Food infection- Brucellosis

 The genus Brucella is named after Sir David Bruce who in 1887 recognized it as the causative organism of undulant fever (brucellosis, Malta fever, Mediterranean fever).

Four species that are human pathogens is associated with a particular animal host,

• B. abortus (cattle)
•  B. melitensis (sheep and goats),
• B. suis (pigs),
• B. canis (dogs)

 Brucellosis is principally contracted from close association with infected animals and is an occupational disease of farmers, herdsmen, veterinarians and slaughterhouse workers. It can also be contracted by consumption of milk or milk products from an infected animal, although the risk is lower.

The illness has been effectively eliminated from the United States, Scandinavia, the UK and other countries by campaigns to eradicate the organism in the national dairy herds through a programme of testing, immunization of young calves and compulsory slaughter of infected cattle.

Brucellosis has sometimes been associated with the consumption of inadequately cooked meat from an infected animal but raw milk or cream are the principal food vehicles. Brucella is readily killed by normal milk pasteurization conditions so there is no risk from pasteurized milk or products made from it. Cheeses made from unpasteurized milk can sometimes pose a problem  since the organism can survive the cheese making processes and subsequent storage in the product.
 

Brucella are Gram-negative, catalase-positive, oxidase-positive, short oval rods, which are non-motile and usually occur singly, in pairs, or, rarely, in short chains. It grows optimally around 37^oC and is killed by heating at 63^oC for 7–10 min. When shed in the milk of an infected animal it can survive for many days if the acidity remains low.

Brucellosis is a protracted (long lasting) and debilitating (causing weakness) illness characterized by an incubation period of from one to six weeks. Symptoms are a chronic, relapsing fever with accompanying lassitude (physical or mental weariness), sweats, headache, constipation, anorexia, pains in the limbs and back, and weight loss. After the temperature has returned to normal for a few days, another bout of fever may ensue and such episodes recur a number of times over several months. Treatment is commonly with a mixture of tetracycline and streptomycin.

It is a facultative parasite and can live intracellularly or in extracellular body fluids. During the febrile stage, caused by circulating endotoxin, the organism may be isolated from the bloodstream but in the majority of laboratory-confirmed cases diagnosis is based on serological tests rather than cultural techniques.

Brucella are quite fastidious organisms and do not grow in conventional laboratory media. Liver infusions or calf serum are normally added. The organism grows slowly and cultures are normally incubated for three weeks before they are considered negative. In view of this, testing foods for the organism is not practically feasible or useful. 

Cattle are tested for the presence of antibodies to the organism in the ‘Ring Test’. Stained antigen is mixed with the test milk, if antibodies to Brucella are present (indicative of infection) then they will cause the antigen to clump and rise with the milkfat on standing to form an intense blue-violet ring at the top of the milk.