Эта статья опубликована в сборнике по материалы первой Международной юбилейной конференции «Актуальные проблемы инфектологии и паразитологии», посвященной 110-летию со дня открытия проф. К.Н.Виноградовым сибирской двуустки у человека (2-5 апреля 2001, г. Томск)

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Изменение величины Biomphalaria glabrata при заражении Schistosoma mansoni

РЕЗЮМЕ

S. mansoni проходит часть своего жизненного цикла в промежуточном хозяине улитках семейства Planorbidae. Biomphalaria glabrata один из наиболее важных промежуточных хозяев шистосом. В этой статье приводятся данные об изменении размеров B. glabrata (Barbula, Венесуэла), заражённых мирацидиями S. mansoni (SM паразитами, обитающими в штате Carabobo, Венесуэла). Улиток группировали по трём размерам: 1 класс (5-8 мм), 2 класс (>8-11 мм) и 3 класс (> 11-14 мм). Каждая улитка заражалась 5 мирацидиями индивидуально. Процент восприимчивости составил: 1 класс - 88,89%, 2 класс - 11,54% и 3 класс - 8,33%. Корреляционный анализ показал отсутствие связи между величиной промежуточного хозяина и продукцией церкариев.

ABSTRACT

Schistosoma mansoni  develops part of its life cycle in intermediate host snails of the family Planorbidae. Biomphalaria glabrata  is one of the most important intermediate host species. This paper reports the influence of size of  Biomphalaria glabrata (Bбrbula, Carabobo estate, Venezuela) were exposed to miracidia of  Schistosoma mansoni (SM parasite, origin of  Carabobo state, Venezuela).  Snails batches (20, B. glabrata in duplicate) of  three different diameter: class I (5-8 mm), class II (>8-11 mm) and class III (>11-14 mm) were  exposed  individually with five  miracidia. The  susceptibility percentage present were: class I 88.89%, class II 11.54% and class III 8.33%. The correlation analysis indicated a not relation between the size of host-intermediary and cercarial production.
Key Words: Biomphalaria glabrata, Schistosoma mansoni, susceptibility, size, Venezuela.

INTRODUCTION
Schistosoma mansoni perform part of its biological cycle on  Planorbid snails of the genus Biomphalaria. Such a association is restricted by ecological, biochemical, geographical  and population  .^{1,2,3,5,11,12}

It has been described that genetic and nutritional factors modulate the good or bad association between snails and parasitic. Other trials have  showed that  the age of the intermediary host is a very important factor in the infectious dynamic, due to the younger host snails are more susceptible than the others. ^8,9,10,13 Richards et al in 1992¹³   determinated  four category of hospedere-intermediate in relation to   response   differences of  susceptibily: Type I, not-susceptibility of snails  in whole life. Type II, juvenile susceptibility/  not-susceptibility when adults. Type III, susceptibily of snails  in whole life. Type IV, young susceptibility/ variable  adults.

Researches have evidenced the roll of ontogenic in relation to cellular responses of host-intermediate snails, demonstrating that juvenile moluscs present a  high valor of positive. The dynamic of immulogical relation between intermediate-host and parasitic involves different strategies: cellular defense and humoral response by snails, encapsulation and destruction  of primary sporocyst and phagocytosis of the miracidia.^1,4,11

In this work, it   is described the influence of size of  Biomphalaria glabrata on the  values of infectivity and the cercarial liberation dynamic when they are experimentally confronted with isolated of Schistosoma mansoni, proceeding  from common geographical area  (Carabobo state, Venezuela).

MATERIALS  AND  METHODS

Protocols experimental. Groups (20 in duplicated) of Biomphalaria glabrata snails of three different  shell diameter: class I (5-8 mm), class II (>8-11 mm) and class III (>11-14 mm)  were exposed  individually with five miracidia of Schistosoma mansoni, which maintained on NMRI mice. The feces was clarified by  washing procedures using isotonic saline solution, helping the  capture of miracidia13 . The eclosion of the eggs was stimulated during 30 minutes  by exposing them to artificial light fountain. Experimental conditions about the infection, evaluation, maintenance of the parasited-snails  and  controls-snails meet detail described in anterior researches.^12,13 Survival reports  were carried  out weekly, for both controls  and parasitized molluscs.  They  were marked with color code up  one to five, using different colors of nail paint.

Statistical analysis. Data was submitted to one-way analysis of variance (p<0.05). Whenever significance was found, the Tunkey test was applied at the same level of significance (p<0.05).    

Geographic origin of  isolated  and  molluscs. The insulating  SM was obtained of patient native of Guacara, Carabobo state, Venezuela.4  The molluscs Barbula are breeding stablishished in Naguanagua, Carabobo state, Universidad de Carabobo, Venezuela 8. The all studied snails were created in the laboratory besides three generations.   

Total  number of  emissions cercarial.  Were carried through registers of the total numbers of eliminate cercariae for each snails ^8,9.

