Enzootic situation and molecular epidemiology of Brucella in livestock from 2011 to 2015 in Qingyang, China

Abstract

A large-scale survey was conducted in domestic animal populations from 2011 to 2015 in Qingyang, China. A total of 448,398 animals from different districts of Qingyang were tested for the presence of Brucella-specific antibodies using the Rose Bengal Plate Test (RBPT) and the Standard Agglutination Test (SAT). From 2011 to 2015, the yearly average positive rates were between 0.04 and 4.75% in the eight counties tested. In addition, the prevalence rates were between 0 and 9.96% in these eight counties. Sheep was the dominant host of Brucella in Qingyang, and the prevalence rate in sheep (2.74%) was higher than those in the other animals tested. Identification of 10 Brucella isolates from sheep confirmed that the epidemic strains were B. melitensis biovar 3 (n = 9) and B. melitensis biovar 1 (n = 1). MLVA-11 (multiple-locus variable-number tandem repeat analysis) analysis of the 10 isolates showed three genotypes: genotype 116 (n = 8), genotype 115 (n = 1) and genotype 136 (n = 1). Furthermore, analysis of the whole-genome sequences of the representative B. melitensis strain QY1 indicated that this isolate was closely related to isolates from China and India. The results of serum epidemiology confirmed that the region of northern Qingyang was a critical Brucella epidemic area and that the disease showed a rising trend, especially from 2013 to 2015. An analysis of the isolate genotypes suggested that sheep brucellosis mainly resulted from conventional B. melitensis (East Mediterranean group), although the external strain (American group) also occurred in Qingyang.

Fig. 1. The seroprevalence of brucellosis in livestock in Qingyang from 2011 to 2015.

a The positive rates of the different counties in each year. b Differential analysis of brucellosis in different counties from 2011 to 2015. Bar, the average seroprevalence from 2011 to 2015; *Significant difference at P < 0.05. c Sample sizes and positively detected cases in the dairy cow brucellosis epidemic in Qingyang from 2011 to 2015. d Dairy cow brucellosis epidemic tendency in Qingyang from 2011 to 2015

Fig. 2. The seroprevalence of brucellosis in sheep from 2011 to 2015.

a The positive rates of the different counties in each year from 2011 to 2015. b Sheep brucellosis epidemic tendency in Qingyang from 2011 to 2015. c Sheep brucellosis epidemic tendencies in different counties in Qingyang from 2011 to 2015. d Differential analysis of the seroprevalence in different counties in Qingyang. Bar, the average seroprevalence from 2011 to 2015; **Very significant difference at P < 0.01

Fig. 3. Geographic distribution of the seroprevalence of brucellosis in livestock in Qingyang, China.

Red area: seroprevalence of 2.71%; yellow area: seroprevalence between 1.50 and 1.90%; gray area: seroprevalence <1%

Fig. 4. AMOS-PCR of isolates in Qingyang.

Lane 1: 100-bp DNA ladder; 2: B. melitensis bv. 1 (16 M reference strain); 3: B. abortus 544; 4: B. suis bv. 1 (S2 vaccine); 5–14: the 10 B. melitensis isolates

Fig. 5. Dendrogram based on the MLVA-11 genotyping assay showing the relationships among the Brucella isolates.

Strain: isolate number; Origin: sample collection area; Host: host from which the strain was isolated; Year: time of isolation

Fig. 6. Phylogenetic tree based on the whole genomes of nine selected B. melitensis strains.

The evolutionary history was inferred using the neighbor-joining method. The optimal tree (with branch length sum = 0.00316668) is shown. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree