34 AN INTRODUCTION TO THE STUDY OF VIRUSES 
normal and when grown in culture does not become organized into 
roots, stems, or leaves, but appears to be capable of indefinite growth 
in its unorganized form. 
Turning now to the consideration of some animal virus diseases, 
one of the most important is foot-and-mouth disease of cattle; the 
disease starts with a rise in temperature during the first two days up 
to 40° or 41°C, and accompanying the initial fever is a rapid pulse 
and diminished appetite. Vesicles then start to appear and the fever 
descends, and usually after 24-36 hours the temperature is normal. 
The lesions in the mouth are commoner in cattle than in sheep, goats, 
and hogs, but the buccal mucous membrane may become sensitive 
and the animal ceases to eat. Lesions develop on the tongue, lip, gums, 
hard palate, and feet, and sometimes on the udder and teats. Saliva 
accumulates in great quantities and falls from the mouth when opened. 
The lesions on the feet may persist for some time, sometimes several 
weeks. Pigs affected in this way are occasionally seen trying to walk 
on their knees, and the lameness may be so acute as to prevent the 
animal from walking. Occasionally cavities develop in the horn of 
the hoof and these may contain viable virus. As a rule recovery takes 
place in two or three weeks, although a severe form of the disease is 
also recognized. In this form, sudden deaths may occur when the 
lesions are beginning to heal. 
There are several strains of the foot-and-mouth disease virus known 
and there appears to be no cross-immunity between them. The 
virus is extremely infectious and the disease is dealt with in Great 
Britain by the slaughter of affected animals and their contacts and 
incineration of the carcasses. 
Dog distemper is a disease which occurs sporadically in every country 
where there are dogs, and at times very severe outbreaks may pass 
over a country taking a serious toll of the canine population. 
Laidlaw (1930) describes the disease as follows: the onset of illness 
in the experimentally induced disease is sharply defined by fever and 
watery discharge from the eyes and nose. The discharge is frequently 
trivial, but may be profuse. Within 24 hours it may become faintly 
purulent and the conjunctivae acutely congested. Crusts, composed 
of half-dried cells, collect at the angles of the eyelids. The temperature 
rises abruptly at the commencement of the illness and may reach 105°F 
or even more within 24 hours. After one or two days it subsides to 
near the normal level of 101-4°F, but it soon rises again. The second 
rise is slower than the first but much more prolonged. Its duration is 
