Risk of rabies introduction by noncommercial movement of pets

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Risk of rabies introduction by non-commercial
movement of pets
P. Have, L. Alban, L.T. Berndtsson, F. Cliquet,
P. Hostnik, S.C. Rodeia and M. Sanaa

• The EFSA Journal (2006) 436 1-54, Assessment of
  the risk of rabies introduction into the UK,
  Ireland, Sweden, Malta, as a consequence of
  abandoning the serological test measuring
  protective antibodies to rabies

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Current legislation

• Regulation (EC) No 998/2003
• valid anti-rabies vaccination
• waiting time of at least 21 days (COM 2005/91/EC)
• derogations for UK, Ireland, Sweden and Malta to
  maintain serological test
• review of derogations at the end of transitory
  period of 5 years following receipt of a
  scientific opinion of EFSA

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EFSA mandate

• Commission requested EFSA to issue a scientific
  opinion on the risk assessment of rabies
  introduction into Ireland, the UK, Sweden and
  Malta, as a consequence of abandoning the
  serological test for antibody titration for rabies

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Mandate cont

• To what extent the abandoning of the individual
  serological test for neutralising
  rabies-antibodies titration could be envisaged
  without increasing the risk
• If the need to maintain the serological test is
  scientifically justified, what is the regime to
  be considered as giving equivalent assurance

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Approach

• Quantitative risk assessment based on
• prevalence of rabies in the country of origin in
  pets
• distribution of incubation periods of rabies
• efficiency of establishing protective immunity by
  vaccination
• specificity of the neutralization test for rabies
  (RFFIT or FAVN)
• length of the waiting period

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Model parameters

• Prevalence (P)
• estimated from annual incidence data by assuming
  a mean incubation time of 38 days
• pet population size estimated from 1.0 dog and
  1.1 cats per 10 people
• P incidence38/(population at risk365)

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Model parameters
Incubation period

• distribution of incubation periods
• Dogs 1 week to several months
• Cats 9 days to 6 months
• described by a log-normal distribution with mean
  38 days

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Model parameters

• Efficiency of vaccination (Ev)
• assessed indirectly by measuring the antibody
  response
• measured after first, single vaccination
• absence of antibodies not always associated with
  loss of protection
• interval between vaccination and testing major
  determinant
• Ev of 98 used tentatively in this study

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Model parameters

• Test specificity (Sp)
• determines whether truly antibody-negative
  individuals are correctly assigned as such or
  whether some individuals are classified as false
  positive
• FAVN test more specific than RFFIT
• a tentative value of 99 is used here

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Model parameters

• Length of waiting period (t)
• Measured from time of vaccination
• t will determine the residual prevalence Pa of
  animals incubating rabies due to pre-vaccination
  exposure

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Risk pathways
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Type A risk
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Type B risk
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Model descriptionPrevalence A and B

• prevalence Pa of animals already incubating
  rabies will gradually decrease as a result of
  developing clinical disease
• Pa P f(t)
• prevalence Pb of animals infected after the time
  of vaccination will gradually replace Pa
• Pb P (1- f(t))

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Model descriptionRisk A and B

• Type A risk
• Type B risk with vaccination
• Type B risk with vaccination and serology

• Ra P f(t) Pa
• Rb (1 - Ev) Pb
• Rb (1 Sp) (1 - Ev) Pb

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Model descriptionTotal risk

• Total risk Rtot that an animal incubates rabies
  at time t after vaccination and serological
  testing is given by the sum of Ra and Rb
• Rtot P f(t) (1 Sp) (1 - Ev) Pb or
• Rtot P (f(t) (1 Sp) (1 - Ev) (1-
  f(t))) or
• Rtot P (1 (Ev Sp - Ev Sp )(1- f(t))

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Model descriptionmodelling f(t)

• Sample an incubation period (ip) from the
  lognormal distribution
• Sample an interval between infection time and
  vaccination (int) from a uniform distribution
  between 0 and 365 days prior to vaccination
• F(t) calculated from positive values of ip-int
• 100.000 iterations

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Model descriptionnon-linear least squares
estimate of f(t)

• f(t) exp(-0.0313t)

Rtot P (1 (Ev Sp - Ev Sp )(1-
exp(-0.0313t))
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Effect of serological testing