From April 29 to May 1, 2020, a brucellosis outbreak was reported in a village in Jingyang County, and as of May 7, 7 symptomatic cases and 6 asymptomatic cases were detected and reported. The investigation on this outbreak was conducted by the Jingyang County CDC and Livestock Center and revealed that Patient 1 and her eight relatives were exposed to their infected dogs, which might have been infected by the pathogen when they ate miscarried lambs that had been buried in the orchard of Patient 10. Patient 10 and the 3 individuals (Patient 11, 12 and 13) with positive test results were infected by contact with their sick sheep. In addition, four suspected Brucella strains (two from infected sheep and two from patients) were isolated and identified by a classical phenotyping method (1) and Abortus, Melitensis, Ovis, and Suis polymerase chain reaction (AMOS-PCR), as described elsewhere (2). MLVA-16 (multiple-locus variable-number tandem repeat analysis) was also performed for epidemiological tracing (3).
Suspected cases were defined as residents of the village and neighbouring villages who developed 2 or more of the following symptoms from March 1 to May 7, 2020: fever (≥37.5 ℃), fatigue, night sweats, and joint pain excluding patients with confirmed diagnosis for other diseases. Confirmed cases were defined as suspected cases with an antibody titer of ≥1∶100 (++) in serum agglutination test (SAT) or positive Brucella isolate according to the guidelines for the Diagnosis of Human Brucellosis (WS 269–2019).
So far, out of the 279 individuals who were tested, 13 met the criteria for diagnosis. The demographic and clinical data of individuals who tested positive during the brucellosis outbreak in the village were shown in Table 1. Among the 13 individuals with positive test results, there were 8 males and 5 females (male to female ratio, 1.6∶1). Their ages covered a wide range of 2 to 64 years. Interestingly, 9 of the individuals (Patients 1–9) were related to each other. Most of the cases were found in the family of Patient 1. Among the 3 asymptomatic individuals, 2 individuals were from the East Third Unit (Patients 11 and 12), and 1 was from the Z Group of the village (Patient 13). They were all sheep farmers and had a history of contact with a sheep that had a miscarriage. They did not live in the same village as the others who tested positive.
No. Sex Age (years old) Occupation Case relationship Results of SAT Date of onset Time of diagnosis 1 Female 46 Beef cattle farmer Initial case 1∶800 (+++) Mar 18 Apr 28 2 Male 15 Student Nephew ≥1∶800 (++++) Apr 26 May 1 3 Male 24 Farmer Son-in-law 1∶200 (+++) Apr 23 May 1 4 Female 2 Scattered child Granddaughter 1∶400 (+++) Apr 26 May 1 5 Female 24 Farmer Daughter ≥1∶800 (++++) May 1 May 1 6 Female 10 Student Niece 1∶800 (+++) Apr 14 May 5 7 Female 20 Student Niece 1∶400 (++) Apr 1 May 5 8 Male 47 Beef cattle farmer Husband 1∶800 (+++) No symptom − 9 Male 62 Farmer Case 8’s older brother 1∶800 (++) No symptom − 10 Male 64 Sheep farmer West No. 2 1∶100 (++) No symptom − 11 Male 75 Sheep farmer East No. 3 1∶200 (++) No symptom − 12 Male 60 Sheep farmer East No. 3 1∶200 (+++) No symptom − 13 Male 67 Sheep farmer Z Group 1∶400 (+++) No symptom − Abreviation: SAT=serum agglutination test.
Table 1. List of individuals who tested positive during the outbreak of brucellosis in Jingyang County, Shaanxi Province, China, 2020.
Patient 1 and her relatives (nine individuals in total) had no history of drinking cow and goat milk and no history of contact with any lamb. The family of Patient 1 currently had 2 dogs, which were sometimes tethered and sometimes let outside their cage, and Patient 1’s husband once fed a stray dog. Starting in 2018, the family raised beef cattle in their backyard under poor sanitary conditions. Patient 1’s husband was responsible for feeding the cattle, but he did not use any protective measures during the feeding process. Jingyang County’s Livestock Center collected blood samples of 10 cattle that belonged to Patient 1, but the test results were negative for all cattle samples. All the individuals who visited Patient 1’s family in her village were investigated, and the results showed that the visits were associated with disease onset. On May 5, the Livestock Center collected blood samples from the two dogs that belonged to Patient 1 and her family, and both tested positive for brucellosis. Given all cases had a suspected exposure to dogs, we concluded that contact with the dogs was a key risk factor of infection.
The family of Patient 10 had 2 long-term residents and live only 2 households away from Patient 1 and her family. Patient 10 had been raising sheep for 10 years by self-breeding and had not purchased any other sheep. By the end of 2019, they had 4 pregnant sheep, two of which miscarried, and the fetuses were buried in their own orchard. However, the miscarried fetuses later disappeared from the burial spot, and it was suspected that they had been taken away by an animal. The family of Patient 10 had 2 lambs in stock, and both tested positive in tests done by the Livestock Center on April 30 indicating that these 2 lambs were the source of infection.
On May 5, the Livestock Center collected samples from 70 sheep in stock at another possibly infected sheep farm in the village, and 4 sheep tested positive. On the same day, Jingyang County CDC collected samples from four employees of the sheep farm and all of them tested negative.
The results of epidemiological investigation and comprehensive analysis indicated that the brucellosis outbreak in Patient 1’s family and the eight relatives was caused by exposure to their infected dogs that were likely exposed due to consuming miscarried lambs buried in Patient 10’s family orchard. Patient 10 and 3 individuals with positive test results in the other groups were infected by contact with their sick sheep. All 4 isolates (2 isolates from sheep belonging to Patient 10 and 1 each from Patient 7 and Patient 11) were identified B. melitensis bv. 3 and showed an identical MLVA profile (1-5-3-13-2-2-3-2-4-40-8-6-4-3-4-5), suggesting the same exposure source.