Mem Inst Oswaldo Cruz, Rio de Janeiro, Vol. 94(6): 841-844, Nov./Dec. 1999

Mauricy A da Motta⁺ , Maria Teresa J Catanho, Ana MMA Melo

Departamento de Biofísica e Radiobiologia, Universidade Federal de Pernambuco, Av. Prof. Moraes Rego s/n^o, 50670-420 Recife, PE, Brasil

Received 31 March 1999
Accepted 5 August 1999

Doses of ⁶⁰Co gamma radiation with 2.5; 5; 7.5; 10; 15; 20; 25; 30; 35; 40; 45; 50; 55; 60; 80; 160; 320 and 640 Gy were applied to 1,080 snails Biomphalaria straminea, an intermediate host of Schistosoma mansoni, divided in groups containing 30 mollusks. In addition, 60 non irradiated snails were kept as control. Fifty percent of the population was kept in colonies (allowing cross fertilization) while the other half was maintained in sexual isolation (allowing self fertilization) and during one month their growth was observed through the daily measurement of the shell diameter. Results showed that after 20 Gy doses the growth in shell diameter of irradiated snails was greater than that of the control group after 30 days. At this dose the snail size was the greatest, among all isolated groups. The 80 Gy doses also induced the final shell diameter of isolated snails to be greater then that observed in the control groups. As this effect was most evident among the isolated snails, a possible hormonal role may have been involved in the observed phenomena, which is under investigation with the objective of identifying any future applications that this could have to schistosomiasis control.

Key words: growth - gamma radiation - Biomphalaria straminea - snails

Schistosoma mansoni is widespread in the northeast of Brazil, reportedly brought from Africa by the slaves (Malta 1994). It also exists in other regions and causes endemic schistosomiasis, a tropical disease that can bring about spleen and liver hypertrophy, digestive bleeding and other circulatory complications known as Banti syndrome.

Biomphalaria are hermaphrodite snails (Paraense & Deslandes 1955, Leal 1976) and their species glabrata, straminea and tenagophila (Paraense & Deslandes 1955) exist in Brazil. These snails are intermediate hosts for S. mansoni with B. straminea being prevalent in many regions of Brazil (Paraense & Deslandes1955, Leal 1976).

The embryological development of snails is particularly affected by ionizing radiation as stated by Liard et al. (1968), Okazaki et al. (1996), Melo et al. (1996) and Melo (1998). A previous report stated that following irradiation with gamma rays, a cessation in the development of the shell may occur (Perlowagora-Szumlewicz & Berry 1964). Snails of different ages showed the same shell diameter as observed by Rey (1967), Leal (1976), Rozenberg (1989) and Carvalho (1992) .

In order to study controversial aspects in B. straminea shell development following exposure to ionizing radiation, the growing behavior of this epidemiological vector of schistosomiasis after exposure to low intensity gamma rays was studied, in colonies (cross fertilization) and sexually segregated (self fertilization) situations.

MATERIALS AND METHODS

A population of 1,080 snails of B. straminea, BH breed obtained from Centro de Pesquisas Aggeu Magalhães-Fiocruz, was exposed to gamma- rays obtained from a ⁶⁰Co source receiving doses of 2.5; 5; 7.5; 10; 15; 20; 25; 30; 35; 40; 45; 50; 55; 60; 80; 160; 320 and 640 Gy, applied to groups of 30 snails per dose. An additional group of 60 non irradiated (0 Gy) snails was kept as control.

The mollusks were bred from eggs laid with a maximal difference of two days and were raised in laboratory for approximately 50 days. All snails were in a reproductive development stage.

Fifty percent of the studied population was kept in colonies and the other half was maintained isolated in sexual segregation, allowing cross fertilization and self fertilization, respectively, in each set. Consequently, the snail in each groups/dose were placed in individual (isolated) and collective (colony) aquariums with the ratio of water volume/snail (50 ml) kept constantly replenished with water (25°C ± 2°C and pH 6.6-6.8) during the period of observation.

The irradiation of the snails by gamma-rays was done with groups of 30 snails held inside glass tubes by dose and placed in gamma-cell irradiator, model RL-60, from Radionics Laboratory, with dose rate of 0.97 Gy per minute. In order to avoid copulation during the exposure time, the snails were separated by a thin cotton cushion, in the isolated set of mollusks. The control group remained in a dark environment for the same time as the maximal (640 Gy) irradiation was applied in order to have the same conditions of temperature and humidity as those used in the irradiation procedure.

The diameter was measured daily by means of a caliper applied to the major shell diameter, with the sites used for measuring marked with white ink.

RESULTS

Another shift in the growth rate was detected after a dose of 80 Gy, but this was less intense than the peak induced by 20 Gy, among both colonized and segregated snails.

The isolated 80 Gy group values were 9.7% greater than those seen in the equivalent control group, while the colonized 20 Gy group showed a development only 2.4% superior to that observed in control snails. However, at the 20 Gy dose the isolated group values surpassed those of the control group by 22.2% and the colonized snails attained a growth 17.2% greater than the control. As stated earlier this suggests the involvement of a window of energy in this phenomenon, as discussed bellow.

DISCUSSION

Notwithstanding, we should consider that an increase of lethality occurring in the high doses groups could cause a variation in the snail mean diameter. Thus, after the death of one snail with a large shell, the mean value of the diameter of its group decreases. This fact could explain some peaks of growth such as those observed in the 15, 20, 25, 40 e 80 Gy groups. However, in the 30, 35, 45, 50, 55 and 60 Gy groups, the shell diameter did not present any significant alteration during the observation time. Such findings were also reported by Perlowagora-Szumlewcz (1964) and Carvalho (1992). We are of the opinion that the error caused in the final observation as irrelevant, taking in account the length of the sampling procedure.

These findings led us to suppose a marked enhancing effect of the 20 Gy gamma-rays doses on the growth of the sexually segregated B. straminea, as compared with the control group. Ongoing studies are in progress to investigate the observed difference between the groups analyzed here, mainly on the sexual isolation and self fertilization hormonal aspects. RIA techniques will be tested in an attempt to define the hormonal role played in this phenomenon, envisaging a possible future application in the epidemiological control of schistosomiasis.

ACKNOWLEDGEMENTS

To CPqAM donation of B. straminea snails and to CNPq for the grants.

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