PATHOGENICITY 827 



1930, 1933), a careful study of the available epidemiological data, and the frequency 

 with which laboratory infections occur, suggest that Br. melitensis is the most 

 pathogenic and Br. abortus the least pathogenic of the three types. The American 

 suis type appears to occupy an intermediate position. The Danish suis type, 

 on the other hand, is probably even less pathogenic than Br. abortus, since there 

 is no record of its ever having been responsible for disease in man. 



Under natural conditions Br. melitensis is pathogenic for goats and sheep, 

 giving rise to an infection which may be acute and accompanied by abortion, but 

 is more frequently chronic and detectable only by bacteriological examination of 

 the milk, blood, or urine, or by allergic skin tests. In areas where infected goats 

 or sheep are numerous, cows may also become infected (Taylor, Vidal, and Roman 

 1934). No symptoms of disease are manifest, the animals do not abort when 

 pregnant, but the organisms are often excreted in the milk. 



Both varieties of Br. suis are pathogenic for pigs, in which they give rise to a 

 disease sometimes accompanied by abortion. Like Br. melitensis, the American 

 type of Br. suis may infect cows and be excreted in the milk (Huddleson 1934, 

 Beattie and Rice 1934). Horses, dogs, and fowls are occasionally infected. 



Br. abortus probably has the widest range of pathogenicity. Besides being 

 responsible for the almost universal and economically important disease of conta- 

 gious abortion in cattle, it occasionally infects other animals, such as horses and 

 dogs, and less often sheep and goats. In the United States it is said to give rise 

 to fairly extensive infection of fowls and other birds (Emmel and Huddleson 1929, 

 1.930, Emmel 1930), though the evidence for this is not wholly satisfactory. Guinea- 

 pigs have also been found infected under natural conditions (Manzullo 1935). 



All three species are infective to a variable extent for laboratory animals. On 

 the whole the guinea-pig appears to be the most susceptible, but rabbits, rats, and 

 mice can often be infected. In the guinea-pig the disease produced by parenteral 

 inoculation with moderate doses is usually chronic and retrogressive. The brunt 

 of the infection is borne by the reticulo-endothelial system. The resulting lesions 

 are relatively inconspicuous, and consist mainly of a non-hypereemic enlargement 

 of the lymphatic glands, some degree of enlargement of the spleen, and the presence 

 of a variable number of circular necrotic foci in the spleen and liver. In male guinea- 

 pigs abscess formation is not uncommon in the testicle or epididymis, and intra- 

 peritoneal inoculation is sometimes followed by a Straus reaction. Occasionally the 

 bones, joints, or other organs may be affected. The lesions are extremely variable 

 in size and number, and may be completely absent on naked-eye inspection. In 

 infections with Br. suis (American variety) the necrotic lesions tend to be few in 

 number, large in size, and purulent in consistency. The lesions in melitensis and 

 abortus infections, on the other hand, are smaller, more numerous, and generally 

 non-purulent, except in the testicle. Numerous statements have been made 

 about the relative virulence of the three main types for guinea-pigs, but workers 

 who have had the widest experience are the most cautious in drawing conclusions. 

 At the moment it is probably safe to conclude that there is no satisfactory method 

 of distinguishing between them on the basis of pathogenicity to laboratory animals. 

 Kristensen (1931) and Bang (1931) regard the Danish suis type as probably the least 

 virulent, while Thomsen (1934) regards it as slightly more virulent than Br. abortus. 

 Its differentiation from the other members is not practicable by guinea-pig inocula- 

 tion. The rough, so-called paramelitensis, para-abortus, and parasuis varieties are 

 comparatively avirulent to guinea-pigs. Kritschewski and Halperin (1934) state 



