Brucella abortus belongs to the family Brucellaceae along with ten other recognised Brucella species. It is the species of Brucella most commonly isolated in South Africa. Although usually associated with cattle, it may also infect sheep, goats, horses, dogs, wild ruminants and humans.
Brucella abortus are very small gram-negative, rod-shaped bacteria that live inside cells. They are partially acid-fast, which means their cell walls retain stain despite attempts at decolourisation by weak acids. This characteristic is employed by the Stamp’s modified Ziehl-Neelsen staining method to detect them. Brucella abortus is difficult to culture due to its requirement for conditions as found inside cells. It grows slowly necessitating an incubation period of 7 to 10 days before cultures are discarded as negative.
Brucella abortus is usually ingested, but due to their small size the bacteria can also be inhaled as aerosol. Upon ingestion or inhalation, they invade cells of the oral or respiratory mucous membranes and/or are taken up by phagocytes (cells able to ingest small particles). They avoid digestion by preventing fusion of the phagosome (containing the Brucellae) and lysosome (containing digestive enzymes). In this environment they can persist for the host’s lifetime as they are protected from the immune system. During the initial phase of infection, bacteria are carried to the regional lymph nodes where they continue to multiply. A phase follows where the bacteria are carried via the bloodstream to the spleen, liver, supramammary lymph nodes, bone marrow and joints. In the pregnant animal, Brucella abortus is attracted to the placenta as its growth is stimulated by erythritol, a carbohydrate that is present in large quantities in the cotyledons and foetal fluids. Bacteria are shed in placenta and foetal fluids as well as urine, milk and semen. Although not known to multiply outside the host, Brucella abortus can survive in the environment for long periods under suitable conditions. It can remain viable in in faeces for up to a year and in dust or soil for up to 125 days. Despite its stability in the environment, Brucellae are sensitive to many disinfectants as well as high temperatures.
OIE Terrestrial Manual. 2016. Chapter 2.1.4. Brucellosis (Brucella abortus, B. melitensis and B. suis) (Infection with B. abortus, B. melitensis and B. suis).
Songer, J.G. & Post, K.W. 2005. Chapter 25: The Genus Brucella, in: Veterinary Microbiology, Elsevier Saunders, China: pp 200-207.