^\mathrm{o}$C). At 30$^\mathrm{o}$C no infections occurred. For both 20$^\mathrm{o}$C and 5$^\mathrm{o}$C, low relative humidity of 20%-35% was most favorable, leading to nearly 100% transmission. At 20$^\mathrm{o}$C the transmission rapidly fell at 5% humidity, rose again to quite high levels at 65%, and was completely blocked at 80%. Such behavior probably reflects the virus stability in aerosols. At 5$^\mathrm{o}$C the transmission was still very efficient at 50% humidity but dropped to 0.5 for higher humidity values. For infected animals housed at 5$^\mathrm{o}$C, the duration of peak shedding was approximately 40 h longer than that of animals housed at 20$^\mathrm{o}$C. This increased shedding probably accounts for the generally enhanced transmission seen at 5$^\mathrm{o}$C.