Brucella spp. is the aetiological agent of brucellosis, a serious contagious disease that results in reproductive failure and that has profound public health significance because of its zoonotic characteristics. This disease also is responsible for a high economic impact associated with the application of prevention, surveillance and test-and-slaughter programmes in animals by national authorities. Brucella spp. infects a large variety of animals and their prevalence is variable worldwide, mainly associated with the presence or absence of control programmes and also with the vaccination of animals against brucellosis. To achieve the control and eradication of brucellosis, the identification of the risk factors of brucellosis that maintain the infection in animals and/or the environment is fundamental. Although several risks have been identified, the most important have been associated with the biology of the bacteria, animal management (age, sex, species or breed), herd management (herd/flock size, number of species, contact with wild animals or type of animal production), farm management (facilities, cleaning and disinfection or veterinary support) and farmers’ knowledge about the disease. Thus, to benefit from proper risk identification of brucellosis, it is essential to put a cost-effective and efficient brucellosis control programme into place.
- risk factors
Brucellosis is a serious contagious disease that results in reproductive failure and has profound public health significance because of its zoonotic characteristics . In animals, brucellosis can be considered as one of the most economically important zoonosis worldwide, resulting in clinical disease, abortion, neonatal losses, increased calving intervals, reduced fertility, decreased milk production, increased culling rates because of metritis and the emergency slaughtering of infected animals and also an impediment to free animal movement and trade [2-4]. However, a high economic impact is associated with the human disease and also by the application of prevention, surveillance and test-and-slaughter programmes in animals by national authorities [4,5].
|Domestic/Farm animals||Alpacas, Cattle, Dogs, Goats, Horses, Llamas, Pigs, Sheep||[8,9,10-14]|
|Wild animals||Bears, Bison, Buffalo, Camelids, Caribou, Deer, Elk, Ferrets, Foxes, Rabbits, Rodents, Wolves||[15,16,17-22]|
|Marine mammals||Dolphins, Dugongs, Manatees, Otters, Sea porpoise||[15,23-29]|
The epidemiology of brucellosis is variable worldwide. In developed countries, brucellosis has been eradicated or presents low individual prevalence due to control programmes and vaccination of animals [30,31]. Currently, the brucellosis status of a country is based on the epidemiology in domestic animals. However, to consider a country free of brucellosis, it may also include epidemiological data regarding brucellosis in both wild animals and in marine animals [15,25].
According to the data available at the World Organisation for Animal Health (OIE) (2009), brucellosis (including
The aim of brucellosis control is both to decrease the impact on human health and avoid economic consequences through reducing exposure to
RBT + I-ELISA
RBT + ELISA
RBT + I-ELISA
RBT + SAT
RBT + I-ELISA
Identification of risk factors of brucellosis has been reported in epidemiological studies [36-41]. Although several risks have been identified, the most important are related to farm management, animal management and knowledge about the disease . Thus, to benefit from proper risk identification of brucellosis, it is essential to put a cost-effective and efficient brucellosis control programme into place.
2. Risk factors of brucellosis in animals
The risk factors can be categorized into those associated with characteristics of animal populations, management and the parasite biology.
2.1. Risk factors associated with the biology of
Dogs, cats and other wild carnivores such as foxes or wolves present an important role in the epidemiology of brucellosis, because they act as mechanical disseminators due to the transportation of infected foetuses or placentas from abortions in infected herds and flocks. Since pigs are susceptible to infection by
Porcine brucellosis is caused by
The infection of a pig herd by brucellosis could be associated with the purchase/entrance of infected animals, contact with wildlife reservoirs, use of contaminated semen or feed  or the use of a lend boar. Other risk factors could be attributed to transmission of the disease by mechanical vectors due to contamination of vehicles, holding equipment or utensils and also to the introduction of infected offal (e.g. placenta and afterbirths) . Serological screening and purchase from brucellosis-free herds should reduce this risk .
