Veterinary Parasitology, 43 (1992) 45-56 Elsevier Science Publishers B.V., Amsterdam
45
Investigations of breakdowns in protection provided by living Babesia bovis vaccine R.E. Bock a, A.J. de Wos a, T.G. Kingston a, I.A. Shielsb and R.J. Dalgliesh ¢ aTick Fever Research Centre, Queensland Department of Primary Industries, Wacol, Qld. 4076, Australia bSilkstone Veterinary Hospital, Blackstone Road, Silkstone, Qld. 4305, Australia CAnimal Research Institute, Queensland Department of Primary Industries, Yeerongpilly, Qld, 4105, Australia (Accepted 15 November 1991 )
ABSTRACT Bock, R.E., de Vos, A.J., Kingston, T.G., Shiels, I.A. and Dalgliesh, R.J., 1992. Investigations of breakdowns in protection provided by living Babesia boris vaccine. Vet. Parasitol., 43: 45-56. Field investigations of protection afforded by live Babesia bovis vaccine in Australia revealed that a ninefold increase in vaccine failures occured in the period from 1985 to 1990. Laboratory trials using 189 experimental cattle were conducted to evaluate the protection afforded by the Babesia boris strain used in the commercial vaccine during this time. Four isolates from clinical cases of babesiosis in vaccinated cattle were assessed. The results showed that the strain used in the vaccine during the 5 year period was poorly protective against three isolates while a recently isolated and prepared vaccine strain was strongly protective. Circumstantial evidence is provided that indicates the vaccine failures were due to change in the field populations of Babesia boris, rather than change in the strain used in the vaccine. Implications of the results for the future ofBabesia bovis vaccines are discussed.
INTRODUCTION
Vaccination of cattle against disease caused by the intra-erythrocytic protozoan parasite Babesia bovis is commonly practised in areas of northern Australia where the tick vector Boophilus microplus is endemic (Callow and Dalgliesh, 1980). From 1964 to the present time a living vaccine produced by infecting splenectomised calves with a Babesia bovis strain of reduced virulence has been used (Callow and Mellors, 1966). During this time, seven strains isolated from natural infections in different areas of Queensland have been modified for use in vaccine (Callow et al., 1979; Callow and Dalgliesh, 1980). Correspondence to: R.E. Bock, Tick Fever Research Centre, Queensland Department of Primary Industries, 280 Grindle Road, Wacol, Qld. 4076, Australia.
© 1992 Elsevier Science Publishers B.V. All rights reserved 0304-4017/92/$05.00
46
R.E. BOCK ET AL.
In general, the vaccine has been very effective. However, in 1968 and again in 1976, the number of confirmed cases of babesiosis in vaccinated cattle in Queensland indicated that the degree of protection provided by the vaccine had decreased. Although there was little evidence to support the concept, a decision was made in 1976 to restrict syringe passaging in splenectomised calves in case this narrowed the immunogenic range of the vaccine (Callow and Dalgliesh, 1980; Anonymous, 1981 ). This was done by returning to a bank of stabilate of a lower passage number once direct passages in splenectomised calves had reached an arbitrary cut-off number of about 30. This decision coincided with the introduction of a new vaccine strain designated Ka strain. One or both factors apparently contributed to a notable improvement in vaccine efficacy at that time. From December 1979 to October 1989, Ka strain was the sole Babesia bovis strain used in the commercial vaccine. However, during 1988-1990 there was again convincing evidence of an abnormal and increasing number of Babesia boris vaccine failures. This paper describes our initial studies in ongoing research to clarify reasons for the vaccine failures and to prevent their reoccurrence. It presents an overview of vaccine failures since 1988. It also describes laboratory experiments designed to verify the failure of Ka strain to protect cattle against certain field strains of Babesia bovis, and to identify a suitable replacement strain for use as a vaccine. MATERIALS AND METHODS
Field investigation of vaccine failure The Tick Fever Research Centre (TFRC), near Brisbane, is responsible for the production and distribution of live Babesia vaccines and for the investigation of suspected outbreaks of babesiosis. Records kept of vaccine supply can therefore be readily correlated with information on disease outbreaks. All cases of suspected vaccine failure are investigated by laboratory staff in collaboration with government field officers and private veterinary practitioners. Failure ofBabesia bovis vaccine on a property is defined as the occurrence of death or acute sickness from laboratory-confirmed infection in one or more cattle immunised at least once with Babesia bovis vaccine. Vaccination and breakdown records were examined for the period 1973-1990. Vaccine failures were investigated in detail on 24 properties since 1988 with 11 of these properties being used to investigate alternative vaccination strains and regimes. Isolates were obtained from six of these properties with four isolates being used in trials described in this paper.
