- Email: [email protected]
Investigations of Acquired Immunity to Trichostrongylus Axei Infections in Seven-Month-Old Lambs
INVESTIGATIONS OF ACQUIRED IMMUNITY TO TRICHOSTRONGYLUS AXEl INFECTIONS IN SEVEN-MONTH-OLD LAMBS By]. G. Ross* Veterinary Research Laboratories, Stormont, Belfast, Northern Ireland
SUMMARY
The immunity acquired by seven-month-old lambs to challenge infection with T. axei is compared following two levels of initial infection in the presence and absence of initial infections at the time of challenge. Low initial infections produced a greater immunity than high initial infections. The absence of a patent infection at the time of challenge infection did not alter the immunity acquired following low initial infections but reduced the immunity acquired following high initial infections. INTRODUCTION
Previous investigations have demonstrated that lambs develop an increasing competence to acquire immunity to reinfection with Trichostrongylus axei between 2 and 5 months of age and that competence develops rapidly between 5 and 7 months of age (Ross, 1970a). In an earlier trial where lambs were initially infected with T. axei at 10 weeks of age and subsequently challenged at 5 months of age, with and without persisting patent infections, the results indicated that the presence of a patent infection at the time of ch allenge did not materially alter the level of immunity acquired (Ross, I970b). In the present investigation the influence of the presence or absence of patent infection at the time of challenge is re-examined in lambs of 7 months of age. The effect of the level of initial infection is also considered. MATERIALS AND METHODS
Eight cross-Blackface lambs were reared parasite-free and at 7 months of age 4 lambs were infected orally with 5000 third-stage T. axei infective larvae and 2 with 50,000 third-stage infective larvae. Four weeks later, 2 of the lambs given 5000 third-stage larvae and I of the lambs given 50,000 third-stage larvae were treated with thiabendazole t (80 mg. jkg.), and two weeks later all of the lambs, plus 2 lambs which were not initially infected, were ch allenged with 100,000 third-stage larvae. All lambs were slaughtered four weeks after challenge infection, and at post-mortem examination the pH of the abomasal • Present address: Ministry of Agriculture, Fisheries & Food, Veterinary Laboratory, Lasswade, Midlothian, Scotland.
t Thibenzole: Merck, Sharp &
Dohme Ltd.
ACQUIRED IMMUNITY TO T. AXEl IN LAMBS
fluid was recorded, the numbers of worms counted and a sample of male and female worms measured as previously described (Ross, 1963). The strain of larvae used was S6B, which was a strain continuously passaged in Blackface and Blackface cross lambs and which had previously been shown to give a consistent p ercentage larval establishment of around 50 per cent and to have a high pathogenicity for lambs (Ross, 1970b). RES U LT S
Details of the treatments and worm burdens at post-mortem are shown in Table I. In the lambs given only one larval infection the numbers of larvae which became established was 50 per cent of that administered. In the two lamb groups initially infected with 5000 larvae the immunity acquired resulted in a 90 per cent reduction in the numbers of challenge larvae which became es tablished compared to the group not initially infected . The immunity acquired following an initial infection with 5000 larvae was therefore similar, irrespective of the presence of a pa tent initial infection at the time of ch allenge, although in the group where the initial infection was removed prior to challenge there was some evidence of a greater vari ation in the expression of immunity. In the lamb in which the 50,000 initial larval infection was removed prior to challenge, the numbers of parasites present at post-mortem was only reduced by 40 per cent of that found in the non-immune lambs. In lamb 95, which was initially infected with 50,000 larvae but in which the initial infec tion was not removed prior to challenge, the worm burden at post-mortem was reduced TABLE I DETAILS OF TREATMENT AND WORM BURDENS I N
Group
Lamb /lumber
Initial larvae i,!!ection (Day I )
Anthelmintic treatment (4th week )
Challenge larvae inJectioll (6th week )
Nil
~il
100,000
89
Control
5
37 Mean 65
Thiabendazole A - - - - -
5,000
Treated
Mean 5 0 ,000
Patent A
5,000
Treated
100,000
N il
100,000
94
Patent B
95
5 0 ,000
Nil
Number of parasites in abomasum ( loth week )
Percelltage a/ immature parasites
43,060
0
59,020
0
5 [ ,040
0
[0,200
0
100,000
93
Thiabendazole B
LAMB GROUPS
[00
100
5,[50
50
29,9 20
2
3,200
0
5,600
0
Mean
4,400
0
100,000
[2 ,800
10
65 0
