Parasitology (1975), 71, 349-355
349
The influence of Nematospiroides dubius on subsequent Nippostrongylus brasiliensis infections in mice D. CONWIL JENKINS Pharmaceutical Research Laboratories, May & Baker Ltd, Dagenham, Essex (Received 18 March 1975) SUMMARY
The fecundity and longevity of Nippostrongylus brasiliensis was prolonged in mice previously infected with Nematospiroides dubius only when the former developed from the larval stage in those mice. Such worms appeared to be less immunogenic than worms which developed in mice never exposed to N. dubius. It is proposed that prolonged fecundity and longevity resulted from an adaptation undertaken by the worms in the face of host antibodies which had been developed against the pre-existing N. dubius infection. INTRODUCTION
Colwell & Wescott (1973) noted that the egg production and longevity of N. brasiliensis was significantly prolonged in mice concurrently infected with N. dubius. They concluded, therefore, that N. dubius changed the threshold level of survival of N. brasiliensis in mice. A plausible explanation for these phenomena was put forward by Bartlett & Ball (1974) who suggested that N. dubius may stimulate the production of blocking antibodies and that the presence of these antibodies may afford protection from the host immune response for both N. dubius and N. brasiliensis. To date, however, no evidence in support of this theory has been published in the literature. The aim of the present work was to look more critically at the circumstances which resulted in the prolongation of the fecundity and longevity of N. brasiliensis. Also it was hoped that the results obtained might help to determine what mechanisms are involved with such phenomena. MATERIALS AND METHODS
Experimental hosts Young sexually mature male albino mice of the M & B Schofield-derived strain and adult male albino Sprague-Dawley rats were used. They were housed in metal cages and fed on a dry ' pellet' feed (Christopher Hill PMD diet) and water both of which were given ad lib.
350
D. CONWIL JENKINS
Parasites Both iV. brasiliensis and JV. dubius were obtained from the Wellcome Laboratories, Euston Road, London in 1958 and 1961 respectively. They were subsequently maintained in our laboratories. Experimental infections Mice were each given, by the oral route, 50 JV. dubius larvae suspended in water. The larvae of JV. brasiliensis were given subcutaneously by injection with a small volume (0-2-0-4 ml) of distilled water. Transfer of worms Worms for transfer to one host from another were placed in 1-0 ml plastic syringes fitted with short (1 cm) wide-bore hypodermic needles. Each recipient mouse was anaesthetized with pentobarbitone sodium and the worms were introduced into the duodenum after laparotomy. These procedures were carried out under clean but not aseptic conditions. Faecal egg counts
Egg counts were taken from the pooled faeces of all the mice in each group and expressed as the number of eggs per gramme of whole faeces. Details of the techniques used are given in a previous publication (Jenkins & Phillipson, 1971). Eggs of JV. brasiliensis were differentiated from those of JV. dubius by their relatively small size. Anthelmintic treatment The drug used was an aqueous solution of pyrantel tartrate (Banminth-Pfizer Ltd). This was dosed by the oral route at the rate of 50 mg/kg bodyweight which was more than adequate to remove all the worms. EXPERIMENTS
Experiment 1 This was designed to determine whether the phenomena of prolonged fecundity and longevity of JV. brasiliensis seen in mice harbouring JV. dubius were also manifest when the population of JV. dubius was removed from the mice prior to infection with JV. brasiliensis. Thirty mice of similar age divided into 3 groups (A, B and C) each consisting of 10 individuals were used. These had previously been treated as follows: group A: each infected with 50 JV. dubius larvae 28 days previously; group B: each infected with JV. dubius as group A but subsequently treated with an anthelmintic on day 25 after infection; group C: not infected with JV. dubius. Each mouse was infected with 200 JV. brasiliensis larvae and daily egg counts were taken from the pooled faeces of each group on days 6-21 after infection. On day 21
Influence o/N. dubius on N. brasiliensis infections
351
Table 1. Egg production of Nippostrongylus brasiliensis in mice from day 6 to 21 after infection (experiment 1) No. N. brasiliensis eggs per gramme faeces Days after infection with N. brasiliensis (200 larvae)
f
~
~~
Group A: mice also harbouring N. dubius
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Group B : mice previously harbouring N. dubius
, Group C: mice never exposed to N. dubius
260
600
860
3460 2800 3400 3130 2130 1300
6460 3800 3000 4150 2730 1860 1600
1660
530
1060 200 660 300 700
900 700 430 600 530
— 130
— 400
16 0 0 0 0 0 0 0 0 0 0 — — 0
Table 2. Mean number adult Nippostrongylus brasiliensis recovered from mice on day 21 after infection (experiment 1) Mean number N. brasiliensis recovered 21 days after infection Group
No. of mice
Male worms
Female worms
Total worms + S.D.
