The use of the field complement fixation test in the diagnosis and control of
contagious bovine pleuropneumonia - the Kenyan experience - successes and failures

S.W. Kairu-Wanyoike, A.K. Bengat and W. Lung’aho

Central Veterinary Laboratory, Kabete, Kenya.

[Editorial Note: This report was not presented at the Consultative Group Meeting on CBPP in Africa Towards sustainable CBPP control programmes for Africa, Rome, 12 - 14 November 2003, but is included in this document for completeness.]

Introduction

The zoning of the country with respect to CBPP has constantly been revised since the first case in Kenya was recorded in 1901 following the pattern of disease outbreaks and the situation of the disease in the neighbouring countries (Kariuki, 1971). Up to 1975 3 zones were recognised (though not mapped) as Category A: Infected or severely threatened districts which included the entire north and eastern districts of Turkana, Marsabit, Mandera, Wajir, Isiolo, Garissa and Tana River; Category B: Areas under threat but with no disease consisted of West Pokot, Baringo, Samburu, Kitui, Kilifi and Maasailand; Category C: Clean areas which included the rest of the country mainly the Central and Western highlands and part of the coast (Ministry of Livestock Development and Fisheries, 1978).

Constant threat of CBPP to West Pokot and outbreaks in the early 80s originating from the neighboring Republic of Uganda saw West Pokot added to Category A districts. Illegal movement of cattle from West Pokot to Busia, Bungoma and Siaya districts in the West led to outbreaks in these districts which were then added to category A districts with the neighbouring districts of Trans-Nzoia and Kakamega being added to Category B districts.

Meanwhile the control policy in Kenya continued along the lines of mass vaccination in enzootic areas, increasing emphasis on field testing with slaughter of reactors within the enzootic areas, rigid control over movement out of the enzootic and suspect areas and rapid stamping out, by testing and possibly vaccination, of any outbreaks that might occur outside the enzootic areas thus the outbreaks in the west were eventually cleaned up (Kane, 1975). Following field-testing with slaughter of reactors and mass vaccination in Maasailand this vast pastoralist area remained free of CBPP from 1968 to 1986. In 1986/87 CBPP reentered Maasailand following the purchase of ex-Garissa and ex-Isiolo cattle probably from Somalia as this also marked the beginning of the Somalia unrest.

Epidemiological studies by Wanyoike (1999) established that CBPP had partly been re-established in Maasailand and was now threatening Ukambani (Makueni, Machakos, Mwingi and Kitui) Nakuru and Thika districts following transhumant cattle movement due to the decrease in the Maasailand dry season grazing area due to human settlements.

Taking into consideration the recommendations of the CBPP Workshop on Control Strategies in Accra, 2003, abattoir surveillance in 2002 and the recommendations of a Consultancy Report on Livestock Health Quarantine Measure Study in 2000/2001, a new zonation of the country into 4 zones and a redefinition of control strategies was put in place in June 2003. The redefining of the control strategies considers the revitalising of the control measures that saw the eradication of the disease in the greater part of the country in the seventies. These control measures are particularly cattle movement control from infected to non-infected areas with identification of cattle by district brands and field-testing with slaughter of reactors. Of increasing importance is the strengthening of epidemiological surveillance networks particularly abattoir surveillance along with extension of both fieldtesting and laboratory confirmation. Vaccination remains an integral part of control of CBPP.

CBPP zones in Kenya 1901 to 2003

Before 1975

1975-1982

1982-2002

June 2003

The history of CBPP field complement fixation test in Kenya

Until 1950 the control of CBPP presented a unique set of problems in Maasailand (and indeed in other pastoralist communities): veterinary cover has never been wholly adequate, information is often difficult to obtain and concealment of infection is common. The Maasai jealously guard their traditional right of unrestricted movement within their territories and indeed transhumance is necessary for survival. The Maasai methods of husbandry include close herding at night in temporary bomas, an ideal situation to perpetuate CBPP. It is against this background that the Kenya Department of Veterinary Services evolved a policy of control based on the removal of infected herds to specially designated quarantine areas (holding grounds) where they remained under maximum possible supervision until free of disease. Difficulties arose in the administration of quarantine because of shortage of water and grazing and farmers visualization of other epizootics as more important than CBPP leading to widespread attempted evasion of quarantine restrictions.

