Risk factors for new bovine brucellosis infections in Colombian herds

Bovine brucellosis is an important zoonosis with worldwide distribution. The infection has been controlled or eliminated in most developed countries, but it remains endemic in Africa, Latin America and Asia [1].

Bovine brucellosis is a chronic, infectious disease caused by Brucella abortus, which has developed mechanisms to live intracellularly and is able to infect cattle for long periods of –time [2, 3]. Some animals are asymptomatic, having latent infection without exhibiting clinical signs, thus maintaining the disease in a herd [4, 5].

Cattle become infected after the ingestion of contaminated milk, food, water or grazing forage; close contact with infected animals; contact with uterine secretions or aborted foetuses; and through vertical and sexual transmission [6, 7]. The disease directly affects cattle and buffalo, which are the main reservoirs of B. abortus, although other domestic and wild species can also act as a reservoir [8, 9]. The main clinical signs are abortion, infertility, stillbirth or the birth of weak calves, along with epididymitis and orchitis in males [10, 11].

The introduction of cattle from infected herds or in contact with false-negative animals can contribute to introducing the infection into brucellosis-free farms [12]. Transmission within and between farms has been associated with different risk factors: the maintenance of positive animals in the herds, large farms, communal pastures, semi-intensive production systems and age (the infection is more prevalent in adults) [13]. Brucellosis is considered a complex disease due to its wide range of hosts and the variable signs at both the individual and population levels. Infected animals can remain infectious after their first abortion and spread the disease [14]. On the other hand, the disease can be misclassified as other reproductive diseases, which is one of the reasons that bovine brucellosis is generally underestimated at the farm level [14, 15]. Only Brucella abortus has been reported in Colombia. There are no studies concerning the epidemiology of brucellosis in the human population in Colombia, but it is assumed that in terms of human health, brucellosis is an under-diagnosed and under-notified disease [16].

The methods to control and eliminate brucellosis from a region are based on vaccination, controlling movements, and testing and removing serologically positive animals. Considerable effort and time are required to achieve disease-free status.

Control efforts against bovine brucellosis in Colombia are led through an official programme. This programme is voluntary, includes 15% of farms and is based on the identification of animals, compulsory vaccination of calves between 3 and 8 months of age (to make this vaccination compatible with further serological tests), control of animal movement, and testing and removal of positive animals older than 24 months (8 months in the case of males). The diagnosis of bovine brucellosis is made by serological serial testing with the Rose Bengal plate test and indirect ELISA and is complemented by competitive ELISA.

Blood sampling is performed by the veterinary service or by authorized veterinarians, and farms are classified as brucellosis-free if all animals are negative in two consecutive serological diagnostic tests. In 2016, 18,996 herds had achieved the status of officially brucellosis-free (4% of the 514,794 cattle farms present in the country).

In a previous study about bovine brucellosis in Colombia, heterogeneity was found in the control efforts and prevalence within the country [17]. In recent years, several farms classified as bovine brucellosis-free have become infected, in most cases without knowledge of the causes of these new infections. The aim of this study is to identify the risk factors associated with new infections of Brucella abortus detected between 2015 and 2016 on cattle farms previously certified as being free of the disease. A secondary objective is to determine the opinion of veterinary services about the possible causes of these new infections.

During the last two years, 98 previously brucellosis-free farms were infected and included in the study. These farms were located in fifteen departments, representing 60% of the Colombian cattle census (Antioquia, Arauca, Bolivar, Boyacá, Caldas, Cesar, Córdoba, Cundinamarca, La Guajira, Nariño, Putumayo, Quindío, Risaralda, Santander and Tolima), and the 93 controls were from the same departments.

The significant variables in the bivariate analysis (p < 0.05) were the type of livestock, direct contact, indirect contact with neighbouring farms through water, infection status of the origin of replacement heifers and bulls, number of cattle introduced into the farm, and status of the mother (for internal replacements).

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.

Three risk factors for the re-introduction of brucellosis into previously brucellosis-free herds were identified: the introduction of replacement animals and bulls from farms with unknown or positive brucellosis status, the use of AI with semen from neighbouring non-brucellosis-negative herds and the identity of a herd as a beef herd.

Governments should reformulate strategies to extend the programme to a larger number of farms and to make the programme mandatory in the medium term. In addition to vaccination and test-and-slaughter strategies, the programme needs to include an awareness campaign that focuses on the cost of the disease, even for beef producers, and on the importance of avoiding risk factors such as the introduction of replacement animals or the use of fresh semen from uncontrolled herds.