(Extracted from Recognizing contagious bovine pleuropneumonia. FAO Animal Health Manual No. 13 (Rev. 1). Rome, 2002)
Investigation leading to a conclusive decision will rely on a combination of the following activities:
(i) epidemiological investigation to obtain a general picture of disease pattern in the herd;
(ii) post mortem examination to observe the characteristic lesions in organs of dead and/or slaughtered animals; and
(iii) laboratory examination to confirm infection.
EPIDEMIOLOGICAL INVESTIGATION
When CBPP is suspected, the questions asked should include the following:
1. What species of animals (e.g. cattle, sheep, goats, pigs, wild animals) are present on the livestock holding facility (or village)? How many of each is present and which species are affected?
If domestic or wild animals other than cattle or water buffaloes are affected, a condition other than CBPP should be considered.
2. What ages of cattle or domestic buffaloes are affected?
Record the various age groups of the animals; e.g. under 6 months; 7 to 18 months; over 18 months. In CBPP, the more severe respiratory forms are observed in adult animals.
3. Have the cattle been vaccinated against CBPP or other epidemic diseases and, if so, when did the last vaccination take place? Which vaccine was used? How many animals were vaccinated? Who conducted the vaccination?
If all the cattle have been vaccinated with a quality-assured CBPP vaccine at the appropriate time intervals they should theoretically not develop the disease. However, CBPP can still occur in non-vaccinated cattle in partially vaccinated herds and even in vaccinated cattle that have not been re-vaccinated as scheduled.
4. When did the first signs of disease appear? Is this the first time that this disease has occurred? If not, what are the approximate dates of previous episodes?
This can help to indicate whether the disease is endemic or newly introduced, and can help to calculate when infection entered the herd.
5. Have other cattle been bought or introduced for any reason during the six months before the disease was first noticed? If so, from where? Did any become sick?
The answer can provide a clue as to how the disease entered the herd.
6. Was the herd exposed to another herd, during the six months before the disease was first noticed? Do nomadic herds pass through the area? If so, when and from where?
Nomadic herds can be a CBPP reservoir. The answers can also provide an explanation of how the disease might have entered the farm or herd.
7. Are grazing lands, water-holes, drinking-troughs or dipping tanks shared with other nomadic or sedentary herds?
This is to indicate possible contacts with animals from other herds, allowing tracing of the origin of outbreak, and therefore helping to provide an early warning signal.
8. Were replacement animals vaccinated against CBPP and other diseases before or after introduction to the herd?
This provides information as to why sickness might be limited to a particular group of animals.
9. Does the community know the disease and does it have a local name?
Pastoralists are often able to provide a useful guide to disease conditions they have encountered in the past.
10. Have the infected animals been treated with antibiotic(s)? If so, which type(s)?
Antibiotics may mask the clinical symptoms of CBPP and alter the progression of disease in a herd. They may also alter the appearance of typical pathological lesions and thereby complicate diagnosis of the disease.
11. What are the signs observed in diseased animals?
Respiratory signs are more evident in adult cattle, whereas enlargement of joints may be present in calves under six months of age.
12. How many animals are clinically sick out of the total?
13. How many animals have died since the outbreak occurred?
14. What is the health status in neighbouring herds?
To decide if CBPP is present in the area, the neighbouring herds should be inspected for evidence of disease.
15. Have any animals been sold, transferred or given on loan, e.g. for ploughing, or as a gift (dowry) in the last six months?
The answer to this question might give important information on spread of the disease and assist in tracing the source of the outbreak.
CLINICAL EXAMINATION
As the clinical appearance of the disease can differ between individuals in a herd depending on the different stages of disease development, it is important to examine as large a number of animals as possible in order to obtain a full clinical picture. A notebook is essential to record all the findings and to refer to this later. The use of pieces of paper is not recommended as these often get lost, and vital information with them.
1. Record the observations of the farmer or animal attendant
Ask for the farmer or animal attendant to describe the disease observed.
Has any treatment been given? Antibiotics such as tylosin and the tetracyclines can be effective in modulating clinical symptoms and progress of disease.