Though quarantine measures, even when allied to vaccination of infected herds, are not calculated to produce spectacular results in terms of disease eradication, the continued application of such measures during the decade (1950-1959) had by the end of that period resulted in a considerable measure of control, in that CBPP was absent from the eastern half of Maasailand but remained endemic in the western half. To consolidate the progress so far made a large-scale vaccination campaign using egg adapted vaccine was attempted in 1957. The campaign was to be abandoned because of severe inoculation reactions. In 1959 the movement of a single infected herd through the east led to the re-infection of the eastern half and about six other districts (Huddart, 1963).

It was in the face of this considerable distribution of the disease that it was decided that possibilities of control based upon increased use of the complement fixation test be investigated. It is towards this end that a modification of the conventional complement fixation test, suitable for large scale field-testing was devised.

Brief outline of the testing operation

Field-testing was designed to be carried out by a mobile unit, transportable, together with its staff of eight assistants in a 3-ton lorry. The important items of equipment include a large tent, portable tables and chairs, an electric generator, centrifuge, refrigerator, water baths, drums of water and petrol and glassware in specially designed containers. Tents for the staff and personal baggage were also carried.

Testing took place at centres equipped with suitable cattle crush facilities (usually at cattle watering points) and the unit travelled to the testing site and set camp on the day before testing was set to begin. It is usually convenient to spend several days at any one site during which time cattle are brought to the crush from within a radius of ten or more kilometres. The testing tent was erected at least a hundred yards from the crush on the windward side of it to avoid dust, one of the greatest enemies of the test in the field.

Today a mobile laboratory (specially designed landrover) transports equipment while another landrover transports the staff hence replacing the 3 ton lorry. Camp is sometimes set up at permanent public facilities such as schools, chiefs’ camps, hospitals and field veterinary offices obviating the need for tents, portable chairs and tables. The sampling team who makes the local runs and delivers the samples to the testing team then uses the second landrover.

When testing is in progress cattle are driven into the crush and there numbered and bled from the tail so as to produce samples of whole blood diluted in saline. Batches of 76 samples at a time are carried to the tent and inactivated to destroy complement, then centrifuged to produce clear diluted plasma that is used in the test. Testing is carried out on plastic agglutination trays incubated by ‘floating’ on the surface of a water bath maintained at 37°C. Preliminary screening test is followed by a more detailed repeat test the whole testing operation taking 3-4 hours to complete. Test results become available at intervals during the day and reactors are identified either the same day or early the next morning. Reactors are the branded ‘PP’ for immediate slaughter under the supervision of a veterinary officer upon which samples are taken.

The testing unit is supplied with preserved test reagents sufficient for two weeks stay in the field though it is usually convenient to renew reagents each week. The samples must be held at 4°C initially otherwise for a days work the reagents are packed in vacuum flasks with a few cubes of ice and adequately padded to resist extreme degrees of vibration in the course of travel over poor road surfaces. The testing capacity of a unit depends greatly on the quality of the crush and local assistance in driving animals and a fair day’s testing under average conditions consists of about six hundred individual tests.

The field CFT has its major advantages over the laboratory CFT in that:

1. It makes use of a method of dispensing suitable for use by relatively untrained laboratory assistants,

2. It makes use of a method of bleeding cattle (through the base of the tail) rapidly and in large numbers so as to produce samples capable of being used immediately in the test,

3. Due to 1 and 2 above, there is high sample throughput.

The testing of animals that was previously carried out at the Central Veterinary Laboratory in Kabete on conventional serum samples is now done by a testing unit in the field resulting in a great saving in time and labour. Field-testing obviates the time consuming jugular bleeding and transport of samples to the laboratory and repeated sampling due to anti-complementary samples.

Application of the field CFT in field control

1. Survey and Quarantine testing

As a survey test, the CFT is used to detect infection in districts and identify infected herds. The usual problem in Kenya has been infection either detected or suspected within an area, commonly involving perhaps ten to twenty thousand head of cattle. Testing for a week or two by a field unit enables the incidence of disease to be mapped out with complete certainty and field officers are able to apply appropriate quarantine measures. This type of operation has been carried out on many occasions and contributes substantially to local control of the disease (Table 1).

