340 ACUTE EXANTHEMATOUS INFECTIOUS DISEASES 



the pox of other animals (ox, horse, swine) are merely modi- 

 fications of these. 



It is extremely probable that in the beginning all forms of 

 variola had a common origin. By continued passage for 

 several generations through different species of animals, how- 

 ever, the virus has become so adapted to the specific species 

 in which it has propagated, that transmissability to other 

 species of animals no longer occurs through natural infection 

 and is difficult to bring about by inoculation. ' 



Etiology. — From recent experiments it has been determined 

 that filtering the virus of the variola of different animals and 

 man does not decrease its virulency unless very fine filters 

 are employed. It is probable, therefore, that the cause is an 

 ultramicroscopic organism. 



The virus is contained in the papule, vesicles, and crusts. 

 It is doubtful whether the /expirium or blood of the varioloid 

 patient is infectious. The secretions and excretions are in- 

 fective when contaminated with the contents of the vesicles. 



Sheep-pox (Variola Ovina). — Definition. — A specific infec- 

 tious disease of sheep characterized by a cutaneous eruption 

 which passes through the stages of papule, vesicle, pustule, 

 and crust. 



Occurrence. — Sheep-pox is not known in the United States. 

 While its home is in Asia, outbreaks have occurred in Europe 

 and Africa. When the disease becomes epizootic it causes 

 considerable' economic losses from death and the damage it 

 causes the wool. 



Etiology. — The cause is probably an ultramicroscopic 

 organism. 



Natural Infection. — The virus enters through the respiratory 

 tract and is carried from the lung alveoli by the blood to the skin 

 and mucous membranes. Intratracheal injections of the con- 

 tents of the vesicles (lymph) readily produce the disease, 

 while feeding experiments have negative results provided 

 inhalation was excluded. Impregnating the air a distance 

 from susceptible sheep with a spray of pox lymph or the 

 dried crusts gave positive results. 



A flock of sheep is usually infected by direct contact with a 

 variolous sheep, one convalescent from the disease, and in 



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