An Intensive Study of 4 Dairy Farms in China to Determine the Incidence Rate of bTB

Research Article

J Bacteriol Mycol. 2018; 5(8): 1087.

An Intensive Study of 4 Dairy Farms in China to Determine the Incidence Rate of bTB

Chen Y1,2,3, Wang J4, Yang L1,3, Robertson ID3,5, Chen H1,2,3 and Guo A1,2,3*

¹Key State Laboratory of Agricultural Microbiology, Huazhong Agricultural University, China

²College of Veterinary Medicine, Huazhong Agricultural University, China

³Hubei International Scientific and Technological Cooperation Base of Veterinary Epidemiology, Key Laboratory of Preventive Veterinary Medicine in Hubei Province, China

4Shanghai Animal Disease Control Center, China

5College of Veterinary Medicine, Murdoch University, Australia

*Corresponding author: Aizhen Guo, Key State Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan City, Hubei Province, China

Received: November 05, 2018; Accepted: November 27, 2019; Published: December 04, 2018


Bovine Tuberculosis (bTB) is endemic in most parts of China, with epidemics occurring in some parts of the country. However little is currently known about the epidemiology of the disease. This research describes the occurrence of bTB in four dairy farms in China. The true animal level prevalence varied from 0.0% (95% CI: 0.0, 0.0) to 59.5% (95% CI: 48.2, 70.0). The incidence rate highlighted the rapid transmission between animals in herds ranging from 0.03 (95% CI: 0.01, 0.05)/cow-month in a closed farm which culled IFN-? release assay testpositive animals to 2.69 (95% CI: 1.59, 3.50)/cow-month in a closed farm which culled animals positive to both the PPD skin test and IFN-? release assay.

Keywords: Incidence Rate; Bovine Tuberculosis; Transmission; China


Bovine Tuberculosis (bTB) is a widely distributed chronic bacterial disease caused by any of the disease-causing mycobacterial species especially Mycobacterium bovis (M. bovis) within the Mycobacterium tuberculosis-complex. Cattle is the natural host of M. bovis, which can also infect other domestic and wild animals as well as humans [1], and can be transmitted among or within different species such as cattle and humans by direct or indirect contact [2].

In 20th Centry 1990s, cattle were State-owned assets in China, at that period of time, bovine Tuberculosis (bTB) was controlled and kept in a very low level. As the privatization of cattle, bTB was epidemic in most area in China. Not until 2012, Chinese government began with the animal infectious diseases eradication program, regarded bTB as the priority animal diseases in China. According to the program, all cattle herds should be routinely tested, positive cattle (reactors) should be slaughtered. Before the eradication program, epidemiological characteristic and dynamics of bTB, including incidence rate and prevalence of bTB would be essential for the controlling strategies.

In many epidemiological research, dynamic modelling was widely used to estimate the bTB infection especially within-herd transmission rates because of the long incubation periods of bTB [3- 5]. But the results varied a lot according to different mathematical models, management practices, species types or other factors.

As different areas had different breeding modes, in the current work, we selected 4 farms of two typical kinds of modes in developed and less developed area in China, which represented closed and open farms, respectively. Traced the farms for 2-7 years, firstly evaluated the bTB prevalence, then used those parameters to estimate the incidence rate of infection, in order to estimate the bTB transmission in Chinese herds. The IFN-? release assay, recommended by the OIE, was used to test for bTB in this study [6].

Materials and Methods

Farms and animals

Four farms were included in this study. Farm A, located in Hubei Province in central China, which regarded as less developed area, purchasing cows from other dairy farms. Test positive cows were removed from the herd within two weeks of obtaining a positive result and were replaced with Single Intradermal Comparative Cervical Test (SICCT, PPD skin test) -negative animals sourced from other commercial dairies. Because of the frequently purchased and selling of the farm, data for Farm A were available for 2010 to 2016, with all cows tested in Feb. 2010, Sep. 2011, Dec. 2012, Jan. 2013, Dec. 2014, July. 2015 and Mar. 2016. The number of animals tested at each point varied, with a starting number of 141 cows on Feb. 2010, then all cows were removed from that farm and 195 cows were newly introduced and tested on Sep. 2011; 49 new cows were introduced after that and 84 in total were tested on Dec. 2012; On Jan. 2013, we found 93 new cows and tested 126 cows in total; then 129 cows were introduced and 236 were tested in total on Dec. 2014; Only 5 were introduced after that and 57 were tested on July. 2015, on Mar. 2016, 212 cows were tested with 16 new comers.

Farms B and C, located in Shanghai City in East China, which regarded as developed area, are only physically separated by a narrow 1-meter wide pathway. They were originally owned by the same company, although were subsequently sold to different farmers. Both farms are closed farms neither purchasing cows nor selling cows from or to other dairies, so the two farms are not really separate epidemiological units. All cows were tested by SICCT on Oct. 2016, Dec. 2016, Feb. 2017 and Sep. 2017 with Farm B initially having 477 cows on Oct. and Dec. 2016, 322 on Feb. 2017 and 206 on Sep. 2017. For Farm C 355 were tested on Oct. and Dec. 2016, 168 on Feb. 2017, 140 on Sep. 2017. No cows were introduced to these two farms during our observation. Before our 1st test (Oct. 2016), ACDC in Shanghai city tested all cows on those two farms by tuberculin skin test and killed all positive-reaction cows.

Farm D, located in Jiangsu Province which was also developed area in East China, also purchased cows directly from other dairy farms. In this study cows were only tested twice, on June 2011 when 477 cows were tested, and on July 2017 when 394 were tested with 124 new cows. They killed all positive animals tested by BOVIGAM.

In those closed farms, positive cows were slaughtered after tested, so cannot be traced for a long time.

Bovine tuberculosis detection

Cows were tested using the commercial BOVIGAM Mycobacterium bovis Gamma Interferon Test Kit for Cattle (Prionics, Switzerland) according to the manufacturer’s instructions. Briefly blood samples were divided into three wells (each well containing 1.5 ml of blood). Then all three wells were mixed with 100μl PPD-B, PPD-A and PBS respectively, and then cultured overnight at 37°C in an incubator containing 5% CO2. The supernatants from each well were then harvested and the presence of IFN-? confirmed with a sandwich ELISA. A result was considered as positive if ODPPD-B-ODPPD-A=0.1 and ODPPD-B-ODPBS=0.1, and negative if ODPPD-B-ODPPD-A<0.1 and ODPPD-B-ODPBS=0.1.

Data analysis

To evaluate the frequency of bTB, the test and real prevalence and incidence rate were calculated. For incidence, only cows that were negative and then became positive on a subsequent test were included in the analyses. 95% confidence intervals were calculated for each parameter.

Apparent (test) Prevalence (AP) = number of cows test positive ÷ total number tested

Real prevalence = (AP+ Specificity -1) ÷ (Sensitivity +Specificity-1)

Incidence rate = new cases ÷ animal time at risk.


Cattle bTB prevalence on individual farms

The test results (AP) of the IFN-? test for bTB in each farm are presented in (Table 1). The AP varied from 8.2% (95% CI: 4.8, 10.3) to 54.8% (95% CI: 43.5, 65.7) on Farm A during the 6 years of testing. As Farms A purchased cows from others, we also calculated the AP of cows that newly introduced to that farms in separated time points. Results showed that only in introduced cows, the highest AP was up to 36.7% (95% CI: 23.4, 51.7) on Dec. 2012 and 30.1% (95% CI: 21.0, 40.5) on Jan. 2013.