Epidemiologic and economic analysis of porcine HP-PRRS outbreak control in Vietnam

開発学研究(Journal of Agricultural Development Studies) Vol.23 No.3, 2013

[研究論文]

ベトナムにおける HP-PRRS（高病原性豚繁殖・呼吸障害症候群）対策に関する経済疫学研究

張海峰a・耕野拓一 b・窪田さと子 b・グエンテイミンホア c・グエンバンフン d

a 帯広畜産大学大学院

b 帯広畜産大学畜産衛生学研究部門

c フエ連合大学フエ経済大学（ベトナム）

d フエ省獣医局（ベトナム）

要旨

本研究はベトナムフエ省における高病原性豚繁殖・呼吸障害症候群（HP-PRRS）発生の特徴を明らかにし, フエ省で

行なった HP-PRRS 対策について SIR モデルと費用・便益分析を用いて, 経済学の視点から評価することを目的とした。

フエ省獣医事務局の疫学データによると, フエ省で HP-PRRS 発生が初めて認められたのは 2008 年４月７日であり,

その後 12 日間で観測地域全体に感染が拡大した。本疾病の感染性が非常に高いなどの観点から感染が拡大したと考えら

れる。一方で, 費用・便益分析により, フエ省で行われた HP-PRRS 対策の費用に対する効果は 2～16 倍であることが試

算され, 経済的効率性が明らかになった。フエ省の事例は, 今後ベトナムにおけて HP-PRRS 対策を行う際に参考になる

と考えられる。

[キーワード] HP-PRRS, SIR モデル, 費用・便益分析、ベトナム

ランニングタイトル：ベトナム PRRS 対策に関する経済疫学的研究

Epidemiologic and economic analysis of porcine HP-PRRS outbreak control in Vietnam

Haifeng ZHANGa, Hiroichi KONOb*, Satoko KUBOTAb, Nguyen Thi Minh HOAc, Nguyen Van HUNGd

a Graduate School of Obihiro University of Agriculture and Veterinary Medicine b Department of Animal and Food Hygiene, Obihiro University of Agriculture and Veterinary Medicine

c Hue College of Economics, Hue University, Vietnam

d Thua Thien Hue Department of Animal Health, Vietnam

*Corresponding Author

Abstract

The objectives of this study were to clarify the transmission ability of HP-PRRS, to analyze the efficiency and

economically evaluate the benefit of the culling program conducted in Hue Province in 2008.

For clarifying the HP-PRRS transmission pattern, data for HP-PRRS outbreak identification was collected from

the Veterinary Department of Hue Province. The disease first occurred on April 7th, 2008 and then transmitted to

entire observational area within only 12 days. Therefore, it is conceivable that one of the reasons for HP-PRRS

dissemination is the highly transmissibility of this disease. On the other hand, according to the result of economic

analysis in this study, the benefit of the culling program was 2.08 to 16.48 times larger than the cost of running the

program. The findings of this study suggest that the culling program is an economically and epidemically efficient

system for controlling HP-PRRS outbreak in Vietnam.

[Keywords] HP-PRRS, SIR, Cost-Benefit Analysis, Vietnam

Running title: Epidemiologic and economic analysis of HP-PRRS control

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（張海峰・耕野拓一・窪田さと子・グエンテイミンホア・グエンバンフン）ベトナムにおける HP-PRRS 対策に関する経済疫学研究

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I. Introduction

Pork is the most important source of animal protein in Vietnam and is consumed by nearly all Vietnamese

households (Tisdell 2009). Pig farming is an important source of cash income for farm families, acting as a “savings

bank” in Vietnam (Huynh et al. 2006). Small-scale pig farms account for approximately 80% of the national pork

production in this country (Van Kinh et al. 2002). Because pork production is the most important meat source in

Vietnam, an outbreak of diseases affecting pigs would have a great impact on the farming sector.

Since 2007, Vietnam pig farming sector has been affected by pig disease, atypical highly virulent strains of

porcine reproductive and respiratory syndrome (PRRS). Significant economic losses from this disease were incurred

due to the high mortality rate of 20 to 100% (Dietze et al. 2011), as this disease quickly spread throughout the

country.

PRRS, also known as “blue-ear pig disease”, is a highly contagious disease of pigs.1) With few exceptions, PRRS

virus is currently endemic in the major swine producing regions of the world. The pathogenic virus is considered to

impose a substantial economic burden on pork producers, although few economic studies have been conducted

(Neumann et al., 2005). For East and Southeast Asia, with the highest pig densities worldwide, the persistence of

this disease poses a growing socioeconomic concern (Dietze et al., 2011). 2) For example, only the pig production in

China and Vietnam account for 50.1% of the total pig production in the world (FAOSTAT 2011). In Vietnam, there is

a close relationship between the poverty level and the pig production, and prevent pig farming from HP-PRRS

outbreak could be considered as a contribution to poverty reduction in Vietnam (Zhang and Kono 2012).