BRITISH VETERINARY JOURNAL, 126, 12
by 75 per cent compared to the non-immune lambs. In both the lambs which received initial infection of 50,000 larvae there was a greater proportion of immature parasites present at post-mortem compared to the other lambs and the mean length of both male and female worms was significantly less than in other lamb groups. DISCUSSION
The results of the present investigation show that lambs possess a high level of immune competence at 7 months of age, and confirm the previous indication that the presence of a patent infection at the time of challenge is not essential to the expression of an acquired immunity (Ross, I970b). Consideration of these results in conjunction with previous investigations illustrates that lambs develop an increasing immune competence between 5 and 7 months and suggest that lambs initially infected at 7 months and subsequently challenged have a greater competence to acquire immunity than lambs initially infected prior to 6 months and subsequently challenged at 7 months of age. The results suggest that the level of initial infection may influence the expression of acq uired immunity to challenge infection, in that lambs initially given a larval infection of 50,000 third-stage larvae had higher worm burdens at post-mortem than lambs given 5000 third-stage larvae, but the small number oflambs used in the present investigation prevent a definite conclusion on this point. There is also evidence of an interaction between the size of initial infection and the presence of a patent infection at challenge, in that higher infections in the absence of patent infections may decrease resistance to challenge, but again this conclusion must be tentative. Previous investigations have suggested that in very young lambs initial infections may result in an immune block to the expression of acquired immunity to challenge and that challenge infections may result in higher larval establishments with higher initial infections (Gibson, 1952; Ross, I970a). There is clearly a need for further investigations of this aspect of acquired immunity to determine the effect of varying levels of initial infection on the subsequent establishment of a challenge infection. ACKNOWLEDGMENTS
I wish to acknowledge the technical assistance of Mr Staff of the Parasitology Department.
J.
Kenny and the
REFERENCES GIBSON, T. E. (1952). J. Helminth., 26, 43· Ross, J. G. (1963) . Vet. R ec., 75, 129. Ross, J. G. ( 1970a).]. Helminth, in press. Ross, ]. G. (1970b). Br. vet. ]., I26, xxi.
Recherches sur l'inununite acquise a suite d'infections par les Trichostrongylus axe; d'agneaux de sept Inois (Ross) Resunle. L'immunite acquise par les agneaux de sept mois testee par une infection provo cat-
ACQUIRED IMMUNITY TO T . AXEl IN LAMBS rice a l'aide de T. axei est comparee apres deux taux d'infection initiale en presence et absence d'infections initiales au moment de la provocation. Des infections initiales faibles produisent une immunite superieure a celie des infections initiales fortes. L'absence d'une infection evidente au moment de l'infection provocatrice n'a pas altere l'immunite acquise apres des infections initiales faibles mais a reduit l'immunite acquise apres des infections initiales fortes. U ntersuchungen tiber erworbene Inununitat gegen Trichostrongylus axeilnfektionen s ieben Monate a lter Lanuner (Ross) Zus anunenfassung . Die erworbene Immunitat sieben Monate alter Lammer gegen T. axei Infektionen wird verglichen nach zwei Arten von erster Infektion bei existierender oder nicht existierender erster Infektion zur Zeit der Attacke. Geringe erste Infektionen brachten eine gross ere Immunitat mit sich als es schwerere erste Infektionen taten. Die Abwesenheit einer deutlichen Infektion zur Zeit der attackierenden Infektion veranderte die erworbene Immunitat nach leichter erster Infektion nicht aber reduzierte die erworbene Immunitat nach schwerer erster I nfektion. Investigaciones sobre Ia imnunidad adquirida a infe cciones par Trichostrongylus axei en corderos de siete lIles es (Ross) R eSUlllen. La inmunidad adquirida por corderos de siete meses a infecciones de desario por T. axei se comparo a continua cion de dos niveles de infecci6n inicial en la presencia y ausencia de infecciones iniciales en el momento de aplicar la de desafio. Las infecciones iniciales bajas produjeron una mayor inmunidad que la obtenida con infecciones iniciales elevadas. La ausencia de una infeccion patente en el momenta de la infecci6n de desafio no a ltero la inmunidad adquirida despues de infecciones iniciales bajas, pero redujo la inmunidad adquirida despues de infecciones iniciales altas.
