Expected net benefit of vaccinating rangeland sheep against bluetongue virus using a modified-live versus killed virus vaccine

Authors: MUNSICK, TRIS - University Of Wyoming; Peck, Dannele; RITTEN, JOHN - University Of Wyoming; JONES, RANDALL - Producer; RANDALL, MICHELLE - Producer; MILLER, MYRNA - University Of Wyoming

Submitted to: Frontiers in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2017
Publication Date: 10/10/2017
Citation: Munsick, T., Peck, D.E., Ritten, J., Jones, R., Randall, M., Miller, M. 2017. Expected net benefit of vaccinating rangeland sheep against bluetongue virus using a modified-live versus killed virus vaccine. Frontiers in Veterinary Science. doi:10.3389/fvets.2017.00166.
DOI: https://doi.org/10.3389/fvets.2017.00166

Interpretive Summary: Bluetongue virus is spread by biting gnats, which sometimes infect domestic sheep in the Intermountain West of the United States, including the state of Wyoming. Bluetongue disease can cause severe illness in sheep, including death. For this reason, producers want to know the benefits and costs of vaccinating sheep against bluetongue disease. We began our study by calculating the cost of a sheep flock in the Big Horn Basin of the state of Wyoming becoming infected with bluetongue. Next, we estimated the cost of buying and giving a bluetongue vaccine to sheep. Finally, we calculated the net benefit of vaccinating—that is, benefits minus cost. This net benefit depends on how often a flock is hit with bluetongue. It also depends on the lamb price, how many sheep in a flock become sick, and how many die. Our results show that the benefits of vaccinating a sheep flock every year against bluetongue should outweigh the costs under many different disease and price conditions in the Big Horn Basin of Wyoming. This is true even if the disease is quite rare—for example, infecting sheep just once every 20 years. We conducted our study for two different types of vaccine: 1) killed virus vaccine, and 2) modified-live vaccine. The killed virus vaccine is seven times more expensive for producers, but is also safer. The modified-live vaccine is much less expensive, but can cause abortions in sheep if not given at the correct time. It can also transfer from vaccinated sheep to the biting gnat, where it sometimes “reverts” to an infectious form, which can then be spread to other sheep and cause illness. Because of these safety concerns, it is hard to get legal permission to use the modified-live vaccine in Wyoming. Even though both vaccines have benefits that are larger than their costs, the killed virus vaccine’s higher cost will keep some producers from adopting this useful disease prevention tool. Policy-makers should weigh this tradeoff between the killed virus vaccine’s greater safety and higher cost when discussing regulations on the two vaccine types.

Technical Abstract: Recurring outbreaks of bluetongue virus in large rangeland sheep flocks in the Intermountain West of the United States have prompted questions about the economic benefits and costs of vaccinating individual flocks against bluetongue disease. We use enterprise budgets and stochastic simulation to estimate the cost of a bluetongue outbreak on a representative rangeland sheep operation in the Big Horn Basin of the state of Wyoming. We then determine the cost of purchasing and administering a bluetongue vaccine, and estimate the expected annual net benefit of vaccinating assuming different outbreak intervals. We incorporate variability in lamb prices, morbidity rates, and mortality rates to simulate a broad range of realistic situations. Expected net benefit is calculated for both a killed virus vaccine and modified-live virus vaccine. Results indicate that, although expected annual net benefit of vaccination decreases as outbreak interval lengthens, vaccination is an economically viable option for a rangeland sheep operation under a wide range of market and disease risk scenarios. There is a significant cost advantage for a modified-live vaccine over a killed virus vaccine. This results in higher expected annual net benefits for administration of a modified-live vaccine. However, both vaccines perform well economically up to at least a 20-year outbreak interval. The modified-live vaccine requires a rigorous authorization process before legal distribution and use can occur in Wyoming. To date, no company has requested to make a modified-live vaccine commercially available for use in Wyoming. The killed virus vaccine is seven times costlier to producers, but also poses less risk to sheep reproduction and less risk of escape. This facilitates the approval process for commercial production. But the tradeoff between the killed virus vaccine’s relative safety and higher cost should be considered whenever policymakers revisit regulations about commercial use of the two vaccine types. Even though both vaccines have positive expected annual net benefit under a wide range of circumstances, the killed virus vaccine’s higher cost will deter some producers from adopting this disease-risk management tool.