zoonoses and communicable diseases common to ... - PAHO/WHO

BRUCELLOSIS 41tions to the monospecific A. (abortus) and M. (melitensis) sera. Thus, B. melitensisis subdivided into three biovars (1–3); B. abortus, into seven (1–7)—biovars 7 and8 were discarded and the current biovar 7 corresponds to 9 in the old classification;and B. suis, into five (1–5). From an epidemiological viewpoint, the taxonomicsystem of the genus Brucella has eliminated confusion arising from the naming ofnew species or subspecies that did not agree with epidemiological reality. Moreover,typing by biovars constitutes a useful research tool in that field. The characteristicsof B. abortus determined by conventional methods vary greatly, such as sensitivityor tolerance to aniline dyes, production of H 2S, and CO 2requirements for growth.Less plasticity is shown by B. melitensis or B. suis (Meyer, 1984). In various partsof the world, strains of B. abortus and, to a lesser extent, of B. suis or B. melitensishave been discovered that are difficult to place within the current scheme, in thatthey differ in some characteristics (Ewalt and Forbes, 1987; Corbel et al., 1984;Banai et al., 1990).However, the genome of the genus Brucella is very homogeneous as shown byVerger et al. (1985) in a DNA:DNA hybridization study. These researchers proposemaintaining a single species, B. melitensis, subdivided into six biogroups, whichwould correspond to the six previous species. For all practical purposes, and especiallyfor epidemiological purposes, the previous scheme that divides the genus intospecies and biovars is still in effect.Geographic Distribution: Worldwide. The distribution of the different species ofBrucella and their biovars varies with geographic areas. B. abortus is the most widespread;B. melitensis and B. suis are irregularly distributed; B. neotomae was isolatedfrom desert rats (Neotoma lepida) in Utah (USA), and its distribution is limitedto natural foci, as the infection has never been confirmed in man or domesticanimals. Infection by B. canis has been confirmed in many countries on several continents,and its worldwide distribution can be asserted. B. ovis seems to be found inall countries where sheep raising is an important activity.Occurrence in Man: Each year about a half million cases of brucellosis occur inhumans around the world (WHO, 1975). The prevalence of the infection in animalreservoirs provides a key to its occurrence in humans. B. abortus and B. suis infectionsusually affect occupational groups, while B. melitensis infections occur morefrequently than the other types in the general population. The greatest prevalence inman is found in those countries with a high incidence of B. melitensis infectionamong goats, sheep, or both species. The Latin American countries with the largestnumber of recorded cases are Argentina, Mexico, and Peru. The same pattern holdstrue for Mediterranean countries, Iran, the former Soviet Union, and Mongolia.In Saudi Arabia, 7,893 human cases of brucellosis were recorded in 1987 (74 per100,000 inhabitants). Brucellosis probably became very important in public healthbecause during the period 1979 to 1987, Saudi Arabia imported more than 8 millionsheep, more than 2 million goats, more than 250,000 cattle, and other animals (buffalo,camels). In Iran, 71,051 cases (13 per 100,000) were recorded in 1988 and it isestimated that 80,000 cases have occurred each year since 1989. In Turkey, 5,003cases (9 per 100,000) were recorded in 1990, an incidence three times higher thanduring the period 1986–1989 (3 per 100,000).Programs for the control and eradication of bovine brucellosis markedly reducethe incidence of disease in humans. For example, in the United States, 6,321 cases