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The influence of thyroxine on the host-parasite relationship of Ancylostoma caninum in swiss albino mice
Infernational Journal for Printed in Great Britain
Parasitology.
Vol.
11, No.
5, pp.
377-379,
198 1. 0
00:~7519/81/050377-03$02.00/0 Pergamon Press Ltd. Scciety Jar Parasitology
1981 Ausfralian
THE INFLUENCE OF THYROXINE ON THE HOST-PARASITE RELATIONSHIP OF ANCYLOSTOMA CANINUM IN SWISS ALBINO MICE ISMAIL BHAI and A. K. PANDEY
Endocrinology
Unit, School of Studies in Zoology, Vikram University, Ujjain, 456010, India (Received 1 October 1980; in revised form 20 January 1981)
Abstract-EM.41 ISMAILand PANDEYA. K. 1981. The influence of thyroxine on the host-parasite relationship of Aneylostoma caninum in Swiss albino mice. International Journal for Parasitology 11: 377-379. Thyroxine treatment, 1 pg/O.3 ml saline/mouse/day for 21 days, significantly increased the susceptibility of female mice (P> 0.001) but not of males to Ancylostoma cuninum infection, compared with their saline-treated controls. This accounted for the loss of the sex difference between the worm populations of male and female mice (O+)Ol
INTRODUCTION THERE is little information on the influence of thyroidal physiology on parasitic infection (von Brand, 1952; Solomon, 1969). Hyperthyroidism may (a) increase the host susceptibility to helminthic infections : Hymenolepis nana in mice (Larsh, 1950); Amplicaecum robertsi in male and female mice (Dobson, 1966); Schistosoma mansoni in Swiss mice (Abdel-Wahab, Warren & Levy, 1971); Schistosomatium douthitti in mice (Cornford, 1974); (b) produce no effect: Ascaridia galli and Heterakis gullinae in chickens (Todd, 1949), or (c) decrease the susceptibility of mice to Poliomyelitis virus infection (Holtman, 1946). Dobson (1966) further reported a sex difference in the susceptibility of male and female hosts to infection, under hyperthyroid condition. The present work was undertaken with Swiss albino mice-Ancylostoma caninum system-to provide more systematic information on host thyroid-parasite relationship.
MATERIALS AND METHODS In the present investigations 140 male and 140 female healthy 3-month old Swiss albino mice, average body weight 25-28 g, were selected from a stock colony. Mice of both sexes were divided into two groups of 80 and 60 mice each, and kept in separate 25 x 15 x 15 cm metal cages (4 mice/cage) over galvanized wire mesh with sterile
paddy husk bedding which was changed regularly every 72 h, and housed in a naturally well-lighted and ventilated room between 25 and 28°C. A balanced diet of mouse cake was given twice a day, with water in drinking-bottles, ad libitum. Each day for 21 days, 80 male and 80 female mice were injected intraperitoneally with 1 rgiO.3 ml L-thyroxine in saline. Thyroxine was prepared by dissolving 10 mg L-thyroxine sodium pentahydrate in 130 ml 0.01 N NaOH and adding 870 ml of 0.9% sterile saline to make thyroxine solution and the pH adjusted between 8.0 and 8.5. Fresh solutions were prepared every 10 days, and stored in a refrigerator. Sixty pairs of mice were given sterile saline injections as controls. Third stage larvae of Ancylostoma cuninum were cultured in Petri dishes in 0.2% sodium chloride solution with O.Ol’% hydrochloric acid as an antibacterial agent adiusted to DH 5 (Ray. Bhooale & Shrivastava, 1972),-using the technique of Sen, joshi & Seth (1965). Mycostatin 3950 pg/mg (50 mg/lOO ml distilled water) was sprayed over the culture mixture to prevent the fungal growth. Mature larvae were collected after 8-10 days, washed repeatedly with distilled water, counted by the dilution technique of Scott (I 928), and an inoculum containing 1000+25 larvae was prepared in 0.25 ml distilled water for each mouse. After 21 days of thyroxine treatment, experimental and control animals were inoculated orally with lOOO+ 25 larvae/mouse, using a 1 ml tuberculin syringe fitted with an 18 gauge needle with the tip removed and the cut end coated with lead to avoid injury. This dosage was found to be nonlethal and effective. Thyroxine treatment was continued on alternate days till the end of experiment. Mice from all groups were killed on 3, 7, 11, 15, 20, 25, 30 and 40 377
378
ISMAILBHAI
and A.K.