SUMMARY
The immunity acquired by seven-month-old lambs to challenge infection with T. axei is compared following two levels of initial infection in the presence and absence of initial infections at the time of challenge. Low initial infections produced a greater immunity than high initial infections. The absence of a patent infection at the time of challenge infection did not alter the immunity acquired following low initial infections but reduced the immunity acquired following high initial infections. INTRODUCTION
Previous investigations have demonstrated that lambs develop an increasing competence to acquire immunity to reinfection with Trichostrongylus axei between 2 and 5 months of age and that competence develops rapidly between 5 and 7 months of age (Ross, 1970a). In an earlier trial where lambs were initially infected with T. axei at 10 weeks of age and subsequently challenged at 5 months of age, with and without persisting patent infections, the results indicated that the presence of a patent infection at the time of ch allenge did not materially alter the level of immunity acquired (Ross, I970b). In the present investigation the influence of the presence or absence of patent infection at the time of challenge is re-examined in lambs of 7 months of age. The effect of the level of initial infection is also considered. MATERIALS AND METHODS
Eight cross-Blackface lambs were reared parasite-free and at 7 months of age 4 lambs were infected orally with 5000 third-stage T. axei infective larvae and 2 with 50,000 third-stage infective larvae. Four weeks later, 2 of the lambs given 5000 third-stage larvae and I of the lambs given 50,000 third-stage larvae were treated with thiabendazole t (80 mg. jkg.), and two weeks later all of the lambs, plus 2 lambs which were not initially infected, were ch allenged with 100,000 third-stage larvae. All lambs were slaughtered four weeks after challenge infection, and at post-mortem examination the pH of the abomasal • Present address: Ministry of Agriculture, Fisheries & Food, Veterinary Laboratory, Lasswade, Midlothian, Scotland.
t Thibenzole: Merck, Sharp &
Dohme Ltd.
ACQUIRED IMMUNITY TO T. AXEl IN LAMBS
fluid was recorded, the numbers of worms counted and a sample of male and female worms measured as previously described (Ross, 1963). The strain of larvae used was S6B, which was a strain continuously passaged in Blackface and Blackface cross lambs and which had previously been shown to give a consistent p ercentage larval establishment of around 50 per cent and to have a high pathogenicity for lambs (Ross, 1970b). RES U LT S
Details of the treatments and worm burdens at post-mortem are shown in Table I. In the lambs given only one larval infection the numbers of larvae which became established was 50 per cent of that administered. In the two lamb groups initially infected with 5000 larvae the immunity acquired resulted in a 90 per cent reduction in the numbers of challenge larvae which became es tablished compared to the group not initially infected . The immunity acquired following an initial infection with 5000 larvae was therefore similar, irrespective of the presence of a pa tent initial infection at the time of ch allenge, although in the group where the initial infection was removed prior to challenge there was some evidence of a greater vari ation in the expression of immunity. In the lamb in which the 50,000 initial larval infection was removed prior to challenge, the numbers of parasites present at post-mortem was only reduced by 40 per cent of that found in the non-immune lambs. In lamb 95, which was initially infected with 50,000 larvae but in which the initial infec tion was not removed prior to challenge, the worm burden at post-mortem was reduced TABLE I DETAILS OF TREATMENT AND WORM BURDENS I N
Group
Lamb /lumber
Initial larvae i,!!ection (Day I )
Anthelmintic treatment (4th week )
Challenge larvae inJectioll (6th week )
Nil
~il
100,000
89
Control
5
37 Mean 65
Thiabendazole A - - - - -
5,000
Treated
Mean 5 0 ,000
Patent A
5,000
Treated
100,000
N il
100,000
94
Patent B
95
5 0 ,000
Nil
Number of parasites in abomasum ( loth week )
Percelltage a/ immature parasites
43,060
0
59,020
0
5 [ ,040
0
[0,200
0
100,000
93
Thiabendazole B
LAMB GROUPS
[00
100
5,[50
50
29,9 20
2
3,200
0
5,600
0
Mean
4,400
0
100,000
[2 ,800
10
65 0
BRITISH VETERINARY JOURNAL, 126, 12
