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The frequency distribution of Hymenolepis diminuta cysticercoids in natural, sympatric populations of Tenebrio molitor and T. obscurus
International Journal for Parasitology, Vol. 9, pp. 85-87. Pergamon Press Ltd. 1979. Printed in Great Britain.
THE FREQUENCY DISTRIBUTION OF HYMENOLEPZS CYSTICERCOIDS IN NATURAL, SYMPATRIC POPULATIONS OF TENEBRIO MOLITOR AND
DZMZNUTA
T. OBSCURUS M. E. RAU Institute of Parasitology,
Macdonald College of McGill University, Macdonald Province of Quebec, Canada H9X TCO
College Post Office,
(Recetved 23 Junuary 1978) Abstract-RAu M. E. 1979. The frequency distribution of Hymenolepis diminuta cysticercoids in natural, sympatric populations of Tenebrio molitor and T. obscurus. International Journal for Parasitology 9: 85-87. Natural, sympatric populations of Tenebrio molitor and T. obscurus were examined for cysticercoids of Hymenolepis diminuta, The distribution of cysticeroids in both species and both sexes conformed to the negative binomial. Cysticeroids were more prevalent and the mean intensity of infection was higher in T. obscurus than in T. molitor. No differences in the intensity of infection were detected between the sexes. L.arvae of both beetle species were always very lightly infected. The significance of these factors in the transmission of the infection to the rat definitive host is discussed. INDEX KEY WORDS: Hymenolepis diminuta; cysticercoids; molitor; Tenebrio obscurus; sympatric populations.
INTRODUCTION THE CYSTICERCOIDSof Hymenolepis diminuta (Rudolphi, 1819), the dwarf tapeworm of rats exhibit an extraordinary lack of specificity at the intermediate host level. Natural hosts include many grain infesting insects, such as the larvae of the meal moth (Pyralis farittalis), earwigs (Anisolobis annulipes), as well as the rat fleas NosopsylIus fasciatus and Xenopsylla cheopis which are infected in the larval stage. The principal natural hosts, however, are considered to be the adults of the meal worm beetle, Tenebrio molitor. Various factors determine the relative importance of an intermediate host species to the transmission of the parasite under natural conditions, not the least of which are the prevalence of infection (number of infected beetles/ total beetles examined), the intensity of infection (number of cysticercoids/infected beetle) and the distribution of the parasite in the intermediate host populations. The present study examines these parameters in two sympatric species of Tenebrio under conditions of approximately equal abundance, in order to gain some insight into the relative significance of these two species in the transmission of H. diminuta to the rodent definitive host population. MATERIALS
frequency
distribution;
Tenebrio
was composed of 102 adults and 18 larvae of T. obscurus and 102 adults and 8 larvae of T. molitor, Nine rats (Rattus rat&s) were lived-trapped in the stable, killed and examined for adults of H. diminuta. Beetles and their larvae were decapitated and dissected in saline under a dissecting microscope, and cysticercoids of H. diminuta were freed of host tissues, and counted. The data were analysed for differences in the prevalence and intensity of infection with cysticercoids in adult and larval beetles, in male and females, and in the two species using the Mann-Whitney U-Test, Fisher’s Exact Test and the Chi-square test where appropriate. The frequency distributions of H. diminuta in the two species of grain beetles, males and females, were tested for goodness of fit against a negative binomial distribution using Fisher’s Maximum Likelihood Method (Fisher, 1953). RESULTS Eight of the 9 rats examined were infected with adults of H. diminuta. The number of cestodes within each rat ranged from 0 to 154 with a mean of 35 and a variance of 6716.6. Six rats harboured fewer than 15 worms (2 = 6.17; range O-14) whilst the remaining three bore 45, 79 and 154 cestodes each (2 = 92.67). Thus, almost one half of the total worm population of 315 was found in a single rat. The distribution of the cysticercoids in the intermediate host populations mirrored the distribution of the adult cestodes in the definitive host. The data fit the negative binomial distribution. Figure 1 shows
AND METHODS
A sample of 204 adult and 26 larva1 grain beetles taken from a riding stable near St. Augustin, Quebec, 85
M. E. RAU
86
I.J.P.