A* Bf CJ
8 10 10
1-6 2-3 0
3-0 7-2 0
4-62 ±4-20 9-50± 10-64 0
* Mice also harbouring adult N. dubius. f Mice previously harbouring adult N. dubius. J Mice never exposed to N. dubius.
all the mice were killed and the number of worms harboured by each mouse was recorded. The results are given in Tables 1 and 2. The N. brasiliensis in the mice from groups A and B continued to produce eggs at a relatively high level for the duration of the experiment. However, those from group C stopped passing eggs by day 8 after infection. Means of 4-6 and 9-5 worms were recovered from the mice of groups A and B respectively whilst no worms were found in any mice from group C. Experiment 2 This was to determine whether the fecundity and longevity of populations of N. brasiliensis that developed to maturity in hosts never exposed to N. dubius were prolonged after they were transferred into mice harbouring N. dubius.
352
D. CONWIL JENKINS
Table 3. Egg production of Nippostrongylus brasiliensis after transfer into mice (experiment 2) No. N. brasiliensis eggs per gramme pooled faeces 1
Days after transfer of N. brasiliensis worm populations (40 3 and 60 $)
Group 1: worms from rats into mice harbouring N. dubius
Group 2: worms from rats into mice never exposed to N. dubius
Group 3: worms from mice into mice harbouring N. dubius
Group 4: worms from mice into mice never exposed to N. dubius
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
1600 1700
1170 1460 1600 1060
4130
3730 3860
260 800 400 430 360 200 400 0 330 130 0 0 0 0
760
1250
330 200 430 0 0 160 0 0 0 0 0 0 0 0
190 360 200 0 0 0 0 0 0 0 0 0 0
730 600 660 200 200 750 400 260 330 530 200 430
Table 4. Mean number of adult Nippostrongylus brasiliensis recovered 21 days after transfer into recipient mice (experiment 2) Mean number N. brasiliensis recovered 21 days after transfer Group
No. of mice
Male worms
Female worms
Total worms + S.D.
1*
9 9 10
2-66 1-66 0-30
5-55 4-11 0-70
10
0-40
0-80
8-22 ±5-53 5-77 + 7-27 1-0 (worms found in 1 mouse only) 1-20 (worms found in 2 mice only)
2t
n
* Worms t Worms % Worms § Worms
from from from from
rats into mice harbouring N. dubius. rats into mice never exposed to N. dubius. mice into mice harbouring N. dubius. mice into mice never exposed to N. dubius.
Mice which had never been exposed to N. dubius and rats were each infected with about 200 N. brasiliensis larvae on day 0. These were the donors. On day 6 after infection their worms were transferred after laparotomy into the small intestine of recipient mice, twenty of which harboured about 40 N. dubius and twenty of which had never been exposed to the latter parasite. Ten of the mice harbouring N. dubius each received 40 male and 60 female N. brasiliensis derived from mice and ten received a similar number of worms derived from rats. Similarly, ten of the mice free of N. dubius received worms from mice and ten worms
Influence o/N. dubius on N. brasiliensis infections
353
Table 5. Egg production of Nippostrongylus brasiliensis after transfer into clean, previously uninfected mice {experiment 3) Number of eggs recovered per gramme pooled faeces Days after transfer of N. brasiliensis worm population (40 cj and 60 $) 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
e
Group A: worms transferred from mice also harbouring N. dubius 1530 1600 1860 1200 1200 1170 730
3300 1060 800 — — 400 700 430 260 700
Group B : worms transferred from mice never exposed to N. dubii 5700 5000 4000 830 760 300 430 60 60 0 — 0 0 0 0 0
derived from rats. The fecundity and longevity of the worms from each of the 4 groups of mice were then compared. The results are given in Tables 3 and 4. Worms derived from mice and those derived from rats were no more fecund or long-lived after transfer into mice harbouring N. dubius than those transferred into mice which had never been exposed to that parasite. Experiment 3 This final experiment was to determine whether populations of N. brasiliensis, which developed to maturity in mice harbouring N. dubius, produced eggs for longer than ' normal' worms after transfer into the intestines of clean, previously unparasitized mice. The egg production of twenty populations of N. brasiliensis each consisting of 40 male and 60 female worms derived on day 6 after infection from mice also harbouring a 28-day-old N. dubius population, and mice which had never been exposed to N. dubius, were compared after transfer into the intestines of two groups each of 10 previously uninfected recipient mice. The results are given in Table 5. Those populations of N. brasiliensis that previously developed to maturity in the presence of N. dubius continued to pass eggs for the duration of the experiment (21 days after transfer into clean mice) whilst those which had developed in mice never exposed to N. dubius stopped producing eggs by day 14.
