Asymptomatic brucellosis patients may develop into brucellosis patients, causing serious damage to their health[14]. Nine farmers and herdsmen in this study may have been exposed to brucella through sheep raising, deliver, contact with aborted lambs and brucella live attenuated vaccine. These work methods give shepherds the opportunity to directly contact the liquid of diseased animals or the partially inactivated brucella, and to be exposed to the polluted environment for a long time, thus leading to infection. During this follow-up, it was found that a total of 7 (46.67%) asymptomatic infections were positive and all showed obvious clinical manifestations of brucellosis, of which 5 patients turned positive in the first month. All 5 patients had an epidemiological history of exposure to diseased animals during the follow-up period, which may make them more likely to be exposed to brucella, it leads to repeated infection and progression from asymptomatic infection to brucellosis. There was one asymptomatic patient showed clinical manifestation after two months of follow-up, but she did not contact brucella directly during the follow-up period, the possible reason was the decline of the body's immune function. In addition. It was also found that two asymptomatic infection adults showed decreased SAT titers and no clinical manifestation. Possible causes were (1) Brucella antibodies had poor persistence in asymptomatic infections (2) Short follow-up time has not been observed the final outcomes of these two people.
Brucellosis reportedly occurs mainly in adult populations[10]. However, there were 6 (40%) minors had asymptomatic infections in this study, most of them (83.3%) had no brucella direct contact history. Possible causes of asymptomatic brucella infection in minors who were not in contact with diseased animals are (1) Minors have less direct contact with brucella, mainly due to ingestion of contaminated food or contaminated environment and aerosol infections[15–17] such as eating unpasteurized dairy products and local traditional foods containing dairy products[18]. (2) Minors, especially children, usually show fewer or milder symptoms than adult patients. However, we also found that only one of the minors had a positive outcome at the seventh month at the same time. This may be due to the lower frequency of contact or contamination with brucella-bearing animals, and the possibility of repeated infection was lower.
We found that it is because of their continuous exposure to pathogenic animals or contaminated environments that asymptomatic infections had progressed from asymptomatic infection to brucellosis. For this asymptomatic brucella outbreak in the laboratory, we supposed that timely inspection and treatment of sources of infection should be performed to prevent asymptomatic infections from re-contacting brucella and reduce the possibility of becoming infected. Similar happened in Thailand[19].They also supposed that the most likely mechanism was air-borne inhalation. At the same time, researchers should strengthen personal protection in the search for the source of infection and terminal disinfection in order to avoid biosecurity issues from happening again. Follow-up observations should be performed on those diagnosed with asymptomatic infection. Fortunately, previous studies had shown that polymerase chain reaction (PCR) testing has an important role in predicting the subsequent outcome of asymptomatic brucella infection[20–23].
There are several advantages in our study. First, the annual incidence rate of brucellosis in qianguoerluosi mongolian autonomous county was high, about 0.36 per thousand. It is a representative area with high incidence of brucellosis in China. Second, we surveyed all members of shepherds. However, only 15 asymptomatic infections were observed in our study, asymptomatic infection rates(5.7%)are lower than previous studies[9, 24]. We supposed that the infection may show obvious clinical manifestation because of the strong virulence of sheep brucella. Previous research has shown that Brucella abortus is less pathogenic and virulent than sheep brucella, leading to asymptomatic and suspected infections in humans after exposure to infected cattle. Our current study also had limitations. First, we only observed 15 asymptomatic infections. Second, the average follow-up time was 10.47 ± 8.47 months, which only reflects the short-term outcomes of asymptomatic infections. Long-term outcomes and influencing factors cannot be obtained. Obviously, more cases and long-term follow-up can get more accurate results.