A resident of Rondevlei on the Garden Route noted an odd sight on February 13 when they spotted a number of dead birds close to Divisional Road 1614.
“The resident alluded to the possibility of the herbicide used by the Garden Route District Municipality: Roads and Transport Planning along the road reserve for Divisional Road 1614 being the cause of the aforesaid bird deaths. Lab tests have indicated otherwise,” Herman Pieters, Senior Communications Officer for the Garden Route District Municipality, said.
Two days later, on February 15, South African National Parks (SANParks) informed the Garden Route District Municipality (GRDM) that the deaths were caused by an uncommon disease called avian botulism.
Avian botulism is a neuromuscular illness in birds caused by a toxin that is produced by the bacterium Clostridium botulinum. Fish-eating birds are poisoned by eating fish that contain the toxin. Ingestion of maggots from the carcass of an infected animal can also cause bird to become infected with avian botulism.
Avian botulism is most prevalent during the summer months. The disease cannot be transmitted to humans, but as a precautionary measure, SANPARK rangers have collected the carcasses daily.
“Various departments in the GRDM responded promptly to establish the cause of the bird deaths and to determine whether the GRDM held any liability therein. Although we could establish that the deaths were not caused by herbicides used by the GRDM: Roads and Transport Planning Services, we will endeavour to exercise caution in the selection of the herbicides that are used for road reserve management and the effect that it may have on the receiving environment,” Pieters said.
“Avian botulism is a fairly common disease all over the word, however, it is neither controlled nor notifiable (which requires that they be are reported to the state vet). For this reason, we do not have data on the number of cases in the province. Our state vet in the George area has noted various numbers of death in water birds, particularly in Summer, over the last five years, due to suspected botulism,” said Bianca Capazorio, spokesperson for the Ministry of Economic Opportunities. “There are no special precautions that need to be taken when handling the carcasses, except those that one would take when handling any dead animal as pathogens can grow in carcasses. Wash hands thoroughly with soap after having handled any dead birds.”
Botulism spores are abundant in habitats where there are low levels of oxygen, and these includes soils, aquatic sediment from wetlands and lakes and the gills and digestive tracts of the fish living in those lakes. The spores can remain within these habitats for extended periods of time, and they do not dry or get affected by temperature changes.
While the spores themselves are harmless, the correct environmental factors may cause them to germinate and begin the process of growing into toxic bacterial cells. The active bacteria that causes avian botulism grows in areas with an abundance of decaying plant or animal material.
Outbreaks of avian botulism only occur when a variety of particular ecological factors play out concurrently.
This typically involves warmer water temperatures, anoxic (oxygen-deprived) conditions and an adequate amount of decaying plants, algae or animal materials.
As average air and water temperatures have risen on a global scale, warmer temperatures and anoxic conditions have been occurring more frequently on wetlands, lakes and ponds, possibly resulting in an increase of avian botulism incidents.
Once these factors lead to production of the toxin in material eaten by fish, the toxin can be passed up the food chain as wild birds consume the contaminated fish or maggots from the carcass of an infected animal. This continues the spread of avian botulism and may be the reason behind large-scale deaths of birds.
Avian botulism typically results in paralysis, and the infected species (in this case, birds) can be seen exhibiting unusual behaviour.
For example, water birds may not be able to hold their head up and as a result, often drown. Gulls can often walk, but not fly. Other birds may drag one or both wings, exhibiting poor posture while standing.
Once infected, fish may flounder or swim erratically near the surface of the water and they may have trouble staying right-side-up. “Breaching” may also occur, during which a fish will float with its head near the surface and tail lowered below. Infected fish usually die quickly.