Since 2006, China’s pig-farming sector has been damaged by a new type of PRRS virus, and it is called highly

pathogenic porcine reproductive and respiratory syndrome (HP-PRRS) and has resulted in huge economic losses in

the Chinese pig industry (An et al. 2010). This disease quickly spread from China to Vietnam during the Beijing

Olympic Games in 2008 (Figure 1).

Figure 1 HP-PRRS outbreak movements from 2006 to 2008 between Vietnam and China

Source: OIE

In 2008, there were 1,077 HP-PRRS outbreaks occurred and 300,906 pigs were infected in Vietnam. Outbreaks

subsided in 2009, but became widespread again in 2010 (OIE 2011). HP-PRRS outbreaks were severe in the North

Central Coast area of Vietnam in 2008 (Table 1). Hue province is located in the North Central Coast area, and this

province is the only province in Vietnam that central slaughter houses has been controlled by government

successfully, and all livestock is slaughtered in these centers. Many governmental officers from other provinces or

other countries also visited Hue Province for educational purposes. The governmental direction system for animal

disease control in Hue Province is considered to be outstanding in Vietnam (Hosono et al. 2007). This province has a

good reputation for animal disease control in Vietnam. Therefore we selected Hue province as our study area.3)

To control HP-PRRS, the Food and Agriculture Organization (FAO) recommended 9 control measures, including

surveillance, vaccination, cleaning, and disinfection. However, conducting all of the measures at pig farms in

Vietnam was considered to be unrealistic due to budget limitations. Vaccination is an important component in

animal disease eradication programs (McLeod & Rushton 2007), but in the case of HP-PRRS, it is reported that the

vaccine that was developed in 2008, is not effective (Zhou & Yang 2010). Although the transmission mechanism of


HP-PRRS remains unknown (An et al. 2010), live

infected pigs are the major source of disease

transmission (Roberts 2002). Culling pigs

infected with HP-PRRS could be considered an

important tool to control HP-PRRS, and the FAO

has recommended that HP-PRRS infected pigs

be culled (Dietze et al. 2011).

To control HP-PRRS, Hue province conducted

a culling program in 2008. The culling program

procedure in Hue Province is as follows: first the farms report pigs suspected of being infected, then the local

veterinarians collect blood samples and cull the test-positive animals, and finally provide the subsidy to the farms.

The subsidy amounts to about 70% of the market value of the culled pigs and it encourage pig farms to report

suspicious pigs and cull infected pigs.4)

It can be hypothesized that if the government did not conduct a culling program, the economic damage due to

HP-PRRS outbreaks would be more severe (i.e. the benefit of culling program is more than the cost of it). To test this

hypothesis, the present study aimed to clarify the transmission ability of HP-PRRS, to analyze the efficiency and

economically evaluate the benefit of the culling program conducted in Hue Province in 2008. Epidemiological (SIR

model) and economic analysis (Cost-Benefit Analysis) was applied to analyze the epidemiological efficiency and

economically evaluate the benefit of the culling program.5) The results clarified that one of the reasons for HP-PRRS

dissemination is the high transmissibility of this disease. On the other hand, the benefit of the culling program was

2.08 to 16.48 times larger than the cost of running the program. The findings of this study suggest that the culling

program is an economically and epidemically efficient system for controlling HP-PRRS outbreak in Vietnam.

Although the impact of an acute outbreak of HP-PRRS on herd performance is easily measured, the economic

impact is not. Little research has been done to assess economic impact of HP-PRRS and evaluate HP-PRRS

elimination programs in developed countries. For example, Yamane (2010) estimated economic impact of HP-PRRS

in Japan. Holtkamp et al. (2012) economically evaluated HP-PRRS elimination programs in U.S.A. To the best of our

knowledge there is no research reports published to date about economically evaluating HP-PRRS control measures

in developing countries. Therefore in this study, we verify the economic effectiveness of an HP-PRRS control

program in a high pig density developing country. The originality of this study is: it combined epidemiological and

economic models together to analyze the efficiency and economic benefit of an animal disease control program. For

economical evaluation of the culling

program conducted in Hue Province,

this study is organized as follows:

first, clarification of the HP-PRRS

transmitting pattern

epidemiologically; second,

evaluation of the costs and benefits

of the culling program conducted in

Hue Province to understand the

economic efficiency of this

HP-PRRS control measure; third,

discussion of the validity of this

program.

