Avian Botulism in Nevada
By: Russell Woolstenhulme –
Migratory Game Bird Biologist, NDOW
During the hot summer months, when many of us dream about cooler temperatures and wonder about the coming waterfowl season, we hope for optimal water conditions and plentiful hunting opportunities. But as one sits sweltering in the high temperatures we have been experiencing it is hard to not think about the recent flooding of Carson Lake Wildlife Management Area (WMA) and consider the potential for another botulism outbreak.
So, what exactly is botulism and how does it impact waterfowl? Avian botulism (botulism Type C) is caused by a soil bacterium. The bacteria are always present year-round in marsh systems. However, during summer months conditions occur that creates the perfect condition for the bacterium to begin reproducing rapidly. This is known as a bloom. While many factors contribute to a bloom, the main culprits include stagnant water, high water temperatures, the level of oxygen in the water (known as dissolved oxygen or D.O.) declining, and a load of protein for the bacteria to feed on. Many of these same conditions can also cause other aquatic life like fish to die, which adds to the problem as the decomposition process further depletes oxygen and the botulism bacteria feed on the proteins of the dead organic material.
As the bacteria causing botulism increases, it begins to accumulate in aquatic invertebrate as they filter feed. Waterfowl and other water birds in turn consume the invertebrates and can become impacted by the amassed bacteria. As birds die from the botulism, they become maggot infested. The maggots that are consuming the dead birds are now accumulating the bacteria, which other birds then feed upon. This creates a botulism cycle.
When ingested by birds, botulism has a paralytic effect. The paralysis typically starts in their feet and legs and works its way towards the head. As land mobility becomes difficult from onsetting paralysis, birds move into the water. Soon even swimming becomes difficult and flightlessness follows. Because the legs have stopped functioning, impacted birds are seen propelling themselves through the water with wing flapping. Finally, these now water bound birds lose the ability to hold their heads up as paralysis moves into their necks. At this point many of the birds fall victim to drowning as they can no longer keep their heads above water.
The susceptibility of waterfowl may differ among species, and certainly ingestion rate of the bacteria will impact susceptibility. When removed early enough from the effected waters and provided proper care and fresh water, many birds can recover. But most often the impacts of botulism will result in death.
There are several remedies to try and combat botulism blooms when they occur. One is to flood old stagnant, warm water with fresh oxygenated and cooler water. Another is to dry up the impacted waters so that birds no longer are drawn to it. When these actions are not possible and an outbreak occurs, one must try and stop the maggot cycle. This requires actively picking up dead birds to remove them from areas where other birds can get to their maggoty carcasses. The complication of this approach is that the bacterium still exists within the invertebrate insect community that birds will continue to eat. It is also unlikely to remove all the dead birds that may be hidden in stands of hard stem in the marshes.
Carson Lake WMA is once again suffering from the effects of flood conditions. During the flood event, damage was sustained to the levee system that impedes the ability to effectively move fresh water into the area. Additionally, other infrastructure difficulties make it impossible to effectively drain and dry the area. These factors make managing the waters to help control an outbreak a challenge. Wildlife Area employees monitor Carson Lake WMA weekly for any signs of an outbreak occurring. Dissolved oxygen levels and water temperatures are monitored to help staff see early risks of an outbreak and the Department of Wildlife staff meet frequently to discuss and plan actions to take in the event of botulism.
The hope is always for the best outcome and that no botulism event will occur during our warm summer months. Perhaps fortune will favor us. Hopefully cool fall weather will come quickly bringing with it flights of waterfowl to greet us at the marsh. I hope to see you there.