B R E A K D O W N S IN P R O T E C T I O N BY B. BOITS VACCINE
47
Experimental cattle At the laboratory, a total of 189 Bos taurus cattle was used in eight separate trials. In seven of the trials, steers aged 15-18 months and derived from an area of Queensland free from Boophilus microplus, were used. Cattle in the remaining trial were 8-month-old steers, seronegative to Babesia bovis and originating from a property experiencing vaccine breakdowns in the tick-infested part of Queensland. The cattle were maintained in grazing paddocks at TFRC under conditions that precluded infestation with ticks. Within each trial, cattle were randomly allotted to treatment groups of six to eight animals per group. Splenectomised Bos taurus calves 2-6 months of age were used to prepare infective blood. The calves originated from the same area as the steers and were housed in concrete-floored pens. All cattle were shown to be seronegative to Babesia bovis in an indirect fluorescent antibody test (IFAT) prior to commencement of trials (Callow et al., 1986).
Strains and isolates ofBabesia bovis For this paper the strains of reduced virulence used as protective inocula in experimental cattle are referred to as vaccine strains and those used for challenge infections as field isolates. Five vaccine strains (Table 1 ) and five virulent field isolates (Table 2 ) of Babesia bovis were used in the study. All were isolated in Queensland or New South Wales over a period of 24 years and stored as frozen stabilates in liquid nitrogen with dimethyl sulphoxide (DMSO) as cryoprotectant (Dalgliesh, 1971 ). Thawed stabilate of each strain or isolate was used to infect splenectomised calves and blood of these calves was then used to prepare experimental vaccines or challenge inocula, respectively. All vaccine strains used in Trials l, 6, 7, 8 and the B strain used in Trial 3, were prepared in frozen form, using glycerol as the cryoprotectant (Jorgensen et al., 1989 ). The actual infecting dose of parasites was, therefore, not known. In the other trials, the strains were prepared as for chilled vaccine (Rodwell et al., 1980), containing 1 × 107 organisms per 2 ml dose.
Inoculation of cattle Cattle in each of the trials were vaccinated with the appropriate vaccine strain and checked for seroconversion by IFAT for Babesia boris. In some trials, groups were revaccinated 1 month later with a second, heterologous strain.
48
R.E. BOCK ET AL.
TABLE l History of strains used as vaccine Strain Date of Origin isolation BL
c. 1963
L
1965
C~
1967
Ticks from outbreak in herd near Springsure in central Queensland
Ka I
1975
Kr L
1975
Blood from chronically infected cow, Maryborough, southeast Queensland As for Ka
T~
1987
Period of use in vaccine (max. passage no. )
Ticks from Beaudesert 1966-1976 district, southeast ( > 90 ) Queensland Blood from field case in Not used northern New South (unknown) Wales
Blood from chronically infected steer from property where vaccine was not used, Townsville, north Queensland
1968-1970 (56) 1976-1979 (30) Dec. 1976-Oct. 1989 ( < 30 ) Oct. 1989-Sept. 1990 ( < 30 )
Sept. 1990 on ( < 30)
Comments
Ref.
First strain attenuated and used in vaccine
O'Sullivan and Callow, 1966
Attenuated by irradiation. Used by CSIRO 2 as research tool Used in rotation with K a f r o m 1976to 1979
Mahoney et al., 1973 Wright et al., 1980 Trueman and Blight, I978; Anonymous, 1981
Sole B. bovis vaccine strain from 1980 to 1989
Callow and Dalgliesh 1980; Anonymous, 1981
Generated by returning Present study to stabilate from passage 15 of original K and reattenuating to passage 20 Present study
~Attenuated by rapid passage in splenectomised calves. 2Commonwealth Scientific and Industrial Research Organisation.
All immunised cattle were challenged after 2 - 4 months by intravenous inoculation with 1 X 108 parasites of an appropriate field isolate.
