Wild boar mortality in Brittany: the H2S hypothesis is highly probable

For a number of years now, green algae has proliferated on certain beaches in Brittany. When it accumulates on the beaches and decomposes, it releases significant emissions of various gases including hydrogen sulfide (H2S). These emissions, because of their toxicity, create a health problem that led the government, in February 2010, to adopt a five-year plan to combat these green algae. As part of this work, ANSES published an Opinion and an Expert Assessment Report in July 2011 on the risks of gas emissions from green algae to the health of surrounding populations, walkers and workers.

In this context, in the summer of 2011, the bodies of several wild animals (thirty-six wild boars, three coypus and one badger) were discovered on the beach near Morieux and on the banks of the estuary of the Gouessant River (Côtes-d'Armor). Tests were performed on these animals and on the surrounding environment.

In light of the results, the Directorate General for Food requested that ANSES issue an opinion on the toxicity of hydrogen sulfide for animals and the possible link between the presence of H2S at the site and the death of the animals. To analyse the available evidence, ANSES set up an Emergency collective expert assessment group bringing together specialists in toxicology, wildlife, animal diseases, laboratory techniques and the environment, after examining their public declarations of interests.

Evidence from the autopsy suggests that the boars had approached the Gouessant estuary to drink after feeding. In addition, the contextual information and the massive, sudden mortality of almost all of the wild boars in the group, in a confined area, strongly indicate an acute intoxication of the animals. Several causes of acute intoxication were discussed and ruled out when the test results and/or autopsy allowed. The autopsy reports found pulmonary oedema in sixteen out of twenty-two of the boars and in all of the three coypus. In addition, in some animals there was a correlation between lesions and significant concentrations of H2S detected on lung samples. This evidence and the absence of other characteristic lesions point to air exposure to H2S, at concentrations sufficient to immobilise the animals.

It should be noted that most of the bodies were recovered in the estuary area and not on the beach. The low tidal ranges recorded during the mortality period do not support the hypothesis that the bodies were washed from the beach to the estuary. Gas emissions from the wide mudflats of the estuary were measured periodically in August 2011, following the health incident, and revealed non-negligible concentrations of H2S, that did not however reach lethal levels. Given the variability of gas emissions from the mud combined with multiple factors, and the ad hoc nature of the samples taken, the possibility of higher concentrations than those found at the time of measurement cannot be ruled out. The animals found dead may therefore have been exposed to H2S concentrations contributing to the lesions and their death, although it is impossible to confirm whether this was the only factor which contributed to these cases of mass mortality. The presence of silt in the upper respiratory tract of most of the animals raises the possibility of their choking in the mud, following their immobilisation.

In order to reach a more formal conclusion on this hypothesis, ANSES recommends that further studies be carried out, including screening for cyanotoxins in stored animal tissue samples.

The Agency emphasises that the health issues associated with the decomposition of green algae, thus far documented for beaches and which have led to management actions, may also, in some cases, apply to mudflats, estuary bottoms and some river beds, although this hypothesis remains to be explored.