Ⅱ. Data and Methods

⒈ Data

For clarifying the HP-PRRS transmission pattern, data for HP-PRRS outbreak identification was collected from

the Veterinary Department of Hue Province. These data can be used to analyze the HP-PRRS outbreak pattern of

the entire Hue Province. For economical evaluation of the effectiveness of the culling program, we focused on one

district as our economical analyzing area, and collected epidemiological and economic data from this district. In Hue

Table 1 Number of HP-PRRS infected farms and pig population in 2008

Number of infected

farms

Pig population

(Units: thousand head)

North East 33 4,626

North West 10 1,302

Red River Delta 81 7,334

North Central Coast 901 3,551

South Central Coast 6 2,328

Central Highlands 23 1,557

North East South 11 2,373

Mekong River Delta 12 3,630

Source: OIE & General Statistics Office of Vietnam

Figure 2 Location of Hue Province

Source: Hue province

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Province, there are eight districts and one downtown city (Figure 2). Huong Tra is the main pig farming area, and

HP-PRRS mainly occurred in this district. Therefore, for evaluating the culling program, we focused on the Huong

Tra district as the study area. The Huong Tra veterinary office provided epidemiological data regarding the culling

program conducted in 2008. These epidemiological data include the number of subsidies provided by local

government to HP-PRRS infected farms, the dates of subsidy provision, the number of infected pigs, the bodyweight

of culled infected animals and the place of infected farms.

To obtain detailed information about the HP-PRRS control program, research interviews were conducted with

local governmental officers and experts of agricultural economics at Hue University. We gained valuable detailed

information from these interviews for understanding the HP-PRRS outbreak and culling program conducted in this

area in 2008.

2. Methods

We evaluated the culling program by comparing the disease cost for the scenario with the culling program to the

disease loss in the scenario without the culling program.6) The former can be calculated from survey data, but an

epidemiological estimation model is needed for the latter, and we employ SIR model.

(1) SIR model

To estimate the total number of HP-PRRS infected pigs in the scenario without the culling program, an

epidemiological analyzing method must be applied. The SIR (Susceptible, Infection and Removed) model is suitable

for this estimation. This model was widely used for estimating the epidemiological damage of a particular disease,

for example, Karstena et al. (2005) estimated the impact of classical swine fever spreading in Germany, and Ward et

al. (2009) simulated the foot and mouth disease spread within an integrated livestock system in Texas in the USA.

Because the length of the HP-PRRS outbreak period is relatively short compared to other animal diseases, a simple

type of the SIR model such as that created by Kermack & McKendrick (1927) is suitable for this study.7)

There are three variables in this model: 𝑆, 𝐼 and 𝑅. Where 𝑆 is the proportion of susceptible animals, 𝐼 is the

proportion of infected animals, 𝑅 is the proportion of removed (culled) animals. The “t” is the time unit (per day).

The total fixed animal population in this model is 100%. The parameter represents the infection rate, and

represents the removed (culled) rate. The rate of new infections per day can be defined as 𝛽𝑆𝐼, and the rate of

removed (culled) per day is 𝛾𝐼 . In other words, the 𝑆 decreases 𝛽𝑆𝐼,per day, 𝐼 changes 𝛽𝑆𝐼 − 𝛾𝐼 per day, and 𝑅

increase 𝛾𝐼 per day. Therefore this SIR model can be written as:

𝑑𝑆

𝑑𝑡= −𝛽𝑆𝐼 (1)

𝑑𝐼

𝑑𝑡= 𝛽𝑆𝐼 − 𝛾𝐼 (2)

𝑑𝑅

𝑑𝑡= 𝛾𝐼 (3)

The parameters and were estimated from previous studies and the local veterinarian’s estimation, and the

initial proportion of susceptible pigs and total pig population were estimated by the local veterinarian.

(2) Cost-Benefit Analysis

To economically evaluate the culling program, an economic evaluation model “Cost-Benefit Analysis (CBA)” is

suitable. CBA is an economic analysis model for comparing the costs and benefits of alternative animal disease

control programs. It has been widely used for evaluating animal disease control programs, for example Berentsen et

al. (1991) evaluated the disease control strategies of Foot and Mouth Disease in the Netherlands, and Nielsen et al.

(1993) evaluated the African Swine Fever eradication program in Spain.

