Etiology-
It is caused by four types of Aphtho virus strain as O, A-22, C and and Asis-1 with marked differences in immunity. Cattle recovered from infection of one type may at once be infected by another type. Thus. one animal may have four attacks of the disease in suceession. The virus is found in the content of vesicle, saliva, milk and in blood during febrile condition.

The disease spreads very commonly by dired or indirect contact through infected feed, fodder, water, manure, pastures, clothes and hands of attendant, milk, etc.
The virus gains entry into the circulating blood of animal through injury in the lining membrane of the mouth, tongue. intestines, clefts of hooves and other similar parts.
In natural infection, the incubation period is 2 to 6 days but it may vary from 1 to 15 days and in artificial infection. it is 1 to 2 days.

Symptoms-
At first, animals are found dull, sluggish, refuse their food and milk yield suddenly falls down in milch animals. There is shivering followed by rise of temperature to 104°F. It soon falls when vesicles appear and susally profuse salivation commences like strings, appetite is reduced. rumination is suspended. The animal frequently smacks its lips, yawns and protrudes its tongue. These signs lead to examine the mouth.

On examination of mouth. the mucous membranes appear red, hot and blisters are found in all stages of development. Some are just formong. some are well formed, some have recently burst and in the oldest the mucous membrane has been cast off from their surface and shallow ulcer-like areas are left. The commonest situations are inner surface of the mouth, dental pad, tongue, lips and hard palate. The blisters of tongue are with thich wall and elsewhere with thin wall containing clear colourless or yellowish or straw-coloured serum. In a day or two, the blisters burst exposing mosit, red, shallow, eroded ulcer-like area. These area of erosion are covented from feeding and drinking. So the animal should be given soft and liquid food.

After 4 to 5 days of moth eruptions, the foot lesions generally appear causing pain and lammeness. On examination, the skin of coronary band, heel, cleft of hoof are found hot, swollen red and painful. In a day or two vesicles form which soon become big containing fluid. These blisters very soon burst discharging infective material. The animal frequently shakes the affected foot.

In milch animals, such as cows, ewes alnd sows, smaller vesicles develop upon their teats or upon the skin of the udder which soon burst by milking or sucking. The affected area remains eroded till healing is established. Milk yield is rapidly reduced. Milk becomes contaminated with discharges and highly infective to suckling young causing death.

The lesions in mouth and teat readily heal up within 2 to 3 weeks but the lesions of feet may get contaminated with pyogenic organisms or bacteria resulting in unhealthy ulcers and necrosis, Infection of maggots is common feature in neglected cases and revovery takes 1 to 3 months. The hooves may be enlarged and sloughed.
In sever cases, the disease may be complicated with the result of pneumonia, pleurisy, septicemia and abortion.

The recovered animals become very weak and debilitated alnd may suffer from panting. They obtain immunity against that strain but not by the other strains. So the animal may get infection with other strains. The duration of immunity in recovered animals is very variable.