days after infection, under ether anaesthesia. Animals which died during the experiments were discarded. Larval recoveries were made according to Soh (1958). The visceral organs and muscles from each mouse were minced, tied in fine muslin cloth, kept in separate beakers containing 100 ml artificial gastric juice, 0.5% pepsin+0.7% HCI in saline, and incubated at 37°C for 4 h. The digested tissues with pepsin solution were transferred to the Baermann’s apparatus. After 18 h, liberated larvae were counted under binocular microscope. Probabilities of significant differences in the mean larval recoveries were determined according to Student’s t-test. Confidence limits (mean+s~.~. tn) were set at P=O.O5, where S.E.M.denotes the standard error of the mean and “tn” denotes the Student’s t-value at 5 % level of significance for n degrees of freedom. RESULTS The thyroxine administration in male and female mice produced dissimilar results (Table 1). Thyroxine injections in female mice increased the survival and retention of infected filariform larvae to a highly significant level (P> O.OOl), as compared to the euthyroid controls. This was revealed by 35.5 62.574 more larval recoveries from hyperthyroid females throughout the experimental period of 40 days post-infection (Fig. 1). However, this
PANDEY
I.J.P.
3 7 11 15 20 25 30 40
Control 782 + 42(5) 716+68(5) 695 + 46(5) 605 + 59(5) 588 t 37(5) 579 * 35(5) 554 + 33(5) 467? 51(6)
0
1
5
10 Day
20
15
past
25
30
35
40
- inoculation
FIG. 1. Mean percentage recovery of 1000 inoculated filariform larvae of Ancylostoma caninum from thyroxine treated male and female mice, during the 40-day postinoculation period.
hormone was only slightly (4-20.6%) effective in male mice, showing significant differences in larval recoveries only on the 3rd (PC 0.01) and 30th (P < 0.005) days of infection. Thus thyroxine treatment eliminated the significant difference (0.001
FEMALE do-DAY
Number of larvae recovered [Mean_+ S.E.M.tn (d.f.)] Male PThyroxine Control value
Female Thyroxine
Pvalue
839*31(6) 699 + 56(6) 732 + 83(6) 655 + 74(6) 614+ 36(6) 602+ 55(6) 668 + 77(6) 515+59(6)
765 + 53(5) 836+111(5) 772 f 94(5) 694 + 39(5) 702 ‘I 54(5) 608 + 57(5) 555 + 83(5) 530 * 37(4)
>OWl > 0.001 > 0.001 >OXtOl > 0.001 > 0.001 > 0.001 >O~OOl
CO.01
co.5 < 0.25 co.1
No sign of further development of filariform larvae towards adulthood was encountered in the thyroxine treated mice in the present studies. The cross comparison (in P-values) between the larval recoveries from hyperthyroid female vs. male mice revealed that females were significantly more susceptible on days 7 (PC 0.01) and 20 (PC 0.005), while males were so on days 3 (P-C 0.01) and 30 (P < 0.025) post-infection. But no significant difference was observed on the 11th, 15th, 25th, and 40th day after infection. When compared with control male mice, recoveries from hyperthyroid females were
11. 1981
00 f
TABLE ~-EFFECT OF THYROXINE TREATMENT ON THE LARVAL RECOVERIES FROM MALE MICE INFECTED WITH 1000 FILARIFORM LARVAE OF Ancvlosroma caninum, DURING EXPERIMENTAL PERIOD
Duration of infection (days)
VOL.
