Print version ISSN 0037-8682
Rev. Soc. Bras. Med. Trop. vol.39 no.5 Uberaba Sept./Oct. 2006
ARTIGO DE OPINIÃO OPINION ARTICLE
Moluscos peçonhentos: riscos de acidentes em humanos pelo molusco Conus (Gastrópode: cunidae) in Brazil
Vidal Haddad JuniorI, II; João Batista de Paula NetoIII; Válter José CoboIV
IFaculdade de Medicina de Botucatu da Universidade Estadual Paulista, Botucatu, SP
IIHospital Vital Brazil do Instituto Butantan, São Paulo, SP, Brazil
IIIPrograma de Assistência à AIDS, Moçambique
IVDepartamento de Biologia da Universidade de Taubaté, Taubaté, SP
Mollusks of the genus Conus present a venomous apparatus composed of radulae, a chitin structure linked to glands, which injects potent neurotoxic peptides, causing serious human envenomation and even death, associated with the blockage of certain receptors and muscular paralysis. No reported envenomation has occurred in Brazil, but certain populations are at risk of accidents.
Key-words: Conus. Venomous animals. Venomous mollusks. Human envenoming. Brazil.
Os moluscos do gênero Conus apresentam um aparato venenoso composto de uma rádula quitinosa ligada a glândulas de peçonha, causando envenenamentos humanos graves e mesmo óbitos pela ação neurotóxica indutora do bloqueio de vários receptores e paralisia muscular. Não há casos descritos de envenenamento no país, mas determinadas populações correm risco de acidentes.
Palavras-chaves: Conus. Animais peçonhentos. Moluscos venenosos. Envenenamentos humanos. Brasil.
Mollusks include some of the most well-known invertebrates. Phyllum species present a specialized chitin tooth-like feeding structure (the radulae) that can grind through the shells of other animals19. The class Gastropoda includes marine, terrestrial and freshwater snails, limpets, nudibranchs, slugs and abalones.
The most specialized gastropods may be the cone snails, genus Conus, in which the radulae is reduced to a few isolated venom-injecting teeth, the so-called toxoglossate radulae, that are linked to venomous glands. The harpoon-like teeth are discharged from the end of a long proboscis that can be ejected rapidly to capture its prey (Figures 1 and 2). The Conus genus presents a unique spiraled shell.
The great majority of cone snails feed on other mollusks and bristle worms. However, some species are piscivorous, presenting the most potent toxins and they can cause fatal envenomation in humans7 13. The most important piscivorous Conus are present in the Indo-Pacific area, such as C. geographus (the species most associated with human deaths), C. aulicus, C. marmoreus, C. striatus C. textile and C. tulipa. There are about two dozen Conus species in Brazil and the most important are C. centurio, C. clench, C. clerii, C. ermineus,, C. jaspidus, and C. regius10. Moreover, species like C. clerii, C. jaspideus and C. regius feed actively on marine worms and present active toxins2. The worm-eating Conus offer less danger to human beings, but some species that provoke serious accidents are not piscivorous, such as C. marmoreus.
VENOM ACTION OF THE CONUS SNAILS
The venom of the Conus species is composed of conotoxins, which are neurotoxins of low molecular weight. Their action is extremely fast, which is compatible with the slowness of the snail in its environment and the consequent difficulty involved in capturing the poisoned prey.
The action of the conotoxins occurs by blockage of muscular and neural receptors7. There are two different toxin effects in the venom. The first, the "lightning-strike" effect, causes immediate immobilization of the injected prey through peptides that inhibit voltage-gated sodium channel inactivation, as well as peptides that block potassium channels. Together, this combination results in a massive depolarization of any axons in the immediate vicinity of the venom injection site, causing an effect similar to electrocuting the prey. The second effect is achieved more slowly and involves total inhibition of neuromuscular transmission through conopeptides, which act at sites remote from the venom injection site, such as neuromuscular junctions. The motor cabal effect, found in all fish-hunting Conus venoms examined so far, includes peptides that inhibit the presynaptic calcium channels that control neurotransmitter release, the postsynaptic nicotinic receptors, and the sodium channels that underlie the muscle action potential20.
Currently, conotoxins are a valuable tool of scientific research, due to the intense pharmacological activity presented by the peptides. One of the drugs in clinical tests is ziconotide, which is a peptide that blocks the neuronal calcium canals with excellent effect in the treatment of chronic and severe painful processes16.
CLINICAL ASPECTS OF THE ENVENOMATION
The exact number of proven deaths caused by envenomation by Conus shells is not known, though about 50 supposed occurrences are cited. Envenomation can simulate other causes of death, like myocardial infarction or cerebral ischemia, and may not always be correctly identified. The index of mortality observed in envenomation by Conus shells reaches 25%7.
A typical accident involving Conus shells, initially presents intense burning pain at the site of the sting, which evolves in about one hour to progressive paralysis of the body muscles without other local symptoms or signs. History of contact with the mollusk associated with intense muscular weakness should raise suspicion of an accident. In later phases, the patient can develop palpebral ptosis, blurry vision and speech and deglutition difficulties, unconsciousness and dyspnoea, with possible evolution to respiratory arrest that can be fatal and occurs 40 minutes to 5 hours after the sting3 7 9 12 13 15 17 18.
In Brazil, three species of Conus of large diameter are potentially dangerous: C. regius, C. centurio and C. ermineus, the last of which is piscivorous3 11. C. ermineus was identified in the Fernando de Noronha Islands6, when studies regarding its high toxicity and the potential risk for human envenomation already existed1 8 4. Currently, however, there is no safe clinical information regarding the species of Conus found in Brazil or the occurrence of possible accidents, although the possibility of envenomation, especially for Biology professionals and shell collectors exists.
Diagnosis of the injuries caused by venomous mollusks is based on clinical observation. There is no antidote for the toxins and severe envenomation should be treated with artificial ventilation, the only effective measure, since no available antivenom exists at present15. Severity depends on the time passed since envenomation and of the full installation of the manifestations, with some consideration given to the sting site. A lack of medical resources also influences the prognosis. Envenomation by the Conus genus is rare, but it is possible in risk populations and the gastrointestinal and neuromuscular symptoms in patients should not be confused with alimentary poisoning.
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Dr. Vidal Haddad Junior
Caixa Postal 557
18618-000, Botucatu, SP, Brazil
Telefax: 55 14 3882-4922
Recebido para publicação em 20/4/2006
Aceito em 10/8/2006