D32.PDF

70 SG/12/CS3 C
Original: English
FOOT AND MOUTH DISEASE DIAGNOSTICS:
REQUIREMENTS FOR DEMONSTRATION OF FREEDOM FROM INFECTION
R.P. Kitching
Director, Canadian Food Inspection Agency, National Centre for Foreign Animal Disease, 1015 Arlington Street
Winnipeg, Manitoba R3E 3M4, CANADA
Summary: Proposed changes to the International Animal Health Code chapter on foot and mouth
disease (FMD) reflect both the introduction of new diagnostic tests to identify animals carrying
live FMD virus, and pressure to make more acceptable the use of vaccination to help control
outbreaks. It is important that those using the Code fully understand the implications of these
changes. One of the proposed changes to the Code is to define the occurrence of FMD virus
infection so as to avoid any ambiguity as to when the OIE should be notified. It is also proposed
that the Code be changed to reduce to 6 months from 12 months the interval required, following
the use of vaccination without slaughter of vaccinated animals, before application can be made to
the OIE for restoration of FMD infection free status. As an aid to Member Countries applying to
the OIE for FMD free or FMD infection free status, the OIE has produced a ‘Guide to the
Establishment or the Regaining of Recognition for a Foot and Mouth Disease Free Zone or
Country’.
1. INTRODUCTION
The OIE International Animal Health Code (Code) is a constantly evolving document, reflecting advances in our
scientific understanding of veterinary diseases of economic importance (5). Its aim is to provide guidelines to
minimise the risk of unintentional transmission of disease across international boundaries during international
trade. The Code does not attempt to adopt the concept of ‘zero risk’ but acknowledges the definable risks
involved and outlines the methods by which these risks can be minimised. The history of the Code chapter on
foot and mouth disease (FMD) provides an example of the evolutionary nature of the document, first with the
introduction of the concept of ‘freedom from FMD with vaccination’ and now with the new proposed categories
for ‘freedom from disease with vaccination’ and ‘freedom from infection without vaccination’. Each change
introduces more risk that FMD could move from an infected country to a free country during legitimate trade,
and while Member Countries may be under political pressure to expand the flow of live animals and animal
products internationally, in the spirit of free trade, those who have the responsibility to maintain the disease
status of their nation’s livestock should be aware of the risks to which their livestock may be exposed, in order to
better formulate their national import regulations. Similarly, Member Countries that have embarked on FMD
control and eradication programmes, either from an FMD endemic situation, or following a recent introduction
of FMD virus, may require guidance on what is required to conform with the different categories of FMD status
now proposed. A supplement to the Code chapter on FMD that covers surveillance guidelines has been produced
for this purpose and will be presented with the report of the FMD and Other Epizootics Commission for approval
by the International Committee. This presentation will relate these guidelines to the new proposed changes to the
Code chapter on FMD.
Until our knowledge of FMD is absolute, which it is never likely to be, the OIE cannot provide guarantees that
the provisions of the Code will adequately cover all eventualities. The Code itself is becoming a more pragmatic
document, attempting to incorporate the requirements of all Member Countries, both importing and exporting,
those with FMD and those without. If one of the objectives of the Code is to encourage the eventual elimination
of FMD from the globe, which is clearly in the interests of those countries that do not have the disease, it cannot
be so prescriptive that it deters Member Countries that have FMD from even starting on the road to FMD
freedom.
During 2000 and 2001 there was a dramatic spread of FMD into countries that had been free from the disease for
many years, for example, South Korea, free since 1934, Japan, free since 1908, and the United Kingdom, free
since 1981 – from whence it spread into France, the Netherlands, Northern Ireland and Eire, all FMD free
countries. Serotype O spread into the Republic of South Africa where it had never been identified previously,
and FMD spread back into the southern cone of South America, large areas of which had only relatively recently
stopped prophylactic vaccination. The response to these outbreaks was heavily influenced by the Code chapter
on FMD, and the slaughter of animals that occurred in Europe in order to eliminate the virus and re-establish
trade was on a previously unseen scale (8). It was made very clear during the International Conference on
Control and Prevention of Foot and Mouth Disease held in Brussels, Belgium, in December 2001, that
alternative policies were required.
This presentation, in addition to introducing the surveillance guidelines that supplement the Code chapter on
FMD, will detail the requirements for recognition of freedom from FMD infection. It will not, however,
specifically address the status of freedom where vaccination is practised.
2. DEFINITION OF INFECTION
In accordance with the 2001 edition of the Code, a Member Country that was recognised as an FMD free country
where vaccination is not practised was required to send a declaration to the OIE that there had been no outbreak
of FMD and no vaccination had been carried out for at least 12 months, together with documented evidence of
an effective import and surveillance programme. An outbreak being defined as the occurrence of clinical signs of
FMD, an obvious loophole was provided such that a country could maintain its free status even where there is
evidence of live FMD virus being present in the absence of reported disease. While this was not in the spirit of
what was intended, it was legally defensible. To correct this anomaly, the new proposals to the Code chapter
require that Member Countries declare that there has also been no evidence of infection found during the past 12
months. This change necessarily requires a definition of FMD virus infection.