VOL.
9. 1979
‘1: molitor
I, h14&.m,~. 0
IO Nunber
20
” 30
n
40
50
n
60
70
80
90
of cysticercoids per beetle
FIG. 1. Frequency distribution of Hymenolepis diminura in 102 adult T. obscurus (above) and T. molitor (below). T. obscurus harbouring from 0 to 83 cysticercoids are presented, as in one value of 1200 cestodes. Twenty-eight beetles, bearing between 84 and 1200 cysticercoids each have been omitted. All T. molitor infections are presented. The corresponding negative binomial distributions for each species are represented by the dotted lines. the relationship between the observed and the fitted negative binomial distribution for T. obscurus and T. molitor. The sample of 102 adults of T. obscurus, 90 of which were infected, bore a mean of 72.7 cestodes. The corresponding k and p values were 0.39109 and 185.8 respectively, and pkq was 13,573. The sample of 102 adults of T. molitor, 48 of which were infected, bore a mean of 4.9 cysticercoids per beetle, while the corresponding k and p values were 0.20069 and 24.5 respectively; the pkq value was 125. Although the frequency distribution of H. diminuta in T. molitor and T. obscurus are overdispersed and are described by the negative binomial equation, their means are highly significantly different according to the Mann-Whitney U-Test (2 = 19.58, P < 0.00003) indicating two distinct populations. Within each species, no significant sex differences in the frequency distribution of cysticercoids were noted. The sample of 51 males of T. obscurus, 44 of which were infected, bore a mean of 84.0 cysticercoids. The k and p values were 0.34744 and 241.7 respectively, while pkq was 20,381. The corresponding values for a sample of 51 females, 46 of which were infected, were as follows: the mean number of cysticercoids was 61.3, k and p were 0.45398 and 135.1 respectively, while the pkq was 8344. Both distributions were described by the negative binomial. For T. molitor the sample of 58 males, 30 of which were infected, bore a mean of only 4.3
cysticercoids. The k and p values were 0.26525 and 16.4 respectively, while pkq was 75. The corresponding values for the sample of 44 females, 18 of which were infected, were as follows: the mean number of cestode larvae was 5.7, k and p were 0.14458 and 39.3 respectively, while pkq was 228. Again, both distributions were described by the negative binomial according to Fisher’s Maximum Likelihood Method. The prevalence of cysticercoids in T. obscurus (88.2%) was highly significantly greater than in T. molitor (47.1 p/,, (x2 = 39.5, P < 0.005). Within species, no significant sex differences in the prevalence of cysticercoids was noted. The prevalence of the infection in male T. obscurus (86.3 %) did not differ significantly from females (90,2x), (x2 = 0.3776, 0.9 < P < 0.5). Similarly, the prevalence of cysticercoids in male T. molitor (51.7%) did not differ from females (40.9x), (x2 = 1.1776,0.5 < P < 0.1). The intensity of infection in T. obscurus (,I?= 82.3 cysticercoids, n = 90) was highly significantly greater than in T. molitor (2 = 10.5 cysticercoids, n = 48), (2 = 5.406, P < 0.00003). Within each species no sex differences were apparent. Thus, infected male T. obscurus with a mean intensity of infection of 97.3 cysticercoids per beetle (n = 44) did not bear significantly more cysticercoids than infected females (X = 67.9 cysticercoids, n = 46), (Z = 0.19, P = 0.4247). Similarly, infected male T.