Measurement of responses to challenge To assess challenge reactions, parasitaemia, fever, depression in packed cell volume (PCV) and the need for specific therapy to control infections were measured according to the methods of Callow and Pepper ( 1974 ) and Timms etal. (1983). Cattle requiring treatment (Timms et al., 1983 ) received quinuronium sulphate (Ludobal, Bayer, Leverkusen, Germany) or imidocarb dipropionate (Imizol, Coopers Animal Health, Australia) both at dose rates of 1 mg kg- ~. No allowance for treatment was made in analysing other data.
49
BREAKDOWNS IN PROTECTION BY B. BOVIS VACCINE
TABLE 2 History of challenge isolates Isolate
Date of isolation
Origin
Comments
Ref.
R
1971
Clinical case, Charters Towers, north Queensland
Trueman and Blight, 1978
W
1988
Clinical case in Ka vaccinated herd, Biloela, central Queensland
Nambour
1988
Dayboro
1990
As for W isolate
Present study
Veresdale
1990
Clinical case in Ka vaccinated herd, Nambour, south east Queensland Clinical case in Ka vaccinated herd, Dayboro, southeast Queensland Clinical case in Ka vaccinated herd, Veresdale, southeast Queensland
Virulent standard challenge isolate used in trials at TFRC ~. Maintained as stabilate following passage in intact calf Virulent. Maintained as stabilate of blood from splenectomised calf infected with original inoculum As for W isolate
As for W isolate
Present study
Present study
Present study
~Tick Fever Research Centre.
The significance of differences between groups in their responses to challenge was determined by analysis of variance. EXPERIMENTS AND RESULTS
Field investigations The number of vaccine failures confirmed annually is shown in Table 3. The actual n u m b e r of failures is undoubtedly much higher owing to under reporting and lack of laboratory confirmation of suspected outbreaks. From these investigations the following information emerged: (i) cattle on all properties were routinely vaccinated at least twice; (ii) failures did not correlate with the time after initial vaccination, which varied from 10 months to 7 years in one outbreak; (iii) only Bos taurus cattle were involved; (iv) some failures occurred on neighbouring properties or on properties in the same ownership; (v) rapidly spreading outbreaks were seen on affected properties
R.E. BOCKET AL.
50 TABLE 3 Vaccine failures (Babesia bovis ) related to vaccine supply 1973-1991 Year
Strain in vaccine
No. of vaccine failures
No. of vaccine orders
Vaccine failures per 1000 orders
1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 j 19912
A and B A and B A and B A and B C and Ka C and Ka C a n d Ka Ka Ka Ka Ka Ka Ka Ka Ka Ka K a a n d Kr Kr T
17 15 10 20 6 6 7 3 6 6 5 7 5 8 10 16 24 33 23
9248 9542 5477 5385 4510 4622 5893 6323 6383 6190 7391 7017 8046 7459 7448 8586 9558 6057 7264
1.84 1.99 1.83 3.76 1.33 1.30 1.19 0.47 0.94 0.97 0.68 1.00 0.62 1.07 1.34 1.86 2.51 5.40 0.27
Lln September 1990 Kr strain was replaced by T strain in the vaccine. Therefore the figures for 1990 are until the end of August only (8 months). 2The figures for 1991 are actually from September 1990 to the end of June 1991 ( 10 months ). -~Vaccine failures for 1991 are for T strain vaccinates only and do not include 40 suspected K strain failures during the same period.
in both resident seropositive cattle and in introduced vaccinated cattle; (vi) on most properties, Ka strain vaccine was used, but Kr vaccine also failed on three properties where its use was closely monitored.
Laboratory study to determine if the protection afforded by Ka strain had altered To achieve this objective a challenge trial was set up using the Centre's traditional challenge isolate (R isolate) against which Ka strain was shown to be protective on a number of occasions during the 1980s (Timms et al., 1983; de Vos et al., 1986 ). The trial design and results are shown in Table 4. The protection afforded by both Ka and Kr strains did not differ significantly from that of the recently isolated T strain.