RESULTS

In  Table 1, the susceptibility percentage obtained to the different shell diameter (mm) of the molluscs Bбrbula, exposed with five miracidia are observed. It observed that 88.89% (size I); 11.54% (size II) and 8.33% (size III) respectively.  Statistical analysis reveleated significantive differences between groups (p*0.05). The comparison of mean (Tukey, p *0.05), confirm that the class I is more susceptible to the experimental infection than the rest of studied size (there are not significant differences between II and III).  In relation to the prepatencia, it was observed that  to the class I   was positive beginning the fourth  week post-exposition;  unlike  the class II and III  had emissions of cercariae after   five weeks (figure 2, 3 and 4).

Means with the same letter are  not significantly  different (Tukey, p <0 .05)

The survival curves are expressed in   figure 1. It is observed  a definitive tendency in three studied size, with  a 50% of died  molluscs among the sixth and ninth week. It was not detected  mortality in association to molluscs shell diameter.
The figure  2, 3 and 4   represents  the  percentage of positive  B. glabrata  evaluated  since the fourth week of  infection. The observed tendency is a high positively of the snails Barbula class I which exposed to  five miracidia of the isolated SM. It observed that the molluscs begin to eliminate cercariae since the fourth till the 11^th week at the beginning of parasitism (figure 2).The figure 3 and 4  shows  minor tendency positively of the molluscs class III and class III respectively.
The  total  number  of   eliminated cercariae   for every animal was variable, from 2 till 1201.  It was carried out a regression and correlation analysis to estimate the possible relation between the total number of eliminated cercariae and the mollusc size. From this analysis is emitted that does not exist the significant relation between these two variables (Y:-37.47X+23.03  r:+ 0.45)

SURVIVAL CURVES FOR THE THREE  SIZE SNAILS OF  BARBULA VS. SM.

Figure 1. 20 B.glabrata (Barbula)  of  diferents shell diameter exposed to 5 miracidia  SM in duplicated.

 class I ♦  ¦    class II  •  -   class III ≡ •   

PERCENTAGE OF SNAILS POSITIVITY

Figure 2. 20  B.glabrata (Barbula) class I (5-8mm) exposed to 5 miracidia SM.

 PERCENTAGE OF SNAILS POSITIVITY

Figure 3. 20  B.glabrata (Barbula) class II (>5-8mm) exposed to 5 miracidia

 PERCENTAGE OF SNAILS POSITIVITY

 Figure 4. 20 B.glabrata (Barbula) class III (>11-14mm) exposed to 5 miracidia

DISCUSSION

According  to the different experiences,   the different  size of B. glabrata, showed differential  patroness in the cellular  reply of the molluscs due to the infection of  S. mansoni with  association to the age,   the research demonstrated  that the ontogeny determine and agree the molluscs  cellular reply to the infection, proving that the younger are less refractory than the older .^10,13   The retardations in the ripeness periods of sporocyst produce  quantitative variations in cercarial  production,  modulated  by  the   interaction molluscs-parasite. The percentage of susceptibily  were exposed in  Table 1. This  support the hypothesis that the bigger molluscs present lower percentage of infection and they are more retarded to the sporocyst ripeness. Other authors point out that the infection is determined  by  the number of settled miracidia that create the primary sporocyst which are the responsible of creating the cercariae. ¹⁰   The dynamics   of cercariae populations  presented in this research are similar to the described by others authors, both in natural environment and the laboratory (Figure 2, 3 and 4).  They point out that this behavior is product of the ripeness of the primary and secondary sporocyst, with the adaptive finality to keep  a  cercarial cycle  and the vertebrate host infection. ^6,15,16,18  It was not the determinated influence between the eliminate cercariae number and the snail size.    

In relation to survival curves, it was not observed a definitive tendency between the influence of size on its hope of life. Independently of the diameter, the lost presented a 50% of survival between the sixth and ninth week post-infection. In independently experiments were detected atrophied sporocyst (date no publicized). Which explain the molluscs mortality  that did not emit cercariae. The infection dynamic is so complex. Some authors point out that the delay of growing is the product of the physiological and biochemical  changes which were originated of the interaction parasite-molluscs.^9,17  Some experiences point out that the refractivity associated to the molluscs age and/or the presence  of lines no susceptible in other condition of the intermediary-host life shows a molecular component ¹⁴ 

In conclusion, this study adds evidences 10 in relations to the influence of shell diameter on susceptibility of  B. glabrata which were exposed to S. mansoni miracidia. This is an important factor in the dynamic relation host-intermediate parasitic.

ACKNOWLEDGMENTS

We express our sincere thanks to Mr. Ysrael Mбrquez  who is professor at the Simуn Rodrнguez University, because of  his English translation of the manuscript.
The technical assistance of Beatriz Moy, Senecio Rivas, Claudia Roman, Josй Castellanos and Jorge Lopez is gratefully  achnowledgments.

REFERENCES 

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