The likelihood of the introduction of the infection from potentially infected wild boar, free-range pigs or hares and its establishment in outdoor and backyard pig populations depends on housing management such as the type of housing (outdoor vs indoor), low levels of biosecurity, direct or indirect contact with infected wild boar, free-ranging pigs or hares, feeding practices (
2.2. Risk factors associated with the host
Age has been referred to as one of the intrinsic factors associated with brucellosis. Higher seroprevalence of brucellosis has been observed in older animals, both in cattle and small ruminants with a prevalence odds ratio (POR) of about 2.0 in cattle over 5 years old and a POR of about 1.7 in small ruminants over 2 years old [43,73-77]. Similar results have been observed in wild boars and camels [78,79]. Brucellosis has traditionally been considered a disease of adult animals since susceptibility increases after sexual maturity and pregnancy . However, variations in the age of sexual maturity among breeds could present differences between age and brucellosis positivity .
The influence of sex in the prevalence of brucellosis has been studied in cattle, small ruminants and wild animals [74,77,79,80,84]. Female ruminants presented a higher odds of brucellosis infection, the same has been observed in female dogs compared to male dogs . Although this is difficult to explain, it could be associated with the intrinsic biology of the microorganisms and its tropism to the foetal tissues as previously described. Since brucellosis infection in males presented clinical signs such as epididymitis and orchitis, the prevalence in males could be lower than females because they may be culled faster . On the other hand, the absence of clinical signs such as abortion or metritis in non-pregnant infected females or the absence of farmers’ observation/identification of abortions in extensive herds may also explain the higher prevalence in females. In addition, in non-pregnant females, brucellosis becomes chronic. This fact has important epidemiological implications since, after an initial immune response, animals may be asymptomatic carriers, the antibodies disappear from circulation and are difficult to detect with traditional serological techniques . Since brucellosis in pigs may affect both males and females equally, sex susceptibility has not been fully demonstrated . Regarding wild boars, the behavior of females living in matriarchal groups could explain the higher prevalence .
2.2.3. Species and breed
The prevalence of brucellosis is variable among species and region as described in Table 2. However, prevalence in farm animals seems to be lower in small ruminants than large ruminants [44,84] and lower in sheep than in goats [45,88,89]. Transmission of brucellosis occurs in ruminants through the excretion of contaminated materials from the female genital tract, which constitutes the main form of transmission to other animals and humans. In most of the circumstances, the main route of spread is the placenta, foetal fluids and vaginal discharges expelled after delivery or abortion. At that time, large numbers of
The phenomenon of latent brucellosis in sheep was observed in lambs born from infected mothers that breast-feed with milk contaminated with
Infected females thus present a high number of abortions with special importance in primiparous females .
In game animals, seroprevalence in wild boars seems to be higher than wild ruminants [8,95-98]. To the best of our knowledge, there is no evidence of higher susceptibility to brucellosis within specific species. In the case of horses, they have usually been considered more resistant to brucellosis than ruminants , but the variation of prevalence reported in endemic areas of brucellosis [99,100] seems to be discussible. The information available about differences of brucellosis infection by species is scarce. In sheep, pregnant dams do not present
Regarding the breed, a higher prevalence of brucellosis has been reported  in cross-breed cattle than local breeds, although other reports indicated no statistical differences among cattle breeds [46,105]. In small ruminants, Maltese and South American sheep breeds seem to present a greater resistance to brucellosis compared to the sheep breeds of Southwest Asia and the Mediterranean, such as the Awassi breed [13,106,107]. Although Husky and Chihuahua dog breeds appeared to be more prone to
Several risk factors of brucellosis have been associated with the host such as age, sex, species or breed. Regarding age, higher seroprevalence of brucellosis is observed in older animals since susceptibility increases after sexual maturity and pregnancy. It could be associated with the tropism of
2.3. Risk factors associated with herds
2.3.1. Herd/flock size
An important risk factor for brucellosis seropositivity is herd size, being higher in large herds and/or flocks. An increased odds ratio for seropositivity has been largely reported in cattle [82,84,104,110] as well as in small ruminants [77,86,111]. In contrast, no statistical differences among goat flocks were observed in the literature [112,113].