563 + 35(5) 530?47(5) 496 _+83(5) 461? 65(5) 518+37(5) 425 t 74(6) 370 + 37(6) 326 + 38(6)
highly significant (0.001 >P< 0.05) except on the 3rd, 25th and 30th day after infection. A higher percentage of mortality was also noted in the thyroxine-treated animals, especially in females. In hyperthyroid females it was 41.25 %, whereas in males it was 30 %, then 15 and 18.33y:, in respective controls, during the entire experimental period of 40 days post-infection. DISCUSSION The present study clearly demonstrates comparison to euthyroid controls, thyroxine
that in admin-
I.J.P. VOL. Ii. 1981
Thyroxine and Ancylostoma caninam
istration si~ifi~~ntly (P> 0301) increased the susceptibility of female mice to the AncyIosroma raninum infection. Percentage mortality was also considerably higher in the hyperthyroid females, but this treatment was poorly effective in males. Thus our results are in general agreement with the observations of Dobson (1966) with Ampiicaecum robertsimouse model. He also noticed that thyroxine treatment increased the worm recoveries signi~cantly more in females than in males. However, thyroxine had no effect on the growth and development of larvae in either of the experiments. However, Todd (1949) reported that protamone treated chickens revealed negligible diflerence in the percentage development of Ascaridia galli or Heterakis gallinne. While Larsh (1950) recovered significantly more cysticercoids of Hymenolepis nana from mice, treated with higher doses of thyroid extract. AbdelWahab ef nl. (1971) and Cornford (1974) observed significantly larger flukes from hyperthyroid mice infected with Se~jstosoma mansoni and Schistosom&urn douthitti respectively. Holtman (1946), however, reported that hyperthyroid condition produced by thyroprotein or thyroid extract decreased the susceptibility of mice to Poliomyelitis virus infection. The mortality rate in the experimental animals was the other parameter used to assess the degree of susceptibility. Mortality was greater in hyperthyroid female mice than in males and euthyroid controls, thus favouring the different levels of larval recovery from male and female mice. Abdel-Wahab et aI. (1971) and Cornford (1974) reported an increased mortality in hyperthyroid animals, but no sex dependent differences. The lack of correlation between earlier reports and present findings may be due to the fact that in some, but not all, host-parasite systems, thyroxine treatment increases the susceptibility of female hosts to a greater extent than that of male. Dobson (1966) pointed out that the discrete abilities of Amplicaecum robertsi larvae to hatch, penetrate and migrate through the tissues of thyroxine-treated and control mice were the factors responsible for differences observed in the worm recoveries. Although Abdel-Wahab et al. (1971) observed no difference in the cercarial penetration of hyperthyroid and euthyroid hosts, but Cornford (1974) considered the possibility that the increased growth seen in schistosomes from hyperthyroid hosts might be related to facilitated penetration of the cercariae, and/or faster migration of the schistosomule. The similar mechanism of differentiated penetration and migration through the host tissues, after oral inoculation of filariform larvae, may be operating in the present model and giving the highly
379
significant differences in the larval recoveries from male and female mice. The circumstantial evidence therefore, suggests that the elevated thyroxine level due to exogenous injections diminishes the sex resistance in female mice. This may provide a favourable environment to the filariform larvae of Ancylostoma caninum for survival over longer periods. However, the mechanism is not known and requires further research.
Acknowftzige~zen~s-The authors are grateful to Professor H. Swamp for critical suggestions and to Professor B. M. Sinha for providing necessary laboratory facilities. Financial support in the form of J.R.F. to (LB.) from the University Grants Commission is also gratefully acknowledged.
REFER~~E~ A~D~L-WA~AB M. F., WARREN K. S. & LEVY R. P. 1911.
