Brucellosis is a globally prevalent zoonotic disease that is caused by several species of the genus Brucella. Over 500,000 cases of brucellosis are reported annually. Despite the low mortality rate associated with brucellosis, acute and chronic infection significantly impacts the patient and healthcare system.
Study: Public and animal health risks associated with spillover of Brucella melitensis into dairy farms. Image Credit: sw_photo / Shutterstock.com
What is brucellosis?
The most common Brucella species that infects animals and humans is B. abortus and B. melitensis, respectively. Humans can also be infected by B. suis, B. abortus, and B. canis.
Previous research has revealed that small ruminants, particularly goats and sheep, are the main hosts of B. melitensis. Humans contract brucellosis through the consumption of unpasteurized dairy products or occupational exposure, such as veterinarians and animal handlers.
In 1984, B. abortus was eliminated by vaccinating cattle with the S19 vaccine and implementing strict regulations in the dairy industry. Despite Rev1 vaccination of small ruminants and practicing control measures, Israel has faced sporadic incidences of B. melitensis over the last decade.
Notably, an increased frequency of B. melitensis has been observed among the Arab population residing in Israel. The Bedouin Arab population in Southern Israel was particularly affected by B. melitensis, with infection rates up to 100-fold higher than the Jewish population residing in the same region.
Continual monitoring of dairy farms in Israel identified an increase in bovine infections with B. melitensis. Considering this report, recent typing methods have been developed based on whole-genome sequencing (WGS) to obtain an in-depth analysis of the origin of the Brucella spp. and their prevalence in endemic and non-endemic settings.
In Israel, every dairy farm is assigned a level of brucellosis risk. Here, it is mandatory to conduct a serological test such as serum complement fixation test (CFT) for Brucella immunoglobulin G (IgG) following abortion episodes in cattle.
About the study
A recent Microbial Genomics study analyzed all B. melitensis outbreaks that affected dairy farms in Israel from 2006 to 2021. The genomic epidemiology of B. melitensis could help elucidate the association between bovine and human infection with B. melitensis.
The current retrospective study identified B. melitensis infection in dairy farms and evaluated the related epidemiological clusters that occurred between 2006 and 2021. The Israeli Brucellosis National Reference Laboratory records were used to obtain relevant data.
B. melitensis isolates were obtained from several veterinary clinical samples including aborted fetal tissue, blood, or milk that were collected during the investigation of late abortion among cattle or human infection due to their exposure to diseased dairy cows. Both CFT for IgG and serum agglutination for Brucella IgM were conducted.
WGS was used to identify the transmission chain of brucellosis that affected dairy cows, small ruminants, and humans.
About 92% of isolates were obtained from 2015 onwards. A total of 23 incidents of brucellosis were investigated in dairy farms and animal husbandries during the study period, from which eighteen epidemiological clusters were derived. These clusters were associated with cattle from 19 different dairy farms that hosted 8,161 cows.
Among the 92 isolates sequenced in this study, 78 were bovine isolates, nine were epidemiologically linked with clinical human isolates, and five originated from small ruminants. The study findings indicate that bovine infections in dairy farms occurred due to spillover from small ruminants. About 72% of the isolates originated from the southern region, whereas the remaining isolates originated from the northern region of Israel.
Phylogenetic analysis revealed that many isolates were closely related, as they were clustered within the same genomic clusters. The genomic data helped provide epidemiological insights in areas of confusion.
For example, WGS helped confirm the source of infection in two farms that previously reported epidemiologically related brucellosis. Notably, traditional epidemiology failed to establish the link between the infection of the studied farms.
Closely related isolates exhibited different biovars, thus indicating that biovar classification is non-specific. The current study helped elucidate the reservoir and transmission between farms, as common and geographically dependent reservoirs, such as sheep and goats infected with B. melitensis, were found to transmit the disease to farm animals.
A low Rev-1 vaccination rate and insufficient surveillance are crucial factors that could result in a brucellosis epidemic. The interconnectedness of dairy farms and local herds, particularly small ruminants in the same region, has been associated with disease transmission. A spillover from the dairy farm to humans has also been observed.
Notably, the current study established a direct epidemiological connection that links nine human cases to bovine cases. Each of these infections were linked with occupational exposures.
The spillover of B. melitensis from its host to dairy herds also suggests the possibility of host adaptation to the pathogen. In the future, more research is needed to determine whether bovines are accidental hosts of B. melitensis.
The current study demonstrated the persistent prevalence of B. melitensis in Israeli dairy farms. Although the robust regional connection between bovine and human brucellosis was established, occupational transmission was prevailed in dairy farms. These findings emphasize the importance of controlling Brucella transmission by regularly monitoring dairy farms and human exposure to the disease.
- Bardenstein, S., Grupel D., Blum, S. E., et al. (2023) Public and animal health risks associated with spillover of Brucella melitensis into dairy farms. Microbial Genomics 9(4). doi:10.1099/mgen.0.001014.