Overview

Brucellosis is a bacterial zoonosis caused by a gram-negative non-spore-forming bacteria of the genus Brucella. It began as an endemic since the discovery of Brucella meritensis, however, currently, Brucella melitensis is responsible for human Brucellosis in different countries. Brucella affects a wide variety of domestic animals (sheep, dogs, horses, camels, cats, cattle, pigs, and goats) as well as wild animals besides humans.^1,2 Bovine Brucellosis is a significant public health concern, particularly in developing countries where it spreads due to inadequate sanitation practices, environmental pollution and limited eradication resources. It is transmitted through direct contact with secretions from infected animals. Brucellosis is still a serious disorder in some developing countries in North and East Africa, and the Middle East because of poor implementation of prevention strategies and high associated costs. It also demands a labour-intensive approach. Some developed countries such as Australia, Japan and the United States, have successfully controlled the spread of brucellosis.³

Pathogens and route of transmission

Brucella species, particularly Brucella melitensis, Brucella suis, Brucella canis, and Brucella abortus are often the main species that infect animals and humans. They are transmitted to horses through skin wounds, ingestion of contaminated food solids, and respiratory exposure.⁴ Brucella abortus is a significant reproductive pathogen that causes abortion and reproductive loss in cattle and buffalo through ingestion of late bacteria in aborted foetuses.⁵

Humans are infected mainly by drinking unpasteurised cow’s milk or eating contaminated cheese.⁴ Moreover, humans can also be infected through direct contact with infected animals and their secretions, particularly in instances where protective measurements have not been implemented. in case no protective measurements were taken.²

Epidemiology 

Infection by Brucella species in horses and other domestic species is detected worldwide. The current situation of Brucellosis in many countries is not well known due to the lack of recent research on this subject. Interestingly, anti-Brucella species antibodies have been examined in horses from Brazil and Eritrea revealing no positive findings. In contrast, Brucella species were documented in horses in Argentina, Australia, and Brazil.

The World Health Organisation (WHO) and the World Organisation for Animal Health (WOAH) have recommended certain measures and strategies to either control or eradicate Brucellosis. Some developed countries have achieved positive outcomes in Brucellosis eradication. However, developing countries, have failed to overcome animal Brucella consequences due to several challenging factors. These countries face challenges due to limited access to resources, making the elimination of Brucellosis hard to achieve, due to high costs, labour-intensive efforts, and a time-consuming process.

According to a systematic review and meta-analysis of the prevalence of Brucellosis antibodies in cows across Latin America and the Caribbean, Venezuela had the highest prevalence (16%). In Central America, the Caribbean islands, and Mexico, a significantly higher percentage of cows tested positive for Brucellosis antibodies (8%) compared to South America. In South America, there are control measures and strategies to eradicate Bovine Brucellosis, in countries like Uruguay and Brazil. Regarding Chile, they are in the medium term. In Columbia, eradication programs for Bovine Brucellosis are carried out. Mexico, and the State of Baja California, and Sonora are free of Brucellosis, 29.16% of the national territory is in the eradication phase. Argentina is free of Bovine Brucellosis. However, Peru is following a control strategy, without any positive effects on the distribution of the infection throughout the country.

According to the WOAH database in 2014, Mexico recorded the highest outbreaks (5514), followed by China (2138), Greece (1268), and Brazil (1142). The main reason for these outbreaks was linked to Brucella abortus which is responsible for Bovine Brucellosis. The risk of infection in Mexico is related to the consumption of contaminated dairy products. Thus, Brucellosis is still a serious problem in many regions including Latin America with higher rates of morbidity in developing countries, such as Mexico.³ 

The horse Brucellosis prevalence depends on several factors such as health measures, sample size of studies, used detection methods in reports, host determinants, and management status.⁴

Clinical manifestations

Clinical finding in animals

Brucellosis appears as a hidden infection in most horses, which is why infected horses do not show any symptoms. Mostly, infected horses don’t show specific symptoms, but they may show localised lesions associated with inflammatory reactions in the joints, ligaments, tendon sheaths, synovium, and others. However, there are certain secondary signs that can be observed, for example:

• Weakness
• Muscle stiffness
• Osteoarthritis
• Osteomyelitis
• Intermittent fever
• Depression

The bursal fluid of the infected horse can be a source of human contamination. Moreover, some animals may exhibit reproductive signs.^1,4

Clinical findings in infected humans

The most reported clinical findings in patients with Brucellosis according to a cross-sectional study with a certain number of participants (n=40) were:⁶

• Fever with/out chills in 90%, number of participants was 36 (n=36)
• Gastrointestinal upset including nausea/vomiting (72.5%,n=29)
• Headache (40%,n=16)
• Malaise (37.5%,,n=15)
• Arthralgia (20%,n=8)
• Abdominal pain (25%,n=10)

Abnormal laboratory parameters in infected humans 

The most reported deranged laboratory parameters in patients with Brucellosis according to a cross-sectional study with a certain number of participants (n=40) were:⁶

• Alkaline phosphatase (ALP) (96%,n=38)
• Aspartate aminotransferase (AST) (62.5%,n=25)
• Leukocytosis (57.5%,n=23)
• Total bilirubin (52.5%,n=21)
• Alanine Transaminase (ALT) (37.5%,n=15)

Diagnosis of brucellosis 

The common detection methods used for Brucellosis diagnosis are blood tests (serology) and culture. Culture and isolation methods can be challenging to regularly use due to difficulty in preparation, testing processes and other factors. Despite that, it is known as the gold standard diagnostic method. Therefore, serological methods are mostly used.