Function of the thyroid and the host-parasite relation in murine schistosomiasis mansoni. Joournalof Infectious Diseases 124: 161-171. BRAND T. VON. 1952. Chemical Physiology of Endoparasitic Animals. Academic Press, New York. CORNFORDE. M. 1974. Schistosomatium douthitti: Effects of thyroxine. Experimental Parasitology 36: 210-221. DOBSONC. 1966. The effects of thiouracil and thyroxine on the host-parasite relationship of Amp~~cae~a~ robertsi &rent and Mines, 1960, in the mouse. Parasitology 56: 425-429. HOLTMANF. 1946. The effect of thiouracil and thyroactive substances on mouse susceptibility to Pofiomyefitis virus. Science, N. Y. 104: 50-51. LARSHJ. E., JR. 1950. The effect of thiouracil and thyroid extract on the natural resistance of mice to Hymenolepis infection. Jourrrul ofParasitology 36: 473-478. RAY D. K., BHOPALEK. K. & SHRIVASTAVA V. B. 1972. Migration and growth of Ancylostoma ceyianicum in golden hamsters Mesacri~efas aarattas. Jourtral of Helminthafog~v 46: 357-362. SCOTT.J. A. 1928. An experimental study of the devetopment of Ancylostoma eaninum in normal and abnormal hosts. American Journal of Hygiene 8: 158-209.
SEN H. G., JOSHI U. N. & SETH D. 1965. Effect of cortisone upon Ancylostorna caninum infection in albino mice, Transactiorzs of the Royal Society of Tropical Medicine and H&ene
59: 684-689.
SOH C. T. 1958. The distribution and persistence of hookworm larvae in the tissues of mice in relation to species and to routes of inoculation. Journal of Parasitology 44: 515-519.
SOLOMONG. B. 1969. Host hormones and parasitic infection. International Rer;iew of Tropicat Medicine 3: 101-15s. TODD A. C. 1949. Thyroid condition of chickens and development of parasitic nematodes. Journal of Parasitology 35: 255-260.
Parasitology.
Vol.
11, No.
5, pp.
377-379,
198 1. 0
00:~7519/81/050377-03$02.00/0 Pergamon Press Ltd. Scciety Jar Parasitology
1981 Ausfralian
THE INFLUENCE OF THYROXINE ON THE HOST-PARASITE RELATIONSHIP OF ANCYLOSTOMA CANINUM IN SWISS ALBINO MICE ISMAIL BHAI and A. K. PANDEY
Endocrinology
Unit, School of Studies in Zoology, Vikram University, Ujjain, 456010, India (Received 1 October 1980; in revised form 20 January 1981)
Abstract-EM.41 ISMAILand PANDEYA. K. 1981. The influence of thyroxine on the host-parasite relationship of Aneylostoma caninum in Swiss albino mice. International Journal for Parasitology 11: 377-379. Thyroxine treatment, 1 pg/O.3 ml saline/mouse/day for 21 days, significantly increased the susceptibility of female mice (P> 0.001) but not of males to Ancylostoma cuninum infection, compared with their saline-treated controls. This accounted for the loss of the sex difference between the worm populations of male and female mice (O+)Ol
INTRODUCTION THERE is little information on the influence of thyroidal physiology on parasitic infection (von Brand, 1952; Solomon, 1969). Hyperthyroidism may (a) increase the host susceptibility to helminthic infections : Hymenolepis nana in mice (Larsh, 1950); Amplicaecum robertsi in male and female mice (Dobson, 1966); Schistosoma mansoni in Swiss mice (Abdel-Wahab, Warren & Levy, 1971); Schistosomatium douthitti in mice (Cornford, 1974); (b) produce no effect: Ascaridia galli and Heterakis gullinae in chickens (Todd, 1949), or (c) decrease the susceptibility of mice to Poliomyelitis virus infection (Holtman, 1946). Dobson (1966) further reported a sex difference in the susceptibility of male and female hosts to infection, under hyperthyroid condition. The present work was undertaken with Swiss albino mice-Ancylostoma caninum system-to provide more systematic information on host thyroid-parasite relationship.
