Etiology and Epidemiology
The type of brucellosis originally studied in Malta and described by Bruce in 1887 is caused by B. melitensis. It is transmitted to human beings by consumption of milk from infected goats; occasional cases due to contamination of skin with infective material have also been observed.
The mode of transmission became established only during the first decade of the twentieth century. In Malta and elsewhere around the Mediterranean littoral, the disease was endemic rather than epidemic during the nineteenth century, its highest incidence occurring during the summer months. Officers of the services, and their wives and children, appeared to be more susceptible than the lower ranks; and likewise among civilians, the professional classes suffered more than the laborers. Records going back to the mid-nineteenth century show very high annual morbidity rates, but low case-fatality rates.Other areas traditionally affected by undulant fever due to B. melitensis show variations in epidemiological patterns. For example, in southeast France where sheep and goats also vastly outnumber cattle, the disease was still widespread in the 1930s. There it had more the character of an occupational disease than of a consumers’ disease. The majority of cases occurred in the farming communities and resulted from direct contact with infected animals and manure, although consumption of goats’ milk and of fresh cheese prepared from goats’ or ewes’ milk also played some part.
Present in all countries around the Mediterranean, brucellosis, caused by B. melitensis, is also known in India, China, South Africa, and South America. In 1918 Alice Evans suggested that the agent of the cattle disease known as contagious abortion was very similar to B. melitensis and might be capable of causing a similar disease in human populations. This was soon confirmed, and cases of brucellosis caused by B. abortus transmitted to human beings by consumption of raw cows’ milk have occurred worldwide, especially in areas of high incidence of epizootic abortion in cattle.
The third major type of undulant fever is due to B. suis. As the name suggests, its natural host is the pig, and the human variety of the disease attacks mainly slaughterers and packers infected by handling contaminated carcasses. In most cases, B. suis invades the human host through skin lesions, although airborne infection is also thought to be possible. The disease in pigs, and in human beings, is far less common than its counterparts in goats and in cattle, and therefore, undulant fever in human beings due to B. suis has been observed mainly in hograising areas of the American Midwest, Brazil, and the Argentine. Sporadic cases of undulant fever in Alaskan Eskimos have resulted from a type of B. suis infecting wild reindeer.