Epidemiologic and economic considerations regarding persistently infected cattle during vaccinate-to-live strategies for control of foot-and-mouth disease in FMD-free regions

Development of a foot-and-mouth disease (FMD) carrier state following FMD virus (FMDV) infection is a well-established phenomenon in cattle. However, the proportion of cattle likely to become carriers and the duration of the carrier state at a herd or population-level are incompletely understood. The objective of this study was to examine the epidemiologic and economic impacts of vaccination-to-live strategy in a disease-free region or country. We developed and simulated scenarios of FMD spread and control in the US livestock population, which included depopulation for a limited period, followed by a vaccinate-to-live strategy with strong biosecurity and movement restrictions. Six scenarios of FMD spread and control were simulated in the InterSpread Plus (ISP) modeling tool. Data on the number of infected and depopulated cattle (by operation types) from ISP model runs were used to estimate the monthly number of infected but not depopulated (potential carrier) cattle after the infection. Using available literature data on the FMD carrier state, we estimated the monthly proportion of carrier cattle (from infected but not depopulated cattle) over time following infection. Among the simulated scenarios, the median (25th, 75th percentile) number of infected cattle ranged from 43,217 (42,819, 55,274) head to 148,907 (75,819, 205,350) head, and the epidemic duration ranged from 20 (11, 30) to 76 (38, 136) days. In general, larger outbreaks occurred when depopulation was carried out through longer periods, and the onset of the vaccination was late (p > 0.05). The estimated proportion of surviving cattle, which were infected and not depopulated and had the potential to become persistently infected ranged from 14 to 35% of total infected cattle. Production losses in beef and dairy sectors were higher when outbreaks started in multiple states simultaneously, but production losses were small compared to trade losses and consumer avoidance losses. These results can be used to inform the consideration of a vaccinate-to-live strategy for FMD outbreaks and the development of appropriate post-outbreak management strategies. Furthermore, this output will enable a more detailed examination of the epidemiologic and economic implications of allowing convalescent cattle to survive and remain in production chains after FMD outbreaks in FMD-free regions.


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Economic estimates of losses to producers and losses to the broader market, as well as costs of 108 government response are outlined below. 109

Production Losses 110
Average daily weight gain (ADG) was assumed to be reduced by 1.4% (1) for each day of clinical 111 disease. Although normal rates of ADG may be resumed at the end of clinical disease, beef cattle sent to 112 slaughter after a typical 180 days on feed would be lighter than normal, reducing the supply of beef available 113 to the market. Alternatively, feedlot owners could feed cattle longer, which increases the cost of production. 114 Milk production in dairy cattle was assumed to reduce by 35% during clinical disease and recover at a 115 uniform rate over a 7 month period post-infection (2). 116 Clinically infected beef and dairy cows were anticipated to abort calves at a rate of (10%) based on 117

Doel (2003) (3) but no evidence was available in the literature to include a long term reduction in fecundity 118
for beef or dairy cows (4). It is difficult to say whether dairy producers would continue to milk lactating 119 dairy cows that aborted or dry them off and subsequently cull them. That choice may depend largely on the 120 market conditions and availability of replacements. Since calving results in the onset of lactation, dairy 121 cows that aborted were assumed to be culled upon recovery/drying out when animal movements resume. 122 No additional culling in beef cows was assumed due to abortions since there is no evidence of reduced 123 fecundity post-recovery. The death rate in unweaned calves was assumed to be 2.8% (5). 124

On-Farm Government Response Costs 125
Total on-farm costs of disease response included surveillance, depopulation and indemnification of 126 Although the US exports a portion of meat and animal production to international partners, the 183 majority of US animal agriculture production is consumed domestically--more than half of US produced 184 pork, beef and dairy products is consumed domestically. Domestic demand has been found to decline in 185 response to an animal health events in the US, but have historically remained relatively small (7). Trade pork demand that was sustained through the outbreak. Recovery was allowed to occur quickly afterward. 196

US Partial Equilibrium Model 197
The impact of production losses, depopulation and death losses, trade embargoes and domestic 198 consumption losses on markets were estimated using the United States Partial Equilibrium Model (USPEM) 199 prices and quantities in the model were primarily driven by revenue shares for each agricultural sector, elasticities, the feed-balance calculations that tie the livestock and grain sectors together, and the exogenous 204 shocks that the user imposes. Each final product was defined by an initial equilibrium that included a price, 205 quantity and elasticities that define the relationship of price and quantity as well as the prices between each 206 of the 11 products. Production losses and demand shocks, as described above, were imposed on the model 207 as exogenous shocks. Changes in quantity supplied and quantity consumed that resulted from price changes 208 were solved endogenously in the model. Output includes market prices and producer welfare, which is 209 defined as the difference between the schedule of prices at which producers are willing and able to supply 210 a good in varying quantities supplied, and the price they actually realize in the market for those quantities 211 supplied. It is different from profit in that producer welfare accounts for fixed, or sunk, costs of production. 212