There are several perspectives for calculating the cost and benefit in the CBA. In the case of subsidies for culling

infected animals, from the pig farm’s or private perspective, those subsidies can be considered a benefit (e.g., Phan

Dang et al. 2007). But from a societal perspective, those subsidies can be considered a cost (e.g., Coelho et al. 2010). In

the current study, we stand at a societal perspective, and consider the subsidies as a cost of the culling program.


In our case, subsidies were the main cost of the culling program, and this study focuses on the economic

evaluation of the subsidies. So the cost of the culling program is the amount of subsidy provided by the government.

If without culling program the HP-PRRS outbreak will spread faster and the disease losses will increase further.

The culling program can prevent the spread of HP-PRRS and reduce the disease losses, and then the benefit of the

culling program is the reduction of disease loss by culling. It can be calculated by disease loss in the scenario without

culling program minus the disease loss in the scenario with culling program (④-③ in Table 2).

The disease loss in the scenario with culling program ③ can be calculated by multiplying Ⓐ, Ⓑ and Ⓒ

together. On the other hand, the disease loss in the scenario without the culling program (④ in Table 2) can be

calculated by multiplying Ⓔ, Ⓕ, Ⓑ, and Ⓒ together (Table 2).

In the scenario with the culling program, Ⓐ and Ⓑ can be found from field survey data, and Ⓒ can be found

from a MARD (Ministry of

Agriculture and Rural Development

in Vietnam) report. In the scenario

without culling program, the value

of Ⓔ can be estimated by an

epidemiological analyzing model

(SIR model), and we can use the

result of the SIR model to calculate

④ . The value of Ⓕ can be cited

from previous studies, and the

value of Ⓑ can be obtained from

field survey data; it is the same Ⓑ

that we used for calculating ③. The

value of Ⓒ can be found from a

MARD report; it is also the same Ⓒ used for calculating ③.

The statistic derived from this analysis is the Benefit-Cost (B/C) ratio, which is the benefit divided by the cost

(Table 2).

Ⅲ. Result

⒈Epidemic Dynamics

The four districts that were

infected by HP-PRRS in Hue

Province in 2008 were examined

(Figure 3). HP-PRRS first occurred

in the Huong Tra District where the

main road passes through this

district. This disease was quickly

transmitted from Huong Tra to

three other nearby districts within

only 12 days. As presented in

Figure 3, the outbreak began in the

area near the main road, which

passes through this province from

north to south, and the disease

spread radially in this province.

Therefore, it is conceivable that one of the reasons for HP-PRRS dissemination is the movement of pigs by the

transportation system in this country. Vietnam and China have a close relationship for live pig trading. According to

the local veterinarians and university researcher witnesses, the main road was busy for live pig transport between

Table 2 Costs and Benefits of culling program

Item Meaning Data source

①Cost = Subsidy provided for culling infected pigs Field research data

②Benefit = ④-③

③Disease loss with culling program =Ⓐ×Ⓑ×Ⓒ

Ⓐ =Total number of pigs culled under culling

program

Field research data

Ⓑ =Average body weight of culled pigs Field research data

Ⓒ =Market price of saleable fattened pig MARD

④Disease loss without culling program =Ⓔ×Ⓕ×Ⓑ×Ⓒ

Ⓔ =Total number of HP-PRRS infected pigs SIR model estimation

Ⓕ =Death rate of HP-PRRS infected pigs Previous researches

Ⓑ =Average body weight of dead pigs Field research data

Ⓒ =Market price of saleable fattened pig MARD

Benefit/Cost Ratio = ②/①

Figure 3 Pattern of the spread of HP-PRRS outbreak in Hue Province

Source: Veterinary Department of Hue Province, 2011.

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China and Hue Province

during the Beijing Olympic

Games in 2008. In the

opinion of local

veterinarians, it is

conceivable that the

HP-PRRS outbreak in

Vietnam was caused by the

trucks coming from China

to collect pigs in Hue

Province. It is assumed

that those trucks coming

from China to Hue carried

the HP-PRRS pathogens,

and then the outbreak

occurred.

From the epidemiological data of the HP-PRRS outbreak in Huong Tra District, we can see that the outbreak

continued for around 5 weeks (Figure 4). A total of 2,441 infected pigs were culled during the outbreak. The

infections reached a peak in the first week, and slowed down during the second and third weeks, and almost

subsided under control in the last 2 weeks (Figure 4).