MATERIALS AND METHODS In the present investigations 140 male and 140 female healthy 3-month old Swiss albino mice, average body weight 25-28 g, were selected from a stock colony. Mice of both sexes were divided into two groups of 80 and 60 mice each, and kept in separate 25 x 15 x 15 cm metal cages (4 mice/cage) over galvanized wire mesh with sterile
paddy husk bedding which was changed regularly every 72 h, and housed in a naturally well-lighted and ventilated room between 25 and 28°C. A balanced diet of mouse cake was given twice a day, with water in drinking-bottles, ad libitum. Each day for 21 days, 80 male and 80 female mice were injected intraperitoneally with 1 rgiO.3 ml L-thyroxine in saline. Thyroxine was prepared by dissolving 10 mg L-thyroxine sodium pentahydrate in 130 ml 0.01 N NaOH and adding 870 ml of 0.9% sterile saline to make thyroxine solution and the pH adjusted between 8.0 and 8.5. Fresh solutions were prepared every 10 days, and stored in a refrigerator. Sixty pairs of mice were given sterile saline injections as controls. Third stage larvae of Ancylostoma cuninum were cultured in Petri dishes in 0.2% sodium chloride solution with O.Ol’% hydrochloric acid as an antibacterial agent adiusted to DH 5 (Ray. Bhooale & Shrivastava, 1972),-using the technique of Sen, joshi & Seth (1965). Mycostatin 3950 pg/mg (50 mg/lOO ml distilled water) was sprayed over the culture mixture to prevent the fungal growth. Mature larvae were collected after 8-10 days, washed repeatedly with distilled water, counted by the dilution technique of Scott (I 928), and an inoculum containing 1000+25 larvae was prepared in 0.25 ml distilled water for each mouse. After 21 days of thyroxine treatment, experimental and control animals were inoculated orally with lOOO+ 25 larvae/mouse, using a 1 ml tuberculin syringe fitted with an 18 gauge needle with the tip removed and the cut end coated with lead to avoid injury. This dosage was found to be nonlethal and effective. Thyroxine treatment was continued on alternate days till the end of experiment. Mice from all groups were killed on 3, 7, 11, 15, 20, 25, 30 and 40 377
378
ISMAILBHAI
and A.K.
days after infection, under ether anaesthesia. Animals which died during the experiments were discarded. Larval recoveries were made according to Soh (1958). The visceral organs and muscles from each mouse were minced, tied in fine muslin cloth, kept in separate beakers containing 100 ml artificial gastric juice, 0.5% pepsin+0.7% HCI in saline, and incubated at 37°C for 4 h. The digested tissues with pepsin solution were transferred to the Baermann’s apparatus. After 18 h, liberated larvae were counted under binocular microscope. Probabilities of significant differences in the mean larval recoveries were determined according to Student’s t-test. Confidence limits (mean+s~.~. tn) were set at P=O.O5, where S.E.M.denotes the standard error of the mean and “tn” denotes the Student’s t-value at 5 % level of significance for n degrees of freedom. RESULTS The thyroxine administration in male and female mice produced dissimilar results (Table 1). Thyroxine injections in female mice increased the survival and retention of infected filariform larvae to a highly significant level (P> O.OOl), as compared to the euthyroid controls. This was revealed by 35.5 62.574 more larval recoveries from hyperthyroid females throughout the experimental period of 40 days post-infection (Fig. 1). However, this
PANDEY
I.J.P.
