86 SHOPE 



disease caused by the pseudorabies virus is uniformly fatal and kills all 

 cattle it infects usually within forty-eight hours of the onset of symp- 

 toms. It is evident therefore that unless some extra-bovine host were 

 involved in the epidemiology of the disease, pseudorabies virus should 

 have died out and been forever lost in the first cow it infected. Fortu- 

 nately for the virus, an intermediate host, capable of perpetuating it in 

 nature and of getting it transmitted from cow to cow, is available. This 

 intermediate and reservoir host is the hog (9). In swine, pseudorabies 

 virus is a contagious disease transmitting readily from pig to pig and 

 persisting in infected swine for upwards of two weeks. Unlike pseudo- 

 rabies in cattle, swine infections are extremely mild. In fact, they are 

 so mild that, though the virus is known to cause extensive infections in 

 Middle Western swine, these infections are seldom recognized clinically. 

 Porcine pseudorabies might therefore be termed a silent epizootic dis- 

 ease and its presence on Middle Western farms is not recognized unless 

 the causative virus escapes from its porcine host and spreads to cattle. 

 Pseudorabies virus is present in and on the noses of infected swine in 

 relatively high concentration and is transmitted from this site to cattle 

 through abrasions on the skin. The epidemiological situation in pseudo- 

 rabies is somewhat reminiscent of that prevailing in the case of lyso- 

 genic cultures of bacteria. If one were to translate the pseudorabies 

 epidemiology into terms of bacteriophagy, the swine in the picture 

 would become the lysogenic carrier strain and the cattle would be anal- 

 ogous to the indicator organisms. The immunity reaction of swine to 

 the pseudorabies virus enters to spoil the complete parallelism of the 

 two pictures. As I mentioned above, pseudorabies virus persists in the 

 noses of infected swine for upwards of two weeks. It disappears then 

 because by this time the pig has become immunized and either frees 

 itself of virus or renders its virus noninfectious. Lysogenic bacterial 

 carrier strains do not have an immunity response to contend with and 

 may therefore remain lysogenic indefinitely. 



d. East African swine fever 



East African swine fever is another virus infection in which the 

 responsible intermediate host plays a role somewhat analogous to a 

 lysogenic organism in spreading its infection from one recognized out- 

 break to the next. In this case the wart hog is the healthy carrier of a 

 virus which it transmits periodically to domestic swine with devastat- 

 ingly fatal consequences to the domestic pigs (10). 



e. Snotsiekte 



Another similar example which may be mentioned is that of Snot- 

 siekte in African cattle (11). In this instance the gnu or wildebeeste 