Serological testing

A quick and simple blood test, known as rapid chromatographic immunoassay, is used on individuals suspected to have Brucellosis. The test helps identify the presence of the Brucella antigen in the patient’s blood.

The serological testing procedure takes the following steps:

• It is a membrane-based immunoassay that detects brucella antigens in human serum.
• This membrane is pre-coated with mouse anti-Brucella antibody
• A reaction between the present Brucella antigen and anti-Brucella antibodies takes place, a dye-conjugate is pre-coated with anti-Brucella on the conjugate pad
• The formed mixture of the reaction migrates upward on the membrane in the test line region.
• The absence of coloured lines in the test line region indicates there is no detected Brucella antigen in the human sample
• There is a control line region with a coloured line that acts as a server for the procedure to confirm its reliability in terms of proper volume of added serum and membrane-wicking occurrence
• The diagnosis can be confirmed by enzyme-linked immunosorbent assays (ELISA) through detecting IgG and IgM using a commercial kit

Relevant laboratory parameters are important to be measured in such cases, including liver function tests (LFTs), as they were found in liver tissue even in the absence of LFTs changes, it is done using a fully automatic analyser, and complete blood count (CBC) by Auto Hematology Analyser. Moreover, molecular diagnostic methods, particularly Real-time polymerase chain reaction (Real-Time PCR) due to its high accuracy and specificity, are being used to detect Brucella species in horses.^4,6 Furthermore, diagnostic imaging methods are considered in patients with spinal Brucellosis including magnetic resonance imaging (MRI) and computed tomography (CT).⁷

Prevention and treatment

Prevention

• The government should focus on the screening of animals, slaughtering infected ones, and the vaccination of seronegative animals.
• An extensive and inclusive team to implement well-organised Brucella control programs among the concerned government sectors including the Ministry of Health.
• It is highly recommended that the family members of patients be screened to enhance the detection time, initiate treatment early, and reduce complications.
• Animal owners should be aware of doing regular checkups of animals due to the high prevalence of Brucellosis among them.
• Improving public awareness programs, prevention strategies and measures through stressing the importance of avoiding all Brucellosis risk factors that increase the infection rates such as drinking raw milk or its products.⁸ 

Treatment and management 

Management of Brucellosis relies on adherence to the recommended treatment strategies which are ⁸: 

• Using a suitable antibiotic which has the ability to act intracellularly and effectively in an acidic medium
• Using combined therapy
• Using antimicrobials for a long period accordingly

Regarding the treatment of Brucella infections, there is a limited number of antibiotics that can be used effectively including rifampicin, doxycycline, gentamicin, streptomycin, ciprofloxacin and trimethoprim-sulfamethoxazole. Moreover, in specific cases, there are commonly used combinations that result in positive outcomes. For example, Doxycycline and TMP/SMX or rifampicin have been shown to have high success rates for children older than 8 years. In serious infections with complications, such as endocarditis, three to five drugs are highly recommended for prolonged periods of up to 12 months. 

Research and future directions

Since the beginning of the 20th century, the research work on the improvement of vaccines includes inactivated vaccines, live attenuated vaccines, and rough attenuated vaccines.

Inactivated vaccines have gradually been replaced by attenuated live vaccines due to higher immune effectiveness. The list of the current live attenuated vaccines that are used includes Rev.1, S19, SR82, S2 and RB51, knowing that RB51 is the only officially approved rough attenuated vaccine.

Although these vaccines play a significant role in the prevention of Brucellosis, they still exhibit some disadvantages, such as causing human infections and the risk of abortion complications in female cattle. 

Interestingly, with the development of DNA technologies, molecular biology strategies, and a better understanding of Brucella mechanisms of invasion to human and animal cells (Brucella pathogenesis), new genetic engineering vaccines are highly expected to replace traditional ones. Moreover, this will enhance targeting of the infected hosts and reduce the side effects.⁹

Summary

Brucellosis is a global bacterial infection caused by Brucella species, it can be asymptomatic in horses and with non-certain clinical manifestations in infected humans. Diagnosis of Brucellosis can be carried out through serological testing mainly using ELISA and AST, in addition to liver function laboratory tests and complete blood count (CBC). Management of Brucellosis is achieved through applying awareness public programs, following prevention strategies, screening family members of patients and doing regular checkups for animals considering vaccination for uninfected equines. Moreover, Brucellosis treatment includes a limited number of antibiotics that are effective, sometimes combination therapy is required for a long period according to the case and presence of complications. Recently, research is heading to find new alternative genetic engineering therapies to the traditional vaccines which exhibit unwanted side effects due to low specificity that might target uninfected host cells as well in addition to the increased risk of abortion in female cattle.