[Conventional understanding is that antibiotic therapy is contra-indicated in outbreaks of CBPP because it is believed that its use leads to the generation of a high proportion of "lungers" (chronic carriers with sequestra in their lungs) in the herd, and that these can later spread infection to susceptible cattle. This may be true, but in most countries in which CBPP occurs, antibiotic therapy is a fact of life. Disagreement over its use should not be allowed to create a barrier between the animal health worker and livestock owner.]
Have any cows aborted?
2. Observe the animals at rest
Before attempting to handle the animals, check if they are alert or depressed, if lameness is present and if body condition is satisfactory for the time of year and type of management system
Do any stand with the neck and head extended, forelegs spread apart, mouth open and panting for air? It is worth remembering that this happens to:
- animals severely affected by CBPP - acute cases; and
- animals with respiratory diseases other than CBPP.
Is breathing difficult, rapid and painful? If breathing is difficult, the nostrils are generally dilated, and clear or bloodstained discharge may be seen from the nostrils.
Check the character and rate of respiration. Is it fast (more than 20 per minute)?
Do any animals cough?
Is there any discharge from the eyes and nose? A clear discharge may be present.
3. Physical examination
Take the rectal temperature: in acute cases it can rise above 40°C.
Check the surface lymph nodes: enlargement is not a feature.
Check the mouth, including the lining of the lips, tongue, cheek papillae and the hard palate - lesions are not found, unlike in rinderpest and FMD, although saliva may dribble from the mouth.
4. Force the animals to run for a few minutes and then examine them again
CBPP symptoms can be more clearly seen after a few minutes exercise - coughing and signs of lameness.
FAO World Reference Laboratory for Contagious Bovine Pleuropneumonia
Centre de Coopération Internationale en Recherche Agronomique pour le Développement, Département d'Elevage et de Médecine Vétérinaire, CIRAD-EMVT
Campus International de Baillarguet
BP 5035
34032 Montpellier Cedex 1
FranceE-mail: [email protected]
Web site: www.cirad.fr
Telephone: (+33) 4-6759 3724
Fax: (+33) 4-6759 3798
FAO Collaborating Centre
Agricultural Research Council - Onderstepoort Veterinary Institute
ARC - OVI
Private Bag X05
Onderstepoort
0110 South AfricaE-mail: [email protected]
Web site: www.ovi.ac.za
Telephone: (+27) 12-529 9101
Fax: (+27) 12-565 4667
OIE Reference Experts and Laboratories
Dr J.L. Martel
AFSSA Lyon, Laboratoire de pathologie bovine
31 avenue Tony Garnier,
BP 7033
69342 Lyon Cedex 07
FRANCE.
Telephone: (+33) 4-7872 6543
Fax: (+33) 4-7861 9145
E-mail: [email protected]Dr J. Regalla
Laboratório Nacional de Investigaçâo Veterinária (LNIV)
Estrada de Benfica 701,
1500 Lisboa
PORTUGAL
Telephone: (+351) 21 711 5339
Fax: (+351) 21 711 5236
E-mail: [email protected]
or: [email protected]Dr A. Pini
CESME, Istituto Zooprofilattico Sperimentale dell'Abruzzo e del Molise
'G. Caporale'
Via Campo Boario,
64100 Teramo
ITALY
Telephone: (+39) 0861 33228
Fax: (+39) 0861 332 251
E-mail: [email protected]
This section has been taken from Appendix 3.8.2 to the OIE
International Animal Health Code 2001, downloaded from
http://www.oie.int/eng/normes/MCode/A_00153.htm.
1. INTRODUCTION
The Ad hoc Group on Contagious Bovine Pleuropneumonia (CBPP) Surveillance Systems held a meeting on 7-9 June 1993 with the purpose of formulating these standards, which describe surveillance systems suited to the declaration of countries and zones free of disease and free of infection. Background information is contained in the report of the meeting. In order to write these standards, the Group reviewed the following:
a) epidemiological and non-disease factors influencing the choice of CBPP surveillance systems;
b) sampling and surveillance strategies;
c) diagnostic methods applicable to CBPP surveillance systems; and
d) the implications of CBPP vaccination for surveillance systems.