I.J.P. VOL.9. 1979
Frequency distribution
of H. diminura cysticercoids
molitor bearing a mean intensity of infection of 8.4 cysticercoids per beetle (n = 30) did not bear significantly more cysticercoids than infected females (2 = 13.9 cysticercoids, n = IS), (2 = 050, P = 0.3085). Adult beetles exhibited a highly significantly greater prevalence of infection (67.6x, n = 204) than did larvae (13.0%, R = 54), (x2 = 51.87, P < 0.001). There was no significant difference in parasite prevalence between the larvae of T. molitor (0 %, n = 8) and T. obscurus (22.2x, IZ = 18) according to Fisher’s Exact Test (P = 0.2047). DISCUSSION There are a number of intriguing discontinuities in the distribution of H. diminuta cysticercoids in the two sympatric intermediate host species, discontinuities that may figure prominently in the transmission of this parasite to the definitive rat host. Thus, while T. obscurus constituted only 50% of the total beetle population at this site, this species accounted for 65.2 ‘A of the infected beetles. This suggests that rats are probably almost twice as likely to acquire the cestode infection from T. obscurus than from T. molitor. Furthermore, the intensity of infection is more than 8 times as high in T. obscurus than in T. molitor. While Chappell & Pike (1976) suggest that all multiple worm infections are characterized either by a loss of worms (15-worm infections and higher) or worm weight loss (5-worm infections), continuous superinfection with large numbers of cysticercoids may, nevertheless, establish a higher plateau of egg passage than continuous superinfection with lower numbers since the major loss in the number of worms occurs after 20-25 days post-infection. Thus, T. obscurus may well contribute more to the transmission of H. diminuta under the present conditions. In the present study, 6 of the 9 rats examined harboured fewer than 15 worms and the remaining three bore 45, 79 and 154 cestodes each. In the light of Chappell & Pike’s (1976) suggestion the latter three infections may be in the process of reduction while the others may be approaching a stable plateau. Non-random distribution of H. diminuta ova may contribute to the negative binomial distribution of cysticercoids in beetle populations. Rat faeces and thus H. diminuta ova are not distributed randomly in the environment of the intermediate host, nor are eggs distributed at random in the faecal pellet itself. Thus, large numbers of H. diminuta eggs remain within an intact proglottid. Beetles are known to feed directly on proglottids as well as rat faeces(Lethbridge, 197l)acquiringmassive infections. Certainly, the data suggest that since Tenebrio of both sexes and species exhibited a
87
similar distribution of cysticercoids, it was not merely the heterogeneity of the beetle population which gave rise to the negative binomial distribution. To what extent other factors listed by Crofton (1971) contribute to this frequency distribution is currently under investigation. The factors responsible for the significantly greater prevalence and intensity of infection in T. obscurus as compared to T. molitor are still unknown. The operation of species specific immunological mechanisms is unlikely since a wide variety of insect intermediate hosts were shown to be incapable of distinguishing cysticercoids of H. diminuta from their own tissues (Lackie, 1976). While the species differences may well have a behavioural and/or physiological basis, it is conceivable that since T. obscurus lives longer and emerges about one month before T. molitor (Cotton, 1929), the former may have had more time to acquire infections. The present data also indicate that larvae of Tenebrio bore very few cysticercoids of H. diminuta. This is in accordance with data published by Lethbridge (1971) who found, in a laboratory study, that the absence of midgut papillae in larval beetles prevented oncospheres from penetrating into the haemocoel. The few larvae that succeeded developed normally. It is thus conceivable that Tenebrio larvae are responsible for the destruction of large numbers of H. diminuta eggs that would otherwise be available to adult beetles. How this and other factors may affect the transmission of H. diminuta is presently under investigation. Acknowledgements- The technical assistance of Miss Karen Smith and Mr. David Gordon is gratefully acknowledged. Research at the Institute of Parasitology is supported by the National Research Council of Canada and the Formation de Chercheurs et d’Action Concert&e du Minis&e de I’Education de Quebec.
REFERENCES CHAPPELLL. H. & PIKE A. W. 1976. Loss of Hymenolepis diminuta from the rat. International Journal for Parasitology 6: 333-339. COTTON R . T. 1929. The meal worms. United States Department of Agricubure Technical Bulletin No. 95. CROFTONH. D. 1971. A quantitative approach to parasitism. Parasitology 62: 179-193. FISHER R. A. 1953. Note on the efficient fitting of the negative binomial. Biometrics 9: 197-200. LACKIEA. M. 1976. Evasion of the haemocytic defence reaction of certain insects by larvae of Hymenolepis diminufa (Cestoda). Parasitology 73: 97-107. LETHRRIDGER. C. 1971. The hatching of Hymenalepis diminuta eggs and penetration of the hexacanths in Tenebrio molitor beetles. Parasitology 62: 4455456.