BREAKDOWNS IN PROTECTION BY B. BOVIS VACCINE
51
TABLE 4 Effect o f v a c c i n a t i o n w i t h v a r i o u s Babesia bovis v a c c i n e s t ra i ns on s u b s e q u e n t challenge w i t h field isolate R Trial
Vaccination history
M e a n max. PCV d e p r e s s i o n (%)
M e a n total parasit, score
M e a n total temp. rise (°C)
No. r e q u i r i n g treatment
T Ka Kr Unvaccinated
35.3 a 37.8 a 33.7 a 55.9 b
6.4 ~ 7.6 a 5.2 a 23.0 b
2.4 a 3.3 a 3.0 a 8.9 b
0/7 0/7 0/6 l 6/7
~One a n i m a l d i d not s e r o c o n v e r t after v a c c i n a t i o n . W i t h i n c o l u m n s m e a n s w i t h o u t c o m m o n superscripts are s i gni fi c a nt l y different ( P < 0.05 ). TA BLE 5 Effect of v a c c i n a t i o n with v a r i o u s Babesia bovis v a c c i n e strains on s u b s e q u e n t challenge w i t h field b r e a k t h r o u g h isolate W Trial
Vaccination h isto ry
M e a n max. PCV depression (%)
M e a n total parasit, score
M e a n total temp. rise (°C)
No. r e q u i r i n g treatment
Ka L B Unvaccinated
40.9 a 43.2 a 47.4 a 53.6 a
12.0 ~ 10.3 a 30.3 b 24.5 b
9.4 ab 7.6 a 10.8 b 11.0 b
1/ 6 2/6 5/6 6/6
T Kr Ka+B Ka
10.5 a 19.6 b 28.2 c 30.6 c
0.6" 5.7 ab 10.6 bc 15.7 c
C Unvaccinated
35.8 c 49.5 d
16.4 b~ 37.0 d
0.4 a 0.8 ab 2.5 b 3.0 be 4.9 ~ 8.2 d
0/7 0/7 0/7 0/7 0/7 6/7
T Ka+B Ka Unvaccinated
28.0" 38.7 bc 41.7 c 56.1 d
5.1 a 14.6 ab 21.1 b 23.0 b
4.1 ab 5.1 bc 4.6 abe 6.2 c
0/7 3/7 3/8 7/7
T Kr Unvaccinated
15.8 a 25.5 a 50.1 b
1.5 a 9.7 b 18.8 ¢
0.8 a 3.5 a 8.7 b
0/6 1/ 6 4/6
For each trial, w i t h i n columns, m e a n s w i t h o u t c o m m o n s upe rs c ri pt s are significantly different (P
45
Investigations of breakdowns in protection provided by living Babesia bovis vaccine R.E. Bock a, A.J. de Wos a, T.G. Kingston a, I.A. Shielsb and R.J. Dalgliesh ¢ aTick Fever Research Centre, Queensland Department of Primary Industries, Wacol, Qld. 4076, Australia bSilkstone Veterinary Hospital, Blackstone Road, Silkstone, Qld. 4305, Australia CAnimal Research Institute, Queensland Department of Primary Industries, Yeerongpilly, Qld, 4105, Australia (Accepted 15 November 1991 )
ABSTRACT Bock, R.E., de Vos, A.J., Kingston, T.G., Shiels, I.A. and Dalgliesh, R.J., 1992. Investigations of breakdowns in protection provided by living Babesia boris vaccine. Vet. Parasitol., 43: 45-56. Field investigations of protection afforded by live Babesia bovis vaccine in Australia revealed that a ninefold increase in vaccine failures occured in the period from 1985 to 1990. Laboratory trials using 189 experimental cattle were conducted to evaluate the protection afforded by the Babesia boris strain used in the commercial vaccine during this time. Four isolates from clinical cases of babesiosis in vaccinated cattle were assessed. The results showed that the strain used in the vaccine during the 5 year period was poorly protective against three isolates while a recently isolated and prepared vaccine strain was strongly protective. Circumstantial evidence is provided that indicates the vaccine failures were due to change in the field populations of Babesia boris, rather than change in the strain used in the vaccine. Implications of the results for the future ofBabesia bovis vaccines are discussed.
INTRODUCTION
Vaccination of cattle against disease caused by the intra-erythrocytic protozoan parasite Babesia bovis is commonly practised in areas of northern Australia where the tick vector Boophilus microplus is endemic (Callow and Dalgliesh, 1980). From 1964 to the present time a living vaccine produced by infecting splenectomised calves with a Babesia bovis strain of reduced virulence has been used (Callow and Mellors, 1966). During this time, seven strains isolated from natural infections in different areas of Queensland have been modified for use in vaccine (Callow et al., 1979; Callow and Dalgliesh, 1980). Correspondence to: R.E. Bock, Tick Fever Research Centre, Queensland Department of Primary Industries, 280 Grindle Road, Wacol, Qld. 4076, Australia.