This last point was the subject of lengthy discussions during the meeting of the OIE Committee in May 1994. A revised text was submitted at the following meeting of the Committee (May 1995), which requested that a small group of experts formulate revised proposals. The present text is the product of their consensus.
2. DEFINITION AND PURPOSES OF SURVEILLANCE
Disease surveillance is necessary to provide evidence that a country or zone is free from a disease or infection.
Disease surveillance should be implemented by both:
a) a system of reporting any signs of disease activity which come to the notice of Veterinary Services or livestock owners; and
b) an active programme of examination of statistically selected samples from host populations in order to detect clinical signs or other indications of the occurrence of disease or transmission of infection.
In either case, suspicion of disease activity should be followed by quarantine, confirmatory diagnostic work and any necessary disease control measures. Surveillance thus implies that official action will follow from the discovery of evidence of disease or infection. It can be contrasted with monitoring, in which the gathering of data from the field takes place similarly, but no official action based on the findings is implied in the data-gathering activity.
Within the context of pleuropneumonia, specific measures need to be implemented, such as an exhaustive inspection of all lungs of bovines throughout the country or zone.
3. STEPS TO BE TAKEN TO DECLARE A COUNTRY FREE FROM CONTAGIOUS BOVINE PLEUROPNEUMONIA
The current goal in CBPP control is to achieve freedom from disease in particular countries and later of entire world regions, with the ultimate aim of achieving global eradication. It is therefore necessary to institute a system for verifying the steps towards these short- and long-term aims, and to assist countries which wish to trade in livestock or livestock products, but face difficulties due to the presence or past occurrence of CBPP.
In conformity with the general principles for assessing disease status developed by the OIE, a four-stage process should be applied:
(i) intention to eradicate pleuropneumonia, the longest phase, depending on prevalence of the disease in the country or zone, geographical, socio-economic and administrative conditions, and the capacity of the animal health infrastructure;
(ii) once a country is free from CBPP and that disease is unlikely to be re-introduced, the country can declare itself provisionally free from disease, provided it meets the criteria listed below;
(iii) declaration of freedom from clinical CBPP, after international verification carried out under the auspices of the OIE; and
(iv) declaration of freedom from CBPP, where a country meets more stringent surveillance and control criteria.
The last three stages are strictly covered by the epidemiological surveillance methods of the OIE.
The sequence of operations differs both in terms of tactics and duration depending on whether or not the country wishing to eradicate CBPP practises vaccination.
'Disease' in the context of declaration of freedom means that the particular pathogenic agent is present and causes significant pathological effects on animals which become infected with the agent. Thus 'freedom from disease' means that there is no evidence in animals within the country or zone of any pathological effects occurring (including clinical signs) due to the presence of the agent, and from all the evidence pathogenic strains of the particular agent have been eliminated.
COUNTRIES PRACTISING VACCINATION
The process is summarized in the following chart.
The specific criteria proposed for each stage of this process are as follows:
(a) Provisional freedom from disease
For a country to declare the whole or a zone of its territory provisionally free from disease, it must fulfill certain conditions, which are:
(i) no clinical or pathological evidence of CBPP should have been detected for at least 3 years;
(ii) There is an effective Veterinary Service which is able to monitor the animal health situation in the country;
(iii) there is effective meat inspection at approved abattoirs, and effective surveillance of populations in which significant numbers of slaughtered susceptible livestock are not subject to meat inspection;
(iv) all evidence suggestive of CBPP is investigated by field and laboratory methods (including serological and microbiological assessment) to refute a possible diagnosis of CBPP;
(v) there is an effective reporting system, both from the field to the central veterinary authority, and by that body to the OIE;
(vi) there is an effective system to prevent the introduction of infection, including appropriate border control, quarantine, etc;
(vii) if vaccination has been used, all vaccination against CBPP has ceased by the date of declaration; the OIE and neighbouring countries having been notified in writing, giving the date from which vaccination was discontinued.