THE FREQUENCY DISTRIBUTION OF HYMENOLEPZS CYSTICERCOIDS IN NATURAL, SYMPATRIC POPULATIONS OF TENEBRIO MOLITOR AND
DZMZNUTA
T. OBSCURUS M. E. RAU Institute of Parasitology,
Macdonald College of McGill University, Macdonald Province of Quebec, Canada H9X TCO
College Post Office,
(Recetved 23 Junuary 1978) Abstract-RAu M. E. 1979. The frequency distribution of Hymenolepis diminuta cysticercoids in natural, sympatric populations of Tenebrio molitor and T. obscurus. International Journal for Parasitology 9: 85-87. Natural, sympatric populations of Tenebrio molitor and T. obscurus were examined for cysticercoids of Hymenolepis diminuta, The distribution of cysticeroids in both species and both sexes conformed to the negative binomial. Cysticeroids were more prevalent and the mean intensity of infection was higher in T. obscurus than in T. molitor. No differences in the intensity of infection were detected between the sexes. L.arvae of both beetle species were always very lightly infected. The significance of these factors in the transmission of the infection to the rat definitive host is discussed. INDEX KEY WORDS: Hymenolepis diminuta; cysticercoids; molitor; Tenebrio obscurus; sympatric populations.
INTRODUCTION THE CYSTICERCOIDSof Hymenolepis diminuta (Rudolphi, 1819), the dwarf tapeworm of rats exhibit an extraordinary lack of specificity at the intermediate host level. Natural hosts include many grain infesting insects, such as the larvae of the meal moth (Pyralis farittalis), earwigs (Anisolobis annulipes), as well as the rat fleas NosopsylIus fasciatus and Xenopsylla cheopis which are infected in the larval stage. The principal natural hosts, however, are considered to be the adults of the meal worm beetle, Tenebrio molitor. Various factors determine the relative importance of an intermediate host species to the transmission of the parasite under natural conditions, not the least of which are the prevalence of infection (number of infected beetles/ total beetles examined), the intensity of infection (number of cysticercoids/infected beetle) and the distribution of the parasite in the intermediate host populations. The present study examines these parameters in two sympatric species of Tenebrio under conditions of approximately equal abundance, in order to gain some insight into the relative significance of these two species in the transmission of H. diminuta to the rodent definitive host population. MATERIALS
frequency
distribution;
Tenebrio
was composed of 102 adults and 18 larvae of T. obscurus and 102 adults and 8 larvae of T. molitor, Nine rats (Rattus rat&s) were lived-trapped in the stable, killed and examined for adults of H. diminuta. Beetles and their larvae were decapitated and dissected in saline under a dissecting microscope, and cysticercoids of H. diminuta were freed of host tissues, and counted. The data were analysed for differences in the prevalence and intensity of infection with cysticercoids in adult and larval beetles, in male and females, and in the two species using the Mann-Whitney U-Test, Fisher’s Exact Test and the Chi-square test where appropriate. The frequency distributions of H. diminuta in the two species of grain beetles, males and females, were tested for goodness of fit against a negative binomial distribution using Fisher’s Maximum Likelihood Method (Fisher, 1953). RESULTS Eight of the 9 rats examined were infected with adults of H. diminuta. The number of cestodes within each rat ranged from 0 to 154 with a mean of 35 and a variance of 6716.6. Six rats harboured fewer than 15 worms (2 = 6.17; range O-14) whilst the remaining three bore 45, 79 and 154 cestodes each (2 = 92.67). Thus, almost one half of the total worm population of 315 was found in a single rat. The distribution of the cysticercoids in the intermediate host populations mirrored the distribution of the adult cestodes in the definitive host. The data fit the negative binomial distribution. Figure 1 shows
AND METHODS
A sample of 204 adult and 26 larva1 grain beetles taken from a riding stable near St. Augustin, Quebec, 85
M. E. RAU
86
I.J.P.
VOL.