354
D. CONWIL JENKINS DISCUSSION
It is evident from the present work that the salient requirement for the prolongation of the fecundity and longevity of populations of N. brasiliensis in mice is that the latter experience a previous infection with JV. dubius. The continuing presence of JV. dubius in the mice on infection with JV. brasiliensis is not, however, essential for these effects to occur. I t is also clear that fecundity and longevity are prolonged only if JV. brasiliensis develops from its larval stages in these mice, because worms which developed previously in mice never exposed to JV. dubius were not more fecund or longer lived after transfer into mice harbouring JV. dubius. Hence Bartlett & Ball's suggestion (1974) that JV. brasiliensis may be protected from the effects of host immunity by blocking antibodies stimulated by the coexisting JV. dubius population is not tenable in the present situation. It can be deduced from experiment 3 that populations of JV. brasiliensis which develop to maturity in mice previously infected with JV. dubius differ in certain important respects from ' normal' worms, because when the former were transferred into hosts which had no experience of JV. dubius their fecundity was nevertheless prolonged. This strongly suggests that the worms were less immunogenic and consequently better adapted to their hosts than' normal' worms. Populations of JV. brasiliensis which become established in rats after challenge infection are adapted to their host's immunity (Ogilvie & Hockley, 1968), and it is believed (Ogilvie, 1969) that this adaptation results from the exposure of the larvae to antibodies during their development in the host. Under certain circumstances this adaptation can considerably prolong both the fecundity and longevity of the worms (Jenkins & Phillipson, 1972a, b) their immunogenicity being significantly reduced (Jenkins, 1972). JV. dubius and JV. brasiliensis stimulate cross-resistance under certain experimental conditions (Jenkins, in preparation) which indicates that they may stimulate similar antibodies. Thus it is plausible to speculate that the prolonged fecundity and longevity of JV. brasiliensis in mice previously exposed to infection with JV. dubius results from adaptation to immunity stimulated in the host by the latter parasite. REFERENCES BARTLETT, A. & BALL, P . A. J . (1974). The immune response of the mouse to larvae and adults of Nematospiroides dubius. International Journal for Parasitology 4, 463—70. COLWELL, D. A. & WESCOTT, R. B. (1973). Prolongation of egg production of Nippostrongylus brasiliensis in mice concurrently infected with Nematospiroides dubius. Journal of Parasitology 59, 216. JENKINS, D. C. (1972). Nippostrongylus brasiliensis: observations on the comparative immunogenicity of adult worms from primary and immune-adapted infections. Parasitology 65, 547-50. JENKINS, D. C. & PHILLIPSON, R. F . (1971). The kinetics of repeated low-level infections of Nippostrongylus brasiliensis in the laboratory rat. Parasitology 62, 457-65. JENKINS, D. C. & PHILLIPSON, R. F . (1972a). Increased establishment and longevity of Nippostrongylus brasiliensis in immune rats given repeated small challenge infections. International Journal for Parasitology 2, 105-11. JENKINS, D. C. & PHILLIPSON, R. F . (19726). Evidence that the nematode Nippostrongylus brasiliensis can adapt to and overcome the effects of host immunity. International Journal for Parasitology 2, 353-9.