(b) Freedom from clinical CBPP
A country which has declared itself or a zone to be provisionally free from disease may be declared by the OIE free from clinical CBPP, provided that the following criteria are met:
(i) no clinical or pathological evidence of CBPP has been detected for at least 5 years;
(ii) no CBPP vaccination has taken place for at least 2 years;
(iii) the country operates surveillance and disease reporting systems for CBPP adequate to detect disease if it were present, and ensures that veterinary staff are adequately trained in the recognition of CBPP;
(iv) all susceptible livestock at recognized abattoirs are subject to meat inspection procedures adequate to detect lung lesions, with diagnostic procedures to refute a possible diagnosis of CBPP;
(v) there has been a programme of surveillance (using serological, pathological and microbiological techniques) for at least 2 years on any populations of susceptible domestic livestock where more than 10% of slaughtering is not subject to adequate meat inspection procedures;
(vi) all evidence suggestive of CBPP is investigated by field and laboratory methods (including serological and microbiological assessment) to refute a possible diagnosis of CBPP;
(vii) there are effective measures in force to prevent re-introduction of the disease.
On meeting these criteria, a country may apply to the OIE for all, or a zone, of its territory to be declared free from clinical CBPP.
An Expert Panel for the Verification of Disease Status of the OIE will evaluate the application and decide whether or not to approve it. In coming to its decision, the Expert Panel will consider evidence presented by the country and will gather information on the extent to which the criteria are met. This information-gathering will usually include sending members of the Panel to make a field visit to the country. The Expert Panel will report its findings to the OIE Foot and Mouth Disease and Other Epizootics Commission. The Commission will report its conclusions annually to the International Committee for endorsement.
To maintain this status, a country must continue to meet these requirements until it is declared free from CBPP, and must report to the OIE an annual summary of developments.
Should there be a localized temporary outbreak of disease due to re-introduction of CBPP to a country which has met, or is within 2 years of meeting, the requirements for a declaration of freedom from clinical CBPP, that country should implement a stamping-out policy, which may be supported by intensive perifocal vaccination, to eradicate the outbreak. In such circumstances if no vaccination was carried out, it will then require at least one year from the date of the last case before the country becomes eligible to apply for a declaration of freedom from clinical CBPP. If vaccination was used, this period is extended to 2 years from the date of the last case or the last vaccination (whichever occurs later). In making an application under these special circumstances, it must be shown that the outbreak did not represent endemic infection, and that the disease has been eradicated by the actions taken.
The declaration of zones to be free from clinical CBPP will not remove the requirement for the country subsequently to meet the criteria for declaration of freedom from clinical CBPP for the country as a whole; if it wishes to achieve that status, it will have to meet all of the requirements specified above before it can apply for a declaration of freedom from clinical CBPP for the entire country.
(c) Freedom from CBPP
A country or a zone of its territory which has within the last 10 years either vaccinated against CBPP, or found clinical or pathological evidence of CBPP, may be declared by the OIE to be free from CBPP if the following criteria are met:
(i) it has been declared free from clinical CBPP at least 2 years earlier, and continues to meet the requirements for this status;
(ii) there has been effective abattoir surveillance for at least 4 years, covering all susceptible domestic livestock;
(iii) use has been made of diagnostic procedures capable of differentiating Mycoplasma mycoides from other bovine Mycoplasma infections in the investigation of respiratory disease, and the findings are consistent with freedom from M. mycoides infection;
(iv) there has been a programme of surveillance, including serological, pathological and microbiological components, for at least 3 years on any populations of susceptible domestic livestock where more than 10% of slaughter stock are not subject to adequate meat inspection procedures.
On satisfying these criteria, a country may apply to the OIE to be declared free from CBPP.
An Expert Panel for the Verification of Disease Status of the OIE will evaluate the application and decide whether or not to approve it. In coming to its decision, the Expert Panel will consider evidence presented by the country and will gather information on the extent to which the criteria are met. This information-gathering will usually include sending members of the Panel to make a field visit to the country.
