256 MANUAL OF MICROBIOLOGICAL METHODS 



animals usually leads to some degree of adaptation. This adaptation 

 may be of little consequence or may be significant, depending upon the 

 object of the investigator or the conclusions he draws. One should 

 assume the possibility of antigenic change in a well-adapted virus and 

 check it against the same strain stored shortly after its isolation. Such 

 antigenic changes have been demonstrated (Hartman et al., 1954), and 

 viruses have been deliberately adapted to altered environmental condi- 

 tions (Jones, 1945) . Since viruses and rickettsiae are quite labile in many 

 of their characteristics, carefully controlled experiments cannot be 

 overemphasized. 



Animal autopsy. Usually it is advisable to autopsy the animal when 

 the first definite signs of an infection are present. If this procedure is 

 neglected, the animal may die after working hours and the tissues become 

 unsatisfactory for pathological study. The bacterial content of the 

 tissue harvested for virus may rise sharply if the animals are not autop- 

 sied shortly before death. 



All the usual precautions are observed when autopsying the laboratory 

 animals. It is a good practice to wet down the fur or hair of the animal 

 with a disinfectant such as a solution containing 1 per cent phenol. The 

 object of the autopsy may vary considerably — from harvesting a specific 

 organ for obtaining a quantity of virus to performing a rather extensive 

 examination to learn something of the nature of the infection. It is a 

 common practice to determine the mass of the tissue excised either gravi- 

 metrically or volumetrically so that some reference point for quantitative 

 work pertaining to the virus can be established. In practice, if one uses 

 the same kind of animal of the same age, an average weight or volume is 

 accepted to save time. For example, the average weight of the lungs 

 from 4-week-old mice will be very close to 0.25 g. An extensive and 

 solidifying infection, of course, may vary the weight somewhat. 



In addition to observing the tissue for signs of gross pathology the 

 investigator may be interested in searching for cellular inclusions. The 

 tissue may be sectioned, and the freshly cut surface smeared on a slide 

 and stained, or the more precise histological methods of fixation, embed- 

 ding, and sectioning may be used. The smear method is more likely to 

 rupture cells and distort relationships. Since many of the viruses are not 

 sufficiently large to be seen by the ordinary microscope and fail to form 

 cellular inclusions, other methods are necessary to determine the presence 

 and extent of viral propagation. When a well-adapted virus is employed 

 to induce an infection, the involved tissue can be serially diluted to deter- 

 mine its potency in establishing an infection, in causing agglutination of a 

 constant concentration and volume of appropriate red cells, or in function- 

 ing as a specific antigen in a satisfactory serological test. The produc- 

 tion of pathology in a given tissue need not necessarily indicate the 



