Natural Infection. 305 



into the superficial vessels of the skin and mucous membranes. 

 The disease may be invariably produced by injecting or atomiz- 

 ing pox lymph into the trachea of susceptible sheep, while the 

 feeding of lymph, when the possibility of itihalation is excluded, 

 is not effective (Nocard). 



The pox virus contained in the vesicles and in the scabs 

 becomes pulverized and disseminated in the air, as a result of 

 which a certain area surrounding the infected animals becomes 

 infectious (according to Gilbert in calm weather for 20-25 paces, 

 in windy weather even further). If the food is contaminated 

 by scabs the virus is inhaled by sheep with the dust disturbed 

 during feeding. 



The disease usually develops in flocks of sheep when they 

 come in contact with affected animals or when sick or not 

 entirely recovered sheep are placed among healthy animals. In 

 this respect sheep which become ill from immunization, es- 

 pecially young lambs, are also dangerous. Susceptible sheep 

 associating with such lambs may also become affected with a 

 generalized, severe form of pox, as the virus used for the im- 

 munization is not attenuated in any way. The frequent occur- 

 rence of the disease among lambs in the spring of the year is 

 often associated with the usual immunization carried out at 

 that season in many localities. The infection may also be trans- 

 mitted by contaminated utensils or attendants, by skins, wool, 

 feed, manure, straw, pasture, highways, transportation wagons, 

 further b}^ shepherds and their clothes, dogs, cats, fowl, etc. 



The infectiousness of a diseased animal is especially great 

 when the eruption becomes matured on the body surface, and 

 to a still higher degree during the stage of scab formation 

 and desquamation. The contagion may remain virulent in the 

 scabs under the wool from 6-8 weeks, and therefore the infection 

 may be communicated by animals which have already passed 

 through the disease. 



The susceptibility of the various breeds of sheep is not uni- 

 form. The most susceptible are the Merino sheep, somewhat 

 less the more hardy, coarse-wooled sheep. In the first the dis- 

 ease develops as a rule in a more severe form than in tlie latter. 

 In northern countries it is less severe and less frequent than 

 in the south, which is probably due to the higher temperature 

 which favors the development of pox. According to Nocard 

 the sheep of Bretagne are not susceptible to pox, and the same 

 opinion is also held by Chauveau concerning the Algerian sheep. 

 Waldteufel & Nocard however found that pox exists continu- 

 ously among the sheep of Algeria, although in a very mild 

 form (if affected Algerian sheep come in contact with French 

 sheep, the latter become severely affected). 



Young sheep are more susceptible to pox -than older 

 animals, and in affected herds the lambs as a rule become 

 severely ill without exception. 



Affected ewes may transmit the disease to their offspring, 



