SOME REPRESENTATIVE VIRUS DISEASES 35 
exceedingly variable, and may be as short as 48 hours in milder cases 
or more than three weeks in more severe. At the onset of the illness 
the dog usually vomits and refuses all food. With the fall of tempera- 
ture, appetite returns for a time, to diminish once more as the secondary 
fever develops. In practically every case there is diarrhoea, and in 
the secondary febrile period this may be profuse. The poor appetite 
coupled with the diarrhoea interferes with nutrition and the animal 
wastes rapidly. 
Symptoms from the respiratory system are usually slight. Some 
cough is common in the period of the secondary fever, but definite 
bronchitis or broncho-pneumonia does not occur. Definite involve- 
ment of the nervous system may take place early or late in the disease, 
and is of very variable occurrence. The earliest symptom is a sudden 
attack of semi-consciousness with vigorous chewing movements, 
accompanied by the secretion of ropy saliva which exudes from the 
angles of the jaws. Laidlaw points out that dog distemper as seen 
in veterinary practice differs somewhat from the experimental disease 
above described, particularly in the incubation period which may be 
as long as three weeks compared with four days in the experimentally 
induced disease. 
Fowls are susceptible to several virus diseases and two are selected 
for discussion here, fowl-pox and fowl-pest. 
The disease of fowl-pox seems to occur in several forms, although 
the exact relationship between them does not appear to be known. 
The two commonest forms are, firstly, the condition known as 
fowl-pox and, secondly, avian diphtheria. In fowl-pox the skin 
alone is involved and the head is first infected. A fine, bran-like, grey 
deposit develops on the comb, ears, and wattles, and later, on parts 
of the body where the feathers are absent. These initial lesions soon 
develop into small nodules; these are at first reddish grey but later 
become greyish yellow. Later still they become brown, dry and firm, 
their surface is warty and they contain horny or fatty degenerated 
epithelial cells. They may coalesce and bleed and thick scabs may form 
over their surface. The margins of the nasal orifices and the eyelids 
become thickened, and the eyes remain shut. Nodules may also develop 
on the mucous membranes of the mouth and throat or on other parts 
of the body. 
In the diphtheritic form there is usually no marked general dis- 
turbance. The condition frequently begins in the mouth with the 
formation of a mucous membrane. The membranes are usually 