© 1992 Elsevier Science Publishers B.V. All rights reserved 0304-4017/92/$05.00
46
R.E. BOCK ET AL.
In general, the vaccine has been very effective. However, in 1968 and again in 1976, the number of confirmed cases of babesiosis in vaccinated cattle in Queensland indicated that the degree of protection provided by the vaccine had decreased. Although there was little evidence to support the concept, a decision was made in 1976 to restrict syringe passaging in splenectomised calves in case this narrowed the immunogenic range of the vaccine (Callow and Dalgliesh, 1980; Anonymous, 1981 ). This was done by returning to a bank of stabilate of a lower passage number once direct passages in splenectomised calves had reached an arbitrary cut-off number of about 30. This decision coincided with the introduction of a new vaccine strain designated Ka strain. One or both factors apparently contributed to a notable improvement in vaccine efficacy at that time. From December 1979 to October 1989, Ka strain was the sole Babesia bovis strain used in the commercial vaccine. However, during 1988-1990 there was again convincing evidence of an abnormal and increasing number of Babesia boris vaccine failures. This paper describes our initial studies in ongoing research to clarify reasons for the vaccine failures and to prevent their reoccurrence. It presents an overview of vaccine failures since 1988. It also describes laboratory experiments designed to verify the failure of Ka strain to protect cattle against certain field strains of Babesia bovis, and to identify a suitable replacement strain for use as a vaccine. MATERIALS AND METHODS
Field investigation of vaccine failure The Tick Fever Research Centre (TFRC), near Brisbane, is responsible for the production and distribution of live Babesia vaccines and for the investigation of suspected outbreaks of babesiosis. Records kept of vaccine supply can therefore be readily correlated with information on disease outbreaks. All cases of suspected vaccine failure are investigated by laboratory staff in collaboration with government field officers and private veterinary practitioners. Failure ofBabesia bovis vaccine on a property is defined as the occurrence of death or acute sickness from laboratory-confirmed infection in one or more cattle immunised at least once with Babesia bovis vaccine. Vaccination and breakdown records were examined for the period 1973-1990. Vaccine failures were investigated in detail on 24 properties since 1988 with 11 of these properties being used to investigate alternative vaccination strains and regimes. Isolates were obtained from six of these properties with four isolates being used in trials described in this paper.
B R E A K D O W N S IN P R O T E C T I O N BY B. BOITS VACCINE
47
Experimental cattle At the laboratory, a total of 189 Bos taurus cattle was used in eight separate trials. In seven of the trials, steers aged 15-18 months and derived from an area of Queensland free from Boophilus microplus, were used. Cattle in the remaining trial were 8-month-old steers, seronegative to Babesia bovis and originating from a property experiencing vaccine breakdowns in the tick-infested part of Queensland. The cattle were maintained in grazing paddocks at TFRC under conditions that precluded infestation with ticks. Within each trial, cattle were randomly allotted to treatment groups of six to eight animals per group. Splenectomised Bos taurus calves 2-6 months of age were used to prepare infective blood. The calves originated from the same area as the steers and were housed in concrete-floored pens. All cattle were shown to be seronegative to Babesia bovis in an indirect fluorescent antibody test (IFAT) prior to commencement of trials (Callow et al., 1986).
Strains and isolates ofBabesia bovis For this paper the strains of reduced virulence used as protective inocula in experimental cattle are referred to as vaccine strains and those used for challenge infections as field isolates. Five vaccine strains (Table 1 ) and five virulent field isolates (Table 2 ) of Babesia bovis were used in the study. All were isolated in Queensland or New South Wales over a period of 24 years and stored as frozen stabilates in liquid nitrogen with dimethyl sulphoxide (DMSO) as cryoprotectant (Dalgliesh, 1971 ). Thawed stabilate of each strain or isolate was used to infect splenectomised calves and blood of these calves was then used to prepare experimental vaccines or challenge inocula, respectively. All vaccine strains used in Trials l, 6, 7, 8 and the B strain used in Trial 3, were prepared in frozen form, using glycerol as the cryoprotectant (Jorgensen et al., 1989 ). The actual infecting dose of parasites was, therefore, not known. In the other trials, the strains were prepared as for chilled vaccine (Rodwell et al., 1980), containing 1 × 107 organisms per 2 ml dose.
