Botulism is an intoxication caused by the botulinum neurotoxin, produced mainly by the bacterium Clostridium botulinum. It is among the most serious food poisonings by humans. At the same time, it is one of the diseases that most significantly affects migratory birds, especially waterfowl and waders, affecting thousands and even millions of individuals of many different species each year. The abundant and widely distributed species surely support the losses by botulism, but a single outbreak can have serious consequences for the viability of those species in danger or with a reduced distribution.
When the spores of C. botulinum present in the sediment of the lagoons are in the ideal conditions, the development of the bacteria takes place, which can arrive directly to the birds that feed by filtering the sediment, or from vectors that accumulate botulinum toxin. During outbreaks of botulism the highest concentrations of the toxin are found in the carcasses of previously affected birds, given that optimal conditions are given because the bacteria grow (high temperature, absence of oxygen and organic matter). The invertebrates that feed on these corpses accumulate the toxin, which when dispersed act as vectors to the birds that eat them. These, when ingested, become intoxicated and die, generating a new substrate in which C. botulinum and its toxin develop again. Thus the cycle feeds back, until the environmental conditions cease to be favorable.
The conditions due to the development of C. botulinum in the sediment are mainly: lack of oxygen, high temperature, pH between 7.5-9, negative redox potential and an increase in organic matter. One of the most important factors is the high temperature, which apart from being necessary for the bacteria to multiply, can affect the previous chemical parameters of the water, with a chain effect due to the increase in the decomposition of organic matter, which has a effect on the pH and decreases the available oxygen and the redox potential.
The first outbreak of botulism in the Parc Natural dels Aiguamolls de l’Empordà occurred in 1998, since then there have been 6 major outbreaks. Those that affected more birds and more different species were the first two, in 1998 and 1999, with 945 and 698 affected birds, respectively.
In the case of the Aiguamolls de l’Empordà, the factors that seem to have triggered the ideal conditions for the development of C. botulinum are diverse. First of all, we must highlight the environmental factors related to weather conditions, both at the time of the outbreak and in the previous months. In this study we have seen that, although there is no significant relationship between temperature, precipitation and the appearance of botulism outbreaks, these factors seem to be of great importance in increasing the risk of outbreaks. Thus, warm and dry years, in which the water of the lagoons decreases and, therefore, also decreases the oxygen level, are susceptible to suffering an outbreak. The years in which these conditions have occurred, but there have been no cases of botulism, it is likely that it could have been avoided thanks to the management that has been made of the water or the influence of other factors. Anyway, you can not claim to have absolute control over the risk of botulism and in fact, some authors already say that some outbreaks of botulism may simply be unpredictable.
Other factors of capital importance are the chemical parameters of water. Als Aiguamolls de l’Empordà, the alteration of the natural water regime and the artificialization of this, may have led to an increase in the concentration of nutrients and organic matter leading to an increase in eutrophication. Over-flooding during the times prior to the first outbreaks could have led to an increase in organic matter and nutrients to water. On the other hand, one of the areas most susceptible to suffering cases of botulism is the Sistema d’Aiguamolls Construïts of the WWTP d’Empuriabrava, where in summer there is a notable increase in the nutrient load, which can also lead to eutrophy and lowering of oxygen in the water.
The reason that may have generated an outbreak of botulism may actually be an interaction of multiple factors, complex and difficult to determine, although they show a set of variables on which we must be especially alert.
Finally, we have seen that both the preventive management and the correction of botulism outbreaks is very complex and involves a series of pros and cons that must be put on the balance when implementing them. From the outset, bird conservation is a key element in this natural park, but it is also important to conserve the rest of the animal and plant species of the wetland system, as well as the water functioning characteristic of the sectors that remain most natural of this system.
Es evidente que los brotes de botulismo pueden tener una gran afectación en cuanto a mortalidad de aves, un efecto muy visible que manifiesta una pérdida muy notoria y que puede tener efectos importantes sobre algunas especies escasas. Pero hay que tener en cuenta que la aplicación de las medidas preventivas de secado durante periodos demasiado prolongados, también comportan un perjuicio importante en en cuanto a fauna y flora, que a menudo puede ser mucho más grave a pesar de ser menos visible. Así, alargar los periodos con agua de algunos sectores, si está disponible y sin alterar de manera importando el ritmo natural de la zona, combinado con una buena vigilancia durante el verano podría ser una buena estrategia para evitar los brotes de botulismo, manteniendo los valores de estos espacios por los cuales se han protegido o incluso creado. También hay que tener en cuenta que, como ya se ha mencionado anteriormente, el verano puede ser el reflejo de la gestión que se ha llevado durante todo el año, de forma que conviene evitar llegar en verano con concentraciones elevadas de nutrientes a los sectores más sensibles.
Even so, we must bear in mind that when we find ourselves in a Mediterranean region where summers are hot and dry, water management becomes an even more difficult task. It is not always possible to create alternatives with highly oxygenated water and most of them sometimes reduce the level of water in such a way that it is impossible to recover it again with quality water. That is why it is key that, in certain places, you are especially alert to maintain and lengthen the levels of water obtained during the spring and to avoid that a sudden descent can not be recovered until the autumn.