?7 VIRUSES AND HUMAN WELFARE 



difficulty with various virus diseases in potato. They have found that a reason- 

 ably effective method of combating these diseases is the importation of the 

 disease-free stock from Scotland and Ireland. Such stock costs the farmers of 

 Britain "between three and four million dollars annually. When it became appar- 

 ent that the disease, curley top, of sugar beets was becoming a serious threat 

 to the sugar beet industry of this country, new varieties resistant to the curly 

 top virus were developed. Thus the industry was saved from that one particular 

 ravage. A similar solution was found for sugar cane mosaic. Varieties of sugar 

 cane have been developed that are relatively resistant to this disease so tnat 

 it no longer constitutes the serious menace to sugar production it once threat- 

 ened to be. Had it not been for the development of virus resistant sugar beets 

 and virus resistant cane sugar, the sugar shortage which annoyed us all during 

 the war might have been much worse than it actually was . Efforts are now being 

 made to control the tobacco mosaic virus by the development of resistant varie- 

 ties. It will be remembered that tobacco mosaic causes a systemic infection in 

 tobacco plants, but local necrotic lesions in Nicotiana glutinosa. if'rom the 

 cross - tobacco with Nicotiana glutinosa - Holmes succeeded in selecting progeny 

 which possessed most of the ordinary properties of tobacco, but which retained 

 the property of Nicotiana glutinosa of producing local necrotic lesions. This 

 tobacco plant is now being crossed with commercial varieties of tobacco. To- 

 bacco plants are available which resemble in most respects those nov/ under cul- 

 tivation, but which differ by developing local lesions rather than systemic in- 

 fections when infected with tobacco mosaic. This seeras to control tobacco mosaic 

 effectively, for the chances for the spread of a virus froia a single necrotic 

 lesion are infinitely less than from a large systemically infected plant. The 

 use of carrier varieties is practiced with some crops. A carrier variety of 

 plant is one which will harbor a serious virus but not show adverse signs. In 

 England there is a prevalent disease called yellow edge of strawberries. A var- 

 iety of -strawberry has been developed by the name of Huxley, which carries yel- 

 low edge virus but which suffers no apparent adverse effects. 



The control of virus diseases of animals and man is effected by tv/o general 

 types of approach. Both of them are preventive or prophylactic measures, i'irst, 

 virus diseases may be controlled by preventing the spread of the causative agent. 

 In those cases in which some intermediate vector, such as an insect, is responsi- 

 ble for the spread of the disease, the control is realized by measures directed 

 toward the destruction of the intermediate. When the virus spreads by direct 

 host contact, then measures are instituted to prevent such contacts. These meas- 

 ures may take the form of quarantine or, when diseases of animals are considered, 

 eradication of infected hosts is often practiced. The other approach to the con- 

 trol of animal and human virus diseases is through the modification of the host 

 by either active or passive immunization. Active Immunization is achieved by 

 vaccination. There are two general types of vaccination. When an active virus 

 is introduced into a susceptible host by an unnatural route, usually a mild dis- 

 ease is produced but, nevertheless, antibodies to the disease are built up in 

 the blood stream. The live virus used for such vaccinating may be the virulent 

 strain normally present in an epidemic, but it is vastly preferable to utilize 

 strains which are of low virulence. The other processes of vaccination consist 

 of the Introduction of a relatively large quantity of inactivated virus. Such 

 virus is incapable of producing disease, but is capable of inducing antibody 

 formation. Passive Immunity is developed by injecting into the blood stream of 

 a susceptible host, antibodies to the virus obtained from a host which was pre- 

 viously immunized. This method of combating virus diseases has been applied 

 successfully in the case of measles. Effective therapeutic agents have not yet 

 been found for virus diseases. No one has ever found a specific agent, such as 

 penicillin or the sulfa drugs, which is of proved value in the treatment of a 

 virus infection. 



By utilizing measures of the sort just outlined, some of the virus diseases 

 of domestic animals have been brought under control. l!any readers are no doubt 



\ 