The following defines the occurrence of FMD virus infection:
1.
FMD virus has been isolated and identified as such from an animal (free-living or domestic), or a product
derived from that animal, or
2.
Viral antigen or viral RNA specific to one or more of the serotypes of FMD virus has been identified in
samples from one or more animals (free-living or domestic) showing clinical signs consistent with FMD, or
epidemiologically linked to a confirmed or suspected outbreak of FMD, or giving cause for suspicion of
previous association or contact with FMD virus, or
3.
Antibodies to structural or nonstructural proteins of FMD virus that are not a consequence of vaccination
have been identified in one or more animals (free-living or domestic) with either epidemiological links to a
confirmed or suspected outbreak of FMD, or showing clinical signs consistent with recent infection with
FMD virus.
The most conclusive evidence of infection is isolation of live FMD virus, but there will be situations in which
this is not possible, particularly if the support of a qualified laboratory is not immediately available. The tests for
FMD virus antigen, as described in the OIE Manual of Standards for Diagnostic Tests and Vaccines (the
Manual), are well validated, and when properly carried out are both sufficiently specific and sensitive to be used
in a fresh outbreak in a naive population. The use of polymerase chain reaction (PCR) technology has been
associated in the past with problems of cross contamination due to its extreme sensitivity, and it is currently
unlikely that a declaration of an outbreak of FMD would be made on the basis of a positive PCR result alone.
Thus it was considered important to link PCR testing with clinical signs of FMD or to link it to a known or
suspected outbreak or contact with FMD virus.
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FMD diagnostics: requirements for demonstration of freedom from infection
The presence of antibodies to FMD virus can also lead to suspicion of disease, but no antibody test for FMD
virus antigens is totally specific, as can be confirmed by anyone involved in routine import/export testing. In
addition, an animal vaccinated against FMD will test positive in the virus neutralisation test, liquid-phase
blocking enzyme-linked immunosorbent assay (ELISA), and the recently introduced solid-phase competition
ELISA. However, vaccinated animals will not test positive in the tests for antibodies to the nonstructural proteins
(NSP) of FMD virus that are produced in response to live replicating virus. Some vaccinated animals,
particularly those that have received regular vaccination, do produce antibodies to the 3D polymerase of FMD
virus (the virus infection associated antigen [VIAA] detected by the agar gel immunodiffusion test), as this
protein is present in the intact inactivated virus used in vaccine formulations; they may also produce antibodies
to other NSPs, depending on the purity of the FMD virus antigen in the vaccine used. However, even a positive
NSP antibody test would not, on its own, be considered sufficient evidence for a declaration of infection, and
there is a requirement that it be linked to evidence of disease or to a confirmed or suspected outbreak. At least
three different NSP antibody test protocols are available (3), and are the subject of an evaluation exercise being
conducted among collaborating laboratories world-wide under a programme run by the Food and Agriculture
Organization/International Atomic Energy Agency, Centre for ELISA and Molecular Techniques in Animal
Disease Diagnosis, Vienna, Austria. None has been fully validated for all FMD susceptible species, under all the
circumstances in which they may be required. A derogation of this requirement for freedom from antibodies
consequent to infection would be necessary for those countries seeking re-establishment of FMD free status,
3 months or longer after an outbreak. It is possible that antibody positive animals would be identified in the postoutbreak surveillance. However, if these animals are found to be negative for evidence of live virus, virus
antigen or virus genome, and immediately slaughtered, it would be reasonable for the OIE to declare the country
free without waiting an additional 3 months. The proposed new Code chapter no longer requires that countries or
zones requesting recognition as ‘FMD free where vaccination is practised’ need ‘have a system of intensive and
frequent surveillance for detection of any viral activity’. However, they must make a declaration that there has
been “no evidence of FMD virus infection for the past 12 months”, that there has been no report in the previous
2 years of an outbreak of FMD, and that vaccination is regularly carried out for the purpose of prevention of
FMD.
This change is intended to be a practical solution to the problem of identifying animals carrying live virus within
the FMD susceptible population. It presupposes that there would be a very low number of these carrier animals;
therefore, it would be necessary to test a large percentage of the total number of susceptible ruminants, in
particular cattle, to make a statistically significant statement as to the infection status of the country/zone.
However, there is the danger that FMD could be circulating in a subclinical form within the vaccinated
population, and even that FMD is being missed in unvaccinated small ruminants because of the frequently mild
clinical signs.
This proposed change decreases the assurance that live FMD virus is not present in the animal population of a
FMD free country or zone where vaccination is practised. Under the proposed change the assurance will be
based only on clinical disease. However, the reduced requirements may encourage more Member Countries to
start an FMD control and eradication programme.
