IMAlUNOLOr.V OF LARVNCOTRACHEITIS • 19 



lead to erroneous conclusions. This point should he made clear by carefully 

 controlled experiments under range conditions, and moreo\'er, further studies of 

 infectious laryngotracheitis are needed to determine the practical value of the 

 laboratory methorls and immunological theories advanced herein under fielil 

 conditions, without complication with chicken pox or any other disease which 

 might lessen \itality. 



In vaccinating a large number of birds, various degrees of reactions are bound 

 to occur. Some cases will be so mild that only temporary immunity results, 

 while others will respond abnormally and develop a disease so severe that some will 

 die and carriers are produced in a few of those that recover. However, evidence 

 is lacking to show that sufficient virus passes into the blood stream from the in- 

 fected bursa of Fabricius to be responsible for clinical cases, although it has been 

 demonstrated that birds do infect themselves after vaccination by picking and rub- 

 bing the affected parts, and probably indirectly from the virus in the droppings. 

 Thus the danger of creating fresh centers of infection, through the agency of vac- 

 cinated birds and carriers, is obvious, in spite of the fact that most ot the birds 

 ma\- respond normalh- and be satisfactorily immunized against the disease. 



SUMMARY 



1. In order to be acceptalole to the majority of poultrymen, treatments for 

 infectious laryngotracheitis must be cheap, easy to administer, and fairly effective. 



2. Since the intravenous and subcutaneous methorls of stimulating immunity 

 did not measure up to this standard, they are not recommended for field practice. 



3. However, the intravenous and subcutaneous studies were conducted under 

 controlled laboratory conditions and the following fundamental immunological 

 points discovered : — 



(a) Birds can be immunized against infectious laryngotracheitis by introduc- 

 ing small doses of virus intermittently into some portion of the body other than 

 the respiratory tract. 



(b) The virus is carried by the blood and may stimulate the production of 

 immune bodies in all tissues directly in touch with the vascular system. 



(c) Large cjuantities of virus released into the blood stream at once may be 

 carried to the respiratory tract and lead to the development of active symptoms. 



4. The inoculation of the bursa of Fabricius appeared to be the most satis- 

 factory way of utilizing the immunological discoveries brought out in this study 

 for immunizing birds against infectious laryngotracheitis. 



5. The best time to vaccinate appeared to be between three and four months 

 of age, or when the bursa of Fabricius had reached its greatest development. 



6. Only birds in the best of health are suitable for vaccination against intectious 

 laryngotracheitis. 



7. The significance of certain traumatic cysts, blow-outs, and adhesions lounfl 

 in the bursa of Fabricius and adjacent parts of some of the birds after vaccination 

 is not understoo 1, in view of the fact that special care was taken to avoid rough 

 handling. 



8. Special care should be exercised in selecting and cultivating tracheal exu- 

 date for bursa of Fabricius vaccination in order to exclude pathogenic bacteria and 

 viruses other than the causative agent in infectious laryngotracheitis. 



0. Infectious laryngotracheitis tracheal exudates desiccated and preserved by 

 the modified Swift method maintain their virulence for several months. 



10. Final judgment should be withheld on th? bursa of Fabricius vaccination 

 until carefully controlled experiments are completed to determine its practical 

 value under range conditions, and the duration of immunity in fowls in heavy pro- 

 duction. 