Influence of N. dubius on N. brasiliensis infections
355
B. M. (1969). Immunity to Nippostrongylus brasiliensis. Symposia of the British Society for Parasitology 7, 31-40. OGILVIE, B. M. & HOCKLEY, D. J. (1968). Effects of immunity on Nippostrongylus brasiliensis adult worms: reversible and irreversible changes in infectivity, reproduction and morphology. Journal of Parasitology 54, 1073-84. OGILVIE,
Printed in Great Britain
349
The influence of Nematospiroides dubius on subsequent Nippostrongylus brasiliensis infections in mice D. CONWIL JENKINS Pharmaceutical Research Laboratories, May & Baker Ltd, Dagenham, Essex (Received 18 March 1975) SUMMARY
The fecundity and longevity of Nippostrongylus brasiliensis was prolonged in mice previously infected with Nematospiroides dubius only when the former developed from the larval stage in those mice. Such worms appeared to be less immunogenic than worms which developed in mice never exposed to N. dubius. It is proposed that prolonged fecundity and longevity resulted from an adaptation undertaken by the worms in the face of host antibodies which had been developed against the pre-existing N. dubius infection. INTRODUCTION
Colwell & Wescott (1973) noted that the egg production and longevity of N. brasiliensis was significantly prolonged in mice concurrently infected with N. dubius. They concluded, therefore, that N. dubius changed the threshold level of survival of N. brasiliensis in mice. A plausible explanation for these phenomena was put forward by Bartlett & Ball (1974) who suggested that N. dubius may stimulate the production of blocking antibodies and that the presence of these antibodies may afford protection from the host immune response for both N. dubius and N. brasiliensis. To date, however, no evidence in support of this theory has been published in the literature. The aim of the present work was to look more critically at the circumstances which resulted in the prolongation of the fecundity and longevity of N. brasiliensis. Also it was hoped that the results obtained might help to determine what mechanisms are involved with such phenomena. MATERIALS AND METHODS
Experimental hosts Young sexually mature male albino mice of the M & B Schofield-derived strain and adult male albino Sprague-Dawley rats were used. They were housed in metal cages and fed on a dry ' pellet' feed (Christopher Hill PMD diet) and water both of which were given ad lib.
350
D. CONWIL JENKINS
Parasites Both iV. brasiliensis and JV. dubius were obtained from the Wellcome Laboratories, Euston Road, London in 1958 and 1961 respectively. They were subsequently maintained in our laboratories. Experimental infections Mice were each given, by the oral route, 50 JV. dubius larvae suspended in water. The larvae of JV. brasiliensis were given subcutaneously by injection with a small volume (0-2-0-4 ml) of distilled water. Transfer of worms Worms for transfer to one host from another were placed in 1-0 ml plastic syringes fitted with short (1 cm) wide-bore hypodermic needles. Each recipient mouse was anaesthetized with pentobarbitone sodium and the worms were introduced into the duodenum after laparotomy. These procedures were carried out under clean but not aseptic conditions. Faecal egg counts
Egg counts were taken from the pooled faeces of all the mice in each group and expressed as the number of eggs per gramme of whole faeces. Details of the techniques used are given in a previous publication (Jenkins & Phillipson, 1971). Eggs of JV. brasiliensis were differentiated from those of JV. dubius by their relatively small size. Anthelmintic treatment The drug used was an aqueous solution of pyrantel tartrate (Banminth-Pfizer Ltd). This was dosed by the oral route at the rate of 50 mg/kg bodyweight which was more than adequate to remove all the worms. EXPERIMENTS
Experiment 1 This was designed to determine whether the phenomena of prolonged fecundity and longevity of JV. brasiliensis seen in mice harbouring JV. dubius were also manifest when the population of JV. dubius was removed from the mice prior to infection with JV. brasiliensis. Thirty mice of similar age divided into 3 groups (A, B and C) each consisting of 10 individuals were used. These had previously been treated as follows: group A: each infected with 50 JV. dubius larvae 28 days previously; group B: each infected with JV. dubius as group A but subsequently treated with an anthelmintic on day 25 after infection; group C: not infected with JV. dubius. Each mouse was infected with 200 JV. brasiliensis larvae and daily egg counts were taken from the pooled faeces of each group on days 6-21 after infection. On day 21
Influence o/N. dubius on N. brasiliensis infections
351
Table 1. Egg production of Nippostrongylus brasiliensis in mice from day 6 to 21 after infection (experiment 1) No. N. brasiliensis eggs per gramme faeces Days after infection with N. brasiliensis (200 larvae)
f
~
~~
Group A: mice also harbouring N. dubius
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Group B : mice previously harbouring N. dubius
, Group C: mice never exposed to N. dubius
260
600
860
3460 2800 3400 3130 2130 1300
6460 3800 3000 4150 2730 1860 1600
1660
530
1060 200 660 300 700
900 700 430 600 530
— 130
— 400
16 0 0 0 0 0 0 0 0 0 0 — — 0
Table 2. Mean number adult Nippostrongylus brasiliensis recovered from mice on day 21 after infection (experiment 1) Mean number N. brasiliensis recovered 21 days after infection Group
No. of mice
Male worms
Female worms
Total worms + S.D.