Analyzing the epidemiological data provided by the Huong Tra District, we discovered that HP-PRRS

transmission rate was very high at the beginning of the infection. At first, only one infected pig was present in this

area, but only 2 days later 306 newly infected animals were observed (Figure 4). Although the pig movement was

the reason for the 2008 HP-PRRS outbreak in Vietnam, the highly transmissibility of HP-PRRS also an important

factor for the rapid disease spread in 2008.

⒉ SIR Model

For analysis of disease loss in the scenario without culling program, an epidemiological estimation using the SIR

model was needed. Because there is no existing research to allow us to clarify the parameter (infection rate) for

HP-PRRS, and although it can be considered that the of HP-PRRS is larger than the of PRRS, in our study we

had to underestimate the parameter , and

set =0.2 based on a PRRS study (Nodelijk

et al. 2000). On the other hand, the

parameter =0.017 was reported by Wills

et al. (2002)8). The total number of pigs in

Huong Tra district is 28,900, which was

estimated by a local veterinarian, and we

assumed that only one pig was infected at

the beginning of the outbreak. Statistical

software R package deSolve9) was used for

running the SIR model.

Figure 5 presents the results of an

estimation done with the parameters

mentioned above. Without the culling

program, all of the pigs would be infected

within 200 days. Furthermore, because the parameters were cited from previous PRRS studies, the parameters

and might have been underestimated, and so we increased the value of parameters and by 5%, 10%, and 20%,

and ran the model again.10) All of the estimates show 100% infection during the outbreak. This result is in

accordance with other studies, which identified that without any control the HP-PRRS infection rate in a herd would

be over 90% (Xiao et al.2010). Therefore in this study, we can consider that without any control, 100% of the pigs

would be infected within one year.

Figure 4 Number of PRRS infected animals in Houng Tra District (Daily totals)

Source: Veterinary Office of Huong Tra District, 2011.

Figure 5 SIR simulation results with no culling program

Note: = 0.2; = 0.017.

0

50

100

150

200

250

300

350

400

450 Number of infected


On the other hand, culling infected animals will actually shorten the infectious period, and that will increase the

value of (is the reciprocal of the infectious

period). According to the local veterinarian’s

estimation, during the culling program the

average infectious period was around 5 to 6

days, so the value of could be set as 0.18.11) We

used this value of parameter=0.18 and ran the

SIR model again. The results show that 10% of

the total pig population will be infected by

HP-PRRS within one year in the scenario under

the culling program (Figure 6). The result of the

SIR model can be well explained from our

epidemiological data collected from Huong Tra

district. Our epidemiological data show that

about 8.5% of the total pig population were

infected by HP-PRRS.12) From the results of the

SIR model, we also found if the infectious period was 5 days or less, an outbreak will not occur. But if the infection

period was larger than 15 days, the HP-PRRS outbreak would continue for more than 6 months and all of the pigs

would be infected.

⒊ Cost-Benefit Analysis

The cost of the culling program is the subsidy provided for farms to cull infected pigs. Vietnamese government

provided subsidies amounting to 25,000VD (Vietnam Dong) per kg of body weight of culled infected animals, as 70%

of the market value. In the HP-PRRS outbreak in the Houng Tra district in 2008, a total of 2,441 infected pigs with a

total body weight of 122,285 kg (average 50.1 kg per head) were culled. Multiplying the body weight by subsidy, we

can calculate that the total subsidy provided by

the government was 3,057 million VD. This is

the cost of culling program. Because this paper

focuses on evaluating the subsidy of the culling

program, in the scenario without culling

program, we can assume that the cost of the culling program is zero (Table 3).

The benefit of the culling program is the reduction of disease loss by culling. The disease loss considered in our

study is the death of infected animals. We can calculate the disease loss by multiplying the market price of fattening

pigs by the body weight of dead animals. The market price was about 38,000 VD per kg in 2008, according to the

MARD.

Table 4 Disease losses

Item Unit With culling program1) Without culling program2)

Death rate 20% Death rate 60% Death rate 100%

Total number of pigs6) head A 28,900 28,900 28,900 28,900

Total number of infected pigs head B 2,441 28,900 28,900 28,900

Total number of dead (culled) pigs head C 2,441 5,780 3) 17,340 4) 28,900 5)

Market price7) VD/kg D 38,000 38,000 38,000 38,000

Average body weight8) kg E 50.1 50.1 50.1 50.1

Total lost body weight kg F =C×E 122,285 289,578 868,734 1,447,890

Disease loss by infection Million VD G =F×D 4,647 11,004 33,012 55,020

Source: Field survey 2011 and the result from SIR model.

Note: 1) Under culling program. Data is from field research.

2) Number of infected animals was estimated from SIR model.

3) 5,780=B(without culling program)×20%.



