Foot and mouth disease virus is one of the most contagious pathogens known to man. The virus infects cloven-hoofed ungulates, including both wild and domestic species, and is arguably the most trade-restricting pathogen globally. In sub-Saharan Africa, African buffalo act as maintenance host for FMD, complicating disease control. However, surprisingly little is known about the dynamics of FMDV transmission in its maintenance host. We estimated the transmission and recovery rates, and the basic reproductive number (R0 ) for FMD in buffalo from empirical data, and used a mathematical model to test the idea that the virus might be maintained as a typical “childhood” infection, circulating through each year’s population of new susceptible calves.
We conducted an experimental study to estimate the transmission and recovery rates from acute FMDV infection; and a cohort study to assess the timing of births in a herd of buffalo in Kruger National Park, South Africa. These data allowed us to parameterize a stochastic, individual-based mathematical model representing the transmission dynamics of acute FMDV infection in African buffalo.
Results/Conclusions
FMD was transmitted readily from acutely infected animals to naïve hosts, and recovery occurred within 4-6 days, resulting in estimates for R0 of 16.4 (strain SAT3), 27.1 (SAT2), and 42.3 for SAT1. Births occurred predominantly between November and March, giving an inter-birth interval of 5 months. Our models suggest that calf-to-calf transmission alone is highly unlikely to support persistence of FMDV in buffalo populations: simulated viral transmission almost invariably burned out in less than a year for realistic herd sizes.
If the loss of maternal immunity in calves is very variable, viral persistence due to calf-to-calf transmission becomes more likely for large populations. Our team is investigating the timing of loss of maternal immunity to FMDV to evaluate this possibility. However, for small buffalo populations, other mechanisms of FMDV transmission are likely to be important for viral persistence. Possibilities include viral transmission from carrier hosts, and loss of effective immune protection in previously exposed animals, due to rapid antigenic change of the virus. Empirical data on viral shedding by carriers and the rate of viral antigenic shift are needed to evaluate the importance of these mechanisms in the dynamics of FMD in natural populations of its southern African wildlife reservoir.