As a quarantine test the field CFT is applied at the end of the outbreak to remove chronically infected animals.

Sometimes it was possible to begin the survey soon after the outbreak was reported while in some instances two to three months elapsed before the survey begun due to lack of funds. In some areas the survey had already begun before an outbreak was reported as it was highly at risk from a neighbouring infected area.

Quarantine was lifted after no more reactors were reported (Kiambu, Embu) or when there were no more reports of disease (Kilifi, Thika). Survey was on both the focus and in-contact herds /farms and was often followed with vaccination to stop the spread of disease hence the need for 3 to 6 or more months interval between tests.

Table 1. The application of survey and quarantine testing in Kenya in recent years.

Graph 1: CBPP survey Kiambu district (Ndeiya/Karai, Kenya, 1989-1992

The progressive removal of reactors coupled with livestock movement control and ring vaccination of about 3,523 cattle in 1991 and 1992 led to the disappearance of disease from this location such that the disease has not been reported again since 1992.

Graph 2: CBPP survey Embu district (Karaba/Evurori), Kenya, 1989-1992

In Embu district the disease was “stamped out” through test and slaughter of the reactors and disease has not been reported since 1993.

Marula estate - Nakuru district

In Marula estate CBPP broke out in January 1991. Attempts to control the disease through test and slaughter were frustrating as each subsequent test yielded more reactors than the previous one. This prompted the manager to attempt treatment but it not being encouraged at the time, a different approach of test and slaughter and vaccination of the remaining animals every 3 months saw the eventual eradication of the disease from the ranch that the disease has not been reported since September 1993.

Graph 3: CBPP screening Machakos district (Yatta, Lukenya/Boming Range), Kenya, 1989-1995

The progressive removal of reactors and ring vaccination of about 31,303 cattle in 1992 and 1995 has not sufficiently controlled CBPP in Machakos district such that the disease persists to date and the affected area is under quarantine since December 1988.

2. Screening testing

In Kenya there exists the frequent necessity for testing large numbers of cattle derived from potentially infected areas and intended for distribution in clean areas. This happens in the case of cattle moved for fattening in ranches and for breeding. Though intended for immediate slaughter, cattle moved from infected areas for slaughter particularly to Nairobi, Kiambu and Mombasa have undergone such screening. Originally cattle were moved only after successfully undergoing consecutive negative CFT tests 2 months apart; 3 for cattle intended for fattening and breeding and 1 for cattle intended for immediate slaughter. Currently only 2 tests 21 days apart are required for cattle moved for fattening and breeding while slaughter cattle are not screened but rather are mouthed and clinically examined.

Field teams

Table 2: The application of screening testing in Kenya in recent years.

Screening to some extent serves as a survey test. Indications over the years are that the prevalence of the disease as detected by the field CFT in infected areas is very low reaching a maximum of only 0.17% (Table 2). The number of cattle screened fell due to the fact that screening of slaughter cattle was put on hold and also because of increased insecurity. However, in the last 2 years it has picked up with improved security and increased movement of cattle into ranches and the establishment of a new screening unit in Garissa.

3. Eradication testing

On a larger scale, survey testing can be applied to establish disease free areas which by further testing can be extended and so form the basis of a large scale eradication scheme.

Eradication testing has been attempted in Kenya by attaching the field teams to vaccination teams during district wide vaccination programs either for CBPP or for FMD. Testing precedes vaccination. Positives are removed and herds in which there are positives are ear marked for subsequent testing. Problems arise due to the need to concentrate testing resources over a lengthy period and lack of full co-operation of livestock owners in removal of reactors.

Problems facing the mobile field teams

Several problems face the field teams as follows; leading to declining operations.

1. Inadequate personnel and operating funds

Following structural adjustments within the civil service, there has been a decline in the number of field personnel leading to the shrinkage of field teams and team members being charged with additional responsibilities. This has greatly affected screening testing of cattle intended for slaughter (up to 150,000 annually) as the numbers are too high for the teams such that this kind of testing has been put on hold. Field testing also requires a near permanent presence in the field, as cattle are moving all the time, which puts a strain on the national budget.