by 75 per cent compared to the non-immune lambs. In both the lambs which received initial infection of 50,000 larvae there was a greater proportion of immature parasites present at post-mortem compared to the other lambs and the mean length of both male and female worms was significantly less than in other lamb groups. DISCUSSION
The results of the present investigation show that lambs possess a high level of immune competence at 7 months of age, and confirm the previous indication that the presence of a patent infection at the time of challenge is not essential to the expression of an acquired immunity (Ross, I970b). Consideration of these results in conjunction with previous investigations illustrates that lambs develop an increasing immune competence between 5 and 7 months and suggest that lambs initially infected at 7 months and subsequently challenged have a greater competence to acquire immunity than lambs initially infected prior to 6 months and subsequently challenged at 7 months of age. The results suggest that the level of initial infection may influence the expression of acq uired immunity to challenge infection, in that lambs initially given a larval infection of 50,000 third-stage larvae had higher worm burdens at post-mortem than lambs given 5000 third-stage larvae, but the small number oflambs used in the present investigation prevent a definite conclusion on this point. There is also evidence of an interaction between the size of initial infection and the presence of a patent infection at challenge, in that higher infections in the absence of patent infections may decrease resistance to challenge, but again this conclusion must be tentative. Previous investigations have suggested that in very young lambs initial infections may result in an immune block to the expression of acquired immunity to challenge and that challenge infections may result in higher larval establishments with higher initial infections (Gibson, 1952; Ross, I970a). There is clearly a need for further investigations of this aspect of acquired immunity to determine the effect of varying levels of initial infection on the subsequent establishment of a challenge infection. ACKNOWLEDGMENTS
I wish to acknowledge the technical assistance of Mr Staff of the Parasitology Department.
J.
Kenny and the
REFERENCES GIBSON, T. E. (1952). J. Helminth., 26, 43· Ross, J. G. (1963) . Vet. R ec., 75, 129. Ross, J. G. ( 1970a).]. Helminth, in press. Ross, ]. G. (1970b). Br. vet. ]., I26, xxi.
Recherches sur l'inununite acquise a suite d'infections par les Trichostrongylus axe; d'agneaux de sept Inois (Ross) Resunle. L'immunite acquise par les agneaux de sept mois testee par une infection provo cat-
ACQUIRED IMMUNITY TO T . AXEl IN LAMBS rice a l'aide de T. axei est comparee apres deux taux d'infection initiale en presence et absence d'infections initiales au moment de la provocation. Des infections initiales faibles produisent une immunite superieure a celie des infections initiales fortes. L'absence d'une infection evidente au moment de l'infection provocatrice n'a pas altere l'immunite acquise apres des infections initiales faibles mais a reduit l'immunite acquise apres des infections initiales fortes. U ntersuchungen tiber erworbene Inununitat gegen Trichostrongylus axeilnfektionen s ieben Monate a lter Lanuner (Ross) Zus anunenfassung . Die erworbene Immunitat sieben Monate alter Lammer gegen T. axei Infektionen wird verglichen nach zwei Arten von erster Infektion bei existierender oder nicht existierender erster Infektion zur Zeit der Attacke. Geringe erste Infektionen brachten eine gross ere Immunitat mit sich als es schwerere erste Infektionen taten. Die Abwesenheit einer deutlichen Infektion zur Zeit der attackierenden Infektion veranderte die erworbene Immunitat nach leichter erster Infektion nicht aber reduzierte die erworbene Immunitat nach schwerer erster I nfektion. Investigaciones sobre Ia imnunidad adquirida a infe cciones par Trichostrongylus axei en corderos de siete lIles es (Ross) R eSUlllen. La inmunidad adquirida por corderos de siete meses a infecciones de desario por T. axei se comparo a continua cion de dos niveles de infecci6n inicial en la presencia y ausencia de infecciones iniciales en el momento de aplicar la de desafio. Las infecciones iniciales bajas produjeron una mayor inmunidad que la obtenida con infecciones iniciales elevadas. La ausencia de una infeccion patente en el momenta de la infecci6n de desafio no a ltero la inmunidad adquirida despues de infecciones iniciales bajas, pero redujo la inmunidad adquirida despues de infecciones iniciales altas.