3. SURVEILLANCE GUIDELINES
The OIE has produced a ‘Guide to the Establishment or the Regaining of Recognition for a Foot and Mouth
Disease Free Zone or Country’. This guide provides a template for countries to follow in order to provide
evidence to the FMD and Other Epizootics Commission of a particular FMD status. It is not intended to be
prescriptive, and alternative strategies can be employed by Member Countries to achieve the same objective. It
does describe the level of testing that is considered necessary, and with reference to the Manual, the type of tests
to employ. For countries wishing to show evidence of freedom from FMD virus infection, a random survey
within the population at risk to give a 95% probability of detecting evidence of FMD or FMD virus infection if it
is present in 1% of the primary sampling units is recommended. A 95% chance of finding a 20% prevalence in
the randomly chosen herds or flocks within each identified stratum of the susceptible population has been
recommended as a defensible compromise between testing an unmanageable number of samples and providing
acceptable evidence of freedom.
The Guide provides a flow diagram of laboratory tests for determining evidence of FMD virus infection, and
there is frequent reference to the tests for NSP antibodies. Similarly the proposed revision to the Code chapter
refers to these tests, in particular in Article 2.1.1.6. A proposed change to this Article, referring to recovery of
free status, has been to reduce from 12 months to 6 months the time between the last case or the last vaccination
(whichever is the latest) and the submission for acceptance of freedom ‘where stamping out, emergency
FMD diagnostics: requirements for demonstration of freedom from infection
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vaccination NOT followed by slaughtering of all vaccinated animals, and serological surveillance are applied,
provided that a serological survey based on the detection of antibodies to nonstructural proteins of FMD virus
demonstrates the absence of infection in the remaining population’. This change would undoubtedly influence
the decision of a previously FMD free country to use vaccine to help control an outbreak of disease, particularly
if it had an important export trade in FMD susceptible animals or animal products.
The consequence of this change is that it becomes more important to have a reliable test to identify vaccinated
animals that may be carrying live FMD virus.
4. DETECTION OF CARRIER ANIMALS
Ruminant animals that have recovered from infection with FMD virus, and vaccinated ruminants that have had
contact with live FMD virus, may retain infection in their pharyngeal region for a variable period of time. A
carrier is defined as an animal from which lives virus can be recovered after 28 days following infection. This is
not an exceptional situation and over 50% of ruminants exposed to live FMD virus become carriers; however,
pigs do not become carriers (1). The duration of the carrier state depends on the species and individual; the
African buffalo may carry virus for over 5 years, cattle for over 3 years, sheep for up to 9 months, and goats and
wild ruminants for shorter periods of time. Carrier animals eliminate the virus eventually (2).
The tests for NSP antibodies can distinguish between animals that are FMD virus antibody positive following
vaccination, and those that are positive following recovery from infection. However, the NSP antibody test will
not necessarily identify the vaccinated animal that has had contact with live virus and become a carrier (4). Some
cattle that have been vaccinated, particularly with a high potency vaccine as might be used in an outbreak in a
previously FMD free country, will fail to develop antibodies to NSPs following contact with live virus. This is
because their level of immunity prevents any significant viral replication, and therefore expression of the NSPs.
Such cattle could, however, become carriers. On a herd basis, even potent FMD vaccine will not protect 100% of
the cattle, and should the herd become exposed to live virus, some will support replicating virus, even though
they may not develop clinical disease, and sero-convert to some of the NSPs, in particular 3ABC. Thus by
testing the whole herd, it would be possible to diagnose the previous encounter with live virus and the potential
for the presence of carriers, assuming that the whole herd was exposed to the same challenge. The test may fail if
only a few animals contacted live virus, as an aerosol from a neighbouring infected farm, and these could all
have been sufficiently immune to prevent sero-conversion to the NSPs. Similarly, if only a random sample of
animals is tested from the herd, those positive for the NSP antibodies could be missed. Consequently, at this
time, the NSP test is considered to be a herd test and should not be used to qualify individual animals as free of
infection.
The alternative to using serology in the post-outbreak surveillance is to sample oesophageal–pharyngeal mucous
for the presence of live virus or virus genome. Such sampling would be impossible on a large scale using current
protocols, but this may change with the introduction of new tests (6, 7). These tests would require a high
sensitivity if they are not to compromise the statistical validity of the sampling regime adopted.
CONCLUSION
A key question for FMD diagnostics and control is to know the importance of carrier animals in causing new
outbreaks of FMD. If that could be answered satisfactorily there may no longer be the requirement to so
assiduously seek out FMD infection, and the fear of the consequences of using vaccination to control an
outbreak of FMD could be lifted. Until we can quantify the risk posed by carriers, however, the mere presence of
live FMD virus in an immune animal will be sufficient to influence decisions on international trade. While the
science is not always available to attach a figure to these risks, the surveillance guidelines developed do provide
some guidance to manage these risks. It is important, however, that all Member Countries be aware of the
background to this guidance in order to develop their own import risk analysis.
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3.
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OFFICE INTERNATIONAL
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SCUDAMORE J. (2001). - Measures Related to the Eradication of FMD in the UK. Paper presented at the
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DES
EPIZOOTIES (2001). - International Animal Health Code. OIE, 12 rue de
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