XI. Usually viruses are separated into several large groups based primarily on

1. Nature of the host

2. Nucleic acid characteristics

3. Capsid symmetry

4. Diameter of the virion

TEXT B

FOOT AND MOUTH DISEASE

VOCABULARY LIST TO TEXT B

Viruses are important pathogens of livestock. Common infections include foot-and- mouth disease and bluetongue.

Pets like cats, dogs, and horses are also susceptible to serious viral infections. For example, dogs may be affected by rabies, canine parvovirus infections (fatal to puppies).Horses may be affected by the Hendra virus that is highly contagious and fatal.

Foot-and-mouth disease (FMD) is a highly infectious, notifiable,vesicular disease of domestic ruminants (cattle, sheep, goats etc.), pigs, other farmed cloven-hoofed mammals and wild ruminants. It is a significant economic disease due to its ability to spread very rapidly and its profound effect on productivity. A very small quantity of the virus is capable of infecting an animal, and the disease could spread rapidly throughout the country if it is not controlled quickly.

The organism, which causes FMD, is an Aphthovirus of the family Picornaviridae. Picornaviruses are non-enveloped, RNA viruses with an icosahedral capsid. Foot-and-mouth disease virus (FMDV), the first animal virus, was discovered in 1897.

There are 7 known types and more than 60 subtypes of the FMD virus. Immunity to one type does not protect an animal against other types or subtypes. Some strains can give rise to high levels of mortality in young animals. In adult animals, the disease is not usually fatal, however it causes severe pain and distress, especially in cattle, and animals may be left permanently lame with reduced productivity following recovery.

The clinical signs of FMD are similar to other vesicular diseases and confirmation of diagnosis can only currently be made following laboratory tests. Cattle with foot-and-mouth disease may develop blister-like sores on the tongue and lips, in the mouth, on the teats and between the hooves.Vesicles rupture and discharge clear or cloudy fluid, leaving raw eroded areas surrounded by ragged fragments of loose tissue. Other clinical signs include: fever, shivering, lameness, slobbering and smacking lips, cows produce less milk. In sheep and goats, however, the disease is less severe or occurs as a sub-clinical infection.Sheep rarely develop blisters in the mouth as a result of foot and mouth disease. Blisters on the hoof are more common. In either location, the blisters tend to be very small and hard to spot.

Signs of foot and mouth disease in pigs include sudden lameness, which may spread quickly among the herd, tendency to lie down and unwillingness to move, reluctance to feed. Pigs don’t usually develop blisters as a result of foot-and-mouth disease. But sometimes blisters do appear on the snout, tongue and on the upper edge of the hoof where the skin and horn meet.

The virus is present in great quantity in the fluid from the vesicles, and it can also occur in the saliva, milk and dung. Contamination of any objects with any of these secretions or excretions is a danger to other susceptible animals. Heat and disinfectants will destroy the virus, whereas cold and darkness tend to keep it alive. Survival of the virus in the environment depends on a range of factors and is highly variable. Under field conditions, this can range from days to months. The virus can be transmitted on fomites (an inanimate object capable of transmitting infectious organisms from one individual to another, for example, vehicles and farm equipment), as well as mechanically by animals and other living vectors. Animals can pick up the virus either by direct contact with an infected animal, or by contact with foodstuffs or other things which have been contaminated by an infected animal, or by eating or coming into contact with some part of an infected carcasses. Airborne spread of the virus can also occur and, under favourable climatic conditions, the disease can spread several miles by this route.

Relevant biosecurity measures should be practiced as a matter of routine. Trucks, lorries, market places, and loading ramps - in or over which infected animals may have travelled - are a disease risk until properly cleansed and disinfected. Roads may also become contaminated, and virus may be picked up and carried on the wheels of passing vehicles. The boots, clothing, and hands of any person who has been in contact with infected animals can spread the disease. Horses, dogs, cats, poultry, wild game and vermin may also carry infection. There is a range of regulatory measures to help reduce the risk of spread of any notifiable disease before it is detected (silent spread) and to help with tracing animals if any incursion was to occur. These include requirements for cleansing and disinfecting livestock vehicles.

Vaccines for FMD are available, but must be matched to the specific type and subtype of virus causing the outbreak. Although these vaccines are very useful as part of eradication campaigns in countries where FMDV is enzootic, it is not feasible to use these products in prophylactic vaccination of entire regions.

EXERCISES TO THE TEXT

Exercise 12. Look through the text and find information about:

- foot-and-mouth disease causative agent;

- animals that are susceptible to foot and mouth disease;

- types of foot-and-mouth disease;

- the ways the FMD virus can be transmitted;

- clinical signs of foot-and-mouth disease in sheep and cattle;

- clinical signs of foot-and-mouth disease in pigs;

- the substances where the FMD virus is present.