2. Breakdown of equipment and vehicles

Of major use in field testing are two 4 wheel drive vehicles, a large centrifuge (to handle up to 144 tubes), 1 or 2 large waterbaths, a portable refrigerator. When these equipment and vehicles breakdown, sometimes replacement is not as fast as should be.

3. Insecurity

Field testing requires the rounding up of cattle and remainder in a holding ground for several days. In areas where rustling is a frequent phenomenon, field operations are threatened and so are the lives of personnel involved.

4. Lack of co-operation from stock owners

Resistance is encountered in presentation of cattle twice for testing such that some cattle are moved only after the first test. Some will resist the slaughter of positives but this is not very common, as there is a salvage value to the animals.

5. Fragmentation in the marketing process

Originally, the Livestock Marketing Division of the Ministry would purchase cattle from the livestock owners and would take responsibility for testing and eventual delivery of the cattle to the slaughter points. Following trade liberalisation, individuals assumed responsibility for testing and marketing of their own cattle making the screening exercise more difficult as screening was now of many small groups instead of a few large groups.

6. Disappearing holding grounds

Recently individuals have taken possession of public utility land and holding grounds have not been spared. In the few cases where they have been spared, fencing and watering and grazing facilities have not been properly maintained and crushes at watering points may also have suffered the same fate.

Conclusion and Discussion

Field testing has proved to be useful in the ‘stamping out’ of the disease from specific enclosed herds such as in ranches and in areas with small plots of private owned grazing land. In certain instances the disease was removed by the test and slaughter policy alone as in Embu district while in Kiambu district vaccination had to be brought in. Where movement control is difficult and herds are not enclosed as in Machakos district, test and slaughter even when coupled with vaccination may not adequately control the disease.

Testing old-standing quarantine herds, with a view to identifying and removing chronic reactors and so enabling the rest of the herd to be discharged from quarantine has achieved little success. Usually the first results in the detection of chronic cases and are removed. A second test six weeks later reveals the presence of a few low titre reactors and even if these are also removed the same thing happens in the third test. This can be ascribed to the presence of persistent low-grade circulating infection, but whether or not this is the case, the practical result is that old standing quarantines cannot be cleared by testing as quickly as would be hoped. Vaccination then becomes of use in preventing the further spread of the infection.

The sensitivity of the field CFT reaches only 40% whilst the specificity reaches 97.8% (Scudamore, 1975). The incubation period of CBPP is 20-40 days (Provost et al., 1987). As the field CFT will not necessarily detect cattle incubating the disease or those with hyperacute or early infections, cattle in Kenya are screened twice at a 21 day interval taking into consideration the incubation period and the economic and practical aspects of how long the cattle can be held for screening.

The screening of trade cattle in Kenya is a workable system in that using the system up to 40,000 cattle per annum have been moved to clean areas without outbreaks in the past (Kane, 1975). Due to lack of economic resources, breakdowns in part of this system have been encountered and all that is required is a revamping of the system to accommodate the 150,000 cattle moved annually.

Large scale survey testing using the laboratory CFT in 2003 involving 6000 samples proved to be rather disappointing since long storage of samples led to massive anti-complementary reactions and false positives probably due to non-specific reactions from probable contamination. With the higher sample throughput of the field CFT and no need for storage of samples, it is possible that a reasonable level of success could be achieved in large scale survey testing for zonal eradication of CBPP and as an epidemio-surveillance tool.

Acknowledgements

Appreciation goes partly to the GTZ, DAAD and the Universities of Berlin and Addis Ababa and the Kenya Veterinary department of the Ministry of livestock Development and Fisheries through which it was possible to collect and process the data used to synthesize this paper.

References

Consultancy report (2001). The Livestock Health Quarantine Measures Study in Kenya by African Technology Development Link.

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Provost, A., Perreau, P., Breard, LeGoff, C., Martel, J. L., Cottew, G. S. (1987). Contagious bovine pleuropneumonia. Rev. Sci. Tech. Off. Int. Epiz. 6, 565-679.

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