MASKING 83 



b. "Masked" swine influenza virus 



The second example of "masked" virus that I am going to outhne 

 for this discussion is that of the swine influenza virus in its lungworm 

 intermediate host. Actually this example of "masking" plays a large 

 role in the second topic that I am to present for discussion at this con- 

 ference, namely, the interepidemic survival of animal viruses. As a 

 consequence this discussion of "masked" influenza virus in the swine 

 lungworm is a natural transition between the two topics. 



The epidemiology of swine influenza is quite simple once an epi- 

 zootic has started. As m.ost of you probably know, this disease has a 

 complex etiology being caused by infection with a bacterium, Hemo- 

 philus influenza suis, and the swine influenza virus acting in concert 

 (3). The disease itself is a highly contagious respiratory infection and 

 the spread from sick to well is rapid and extensive. Once a case has 

 appeared on a farm or in a community, orthodox epidemiological con- 

 siderations are all that are required to explain its widespread dissemi- 

 nation. However, getting that first case of swdne influenza started from 

 "scratch," so to speak, is a stickler. I might point out in passing that 

 the problem of accounting for the first case in an epidemic is not unique 

 to the case of swine influenza, but is shared by most, if not all, of the 

 other epidemic diseases of man or animals. 



Characteristically swine influenza epizootics are of annual occur- 

 rence in the Middle West and begin explosively usually the last week 

 in October or the first week in November (4) . The build up of cases is 

 extremely rapid and one gains the impression that either the disease 

 spreads wdth unbelievable rapidity or that it has arisen at many differ- 

 ent foci simultaneously. After the initial widespread outbreak, fresh 

 swine droves are infected in smaller and smaller numbers until by late 

 December or early January, as a rule, the epizootic appears to have run 

 its course and swine influenza disappears as a farm infection until the 

 following October or November. Consideration of the epidemiological 

 situation made it appear quite obvious either that the causative agents 

 arose de novo each year or that a reservoir host mechanism existed 

 which was capable of preserving the agents throughout the nine months 

 of the year that the disease is not active. Persistence of the bacterial 

 component Hemophilus influenzae suis for long periods of time in the 

 respiratory tracts of recovered animals can be demonstrated. However, 

 similar persistence of the swine influenza virus cannot be shown and 

 ordinarily it disappears from the respiratory tracts of recovered animals 

 on about the eighth day after their initial infection. 



As it turns out, there is an intermediate host capable of preserving 

 the swine influenza virus in nature for long periods of time. This inter- 



