Bovine brucellosis is endemic in Colombia [17] and, over the last 20 years, has been subject to an official control programme based on the compulsory vaccination of all bovine and bubaline females between 3 and 8 months of age and on a voluntary programme of testing and removing positive animals. Despite this programme, the incidence of brucellosis has remained stable, with the percentage of positive farms and positive animals averaging 22–23% and 4.7–4.6%, respectively, between 2006 and 2012 [17]. One of the reasons for the lack of progress in brucellosis control is the detection of new infections in herds that had previously been brucellosis-free. Between January 2015 and December 2016, 98 farms became infected. In this case-control study, we determined the possible factors related to these new infections. When these farms were compared with 93 herds that remained brucellosis-free, three factors were statistically significant: the origin of replacement animals, type of animal (dairy, beef or dual purpose) and use of AI or natural mating.
One of the more important causes of the introduction of diseases into non-affected populations is the introduction of infected animals: The acquisition of replacement animals from farms with an unknown status or where brucellosis is present carries a significant risk of introducing the infection [12, 19]. In Colombia, the origin of replacement animals was also the most significant factor associated with the new infections, with an OR of 4.8 (32% of the herds that became infected had brought in animals from non-brucellosis-free farms, compared with 9% of the controls).
Beef cattle farms had a higher risk of becoming infected than farms with dairy or dual-purpose herds (OR = 3.6, using dairy cattle as the reference). In general, beef cattle producers are more reluctant than dairy producers to apply control measures because they do not appreciate the benefits deriving from a brucellosis-free herd: The disease does not affect prices, and they do not understand the importance and the cost of reproductive disorders. Additionally, animals are distributed over large, extensive grazing areas that are usually difficult to access, where the control measures are more difficult to apply, and the animals are prone to greater contact with uncontrolled contaminated sources.
In contrast, dairy cattle producers are fully aware of the loss of production due to abortions and infertility and the subsequent important milk losses [20]. Furthermore, brucellosis-free herds can obtain more competitive prices for milk than non-certificated farms, because brucellosis is a concern for the milk industry and customers pay a bonus to brucellosis-free herds. Thus, dairy farmers are more prone to adopt the official control programme and to apply prevention and control measures more strictly than beef producers do.
The third variable that was significantly different between the cases and controls was the origin of the bulls. As expected, the entry of bulls from herds with a positive or unknown brucellosis status was a risk factor for the introduction of the infection in previously brucellosis-free herds. Surprisingly, the use of AI carries a higher risk than natural mating with bulls derived from herds certified as brucellosis-free (p = 0.004); these differences are probably due to the common practice on some farms of using fresh semen obtained from bulls on neighbouring farms. The use of artificial insemination with refrigerated semen obtained from a farm’s own bulls is a common practice in Colombia, and some farmers sell semen to neighbouring farms without any previous sanitary control. In our opinion, this lack of sanitary control is the reason that farms using AI have a higher risk than those that use natural mating with bulls from brucellosis-free herds. Unfortunately, we could not differentiate between insemination with refrigerated semen and insemination with frozen semen. Frozen semen comes from insemination centres certified as brucellosis-free by veterinary services.
In this study, the role of neighbouring herds was not observed except for the use of fresh semen, as mentioned above. Brucella infection has been associated with the spread of infection through direct contact with cattle from neighbouring herds and through the exchange of bulls between farms for mating [21]. Indirect contact with neighbouring farms, such as through shared drinking water access points and grazing lands, also constitutes a risk for the transmission of brucellosis between herds [7, 21].
The presence of mixed farms, including interactions with small ruminants, has also been associated with high prevalence of brucellosis [22] and with a resurgence of the disease in cattle herds [14]. A relationship between these species and brucellosis was not observed in our study, despite traditional husbandry practices in Colombia, including rearing different species in the same habitat.
The results of this study contribute to understanding new infections of previously infection-free herds and may help decision makers in Colombia and other tropical countries with a similar situation to improve control strategies for bovine brucellosis.
The factors described here are well known. Indeed, veterinarians recommend the need to introduce only animals from brucellosis-free herds and to require a brucellosis-free certification for semen, but veterinary officers believe that many farmers do not comply with such recommendations. Consequently, it is important to emphasize to cattle owners the risk involved in the introduction of animals and semen from farms of unknown health status. Furthermore, control measures for the introduction of animals into brucellosis-free herds, including a quarantine of newly acquired animals to assure that these animals are negative, should be strictly applied.
As the prevalence of Brucella is relatively low, culling positive animals should be mandatory and reinforced with an educational programme for farmers to highlight the importance of brucellosis as a zoonotic disease [17, 21]. This campaign should also include the importance of biosecurity measures to reduce the spread of brucellosis between and within herds.
Finally, when the risk factors were compared with the veterinary officers’ opinions, there was substantial agreement on the consequences of introducing infected animals into the herd but not on other significant variables such as the role of artificial insemination with semen from non-controlled herds. Additionally, other variables that can also play a role in reintroducing brucellosis were cited. This analysis reflects the fact that there are distinct perceptions of possible causes for the introduction of brucellosis into herds and highlights a lack of knowledge regarding some of these causes.