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Virus Conc.
Log10 I.D.50 (titre/litre)
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Milk from subclinically infected cow
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9.6
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Milk mixed in bulk tank with milk from uninfected
cattle (dilution x 10)
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8.6
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Milk mixed in milk tanker with uninfected milk
from other farms (dilution x 5)
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7.9
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Milk filtered at dairy plus further mixing with
uninfected milk (dilution x 10)
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6.9
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High-temperature-short-time (HTST = 72ºC for 15
s) pasteurisation
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1.9 - 2.9
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Infectious Dose (ID) for a pig by mouth is log10
5 i.e., a pig would have to consume 125-1250 litres of this milk
to obtain an infective dose of FMD virus.
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Infectious dose for a calf by mouth is log10
6 i.e., a calf would need to consume 1250 - 12 500 litres of this
milk to become infected.
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Infection by the aerosol route requires less FMD
virus. If the calf or pig inhaled an aerosol of the infected milk
while feeding (insufflation), they would require to inhale 12.5-125
ml of milk (calf) or 500 - 5000 ml of milk (pig).
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However, these figures are average figures
for the minimum infectious dose. The susceptibility of a large number
of individual animals probably follows a binomial distribution, i.e.,
some animals will be very susceptible to infection with FMD and others
less so, and in some animals a low dose of virus is introduced into
an environment where it can replicate and spread (e.g., epithelial cells
of the tongue), whereas in other equally susceptible animals, the virus
is not given the opportunity to multiply.
From the above example with infected milk, what is the probability of
infecting one of 100 pigs fed 1 litre of milk?
The probability (P) of one pig becoming infected :
= 1 - (1-q)n
where 'q' is the probability that approximately 500 ID50 in a litre
of milk will start an infection in a pig, and 'n' is the number of pigs.
There is a 30% chance that if 100 pigs are each fed 1 litre of the infected
milk, one will become infected. Once one is infected, infection will
spread to the others in the group and an outbreak will occur (Sutmoller
and Vose 1997).
