Neurotoxins
Botulism is a neurological disease that affects humans and other animal species and is linked to different types of neurotoxins produced by bacteria of the Clostridium genus, usually attributable to C. botulinum. Among these neurotoxins, some are responsible for human botulism, for example that linked to canned food, others affect wild birds. In these birds, avian botulism is the cause of mortality episodes recorded in Italy and in the world sometimes with serious impacts on bird populations as occurred during the 1932 die-off in the Great Salt Lake in North America which led to the death of approximately 250.000 animals. On 5 September 2019 in the Valle Mandriole area, in Po Delta Regional Park Emilia-Romagna in the province of Ravenna, some bird carcasses were found which, when analysed at the laboratories of the Experimental Zooprophylactic Institute of Lombardy and Emilia Romagna (IZSLER), showed the presence of clostridia producing neurotoxins.
Disease transmission
Among the first measures taken by the technical table set up to manage the avian botulism emergency - a table coordinated by the Municipality of Ravenna, with the participation of technicians and representatives of numerous local authorities and stakeholders, including the Po Delta Park Authority, the veterinary service of the AUSL of Romagna, the Emilia and Romagna Region - there was the recovery of the poisoned subjects still alive and the prompt removal of the carcasses of the deceased birds. This action is very important because the larvae of blowflies develop in the carcasses, which are biological accumulators of the neurotoxin present in the dead animals and when they are in turn preyed upon by other birds they poison them, giving rise to a cycle that sustains and amplifies the effects and possibilities of further transmission of the disease.
Wetlands
Another intervention decided to block the life cycle of the toxigenic strains of Clostridium spp. was to act on the water levels of the valley. In humid areas, especially freshwater, these bacteria develop when organic material of plant or animal origin decomposes in conjunction with high temperatures and a lack of oxygen in the water (anoxia), exactly the conditions that, together with the scarcity of rainfall, occurred in the summer of 2019. There were two viable alternatives to address this emergency situation: flooding or drying out the Mandriole Valley. Both actions can help reduce the presence of the bacterium and its negative impact, respectively by reducing its multiplication and by driving away the aquatic birds that were the first victims of the disease. It was decided to proceed with drying out to prevent aquatic birds from continuing to land in the few remaining pools. The strategy was effective and most of the 2158 birds that were victims of the disease died in the first few days after the onset of the disease and before the countermeasures in place.
Containment measures
The cases of avian botulism that occurred during the primary event between the end of summer and the beginning of autumn 2019 and in the following three years have been documented and recently published in the journal Animals. In the article, written by ISPRA researchers with the collaboration of veterinarians and biologists from the Municipality of Ravenna, the AUSL of Romagna and the IZSLER, the sequence of events is analyzed in relation to the environmental conditions and the management measures of containment, monitoring and prevention derived from this experience and to what happened in the three years following the recurrence of favorable conditions for avian botulism. The survival of Clostridium spp. spores in a quiescent state in the soil and in the substrate of wetlands, and also the winter persistence of toxins from overwintering blowfly pupae underline the need to monitor the areas affected by botulism, since if suitable conditions occur the disease can recur even years after the first event.
Climate changes
Valle Mandriole, a site of international importance under the Ramsar Convention included among the regional protected areas and ZSC/ZPS of the Natura 2000 Network, is like other wetlands among the most fragile ecosystems and most exposed to climate change of anthropogenic origin. The development of Clostridium spp. spores, favored by conditions of anoxia and decomposition of organic biomass in the presence of high summer temperatures, can become a major mortality factor for aquatic birds that can ingest and therefore spread botulinum neurotoxin. The study and management of this environmental emergency has linked the life cycle of Clostridium botulinum, the water management of the basins linked to water anoxia, climate change, the state of health of sedentary and migratory birds, making explicit the complexity of natural systems and the need for adequate prevention and management actions of emergency events (source: ISPRA).
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