3 7 11 15 20 25 30 40
Control 782 + 42(5) 716+68(5) 695 + 46(5) 605 + 59(5) 588 t 37(5) 579 * 35(5) 554 + 33(5) 467? 51(6)
0
1
5
10 Day
20
15
past
25
30
35
40
- inoculation
FIG. 1. Mean percentage recovery of 1000 inoculated filariform larvae of Ancylostoma caninum from thyroxine treated male and female mice, during the 40-day postinoculation period.
hormone was only slightly (4-20.6%) effective in male mice, showing significant differences in larval recoveries only on the 3rd (PC 0.01) and 30th (P < 0.005) days of infection. Thus thyroxine treatment eliminated the significant difference (0.001
FEMALE do-DAY
Number of larvae recovered [Mean_+ S.E.M.tn (d.f.)] Male PThyroxine Control value
Female Thyroxine
Pvalue
839*31(6) 699 + 56(6) 732 + 83(6) 655 + 74(6) 614+ 36(6) 602+ 55(6) 668 + 77(6) 515+59(6)
765 + 53(5) 836+111(5) 772 f 94(5) 694 + 39(5) 702 ‘I 54(5) 608 + 57(5) 555 + 83(5) 530 * 37(4)
>OWl > 0.001 > 0.001 >OXtOl > 0.001 > 0.001 > 0.001 >O~OOl
CO.01
co.5 < 0.25 co.1
No sign of further development of filariform larvae towards adulthood was encountered in the thyroxine treated mice in the present studies. The cross comparison (in P-values) between the larval recoveries from hyperthyroid female vs. male mice revealed that females were significantly more susceptible on days 7 (PC 0.01) and 20 (PC 0.005), while males were so on days 3 (P-C 0.01) and 30 (P < 0.025) post-infection. But no significant difference was observed on the 11th, 15th, 25th, and 40th day after infection. When compared with control male mice, recoveries from hyperthyroid females were
11. 1981
00 f
TABLE ~-EFFECT OF THYROXINE TREATMENT ON THE LARVAL RECOVERIES FROM MALE MICE INFECTED WITH 1000 FILARIFORM LARVAE OF Ancvlosroma caninum, DURING EXPERIMENTAL PERIOD
Duration of infection (days)
VOL.
563 + 35(5) 530?47(5) 496 _+83(5) 461? 65(5) 518+37(5) 425 t 74(6) 370 + 37(6) 326 + 38(6)
highly significant (0.001 >P< 0.05) except on the 3rd, 25th and 30th day after infection. A higher percentage of mortality was also noted in the thyroxine-treated animals, especially in females. In hyperthyroid females it was 41.25 %, whereas in males it was 30 %, then 15 and 18.33y:, in respective controls, during the entire experimental period of 40 days post-infection. DISCUSSION The present study clearly demonstrates comparison to euthyroid controls, thyroxine
that in admin-
I.J.P. VOL. Ii. 1981
Thyroxine and Ancylostoma caninam
istration si~ifi~~ntly (P> 0301) increased the susceptibility of female mice to the AncyIosroma raninum infection. Percentage mortality was also considerably higher in the hyperthyroid females, but this treatment was poorly effective in males. Thus our results are in general agreement with the observations of Dobson (1966) with Ampiicaecum robertsimouse model. He also noticed that thyroxine treatment increased the worm recoveries signi~cantly more in females than in males. However, thyroxine had no effect on the growth and development of larvae in either of the experiments. However, Todd (1949) reported that protamone treated chickens revealed negligible diflerence in the percentage development of Ascaridia galli or Heterakis gallinne. While Larsh (1950) recovered significantly more cysticercoids of Hymenolepis nana from mice, treated with higher doses of thyroid extract. AbdelWahab ef nl. (1971) and Cornford (1974) observed significantly larger flukes from hyperthyroid mice infected with Se~jstosoma mansoni and Schistosom&urn douthitti respectively. Holtman (1946), however, reported that hyperthyroid condition produced by thyroprotein or thyroid extract decreased the susceptibility of mice to Poliomyelitis virus infection. The mortality rate in the experimental animals was the other parameter used to assess the degree of susceptibility. Mortality was greater in hyperthyroid female