The Expert Panel will report its findings to the OIE Foot and Mouth Disease and Other Epizootics Commission. The Commission will report its conclusions annually to the International Committee for endorsement.
In the special case of a country or zone which has been considered to be continuously free from CBPP for at least 10 years, and meets all of the following requirements:
(v) has not vaccinated against CBPP for at least 10 years;
(vi) throughout that period found no clinical or pathological evidence of CBPP infection;
(vii) had throughout that period, and undertakes to maintain permanently, an adequate disease surveillance and reporting system, covering all susceptible domestic livestock;
(viii) in appropriate circumstances, made use of diagnostic procedures capable of differentiating Mycoplasma mycoides from other bovine Mycoplasma infections in the investigation of respiratory disease, with findings consistent with freedom from M. mycoides infection;
the country or zone may be declared by the OIE to be free from CBPP without the necessity to proceed through the normal intermediate steps. This declaration will be based on the conclusions of the Expert Panel for the Verification of Disease Status.
Declaration of freedom from CBPP can be made for the country as a whole, or for zones within a country.
Should there be a localized temporary outbreak of disease due to re-introduction of CBPP to a country which has met, or is within one year of meeting, the requirements for a declaration of freedom from CBPP, that country may take special measures (excluding the use of vaccination) to eradicate the outbreak. In such circumstances, it will then require at least 2 years from the date of the last case before the country becomes eligible to apply for a declaration of freedom from CBPP. In making an application under these special circumstances, the country must demonstrate that the outbreak did not represent endemic infection, and that the disease has been eradicated by the actions taken.
In order to maintain this status, the country must continue to operate an efficient disease surveillance and reporting system, which would detect CBPP if it occurred.
COUNTRIES NOT PRACTISING VACCINATION
These are generally countries with a solid animal health infrastructure (with a system for individually identifying animals) where CBPP has been accidentally introduced.
The accelerated eradication process is summarized in the following chart:
The specific criteria proposed for each stage of this process are as follows:
a) Provisional freedom from disease
A country may declare the whole or a zone of its territory provisionally free from disease one year after the last infected herds and in-contact herds have been slaughtered, on condition that:
(i) there has been no vaccination in the country or zone for at least 2 years;
(ii) all treatment against CBPP is prohibited for sick animals or suspected cases;
(iii) a stamping-out policy is implemented after any CBPP outbreaks. Within the framework of the declaration, a minimum period of 12 months will be required after the last sick or in-contact herd has been slaughtered;
(iv) an epidemiological investigation, including serological tests, has been carried out to determine the prevalence of the disease in the country or infected zone. Special attention should be given to screening animals transported into or out of the infected herds during the 6 months preceding detection of the outbreak(s);
(v) a system of livestock identification and movement control has been set up in the country or zone for the purposes of CBPP control and surveillance as follows:
- all herds are officially registered and all animals of susceptible species aged over 12 months are individually identified;
- before being moved, other than for immediate slaughter, all animals of susceptible species are to be clinically inspected and serologically tested for CBPP;
(vi) all animals of susceptible species in herds or establishments within a 3-km radius of an outbreak, and any animals with a possible epidemiological link, are individually identified, placed in quarantine for at least 6 months, and
- all animals of susceptible species in the aforementioned herds or establishments are serological tested on two occasions at an interval of 2 to 8 weeks; microbiological investigations are to be carried out on any serologically positive animal;
- during the quarantine period, animals in the aforementioned herds or establishments are not to be moved other than to an officially approved abattoir, where they are to be immediately slaughtered and subjected to sanitary inspection after slaughter;
- microbiological tests should be carried out on animals presenting lesions suggestive of CBPP;
(vii) surveillance is carried out in abattoirs in the contaminated country. Any lesion suggestive of CBPP should be examined microbiologically and, if the result is positive, the herd of origin must be found and subjected to serological testing;
(viii) the diagnostic tests used in the country or zone comply with OIE standards and are conducted in a nationally approved laboratory.
b) Freedom from contagious bovine pleuropneumonia
A country or zone may be declared by the OIE to be free from CBPP 2 years after the last infected and in-contact herds have been slaughtered if the conditions listed in paragraphs (a)(i) to (a)(viii) continue to be met.