9. 1979
‘1: molitor
I, h14&.m,~. 0
IO Nunber
20
” 30
n
40
50
n
60
70
80
90
of cysticercoids per beetle
FIG. 1. Frequency distribution of Hymenolepis diminura in 102 adult T. obscurus (above) and T. molitor (below). T. obscurus harbouring from 0 to 83 cysticercoids are presented, as in one value of 1200 cestodes. Twenty-eight beetles, bearing between 84 and 1200 cysticercoids each have been omitted. All T. molitor infections are presented. The corresponding negative binomial distributions for each species are represented by the dotted lines. the relationship between the observed and the fitted negative binomial distribution for T. obscurus and T. molitor. The sample of 102 adults of T. obscurus, 90 of which were infected, bore a mean of 72.7 cestodes. The corresponding k and p values were 0.39109 and 185.8 respectively, and pkq was 13,573. The sample of 102 adults of T. molitor, 48 of which were infected, bore a mean of 4.9 cysticercoids per beetle, while the corresponding k and p values were 0.20069 and 24.5 respectively; the pkq value was 125. Although the frequency distribution of H. diminuta in T. molitor and T. obscurus are overdispersed and are described by the negative binomial equation, their means are highly significantly different according to the Mann-Whitney U-Test (2 = 19.58, P < 0.00003) indicating two distinct populations. Within each species, no significant sex differences in the frequency distribution of cysticercoids were noted. The sample of 51 males of T. obscurus, 44 of which were infected, bore a mean of 84.0 cysticercoids. The k and p values were 0.34744 and 241.7 respectively, while pkq was 20,381. The corresponding values for a sample of 51 females, 46 of which were infected, were as follows: the mean number of cysticercoids was 61.3, k and p were 0.45398 and 135.1 respectively, while the pkq was 8344. Both distributions were described by the negative binomial. For T. molitor the sample of 58 males, 30 of which were infected, bore a mean of only 4.3
cysticercoids. The k and p values were 0.26525 and 16.4 respectively, while pkq was 75. The corresponding values for the sample of 44 females, 18 of which were infected, were as follows: the mean number of cestode larvae was 5.7, k and p were 0.14458 and 39.3 respectively, while pkq was 228. Again, both distributions were described by the negative binomial according to Fisher’s Maximum Likelihood Method. The prevalence of cysticercoids in T. obscurus (88.2%) was highly significantly greater than in T. molitor (47.1 p/,, (x2 = 39.5, P < 0.005). Within species, no significant sex differences in the prevalence of cysticercoids was noted. The prevalence of the infection in male T. obscurus (86.3 %) did not differ significantly from females (90,2x), (x2 = 0.3776, 0.9 < P < 0.5). Similarly, the prevalence of cysticercoids in male T. molitor (51.7%) did not differ from females (40.9x), (x2 = 1.1776,0.5 < P < 0.1). The intensity of infection in T. obscurus (,I?= 82.3 cysticercoids, n = 90) was highly significantly greater than in T. molitor (2 = 10.5 cysticercoids, n = 48), (2 = 5.406, P < 0.00003). Within each species no sex differences were apparent. Thus, infected male T. obscurus with a mean intensity of infection of 97.3 cysticercoids per beetle (n = 44) did not bear significantly more cysticercoids than infected females (X = 67.9 cysticercoids, n = 46), (Z = 0.19, P = 0.4247). Similarly, infected male T.
