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Aphthovirus (causes
foot-and-mouth disease) (Life; viruses;
Family: Picornaviridae)
Foot and Mouth Disease
Foot-and-mouth disease is one of the most contagious
animal diseases and causes death mainly to young animals through inflammation of
the heart muscle (i.e. myocarditis). It infects animals in the families Bovidae
(cattle, sheep, goats, antelope) and Suidae
(pigs, warthogs). Symptoms include the formation of blisters in the mouth
and between the claws, hence the name. People can be infected through skin
wounds or the mucous membranes in the mouth by handling diseased stock, handling
the virus in the laboratory, or by drinking infected milk, but not by eating
meat from infected animals. However, infection of humans is temporary and mild
and foot-and-mouth disease is not considered a public health problem.
Transmission. Infection can spread all too easily
through direct or indirect contact. This can be through movement of humans,
animals, vehicles and implements, and through the air, especially in temperate
zones (up to 60 km overland and 300 km by sea). Cattle are mainly infected by
inhalation, often from pigs, which excrete large amounts of virus through their
breathing and are considered highly important in disease spread. Aphthovirus,
which causes this disease, is in the same family of viruses as the Common Cold
viruses (Rhinovirus) and you know just how easily they can spread! Newly
infected animals start spreading the virus before symptoms appear so it is often
difficult to control this disease before it has spread too far. African Buffalo
are important carriers of foot-and-mouth disease. Recovered cattle may be
carriers for 18 to 24 months; sheep for 1 to 2 months, while pigs are not
carriers.
Distribution. Foot-and-mouth disease is endemic in
parts of Asia, Africa, the Middle East and South America with sporadic outbreaks
in free areas. Foot-and-mouth disease was last reported in 1929 in
the U.S.A., 1952 in Canada, and 1954 in Mexico. There have been a notably high
number of outbreaks recently, the most publisised being that in the UK. Foot-and-mouth disease was
reported from the following countries from 1999 to March 2001: Argentina,
Bhutan, Brazil, Colombia, Egypt, France, Georgia, Greece, Iran, Israel, Japan,
Kazakhstan, Korea, Kuwait, Malawi, Malaysia, Mauritania, Mongolia, Namibia,
Peru, Philippines, Russia, Saudi Arabia, South Africa, Swaziland, Taipei China,
Tajikistan, Turkey, United Kingdom / Great Britain, Northern Ireland (isolated
case stemming from outbreak in England), Uruguay, Zambia and Zimbabwe (information
from OIE). The outbreak in KwaZulu-Natal in South Africa was first detected
on 12 September 2000.
Symptoms. Fever (pyrexia), lack of appetite
(anorexia), reduction in milk production for 2-3 days. The blisters that
form in the mouth and nasal cavities make the animal smack its lips, grind its
teeth and drool. The blisters that form between the claws cause lameness,
stamping and kicking of feet. In sheep and goats the blistering is less
pronounced.
Prevention and Control.
- Animal movement control through construction of border
fences and surveillance of trucks, etc where roads and railways pass through
these borders.
- Slaughter of infected, recovered, and susceptible
contact animals.
- Disinfection of premises and infected materials
(implements, cars, clothes, etc.).
- Destruction of cadavers and susceptible animals
products in the infected areas.
- Quarantine measures. The incubation period is 2-21 days
(average 3-8 days) so animals passing out of infected areas would presumably
need to be held in quarantine for at least 14 days before the disease could
be reliably tested.
- Vaccination. Tests for foot-and-mouth disease
involve testing for antibodies to this disease and as both the real disease
and the vaccination lead to the build up of antibodies, vaccinated animals
are excluded because of the risk of bringing in animals that actually are
infected. However, new tests are currently being developed that will enable
differentiation of vaccinated and infected livestock. The rationale
behind this method is as follows: (1) the vaccination involves injecting the
animal with inactivated viral particles made up of structural proteins; (2)
the animal develops antibodies against these structural proteins; (3) the
live viral particles include structural proteins as well as non-structural
proteins which are produced during viral replication; (4) the animal
develops antibodies against both these forms of proteins when infected by
the live virus; (5) the test therefore distinguishes each of these main
types of antibodies - if only the antibodies against structural proteins are
detected, then there can be certainty that it is vaccinated and not
infected.
Links
References
Text by Hamish G. Robertson
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