4. EPIDEMIOLOGICAL METHODS
(a) Surveillance systems
In demonstrating that a country or zone is free of disease, it is necessary to conduct a surveillance programme which would have a very high probability of detecting the disease if it were present. Surveillance for CBPP will include a combination of clinical, pathological, serological and microbiological methods, built around an epidemiological surveillance approach. The mix of procedures used will depend on the specific circumstances of the country or zone.
The most efficient means of detecting CBPP is through effective meat inspection procedures at abattoirs followed by laboratory examination of suspect lesions. Where a very high proportion of susceptible domestic livestock are slaughtered in controlled abattoirs, this will provide a very sensitive surveillance system covering the whole population. It is possible that structured investigation of a statistical sample of carcasses might be used to augment the routine meat inspection procedures.
Where large numbers of susceptible livestock are exported for slaughter, it may be necessary to obtain meat inspection data from the importing country.
Where a significant proportion of susceptible domestic livestock are not subject to meat inspection at the abattoir, then it will be necessary to use alternative surveillance methods based on the examination of samples of herds so as to achieve a standard probability of detection. Animals in sampled herds would be subjected to clinical examination for signs of CBPP, but not all infected animals exhibit clinical signs. Serological testing can be useful in identifying infected herds, but due to the limitations of the currently available serological tests, and the possibility that the disease may be present at very low prevalence, such surveillance systems are not very efficient in proving freedom from disease, and require large numbers of herds to be sampled.
(b) Definition of sampling units
A sampling unit for the purposes of disease investigation and surveillance is defined as a group of animals in sufficiently close contact that individuals in the group would be at approximately equal risk of coming into contact with the disease agent if there were an infectious animal within the group. In most circumstances, the sampling unit would be a herd which is managed as a unit by an individual or a community, but there may be other epidemiologically appropriate groupings which are subject to regular mixing, such as all the animals belonging to residents of a village. Sampling units should normally be defined so that the majority of units contain between 50 and 1000 animals.
(c) Criteria for the stratification and sampling of host populations
Serological surveillance would only be adopted for CBPP in circumstances where the preferred slaughterhouse surveillance system described in item 3(c) of this document could not be carried out on an adequate scale because too low a proportion of animals was slaughtered in a slaughterhouse. Thus the following system would be used as an exceptional case, rather than as the usual procedure.
Any disease surveillance activities must be conducted on populations stratified according to disease risk, which depends principally upon the environment and management system. The cattle production systems of most countries would be categorized into between two and six strata.
Annual sample sizes must be sufficient to provide 95% probability of detecting evidence of CBPP if it were present at a prevalence of 1% of herds or other sampling units. Given perfect sensitivity of the within-herd testing procedure, this would require the examination of 300 herds from each stratum per year. However, the currently available serological tests have rather low sensitivity. The sensitivity of the test procedure at herd level is further reduced when only a sample of the herd is tested. It is possible to compensate for lower sensitivity by increasing the numbers of herds examined. The required sample size is determined by adjusting the prevalence to allow for the lack of sensitivity. For example, if there was 50% probability of detecting a sampled infected herd (sensitivity 0.5), then a true disease prevalence of 1% of herds would result in a detectable prevalence of 0.5%, and this detectable prevalence would be used to determine the required sample size.
Herds, or other sampling units, must be selected from each stratum by proper random methods, which are described in the Guide to Epidemiological Surveillance for Rinderpest published by the OIE. Any randomly selected herd must be examined in order to achieve the required probability of detection. However, this probability can often be increased by an important but unquantifiable margin by sampling additional herds based on subjective assessment of risk, or information gained during field work.
5. CONTAGIOUS BOVINE PLEUROPNEUMONIA VACCINES
T1 strain (and its streptomycin-resistant variant) is the recommended vaccine, and the following facts are relevant to disease surveillance activities:
Current vaccines do not induce life-long immunity; the duration of protection after vaccination is about one year.