I.J.P. VOL.9. 1979
Frequency distribution
of H. diminura cysticercoids
molitor bearing a mean intensity of infection of 8.4 cysticercoids per beetle (n = 30) did not bear significantly more cysticercoids than infected females (2 = 13.9 cysticercoids, n = IS), (2 = 050, P = 0.3085). Adult beetles exhibited a highly significantly greater prevalence of infection (67.6x, n = 204) than did larvae (13.0%, R = 54), (x2 = 51.87, P < 0.001). There was no significant difference in parasite prevalence between the larvae of T. molitor (0 %, n = 8) and T. obscurus (22.2x, IZ = 18) according to Fisher’s Exact Test (P = 0.2047). DISCUSSION There are a number of intriguing discontinuities in the distribution of H. diminuta cysticercoids in the two sympatric intermediate host species, discontinuities that may figure prominently in the transmission of this parasite to the definitive rat host. Thus, while T. obscurus constituted only 50% of the total beetle population at this site, this species accounted for 65.2 ‘A of the infected beetles. This suggests that rats are probably almost twice as likely to acquire the cestode infection from T. obscurus than from T. molitor. Furthermore, the intensity of infection is more than 8 times as high in T. obscurus than in T. molitor. While Chappell & Pike (1976) suggest that all multiple worm infections are characterized either by a loss of worms (15-worm infections and higher) or worm weight loss (5-worm infections), continuous superinfection with large numbers of cysticercoids may, nevertheless, establish a higher plateau of egg passage than continuous superinfection with lower numbers since the major loss in the number of worms occurs after 20-25 days post-infection. Thus, T. obscurus may well contribute more to the transmission of H. diminuta under the present conditions. In the present study, 6 of the 9 rats examined harboured fewer than 15 worms and the remaining three bore 45, 79 and 154 cestodes each. In the light of Chappell & Pike’s (1976) suggestion the latter three infections may be in the process of reduction while the others may be approaching a stable plateau. Non-random distribution of H. diminuta ova may contribute to the negative binomial distribution of cysticercoids in beetle populations. Rat faeces and thus H. diminuta ova are not distributed randomly in the environment of the intermediate host, nor are eggs distributed at random in the faecal pellet itself. Thus, large numbers of H. diminuta eggs remain within an intact proglottid. Beetles are known to feed directly on proglottids as well as rat faeces(Lethbridge, 197l)acquiringmassive infections. Certainly, the data suggest that since Tenebrio of both sexes and species exhibited a
87
similar distribution of cysticercoids, it was not merely the heterogeneity of the beetle population which gave rise to the negative binomial distribution. To what extent other factors listed by Crofton (1971) contribute to this frequency distribution is currently under investigation. The factors responsible for the significantly greater prevalence and intensity of infection in T. obscurus as compared to T. molitor are still unknown. The operation of species specific immunological mechanisms is unlikely since a wide variety of insect intermediate hosts were shown to be incapable of distinguishing cysticercoids of H. diminuta from their own tissues (Lackie, 1976). While the species differences may well have a behavioural and/or physiological basis, it is conceivable that since T. obscurus lives longer and emerges about one month before T. molitor (Cotton, 1929), the former may have had more time to acquire infections. The present data also indicate that larvae of Tenebrio bore very few cysticercoids of H. diminuta. This is in accordance with data published by Lethbridge (1971) who found, in a laboratory study, that the absence of midgut papillae in larval beetles prevented oncospheres from penetrating into the haemocoel. The few larvae that succeeded developed normally. It is thus conceivable that Tenebrio larvae are responsible for the destruction of large numbers of H. diminuta eggs that would otherwise be available to adult beetles. How this and other factors may affect the transmission of H. diminuta is presently under investigation. Acknowledgements- The technical assistance of Miss Karen Smith and Mr. David Gordon is gratefully acknowledged. Research at the Institute of Parasitology is supported by the National Research Council of Canada and the Formation de Chercheurs et d’Action Concert&e du Minis&e de I’Education de Quebec.
REFERENCES CHAPPELLL. H. & PIKE A. W. 1976. Loss of Hymenolepis diminuta from the rat. International Journal for Parasitology 6: 333-339. COTTON R . T. 1929. The meal worms. United States Department of Agricubure Technical Bulletin No. 95. CROFTONH. D. 1971. A quantitative approach to parasitism. Parasitology 62: 179-193. FISHER R. A. 1953. Note on the efficient fitting of the negative binomial. Biometrics 9: 197-200. LACKIEA. M. 1976. Evasion of the haemocytic defence reaction of certain insects by larvae of Hymenolepis diminufa (Cestoda). Parasitology 73: 97-107. LETHRRIDGER. C. 1971. The hatching of Hymenalepis diminuta eggs and penetration of the hexacanths in Tenebrio molitor beetles. Parasitology 62: 4455456.