Multilevel model for airborne transmission of foot-and-mouth disease applied to Swedish livestock

Abstract

The foot-and-mouth disease is an ever-present hazard to the livestock industry due to the huge economic consequences following an outbreak that necessitates culling of possibly infected animals in vast numbers. The disease is highly contagious and previous epizootics have shown that it spreads by many routes. One such route is airborne transmission, which has been investigated in this study by means of a detailed multilevel model that includes all scales of an outbreak. Local spread within an infected farm is described by a stochastic compartment model while the spread between farms is quantified by atmospheric dispersion simulations using a network representation of the set of farms. The model was applied to the Swedish livestock industry and the risk for an epizootic outbreak in Sweden was estimated using the basic reproduction number of each individual livestock-holding farm as the endpoint metric. The study was based on comprehensive official data sets for both the current livestock holdings and regional meteorological conditions. Three species of farm animals are susceptible to the disease and are present in large numbers: cattle, pigs and sheep. These species are all included in this study using their individual responses and consequences to the disease. It was concluded that some parts of southern Sweden are indeed preconditioned to harbor an airborne epizootic, while the sparse farm population of the north renders such events unlikely to occur there. The distribution of the basic reproduction number spans over several orders of magnitudes with low risk of disease spread from the majority of the farms while some farms may act as very strong disease transmitters. The results may serve as basic data in the planning of the national preparedness for this type of events.

Multiple species animal movements: network properties, disease dynamics and the impact of targeted control actions

“…This is because the connectivity of single host farm networks allows for the spread of disease quickly, infecting more farms of the specific host species in the 30-day time frame than when the disease is started in a multi-host farm. Similar patterns have been observed in past epidemics, such as the FMD [ 14 , 15 , 26 , 42 , 43 ] where multi-host farms had a role in infection propagation.…”

“…Furthermore, network analysis approaches have been widely used recently; the great majority centered on describing the contact networks of a single species, here we cite just some relevant examples [ 2 , 18 , 32 , 39 , 40 , 46 – 48 ], however, only a fraction of studies have explicitly considered the interaction of possible contacts among more than one species while constructing their transmission networks [ 7 – 9 , 11 – 13 , 15 , 16 , 49 ]. While single host networks are informative, they are likely to underestimate the epidemic propagation of pathogens capable of infecting multiple species [ 26 , 50 – 53 ]. Our results reinforce the relevance of including data, when available, of all susceptible host species while developing regional disease preparedness activities into disease surveillance systems and agree with previous literature which reported better performance of temporal network measures in identifying farms that could be targeted to reduce the secondary number of infections [ 39 ].…”

“…Here, we implemented analysis that allows for the description of the possible risks of animal movements, and identify key farms in the contact network using a multispecies approach. Social network analysis and mathematical models considering more than one host can deliver insights into the quantification of potential outbreak paths and provide useful data for the identification of key farms that can be used to break the direct contact transmission of animal diseases [ 8 , 14 , 26 – 29 ]. These insights are invaluable in supporting the design and implementation of efficient control and eradication strategies.…”

Causes of delayed outbreak responses and their impacts on epidemic spread

“…Slower response to large farms raises a potentially important concern for global agribusiness and livestock production: while large farming units may yield higher productivity, they could also create a logistical bottleneck for control actions in the event of an outbreak. This adverse effect further suggests that large farms not only have a dominating role in the risk of onward transmission [48,49], but may also increase the potential for pathogen exposure on susceptible premises due to infective hosts (i.e. live animals or carcasses) being kept longer onsite.…”

Therapeutic and Prophylactic Potential of FMD Virus-specific Polyvalent Immunoglobulins during an FMD Outbreak in Cattle

“…Although emergency vaccination to control FMD outbreaks is essential, the protection takes at least 7 days to protect susceptible animals (Salt et al 1998). Within this time, the disease can spread further where the vaccinated animals might transmit the FMD virus to contact susceptible animals during the negative phase of the vaccination process (4-7 days), wherever the airborne transmission route between farms is at highest risk approximately 5-20 days after the disease becomes detectable at the farm (Björnham et al 2020). Therefore, there is a need for therapies that provide not only rapid protection against FMD but reduce shedding and prevent the spread of the virus.…”