mice than in males and euthyroid controls, thus favouring the different levels of larval recovery from male and female mice. Abdel-Wahab et aI. (1971) and Cornford (1974) reported an increased mortality in hyperthyroid animals, but no sex dependent differences. The lack of correlation between earlier reports and present findings may be due to the fact that in some, but not all, host-parasite systems, thyroxine treatment increases the susceptibility of female hosts to a greater extent than that of male. Dobson (1966) pointed out that the discrete abilities of Amplicaecum robertsi larvae to hatch, penetrate and migrate through the tissues of thyroxine-treated and control mice were the factors responsible for differences observed in the worm recoveries. Although Abdel-Wahab et al. (1971) observed no difference in the cercarial penetration of hyperthyroid and euthyroid hosts, but Cornford (1974) considered the possibility that the increased growth seen in schistosomes from hyperthyroid hosts might be related to facilitated penetration of the cercariae, and/or faster migration of the schistosomule. The similar mechanism of differentiated penetration and migration through the host tissues, after oral inoculation of filariform larvae, may be operating in the present model and giving the highly
379
significant differences in the larval recoveries from male and female mice. The circumstantial evidence therefore, suggests that the elevated thyroxine level due to exogenous injections diminishes the sex resistance in female mice. This may provide a favourable environment to the filariform larvae of Ancylostoma caninum for survival over longer periods. However, the mechanism is not known and requires further research.
Acknowftzige~zen~s-The authors are grateful to Professor H. Swamp for critical suggestions and to Professor B. M. Sinha for providing necessary laboratory facilities. Financial support in the form of J.R.F. to (LB.) from the University Grants Commission is also gratefully acknowledged.
REFER~~E~ A~D~L-WA~AB M. F., WARREN K. S. & LEVY R. P. 1911.
Function of the thyroid and the host-parasite relation in murine schistosomiasis mansoni. Joournalof Infectious Diseases 124: 161-171. BRAND T. VON. 1952. Chemical Physiology of Endoparasitic Animals. Academic Press, New York. CORNFORDE. M. 1974. Schistosomatium douthitti: Effects of thyroxine. Experimental Parasitology 36: 210-221. DOBSONC. 1966. The effects of thiouracil and thyroxine on the host-parasite relationship of Amp~~cae~a~ robertsi &rent and Mines, 1960, in the mouse. Parasitology 56: 425-429. HOLTMANF. 1946. The effect of thiouracil and thyroactive substances on mouse susceptibility to Pofiomyefitis virus. Science, N. Y. 104: 50-51. LARSHJ. E., JR. 1950. The effect of thiouracil and thyroid extract on the natural resistance of mice to Hymenolepis infection. Jourrrul ofParasitology 36: 473-478. RAY D. K., BHOPALEK. K. & SHRIVASTAVA V. B. 1972. Migration and growth of Ancylostoma ceyianicum in golden hamsters Mesacri~efas aarattas. Jourtral of Helminthafog~v 46: 357-362. SCOTT.J. A. 1928. An experimental study of the devetopment of Ancylostoma eaninum in normal and abnormal hosts. American Journal of Hygiene 8: 158-209.
SEN H. G., JOSHI U. N. & SETH D. 1965. Effect of cortisone upon Ancylostorna caninum infection in albino mice, Transactiorzs of the Royal Society of Tropical Medicine and H&ene
59: 684-689.
SOH C. T. 1958. The distribution and persistence of hookworm larvae in the tissues of mice in relation to species and to routes of inoculation. Journal of Parasitology 44: 515-519.
SOLOMONG. B. 1969. Host hormones and parasitic infection. International Rer;iew of Tropicat Medicine 3: 101-15s. TODD A. C. 1949. Thyroid condition of chickens and development of parasitic nematodes. Journal of Parasitology 35: 255-260.