A* Bf CJ
8 10 10
1-6 2-3 0
3-0 7-2 0
4-62 ±4-20 9-50± 10-64 0
* Mice also harbouring adult N. dubius. f Mice previously harbouring adult N. dubius. J Mice never exposed to N. dubius.
all the mice were killed and the number of worms harboured by each mouse was recorded. The results are given in Tables 1 and 2. The N. brasiliensis in the mice from groups A and B continued to produce eggs at a relatively high level for the duration of the experiment. However, those from group C stopped passing eggs by day 8 after infection. Means of 4-6 and 9-5 worms were recovered from the mice of groups A and B respectively whilst no worms were found in any mice from group C. Experiment 2 This was to determine whether the fecundity and longevity of populations of N. brasiliensis that developed to maturity in hosts never exposed to N. dubius were prolonged after they were transferred into mice harbouring N. dubius.
352
D. CONWIL JENKINS
Table 3. Egg production of Nippostrongylus brasiliensis after transfer into mice (experiment 2) No. N. brasiliensis eggs per gramme pooled faeces 1
Days after transfer of N. brasiliensis worm populations (40 3 and 60 $)
Group 1: worms from rats into mice harbouring N. dubius
Group 2: worms from rats into mice never exposed to N. dubius
Group 3: worms from mice into mice harbouring N. dubius
Group 4: worms from mice into mice never exposed to N. dubius
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
1600 1700
1170 1460 1600 1060
4130
3730 3860
260 800 400 430 360 200 400 0 330 130 0 0 0 0
760
1250
330 200 430 0 0 160 0 0 0 0 0 0 0 0
190 360 200 0 0 0 0 0 0 0 0 0 0
730 600 660 200 200 750 400 260 330 530 200 430
Table 4. Mean number of adult Nippostrongylus brasiliensis recovered 21 days after transfer into recipient mice (experiment 2) Mean number N. brasiliensis recovered 21 days after transfer Group
No. of mice
Male worms
Female worms
Total worms + S.D.
1*
9 9 10
2-66 1-66 0-30
5-55 4-11 0-70
10
0-40
0-80
8-22 ±5-53 5-77 + 7-27 1-0 (worms found in 1 mouse only) 1-20 (worms found in 2 mice only)
2t
n
* Worms t Worms % Worms § Worms
from from from from
rats into mice harbouring N. dubius. rats into mice never exposed to N. dubius. mice into mice harbouring N. dubius. mice into mice never exposed to N. dubius.
Mice which had never been exposed to N. dubius and rats were each infected with about 200 N. brasiliensis larvae on day 0. These were the donors. On day 6 after infection their worms were transferred after laparotomy into the small intestine of recipient mice, twenty of which harboured about 40 N. dubius and twenty of which had never been exposed to the latter parasite. Ten of the mice harbouring N. dubius each received 40 male and 60 female N. brasiliensis derived from mice and ten received a similar number of worms derived from rats. Similarly, ten of the mice free of N. dubius received worms from mice and ten worms
Influence o/N. dubius on N. brasiliensis infections
353
Table 5. Egg production of Nippostrongylus brasiliensis after transfer into clean, previously uninfected mice {experiment 3) Number of eggs recovered per gramme pooled faeces Days after transfer of N. brasiliensis worm population (40 cj and 60 $) 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
e
Group A: worms transferred from mice also harbouring N. dubius 1530 1600 1860 1200 1200 1170 730
3300 1060 800 — — 400 700 430 260 700
Group B : worms transferred from mice never exposed to N. dubii 5700 5000 4000 830 760 300 430 60 60 0 — 0 0 0 0 0
derived from rats. The fecundity and longevity of the worms from each of the 4 groups of mice were then compared. The results are given in Tables 3 and 4. Worms derived from mice and those derived from rats were no more fecund or long-lived after transfer into mice harbouring N. dubius than those transferred into mice which had never been exposed to that parasite. Experiment 3 This final experiment was to determine whether populations of N. brasiliensis, which developed to maturity in mice harbouring N. dubius, produced eggs for longer than ' normal' worms after transfer into the intestines of clean, previously unparasitized mice. The egg production of twenty populations of N. brasiliensis each consisting of 40 male and 60 female worms derived on day 6 after infection from mice also harbouring a 28-day-old N. dubius population, and mice which had never been exposed to N. dubius, were compared after transfer into the intestines of two groups each of 10 previously uninfected recipient mice. The results are given in Table 5. Those populations of N. brasiliensis that previously developed to maturity in the presence of N. dubius continued to pass eggs for the duration of the experiment (21 days after transfer into clean mice) whilst those which had developed in mice never exposed to N. dubius stopped producing eggs by day 14.