Inoculation of cattle Cattle in each of the trials were vaccinated with the appropriate vaccine strain and checked for seroconversion by IFAT for Babesia boris. In some trials, groups were revaccinated 1 month later with a second, heterologous strain.
48
R.E. BOCK ET AL.
TABLE l History of strains used as vaccine Strain Date of Origin isolation BL
c. 1963
L
1965
C~
1967
Ticks from outbreak in herd near Springsure in central Queensland
Ka I
1975
Kr L
1975
Blood from chronically infected cow, Maryborough, southeast Queensland As for Ka
T~
1987
Period of use in vaccine (max. passage no. )
Ticks from Beaudesert 1966-1976 district, southeast ( > 90 ) Queensland Blood from field case in Not used northern New South (unknown) Wales
Blood from chronically infected steer from property where vaccine was not used, Townsville, north Queensland
1968-1970 (56) 1976-1979 (30) Dec. 1976-Oct. 1989 ( < 30 ) Oct. 1989-Sept. 1990 ( < 30 )
Sept. 1990 on ( < 30)
Comments
Ref.
First strain attenuated and used in vaccine
O'Sullivan and Callow, 1966
Attenuated by irradiation. Used by CSIRO 2 as research tool Used in rotation with K a f r o m 1976to 1979
Mahoney et al., 1973 Wright et al., 1980 Trueman and Blight, I978; Anonymous, 1981
Sole B. bovis vaccine strain from 1980 to 1989
Callow and Dalgliesh 1980; Anonymous, 1981
Generated by returning Present study to stabilate from passage 15 of original K and reattenuating to passage 20 Present study
~Attenuated by rapid passage in splenectomised calves. 2Commonwealth Scientific and Industrial Research Organisation.
All immunised cattle were challenged after 2 - 4 months by intravenous inoculation with 1 X 108 parasites of an appropriate field isolate.
Measurement of responses to challenge To assess challenge reactions, parasitaemia, fever, depression in packed cell volume (PCV) and the need for specific therapy to control infections were measured according to the methods of Callow and Pepper ( 1974 ) and Timms etal. (1983). Cattle requiring treatment (Timms et al., 1983 ) received quinuronium sulphate (Ludobal, Bayer, Leverkusen, Germany) or imidocarb dipropionate (Imizol, Coopers Animal Health, Australia) both at dose rates of 1 mg kg- ~. No allowance for treatment was made in analysing other data.
49
BREAKDOWNS IN PROTECTION BY B. BOVIS VACCINE
TABLE 2 History of challenge isolates Isolate
Date of isolation
Origin
Comments
Ref.
R
1971
Clinical case, Charters Towers, north Queensland
Trueman and Blight, 1978
W
1988
Clinical case in Ka vaccinated herd, Biloela, central Queensland
Nambour
1988
Dayboro
1990
As for W isolate
Present study
Veresdale
1990
Clinical case in Ka vaccinated herd, Nambour, south east Queensland Clinical case in Ka vaccinated herd, Dayboro, southeast Queensland Clinical case in Ka vaccinated herd, Veresdale, southeast Queensland
Virulent standard challenge isolate used in trials at TFRC ~. Maintained as stabilate following passage in intact calf Virulent. Maintained as stabilate of blood from splenectomised calf infected with original inoculum As for W isolate
As for W isolate
Present study
Present study
Present study
~Tick Fever Research Centre.
The significance of differences between groups in their responses to challenge was determined by analysis of variance. EXPERIMENTS AND RESULTS
Field investigations The number of vaccine failures confirmed annually is shown in Table 3. The actual n u m b e r of failures is undoubtedly much higher owing to under reporting and lack of laboratory confirmation of suspected outbreaks. From these investigations the following information emerged: (i) cattle on all properties were routinely vaccinated at least twice; (ii) failures did not correlate with the time after initial vaccination, which varied from 10 months to 7 years in one outbreak; (iii) only Bos taurus cattle were involved; (iv) some failures occurred on neighbouring properties or on properties in the same ownership; (v) rapidly spreading outbreaks were seen on affected properties
R.E. BOCKET AL.
