Causes and symptoms of foot and mouth disease

The deadly disease chasing dairy farmers out of business

Foot and mouth disease FMD has run many farmers out of dairy. Actually out of 10 farmers 8 farmers have lost their lactating cows in every outbreak. Others have lost all their herd to the disease and are left with empty shades never to recover again. This happens despite frequent vaccination.

But what causes FMD? What are the signs?

Foot and mouth disease is caused by strains of seven immunologically distinct serotypes of the virus and consequently, recovery from infection with a strain of one serotype does not protect against strains of the other six serotypes.

In many of the FMD endemic regions, more than one serotype may circulate, creating waves of infection as different serotypes enter, infect the susceptible animals and then move on to reappear a few years later as a new susceptible population becomes established. Alternatively, one serotype may persist in a region and rarely appear clinically, producing only mild infection in the young stock as they lose their

Calves drinking infected milk can be infected by the insufflation of milk droplets as they drink. Of the domesticated susceptible species, cattle are the most likely to be infected by aerosol virus generated by other infected animals, particularly pigs, because of their larger respiratory volume when compared with small ruminants and their higher susceptibility to this route of infection compared with pigs

The transmission of the FMD virus within an unvaccinated herd is usually rapid. Even within a vaccinated herd, the aerosol production of the virus from a single infected animal can overcome the immunity of others in the herd resulting in a further increase in the level of challenge and the appearance of clinical disease.

Milk and semen from infected cattle may contain the virus up to four days before the onset of visible signs. More than 50% of cattle that have recovered from infection with the FMD virus and vaccinated cattle that have had contact with the live virus become carriers.

Clinical signs

The incubation period for FMD in cattle is between two and fourteen days, depending on the infecting dose, the strain of the virus and the susceptibility of the individual host. Typically, between-farm transmission has a longer incubation period, but once the quantity of virus in the environment increases on an infected farm, the incubation period reduces.

Cow with ruptured tongue vesicle, two days after start of
clinical signs of foot and mouth disease
Cow with ruptured tongue vesicle, two days after the start
of clinical signs of foot and mouth disease IMAGE: KITCHING CATTLE

A variable number of vesicles develop on the tongue, hard palate, dental pad, lips, gums, muzzle, coronary band and interdigital space. Vesicles may also be seen on the teats, particularly of lactating cows. Young calves may die before the appearance of vesicles because of the predilection of the virus to invade and destroy cells of the developing heart muscle.

Young bovine with foot and mouth disease
Young bovine with foot and mouth disease Image: Kitching cattle

Acutely infected cattle salivate profusely and develop a nasal discharge, at first mucoid and then mucopurulent, which covers the muzzle. They stamp their feet as they try to relieve the pressure on first one foot and then another. They may prefer to lie down and resist attempts to raise them. Lactating cattle with teat lesions are difficult to milk and the ruptured vesicles frequently become infected, predisposing to secondary mastitis.

The vesicles in the mouth rupture rapidly, usually within 24 hours, leaving a shallow erosion surrounded by shreds of epithelium. Vesicles on the tongue frequently coalesce and a large proportion of the dorsal epithelium may be displaced. The vesicles on the feet may remain intact for two or three days before rupturing, depending on the terrain or floor surface of the cattle accommodation.

Why infection despite vaccination?

Intensive vaccination does not always prevent the appearance of clinical FMD. Because of the severe challenge originating predominantly from the nomadic herds of sheep, goats and cattle which graze freely, therefore the introduction of the virus into the dairies is inevitable. When these dairy cattle become infected, they frequently exhibit a very severe form of the disease, in which the tongue swells and protrudes from the mouth, and the majority of the tongue epithelium is shed.

Agricultural consultancy
Agricultural consultancy

The data show that in herds that had only recently been vaccinated, the disease usually first appeared in a large number of infected animals because the high level of immunity within the herd kept the clinical disease suppressed and the virus circulated sub-clinically until the level of viral challenge had reached a sufficiently high level.

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This level of the virus within the herd environment overcame the vaccinal immunity of a large group of animals throughout the herd. In herds with lower levels of immunity, the first appearance of disease was frequently only in a single animal and if surveillance within the herd was good, a rapid response by re-vaccinating the herd would bring the outbreak under control.

Reference: Clinical variation in foot and mouth disease: cattle-R.P KITCHING

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