6) Total pig numbers in Houng Tra District, estimated by local veterinarian.

7) Average market price during 2008, MARD.

8) Field research data.

In the scenario with culling program, the body weight of dead (disposed of) animals was 122,285 kg (Table 4).

Therefore the total loss by death of infected pigs can be calculated at 4,647 million VD (=38,000×122,285 VD; in

Figure 6 SIR simulation results with culling program

Note: β= 0.2; = 0.18.

Table 3 Cost of culling program

Item Unit With control Without control

Subsides VD/kg ① 25,000 0

Total lost body weight kg ② 122,285 ---

Total subsides (cost) Million VD =①×② 3,057 0

Source: Field survey 2011.

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Table 4).

On the other hand, from the SIR model estimation, without any control treatment, all of the pigs will be infected.

The death rate of infected pigs for all ages was estimated at 20 to 100%; for young piglets the death rate is nearly

100%, but for adult pigs the death rate is around 20% (Zhou & Yang 2010). To sensitively analyze the disease loss in

the scenario without culling program, we set three alternative assumptions with regard to the death rates of

HP-PRRS infected pigs at 20%, 60% and 100% respectively. The death rate is highest in the case of piglets (100%),

and lowest in the case of adult pigs (20%), while 60% is the middle point between 100% and 20%. We assumed that

the total pig population in the study area was 28,900, and a homogeneous distribution of pigs.13) The body weight of

died pigs was obtained from the survey data. In survey data, the average body weight of culled pigs was 50.1 kg. So

the total body weight of died pigs in the case of a death rate of 20% can be calculated as: 28,900×20%×50.1=289,578

kg. The market price of a fattened pig is 38,000 VD per kg. Therefore the disease loss in this case is 11,004 million

VD (=289,578kg ×38,000 VD; in Table 4). On the other hand, in the case of a death rate of 100%, without a culling

program all of the pigs will be infected and all of them will die. Therefore, the disease loss in this case is 55,020

million VD (=28,900×100%×50.1kg×38,000VD). In the case of a death rate of 60%, the disease loss is 33,012 million

VD (=28,900×60%×50.1×38,000VD). In Vietnam small-scale farms accounting for 80% of total pig production, and in

those farms, sows, piglets and fattening pigs are reared together. So this case (Death rate of 60%) can be considered

as the most rational estimation for calculating disease loss from an HP-PRRS outbreak in Vietnam.

Table 5 Benefit/Cost ratio of culling program

Item Unit With culling program Without culling program

Death rate=20% Death rate=60% Death rate=100%

Cost 1) Million VD Ⓐ3,057 0 0 0

Disease losses 2) Million VD ①4,647 ②11,004 ③33,012 ④55,020

Benefit

Death rate 20% Million VD Ⓑ6,357 =②-①

Death rate 60% Million VD Ⓒ28,356 =③-①

Death rate 100% Million VD Ⓓ50,373 =④-①

Benefit-cost ratio

Death rate 20% 2.08 =Ⓑ/Ⓐ

Death rate 60% 9.28 =Ⓒ/Ⓐ

Death rate 100% 16.48 =Ⓓ/Ⓐ

Source: Field survey 2011 and the result from SIR model.

1) Table 3

2) Table 4

From the estimates made in this study, without a culling program the disease loss from an HP-PRRS outbreak is

between 11,004 and 55,020 million VD. But by conducting a culling program disease loss is only 4,647 million VD.

Subtracting the disease loss with culling program from disease loss without culling program, we can obtain the

benefit of the culling program, which is around 6,357 to 50,373 million VD (Table 5). The cost of culling program is

3,057 million VD, therefore, the B/C ratio of the culling program in this study is around 2.08 to 16.48 (Table 5). That

means the benefit of the culling program is 2.08 to 16.48 times larger than the cost of running the program.14)

Ⅳ. Discussion

1. Effectiveness of culling program

From this study, it is conceivable that the subsidy system in culling program has two effects for controlling

HP-PRRS. The first is that the subsidy encourages farms to report and cull infected pigs, so the infection period is

shortened. The second one is that the subsidy reduces the economic damage to infected farms.

In the scenario where there is no efficient medical cure or effective vaccine for controlling an HP-PRRS outbreak,

as in 2008, culling the infection source (infected pigs) is the most important HP-PRRS outbreak control measure.

From the result of our SIR model, we found if the infectious period was 5 days or less, an outbreak will not occur.

