Date Published: June 13, 2019
Publisher: Public Library of Science
Author(s): Jonggul Lee, Youngsuk Ko, Eunok Jung, Daniel Becker.
During the winter of 2016-2017, an epidemic of highly pathogenic avian influenza (HPAI) led to high mortality in poultry and put a serious burden on the poultry industry of the Republic of Korea. Effective control measures considering spatial heterogeneity to mitigate the HPAI epidemic is still a challenging issue. Here we develop a spatial-temporal compartmental model that incorporates the culling rate as a function of the reported farms and farm density in each town. The epidemiological and geographical data of two species, chickens and ducks, from the farms in the sixteen towns in Eumseong-gun and Jincheon-gun are used to find the best-fitted parameters of the metapopulation model. The best culling radius to maximize the final size of the susceptible farms and minimize the total number of culled farms is calculated from the model. The local reproductive number using the next generation method is calculated as an indicator of virus transmission in a given area. Simulation results indicate that this parameter is strongly influenced not only by epidemiological factors such as transmissibility and/or susceptibility of poultry species but also by geographical and demographical factors such as the distribution of poultry farms (or density) and connectivity (or distance) between farms. Based on this result, we suggest the best culling radius with respect to the local reproductive number in a targeted area.
During the 2016-2017 winter season, the epidemic of highly pathogenic avian influenza (HPAI) in the Republic of Korea led to high mortality rates in domestic poultry and put a serious economic burden on the poultry industry. By April 4, 2017, 383 farms were reported to be infected by the HPAI virus (subtype H5N6 and H5N8), and approximately 3.7 million poultry (3154 thousand chickens and 332 thousand ducks) from 946 farms were culled (i.e., depopulated) . As most of the culled chickens are layer chickens (2518 thousand), it disrupted the egg supply and led to a surge in the egg price [2, 3].
Using farm-to-farm transmission dynamics incorporating the two poultry types, chicken and duck, we estimated the spread of the HPAI outbreak in the Republic of Korea in 2016-2017. We found that from modeling result the transmissibility between duck farms was higher than that between either chicken farms or different type of farms. Ducks can carry and shed the AI virus without symptoms and have low mortality while chickens have high pathogenicity and mortality . In addition, the outbreak started at a duck farm in high farm density area. These epidemiological and spatial factors amplified the virus and allowed it to spread easily surrounding poultry farms. Therefore, duck farms played an important role in the spread of the HPAI virus in Eumseong-gun and Jincheon-gun. This is the first study on estimating the basic reproductive number of the 2016-2017 HPAI epidemic in the Republic of Korea and introducing the local reproductive number as an indicator of the virus transmission in a given area. There were six towns in which the local reproductive number is greater than 1: Maengdong (R0(0)=1.6095), Daeso (R0(3)=1.5144), Deoksan (R0(1)=1.3913), Iwol (R0(8)=1.2940), Samseong (R0(10)=1.0926), and Jincheon (R0(7)=1.0277). These towns seem to be either relatively close to Maengdong (Deoksan, Geumwang and Daeso) or have high farm density (Maengdong, Deoksan and Samseong). The distance from Maengdong to Deoksan, Geumwang and Daeso are 7.76 km, 9.77 km and 9.95 km, respectively, which are almost half of mean distance (18.31 km) to Maengdong. Since the transmission rate depends on the distance between towns by the kernel, K(i,j)=e−d(i,j)r0, it is clear that the local reproductive number is affected by the distance. The high farm density in Deoksan and Samseong (1.31 and 1.60, respectively) might allow the virus to spread easily surrounding poultry farms via movement of humans (farm personnel and visitors) and vectors (rodents), or contaminated environment (air and water) .
In this paper, we presented mathematical modeling for the spatial-temporal transmission dynamics of HPAI using geographical and epidemiological data of the 2016-2017 AI epidemic in the Republic of Korea. To consider spatial heterogeneity, we introduced the PE culling rate as a function of the number of reported farms  with different coefficients based on the farm density in each town. As the exact location of the poultry farms affected by the AI virus was not available, a metapopulation framework has been adopted with the two types of poultry farms assuming random mixing between farms in a patch and nonrandom contact by the transmission kernel [19, 41] between farms in different patches. We introduced the local reproductive numbers using the next generation method [33, 34] in the metapopulation modeling framework. This quantified parameter allowed one to assess the transmissibility in each town. For total and even both types of farms, the model predictions and data about the AI-reported farms are in good agreements. The estimated transmission rates showed that the transmission between duck farms played an important role in the spread of the HPAI virus in Eumseong-gun and Jincheon-gun.