84 SHOPE 



mediate host is the swine lungworm. Since the lungworm cycle is a 

 rather comphcated one, it probably should be outlined briefly at this 

 point for the sake of clarity (5). 



The lungworm is a nematode parasitic in the bronchioles of the 

 bases of the lungs of swine. The female lungworm lays fully embry- 

 onated eggs in the bronchi of the swine she infests. These are coughed 

 up, swallowed, and reach the outer world in the feces. Their further 

 development then is dependent upon their being ingested by an earth- 

 worm. Once within the earthworm, the lungworm eggs hatch and the 

 larvae develop to the third or infective stage, usually localizing in the 

 calciferous glands and hearts of the parasitized earthworm. They per- 

 sist in this stage until the earthworm is eaten by a pig. In the pig the 

 lungworms undergo two further developmental stages finally reaching 

 the respiratory tract, by way of the blood stream and lymphatics, where 

 they become adults. The whole of this cycle can occupy a span of 

 several years for its completion, or, under the most favorable conditions, 

 can be completed in a little more than a month. 



In the above cycle the larvee developing from lungworm ova laid 

 during the time the host pig is undergoing an attack of swine influenza, 

 or even from those laid for at least a short period after recovery, will 

 be carriers of swine influenza virus (6). A most puzzling feature of 

 this situation, however, is that the virus cannot be detected by direct 

 means either in the larvee in their intermediate hosts or in the adult 

 lungworms after transmission to their definitive host, the pig. It appears 

 to be present in an occult or "masked" form and evidence of its presence 

 is furnished only by its subsequent behavior under very specialized 

 conditions. Swine that have become parasitized with lungworms known 

 by the subsequent course of events to be carriers of "masked" virus do 

 not come down directly wdth swine influenza. Instead they remain to 

 all outward appearances perfectly normal pigs. However, actually they 

 are in a very precarious situation so far as their eventual well-being is 

 concerned because all that is required to bring them down with a severe 

 or even fatal influenza infection is the application of some stimulus, of 

 itself completely harmless. Several such provocative stimuU have been 

 used, but the one that has proven most regularly effective consists in 

 the administration of multiple injections of the bacterium, H. influenzae 

 suis. I should add that the provocation of "masked" influenza virus 

 succeeds under experimental conditions only in the period between 

 September and April. It has uniformly failed in the months of May, 

 June, July, and August. 



The nature of the "masking" process of swine influenza virus in the 

 lungworm is not at all known and even the indirect means which have 