Therefore, for controlling an HP-PRRS outbreak, it is necessary to reduce the infectious period. For reducing the

infectious period, it is very important that farmers report suspected infected pigs and local veterinarians identify the

HP-PRRS infection quickly. Vietnamese pig farms had incentives to report suspected infected pigs, because the

government provided subsidies to cull infected pigs.


The important point to note is the quantity of subsidy provision. In our study area, the subsidies in the culling

program covered 70% of the market value to farms from the HP-PRRS outbreak, and therefore the subsidy was

effective. If the subsidy cannot cover the major disease loss from an HP-PRRS outbreak, the culling program cannot

even be conducted. Small pig farms supply 80% of the total pig production in Vietnam (Kinh et al. 2002). Without

subsidy for culling infected pigs, an outbreak would severely damage the most pig farmers’ livelihoods. The FAO also

has recommended avoiding culling infected pigs when the subsidy for culling is not enough (Dietze et al. 2010). But

in some other developing countries, for example in China, the subsidy for culling HP-PRRS infected pigs is much

lower than in Vietnam.15) This is why the culling measure was not effective in China, and the outbreak continued for

6 years.

The culling program not only reduces the infectious period, but also reduces the economic loss from HP-PRRS

outbreaks. HP-PRRS has a high death rate. If pig farms are infected the economic loss will be huge in the scenario

without any government interventions for HP-PRRS control. In the case of the study area, if the culling program

was not conducted, the economic loss from the HP-PRRS outbreak would increase from 4,647 million VD to around

11,004 to 55,020 million VD.16)

The findings of this study suggest that the culling program is an economically and epidemically efficient system

for controlling HP-PRRS outbreak in developing countries. But in the scenario without the culling program, the

economic damage might be more severe than our results indicate. In the scenario without the culling program, even

if some pigs can recover after an outbreak, the farms would have to devalue their pig for selling, because the disease

severely damages the quality of pig meat (Tian et al. 2007). Moreover, in Vietnam, there is a close relationship

between pig farming and poverty in the population (Zhang & Kono 2011). An HP-PRRS outbreak would not only

reduce the farmers’ incomes and damage livelihoods, but it might also cause social instability.

2. Importance of surveillance system

To shorten the infectious period and control HP-PRRS outbreaks in Hue province, the subsidy provided by the

government encouraged farmers to report infected pigs as soon as possible, and the existing animal disease

inspection system in Hue province also contributed to the conducting of inspections quickly.

Animal disease surveillance system is very important in the control and prevention of emerging disease (WHO

2000). But in developing countries, their surveillance systems are usually imperfect (Frerichs 1991). This can be

considered as one of the reasons that HP-PRRS have occurred more than 4 years in Vietnam since 2007. The success

of HP-PRRS control in Hue was due to not only the appropriate subsidy provision but also the existing outstanding

animal disease surveillance system in this province. In 2008, Hue government paid a great attention on HP-PRRS

control, and the outstanding governmental direction system for animal disease control in Hue also enhanced the

efficiency of reporting and inspecting HP-PRRS outbreak under surveillance system. Therefore the infectious period

was shortened and HP-PRRS outbreak was under control in Hue province.

To maintain appropriate subsidy provisions and surveillance system for animal disease control, a great deal of

governmental assistance is needed. However, as for HP-PRRS, it is conceivable that other severe new animal

diseases might appear in the future. Budgetary expenditures to develop animal disease prevention and control

systems will be valuable for developing countries where stockbreeding is an important source of cash income for

farm families.

ⅤConclustion

From the epidemiological data of Hue, we understand HP-PRRS is a highly epidemic disease with highly

infectious speed. The pathogen of HP-PRRS could transmit from one village to other 7 villages within 12 days.

The culling program conducted in Hue for controlling HP-PRRS was successful. If without any disease control

treatment, all of the pigs would be infected, and the economic losses will be much larger. In the situation that there

was no effective treatment and vaccination in 2008, culling infected animals to prevent infection could be considered

as an effective way for HP-PRRS control. The estimated Benefit-Cost ratio for investment in a culling program range

from 2.08 to 16.48, indicating the culling program to be an economically viable investment.

Notes

1) HP-PRRS is one of the OIE listed diseases in 2012. However, It is a disease of pigs, no other species have been

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found susceptible to HP-PRRS virus.

2) A number of economic researchers have yet focused on the survey on pig disease of FMD (Food and Mouth

Disease), CSF (Classical Swine Fever). However, on the basis of severe HP-PRRS outbreak occurrences in recent

years, Dietiz et al. (2011) concluded that HP-PRRS is considered to be the most economically important viral

disease of pig farms in Asia, Europe and North America.

