INFECTIOUS LAKVX(!<)THACIIEIT1S CAHHIEH8 167 



The number of days the 14 chronic carriers eUminated sufficient virus at 

 weekly tests to inoculate susceptible chickens, up to the date this paper was 

 written, were as follows: 



* V 2270 was still a clirouie ranici- at the tinio this paijn- was written. 



The results of the above experiments indicate that a small number of all 

 birds in a flock sm-viving infectious laryngotracheitis are liable to become chronic 

 carriers for an indefinite i)eriod. The longest period was not determined, but the 

 shortest was 67 days. The less \-irulent Massachusetts virus jiroduced the 

 greatest number of carriers and the most virulent New Jersey virus the least. 

 It is evident that the more virulent strains produced the smaller number of chronic 

 carriers. The reason for this is unknown, although it may be readily supposed 

 that these strains killed some of the birds that would have become chronic 

 carriers if they had been infected with a less virulent virus. When a bird once 

 became a carrier it invariably remained so for two months or longer up to an 

 indefinite period. Chronic carriers for shorter periods were not found in these 

 experiments, although birds harboring the virus may have existed and not been 

 detected by the lal)oratory methods used. 



Two Types of Chronic Carriers 



Two tyjx's of chronic carriers, characterized in this bulletin b\- the terms 

 "regular" and "irregular," were found among the birds examined. The regular 

 carriers eliminated the virus at weekly tests as long as they remained carriers, 

 while the irregular carriers failed to eliminate virus at two or more tests during 

 the carrier stage. Possibly the birds should have been classified as constant and 

 intermittent carriers, but on account of the cost involved in providing chickens 

 for making the tests it was impossible to study the constancy and the inter- 

 mittency of the elimination of the virus more closely. The number of regular 

 and irregular carriers as detected by weekly inoculations of tracheal exudates 

 from the chronic carriers into the tracheas of susceptible chickens were as follows: 

 \'irus Carriers 



Regular Irregular 



Massachusetts strains 7 2 



California strains 2 1 



New Jersev strains 2 