A significant proportion of vaccinated animals do not develop a serological response detectable by currently used techniques, although such animals may be protected against challenge. Where the serological response to vaccination is detectable by the complement fixation test, it usually persists for less than 3 months.
As their immunity wanes, vaccinated cattle are more likely to develop chronic lesions (sequestra) after infection.
6. DIAGNOSTIC METHODS
The diagnosis of CBPP depends on: (a) clinical signs in the live animal; (b) gross pathological findings; (c) serological tests; and (d) culture and identification of the causative organism.
(a) Clinical diagnosis
The clinical signs of CBPP may be slight or non-existent. Furthermore, the use of anti-microbial or anti-inflammatory drugs can mask the clinical expression of the disease. For these reasons, clinical signs are an unreliable indicator of the presence of the disease. However, if respiratory disease is observed in a livestock population, then the diagnosis of CBPP should be considered and confirmed or rejected on the basis of further pathological, microbiological or serological investigations.
(b) Gross pathology
The lung lesions of CBPP are distinctive. Consequently, abattoir meat inspection is the most practical single method for maintaining CBPP surveillance. The pleura and lungs should be examined by palpation and section. A mixture of acute lesions and chronic lesions (sequestra) may be found in the same herd or even the same animal. In case of chronic infection, post-mortem diagnosis may be the only way of detecting asymptomatic animals, which may not react to serological tests.
(c) Serological diagnosis
The serological test of choice is the complement fixation test (CFT). The specificity of this test can be as high as 99.5%, but the frequency of false positive reactions may temporarily be higher in certain herds. The sensitivity of the test is limited, and it may fail to identify four classes of animals:
(i) animals in the very early stages of the disease;
(ii) animals in the very late stages of the disease (the CFT appears to fail to detect 30% of animals containing sequestra);
(iii) animals with massive lesions, where the antibodies produced are overwhelmed by the antigen;
(iv) animals which have been treated in the early stages of the disease may fail to develop a detectable serological response.
Despite these limitations, the CFT is a useful herd test. The CFT reaction after vaccination is inconstant and short-lived (generally less than 3 months).
An indirect enzyme linked immunosorbent assay (ELISA) is under field evaluation in several countries. It is at least as sensitive as the CFT, but as with other ELISA systems, increased sensitivity can only be achieved at the expense of specificity, and vice versa. It is a useful tool to measure the efficacy of vaccination programmes, as the detectable response is more reliable than the CFT, and may persist for as long as one year after vaccination.
Monoclonal and competitive ELISA systems are being developed and should offer higher specificity.
The passive haemagglutination test, while not used routinely, may have a place in serological diagnosis. It is more sensitive than CFT in early and late stages of disease, but with lower specificity, and has a potential role as a screening test.
The slide agglutination test is simple to perform and could be used as a pen-side test. It is more sensitive than the CFT in the early stages of the disease, but it lacks specificity.
(d) Culture and identification of the causative organism
It is desirable that all diagnoses are confirmed by isolation of the causative organism. It may prove difficult to isolate Mycoplasma from chronic lesions and also after animals have been treated with anti-microbial drugs.
The causative organism is normally identified by growth inhibition tests and/or the immunofluorescence test. Closely related Mycoplasma may cause cross-reactions in these tests. Several new techniques which may overcome this problem are being developed, and these include immunobinding, immunoperoxidase and polymerase chain reaction (PCR) tests. These need further evaluation.
(e) Testing imported animals
In formulating its recommendations for a system of declaration of freedom, the Group acknowledged that existing serological tests for CBPP are quite variable in sensitivity and specificity. Hence serological methods alone are unlikely to prevent the introduction of infection if live animals are imported from CBPP-infected countries. The chronic course of the disease may mean that diagnosis following introduction of CBPP may be delayed by a number of years. In the longer term there is a need for more sensitive and specific diagnostic tests. Pending the development of such tests, serological methods are necessary, but not sufficient to prevent introduction of the disease in live animals.