The higher prevalence of brucellosis in large herds and/or flocks has been associated with several factors, such as a higher number of animals tested in larger herds means the probability of detecting at least one seropositive animal is greater  or the higher number of animals increases the likelihood of transmission of the disease by contact among them . The low prevalence of brucellosis in small-sized herds could also be associated with the herd and/or farm management . Thus, small-sized flocks usually graze at pastures near or contiguous to the farm, avoiding contact with other flocks or utilization of common paths and/or roads. Because premises for small herds or flocks are smaller, cleaning, disinfection and manure removal procedures are easier and less time consuming to the farmer. Disinfection is also facilitated by the low resistance of
The health status of a flock may influence the predisposition to brucellosis infection. Thus, in small-sized herds, farmers can easily identify sick animals and veterinary and preventive treatments are usually carried out at low financial cost. Regarding the official control of brucellosis by the official veterinary authority, small-sized flocks are easily controlled and in the case of a positive finding, most farmers agree to cull the whole flock to maintain the brucellosis-free status and also to avoid a zoonotic infection [116,117]. In addition, the vaccination coverage of young animals with RB-51 or Rev-1 is more easily achievable in these herds.
On the other hand, the higher prevalence of brucellosis observed in large flocks may be also associated with the utilization of communal pasture areas, utilization of common paths and/or roads and due to contact with others flocks . Cleaning and disinfection procedures of premises and manure removal in large-sized flocks is more difficult than in medium or small flocks because it requires the availability of mechanical equipment and consequently a higher financial cost. An increased prevalence of brucellosis associated with decreased of proper manure removal, cleaning and disinfection procedures has been described . The control of reproductive management is difficult in large flocks, where parturitions on grazing areas are frequent and abortions are a source of pasture contamination. In addition, animal movement in large herds is frequent, both for replacement and/or trade, thereby increasing the risk of infection by brucellosis. Due to the higher cost of veterinary treatments and/or application of preventive programmes, animals in large flocks may be more susceptible to brucellosis infection. Moreover, associated with high numbers of animals unvaccinated and/or non-blood sampled animals may occur and remained unprotected and susceptible in case of infection. In addition, these animals act as a source of brucellosis contamination to the rest of the herds [74,118] and in the case of positive animals, farmers hesitate to slaughter the entire flock.
In dogs, the risk of transmission increases in kennel environments due to the high interaction among the animals and reduced space, which infected dogs share with other healthy ones to play, defecate or urinate . Kennels with a history of abortion are 13 times more likely to be seropositive than kennels without this record .
Transmission studies have demonstrated that the exposure of healthy dogs to abortion products is an easy way for
2.3.2. Number of species
Farming several species in the same herd has been described as a risk factor [78,80,84], although there is no evidence of higher susceptibility of brucellosis in specific species. Thus, an increase in prevalence where several species intermingle is difficult to explain but could be associated with higher chances of being
It has been suggested that brucellosis is transmitted only rarely from sheep and goats to cattle, or among cattle . However, the higher risk for cattle on farms which also had sheep or goats suggests that some of the cattle infections may have originated from small ruminants since
In regions where
As previously described, horses present a certain resistance against brucellosis, however, seropositivity has been associated with horses in areas without brucellosis control programmes for large and small ruminants. In addition,
The presence of swine could be a risk for brucellosis transmission to cattle  and is a public health concern. However, recent studies showed that cattle intermingling with pigs in the same area do not seem to be infected by
The practice of mixing cattle, either through grazing or sharing watering points, is a significant risk factor for brucellosis [104,127,128]. Community pastures should be treated as livestock unit and control measures must be applied to all animals .
Other researchers  found that the disease is easily transmitted in areas where extensive production systems predominate, based on grazing and the high mobility of herds, the mixture of species in the same herd and where sharing pastures, roads and water sources occurs. Mobility increases the likelihood of contact with other potentially infected herds or wild animals that are reservoirs of disease.
The presence of dogs has been described as a risk for brucellosis infection in farm animals  and represents a potential epidemiological threat in endemic and/or brucellosis areas without brucellosis control programmes. However, dogs are a potential risk in the diffusion of brucellosis, acting as mechanical disseminators by feeding on aborted foetuses, dragging them along and spreading the bacteria .