354
D. CONWIL JENKINS DISCUSSION
It is evident from the present work that the salient requirement for the prolongation of the fecundity and longevity of populations of N. brasiliensis in mice is that the latter experience a previous infection with JV. dubius. The continuing presence of JV. dubius in the mice on infection with JV. brasiliensis is not, however, essential for these effects to occur. I t is also clear that fecundity and longevity are prolonged only if JV. brasiliensis develops from its larval stages in these mice, because worms which developed previously in mice never exposed to JV. dubius were not more fecund or longer lived after transfer into mice harbouring JV. dubius. Hence Bartlett & Ball's suggestion (1974) that JV. brasiliensis may be protected from the effects of host immunity by blocking antibodies stimulated by the coexisting JV. dubius population is not tenable in the present situation. It can be deduced from experiment 3 that populations of JV. brasiliensis which develop to maturity in mice previously infected with JV. dubius differ in certain important respects from ' normal' worms, because when the former were transferred into hosts which had no experience of JV. dubius their fecundity was nevertheless prolonged. This strongly suggests that the worms were less immunogenic and consequently better adapted to their hosts than' normal' worms. Populations of JV. brasiliensis which become established in rats after challenge infection are adapted to their host's immunity (Ogilvie & Hockley, 1968), and it is believed (Ogilvie, 1969) that this adaptation results from the exposure of the larvae to antibodies during their development in the host. Under certain circumstances this adaptation can considerably prolong both the fecundity and longevity of the worms (Jenkins & Phillipson, 1972a, b) their immunogenicity being significantly reduced (Jenkins, 1972). JV. dubius and JV. brasiliensis stimulate cross-resistance under certain experimental conditions (Jenkins, in preparation) which indicates that they may stimulate similar antibodies. Thus it is plausible to speculate that the prolonged fecundity and longevity of JV. brasiliensis in mice previously exposed to infection with JV. dubius results from adaptation to immunity stimulated in the host by the latter parasite. REFERENCES BARTLETT, A. & BALL, P . A. J . (1974). The immune response of the mouse to larvae and adults of Nematospiroides dubius. International Journal for Parasitology 4, 463—70. COLWELL, D. A. & WESCOTT, R. B. (1973). Prolongation of egg production of Nippostrongylus brasiliensis in mice concurrently infected with Nematospiroides dubius. Journal of Parasitology 59, 216. JENKINS, D. C. (1972). Nippostrongylus brasiliensis: observations on the comparative immunogenicity of adult worms from primary and immune-adapted infections. Parasitology 65, 547-50. JENKINS, D. C. & PHILLIPSON, R. F . (1971). The kinetics of repeated low-level infections of Nippostrongylus brasiliensis in the laboratory rat. Parasitology 62, 457-65. JENKINS, D. C. & PHILLIPSON, R. F . (1972a). Increased establishment and longevity of Nippostrongylus brasiliensis in immune rats given repeated small challenge infections. International Journal for Parasitology 2, 105-11. JENKINS, D. C. & PHILLIPSON, R. F . (19726). Evidence that the nematode Nippostrongylus brasiliensis can adapt to and overcome the effects of host immunity. International Journal for Parasitology 2, 353-9.
Influence of N. dubius on N. brasiliensis infections
355
B. M. (1969). Immunity to Nippostrongylus brasiliensis. Symposia of the British Society for Parasitology 7, 31-40. OGILVIE, B. M. & HOCKLEY, D. J. (1968). Effects of immunity on Nippostrongylus brasiliensis adult worms: reversible and irreversible changes in infectivity, reproduction and morphology. Journal of Parasitology 54, 1073-84. OGILVIE,
Printed in Great Britain