50 TABLE 3 Vaccine failures (Babesia bovis ) related to vaccine supply 1973-1991 Year
Strain in vaccine
No. of vaccine failures
No. of vaccine orders
Vaccine failures per 1000 orders
1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 j 19912
A and B A and B A and B A and B C and Ka C and Ka C a n d Ka Ka Ka Ka Ka Ka Ka Ka Ka Ka K a a n d Kr Kr T
17 15 10 20 6 6 7 3 6 6 5 7 5 8 10 16 24 33 23
9248 9542 5477 5385 4510 4622 5893 6323 6383 6190 7391 7017 8046 7459 7448 8586 9558 6057 7264
1.84 1.99 1.83 3.76 1.33 1.30 1.19 0.47 0.94 0.97 0.68 1.00 0.62 1.07 1.34 1.86 2.51 5.40 0.27
Lln September 1990 Kr strain was replaced by T strain in the vaccine. Therefore the figures for 1990 are until the end of August only (8 months). 2The figures for 1991 are actually from September 1990 to the end of June 1991 ( 10 months ). -~Vaccine failures for 1991 are for T strain vaccinates only and do not include 40 suspected K strain failures during the same period.
in both resident seropositive cattle and in introduced vaccinated cattle; (vi) on most properties, Ka strain vaccine was used, but Kr vaccine also failed on three properties where its use was closely monitored.
Laboratory study to determine if the protection afforded by Ka strain had altered To achieve this objective a challenge trial was set up using the Centre's traditional challenge isolate (R isolate) against which Ka strain was shown to be protective on a number of occasions during the 1980s (Timms et al., 1983; de Vos et al., 1986 ). The trial design and results are shown in Table 4. The protection afforded by both Ka and Kr strains did not differ significantly from that of the recently isolated T strain.
BREAKDOWNS IN PROTECTION BY B. BOVIS VACCINE
51
TABLE 4 Effect o f v a c c i n a t i o n w i t h v a r i o u s Babesia bovis v a c c i n e s t ra i ns on s u b s e q u e n t challenge w i t h field isolate R Trial
Vaccination history
M e a n max. PCV d e p r e s s i o n (%)
M e a n total parasit, score
M e a n total temp. rise (°C)
No. r e q u i r i n g treatment
T Ka Kr Unvaccinated
35.3 a 37.8 a 33.7 a 55.9 b
6.4 ~ 7.6 a 5.2 a 23.0 b
2.4 a 3.3 a 3.0 a 8.9 b
0/7 0/7 0/6 l 6/7
~One a n i m a l d i d not s e r o c o n v e r t after v a c c i n a t i o n . W i t h i n c o l u m n s m e a n s w i t h o u t c o m m o n superscripts are s i gni fi c a nt l y different ( P < 0.05 ). TA BLE 5 Effect of v a c c i n a t i o n with v a r i o u s Babesia bovis v a c c i n e strains on s u b s e q u e n t challenge w i t h field b r e a k t h r o u g h isolate W Trial
Vaccination h isto ry
M e a n max. PCV depression (%)
M e a n total parasit, score
M e a n total temp. rise (°C)
No. r e q u i r i n g treatment
Ka L B Unvaccinated
40.9 a 43.2 a 47.4 a 53.6 a
12.0 ~ 10.3 a 30.3 b 24.5 b
9.4 ab 7.6 a 10.8 b 11.0 b
1/ 6 2/6 5/6 6/6
T Kr Ka+B Ka
10.5 a 19.6 b 28.2 c 30.6 c
0.6" 5.7 ab 10.6 bc 15.7 c
C Unvaccinated
35.8 c 49.5 d
16.4 b~ 37.0 d
0.4 a 0.8 ab 2.5 b 3.0 be 4.9 ~ 8.2 d
0/7 0/7 0/7 0/7 0/7 6/7
T Ka+B Ka Unvaccinated
28.0" 38.7 bc 41.7 c 56.1 d
5.1 a 14.6 ab 21.1 b 23.0 b
4.1 ab 5.1 bc 4.6 abe 6.2 c
0/7 3/7 3/8 7/7
T Kr Unvaccinated
15.8 a 25.5 a 50.1 b
1.5 a 9.7 b 18.8 ¢
0.8 a 3.5 a 8.7 b
0/6 1/ 6 4/6
For each trial, w i t h i n columns, m e a n s w i t h o u t c o m m o n s upe rs c ri pt s are significantly different (P
