CONTROL OF VIRUS DISEASES 87 
applicable in a few cases. For example, horses can be kept free of 
African horse sickness by allowing them to graze in the open only 
during certain hours of daylight when the mosquito vector is quiescent 
and by stabling them at sundown in mosquito-proof buildings. 
In certain parts of S. America the virus of a form of rabies is carried 
by “‘vampire” or blood-sucking bats, and here again the screening of 
windows and the protection of cattle is necessary. 
Quarantine. Rigid quarantine regulations can be efficacious in 
preventing the entrance of a virus into a country. The almost complete 
exclusion of rabies from England by very strict regulations governing 
the import of dogs is a case in point. Again, foot-and-mouth disease 
is absent in Australia and this is apparently due to the embargo on 
the entrance of cattle from other countries. 
Incidentally it is of interest in this connexion to notice the appearance 
in England of a new virus disease of dogs called hard-pad, because it is 
accompanied by thickening of the horny layer of the skin of the pads. 
It is transmissible to ferrets which also develop corns (MacIntyre et al., 
1948). This virus may possibly have entered this country in dogs 
smuggled in by allied soldiers during the late war. 
2. Measures Directed against Virus Vectors 
Attacking the arthropod vectors of a virus is not likely to lead to 
the complete control of a virus disease but may prove extremely 
successful if supplemented by other measures. 
The classic case of this type of control measure was the attack on 
the mosquito in the Panama Canal zone. The early attempts to 
construct this canal had failed owing to the ravages of yellow fever, 
and it was only after the breeding grounds of the mosquito vector 
had been eliminated that success was achieved. During the trench 
warfare of the first World War, the louse-transmitted disease of 
trench fever caused great wastage of troops. Towards the end of the 
war, systematic attempts were made to keep the louse under control 
by steam sterilization of clothes and bedding. 
The discovery of D.D.T. and other new insecticides now gives 
greater promise of success for this method of control. In 1944 an 
outbreak of typhus in Naples was stopped completely owing to the 
lethal effect of D.D.T. on the louse which transmits the disease. 
Because of the toxic action of this substance on mosquitoes and with 
the development of aerial distribution, some progress in the elimination 
of this insect vector may be expected. 