3) Southeast Asia has high pig density, and HP-PRRS mainly occurred in this area. Hue province has high pig

density and HP-PRRS severely occurred in this province. Moreover, small-scale pig production system in Hue is

quite similar to other Southeast Asia countries, and according to a local veterinarian’s interview, the pig density in

Hue is 3 to 5 heads per farm, and it is similar to the average pig density in Vietnam. So, some of the results can be

generalized to other Asian developing countries. Therefore, we choose Hue as our study area.

4) On account of several animal diseases such as FMD and HP-PRRS with severe outbreaks in 2008, the Vietnam

government provided subsidies for animal disease control. According to governmental regulation No. 719

(established on June 5, 2008), for culling pigs, the subsidy amounts to 70% of the market value of the culled pigs.

The culling program in Hue province was conducted under this government regulation.

5) Epidemiological and economical analysis is also called “Animal health economics” or “Veterinary epidemiology

and economics”. It is a new area of economics that applies the principles and methods of economic analysis to

animal health problems (Mlangwa & Samui 1996). Both epidemiology and economic skills are needed for the

research in this area. To the best of our knowledge this is the first study which applied the SIR model and

Cost-Benefit Analysis to evaluate HP-PRRS control measure. Epidemiological and economic analyzing of animal

disease is a new scientific field in Asia, and this study is also the first study for economically evaluating HP-PRRS

control measures in developing countries. Moreover, little research has been done to epidemiological and

economical analysis for animal disease control to date in Japan. Therefore, this study also has scientific value from

this point.

6) We assumed pig population, pig farm numbers and HP-PRRS infectivity are same between those two scenarios.

7) Although there are other epidemiological models, for example, the SEIR (Susceptible, Latent period, Infection and

Removed) model, the characteristic of HP-PRRS outbreak is short period and that the infected pigs are

immediately infectious, so we do not consider the latent period in our simulation model. Furthermore, it also would

be desirable to use other epidemiological models such as “SIR model including birth and mortality (Oli et al. 2006)”.

However, for scarcity of available data about newly born piglets, abortion number and stillbirth number etc., and

for ease of interpreting epidemiological results into economical model, the authors continued to rely on the simple

SIR model. May (2004) also recommends that epidemiological models should be as simple as possible and not

simper. Our SIR model is used under the following assumptions (Vynnychy and White 2010): ①the population is

fixed, ②age, sex, and race do not affect the probability of being infected, ③there is no inherited immunity, ④the

number of the population mix homogeneously (also see Notes 13).

8) In this article, the length of infectious period was estimated to be 60 days. Taking the reciprocal gives 0.017 or

(=1 60⁄ ).

9) This simulation program was developed by Soetaert, et al. (2010) under the R computing environment (R

Development Core Team, 2009).

10) We used parameter sets [= 0.21; = 0.0178], [=0.22; =0.0187] and [= 0.24; = 0.0204] to run the simulation

again, and all of the simulation results showed that all of the pigs will be infected within 250 days.

11) 0.18=1 ((5 + 6) 2⁄ )⁄ .

12) 8.5%=2,441/28,900. Total pig population (28,900) is estimated by the local veterinarian. Total number of infected

and culled pigs (2,441) are calculated from field research data.

13) According to the local veterinarian’s estimation, the total number of pigs in the Huong Tra District in 2008 was

around 28,900. It was impossible to classify the pigs in this area into several categories, for example, piglet,

fattening pigs and sows, because those veterinary data do not exist, so we assume the distribution of pigs to be

homogeneous with an average body weight of 50.1 kg.

14) Randolph et al. (2002) estimated B/C ratio of the FMD eradication program in the Philippines range from 1.6 to

12.0. The culling program for controlling HP-PRRS in Vietnam is more economically efficient than FMD

eradication program in the Philippines.

15) In some provinces, the local government provided subsidies for culling HP-PRRS infected pigs. The subsidy is


100 Yuan (329,457 VD) per head, and it is much lower than the subsidy given in Vietnam (1,252,500 VD =

25,000VD ×50.1 kg).

16) The result of disease loss might be underestimated, because our SIR model did not consider the damage of pig

reproductive failure (abortion, stillbirth etc.) caused by HP-PRRS infection. If without culling program, the

economic loss from the HP-PRRS outbreak might be much larger than our estimation result. If we could consider

the disease loss of reproductive failure caused by HP-PRRS in our model, the disease loss will become larger

without culling program, and the B/C ratio of the culling program will become relatively larger.

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Epidemiologic and economic analysis of porcine HP-PRRS outbreak control in Vietnam

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