Canine brucellosis is usually caused by
2.4. Risk factors associated with farm management and environment
Several risk factors of brucellosis associated with farm management and environment have been referred to in the literature as presented in Table 3.
|Absence of calving paddock||[14,43]|
|Cleaning and disinfection||[40,112,114,130]|
|Commingling with other animals||[14,88,114,131]|
|Contact with wildlife||[36,74,104]|
|Handling of aborted material||[43,80]|
|Main animal production (beef /dairy)|||
|Purchase/entrance of new animals||[39,112]|
|Handling of aborted material||[43,80,134]|
Regarding the main seroprevalence, dairy animals have a much greater chance of not only contracting brucellosis but also of spreading it faster than beef animals. The reason is not a genetic or physiological factor but due to husbandry. Animals that live in concentrated smaller areas come into close contact when they are grazing and when they are milked . The zoonotic transmission of brucellosis by improper milking procedures was observed  associated with skin lesions in hands. Thus, transmission through skin lesions of the udder is not a neglectable source of infection. In addition, it is considered that dairy animals are subjected to more stress conditions on farms, leading to a higher susceptibility to brucellosis infection . The persistence of the infection of the udder and supramammary lymph nodes leads to a constant or intermittent excretion in milk in successive lactations. This fact constitutes an important source of infection for humans and for the young animals .
Animal purchase has been considered as a risk for brucellosis. Purchasing in larger herds has usually been associated with more animal movements on and off the farm, and this practice increases the risk of introducing an infected animal into a herd . Introduction of animals from market fairs also presents a higher risk of infection. The majority of infections or reinfection in disease-free herds starts through buying infected animals of unknown status . This has a higher importance in those endemic areas or countries where there is an absence of control programmes. However, in countries with test-and-slaughter control programmes, the movement of cattle are subjected to a compulsory pre-movement test that consists in the serological brucellosis diagnostics before an animal leaves the farm . Moreover, animal movement restriction measures are applied in brucellosis positive herds to avoid spreading the disease .
The proximity to other infected herds or flocks has also been described an infection risk, although, small ruminant contact with other flocks was reported to have no impact on
The influence of the agro-ecological zone has been also referred to as a brucellosis risk factor, having a higher prevalence in dry zones . Since pasture areas are scarce in dry zones, animals must seek pastures over large areas implying an unrestricted animal-to-animal contact with potential transmission as previously described. In addition, transmission due to aerosol inhalation of contaminated dust from foetal discharges or abortions is possible . In contrast, a lower prevalence of brucellosis in these areas has been proposed by other authors  due to lower survival of
Larger herds might be expected to be associated with intensive management practices that are typically more difficult to control and allow for closer contact between animals and their environment, which increases the potential for exposure to infectious excretions . In addition, the stressful conditions of animals subjected to intensive production may make them more susceptible to the infection, as previously described. On the other hand, extensive management may also imply a risk of brucellosis and higher prevalence has been reported in small ruminants. Although difficult to explain, it could be associated with controlling abortions, observation of sick animals or contact with animals, among others . Since extensive management implies rearing a large number of animals in large areas and/or sharing communal pastures, the contamination of pastures with placentas or abortions is a source of infection to other animals in the herds, as we described previously in the risk factors of brucellosis by the herd size.
Animal handling and environmental conditions are risk factors which influence the transmission of infection, such as births and breeding in semi-dark settings, confinement in closed spaces and high animal densities . Another risk of intensive systems could be associated with airborne dust transmission indoors .
The season also has an impact on herd management and animal nutrition, mainly in production systems involving transhumance or nomadic practices . Rainfall affects the development and the nutritional state of the pasture. These factors influence the reproduction of animals kept in extensive systems and thus the time of delivery/miscarriages. In intensive systems, isolation of post-parturient animals in maternity facilities reduces the spread of infection to the rest of the herd or flock .
Cleaning and disinfection of farm facilities and proper manure removal have been described as a protective factor against brucellosis infection [114,118,130]. This fact is associated with the low resistance of
Insect rodents on dogs could act as a mechanical vector of brucellosis. Blood-sucking insects have been reported as disseminators of brucellosis.
Environmental factors that affect the ability of
The survival of
2.5. Other factors associated with brucellosis
The role of farmers’ knowledge about brucellosis has been discussed in the literature. It was noted that knowledge ages equal to or older than 55 years was a protective factor for brucellosis prevention [40,42]. This observation is difficult to explain and may be due to younger farmers’ lack of experience. Older farmers have more familiarity with recognizing the clinical signs of the disease or the main route of transmission and can be more aware of the importance of preventive measures [67,144,145]. Farmers who had previously experienced brucellosis in their herds had a higher probability of having greater knowledge of bovine brucellosis, which is consistent with having experience with the disease. Producer’s associations, education and veterinary support have been recognized as protective factors [42,118]. Farmer’s lack of awareness about brucellosis, improper handling of aborted materials and the habit of consuming raw milk, among other factors, might contribute to further spread of brucellosis in their livestock and expose the community to a public health hazard .
