Infectious Diseases of Animals. 255 



through a herd, and only ceases when the last animal has been attacked. In 

 other cases, apparently when the conditions are precisely the same, the disease 

 attacks one or two animals of the herd and then ceases. Under those circum- 

 stances it is difticult to decide whether or not the cessation of the disease is 

 due to measures which have been employed to arrest its spread. This difli- 

 culty becomes most apparent in considering the effects of inoculation. 



Pleuro-pneumonia, it has been already stated, cannot be produced by the 

 introduction of matter from the diseased lungs ; but a knowledge of the fact 

 of the immunity which attends animals which have once suffered from a con- 

 tagious malady, even in its mildest form, led Dr. AVillems in 1852 to try the 

 effect of the introduction of some fluid from a diseased lung into the system of 

 a healthj^ animal. After numerous experiments Dr. Willems came to the 

 conclusion that animals which had suffered from the development of the 

 local effects of this inoculation were, as a rule, secure from the natural attack 

 of the disease. He accordingly made known his discovery to the Belgian 

 Government. 



Dr. Willems's discovery attracted considerable attention, and several com- 

 missions were subsequently appointed to investigate and report as to its 

 efficacy. The Dutch commission appointed in 1852 declared that inoculation 

 possesses the power to temporarily prevent infection, but they declined to 

 express any opinion as to the permanence or otherwise of the security thus 

 afforded. 



The Belgian commission appointed in 1853 reported that inoculation was 

 not a certain preventive of pleuro-pneumonia, that the effects of inoculation 

 are manifested several times in the same animal whether it has had pleuro- 

 pneumonia or not ; that they are developed even when the inoculated animal 

 is suffering from the natural disease. The commission gave no opinion as to- 

 the protective effect of the operation. 



Professor Simonds, who was commissioned by the Eoyal Agricultural Society 

 to visit Belgium in 1853, reported that inoculation made by superficial punc- 

 tures and small abrasions of the skin failed to produce any local effects ; that 

 deep punctures were followed by the ordinary phenomena only of such wounds 

 when some highly irritating agent is introduced into them; that when bad 

 decomposed matter is employed, and the incision is roughly made, ulceration 

 and gangrene follow, sometimes causing the death of the patient ; that inocula- 

 tion with irritating agents was followed by similar phenomena to those observed 

 in inoculation with the exuded serum from a diseased lung ; and that pleuro- 

 pneumonia occurs at various periods of time after the so-called successful 

 inoculation — the severity of the disease being in no way modified by the 

 oiJeration. 



The French commission ap]iointed in 1854 reported that inoculation with 

 fluid obtained from a diseased lung has a preventive influence. It imparts to 

 the organism of the great majority of animals on which the operation is per- 

 formed a power of resisting infection for a period which is as yet unde- 

 termined. 



These conflicting opinions have not been in any way reconciled bj^ the 

 results of subsequent investigations, and there are still two sides to the ques- 

 tion of the efficacy of the operation. On the one hand it is contended that it 

 invariably an-ests the spread of the disease, but on the other it is held that 

 inoculation is entirely inoperative, that the disease progresses in spite of it, or 

 that it ceases in obedience to the influence of certain conditions which are not 

 understood. 



The fact remains that in many instances an outbreak of pleuro-pneumonia 

 is occasionally arrested in herds on which inoculation has been performed. It 

 is also true that it ceases in many instances when no such operation is had 

 recourse to ; but it is certainly the case that the records of instances of the 