The risk factors of brucellosis associated with the herd are size and the number of animal species. The higher prevalence of brucellosis in large herds could be explained by the higher odds of detecting at least one seropositive animal, the increase of the transmission of the disease by contact among them, utilization of communal pasture areas or improper cleaning and disinfection procedures in large farms. Farming several species in the same herd has been described as a risk of infection due to multiple sources of infection. Thus, presence of dogs in large herds may spread
Dairy animals have a much greater chance of not only contracting brucellosis but also of spreading it faster than beef animals. Because animals that live in concentrated smaller areas come into close contact when they are grazing and when they are milked. In addition, it is considered that dairy animals (intensive production) are subjected to more stress conditions on farms, leading to a higher susceptibility to brucellosis infection. Purchasing in larger herds has usually been associated with more animal movements on and off the farm, and this practice increases the risk of introducing an infected animal of unknown status with special importance in areas with absence of control programmes.
Also the influence of the agro-ecological zone has been also referred as a brucellosis risk factor. High humidity, low temperatures and absence of direct sun light may favour the survival of
2.6. Brucellosis in wild animals — A threat to farm animals
With regards to elk and bison, artificial feeding management during winter results in significant congregations in the feeding grounds and increases the risk of elk being exposed to
Rangiferine brucellosis (brucellosis in reindeer and caribou) is caused by
2.7. Brucellosis in marine mammals — New threat?
The transmission of brucellosis in marine mammals is not totally understood . The dilution of the agent in sea water may make transmission difficult due to a low infecting dose. It is likely that the mode of transmission is through close contact between hosts, such as sexual intercourse or maternal feeding, contact with aborted foetuses and placental tissues or through fish or helminth reservoirs . A second alternative corresponds to vertical transmission from mother to foetus, which is feasible since foetuses and placenta from infected animals have been found to contain large quantities of
2.8. Animal brucellosis and zoonotic risk
In endemic regions without brucellosis eradication programmes, zoonotic risk still represents an important public health threat . Infection happens due to contact with infected animals or consumption of their products, mostly unpasteurized milk and milk products of sheep and goats . It presents special importance in those regions where trading of raw milk and raw milk products is a common practice among farmers . The survival of
Although zoonotic brucellosis is mainly associated with farmers in high prevalence areas, even in low prevalence countries brucellosis represents an important threat as a work-acquired infection among dairy farmers, butchers, veterinary practitioners, meat inspectors, slaughterhouse personnel or artificial inseminators who do not take adequate biosafety precautions while performing their jobs [174-176]. In addition, brucellosis vaccines such as Rev-1 and RB51 are live dried living vaccines. Thus, needlestick accidents during their preparation or administration could also be a risk factor for human infection. Close contact with animals may occur when farmers or veterinarians assist animals during parturition or abortion or handling of stillbirth. In some parts of the world it is also common practice for farmers to separate the placenta manually, thereby increasing their exposition to tissues infected with
Dairy farmers who milk with bare hands have a greater chance of becoming infected from
Zoonotic brucellosis from marine mammal includes individuals in traditional communities where products from whales and seals are still an important part of their diet . In addition, occupational acquired infection in people handling stranded marine mammals, whale and seal hunters, marine researchers and other people handling raw products from the ocean could be exposed [25,178]. Also, it is suggested that marine avian species may harbour
Wild animals have been referred as reservoir of brucellosis and represent an important risk of infection to farm animals, particularly in extensive breeding systems.
The prevalence of brucellosis in wildlife varies worldwide and several species such as bison, reindeer, caribou or wild boar have been described as potential source of infection of livestock. However, their role as risk factors of infection is still discussed since the microbiological isolation of
Foodborne brucellosis is an important biological hazard associated with dairy products. However, the presence of
The work was supported by the strategic research project PEst-OE/AGR/UI0772/2014 financed by the Foundation for Science and Technology [FCT]. This study was supported by the Foundation for Science and Technology of Portuguese Ministry of Education and Science [FCT - Fundação para a Ciência e Tecnologia do Ministério Português para a Educação e Ciência] by a research grant SFRH/BD/85118/2012 given to the author J. García Díez.