What is Botulism?

By Erica A. Miller, DVM, and Deborah Comings, MS

The Ohio Rehabilitator, Issue 3, 2009

Botulism is caused by exposure to botulism toxin. This toxin is the most poisonous substance known. Waterfowl, particularly mallards, wood ducks, and teal are very sensitive to the toxin. Shorebirds, herons and gulls are also commonly affected. However, all animals (including people) are susceptible. Outbreaks are usually in late July/August, when water levels are low, but can occur in the winter when areas of open water are scarce. Maggots and mussels concentrate the toxin.

What Causes Botulism?

Botulism is caused by a toxin produced by the Gram-positive bacterium, Clostridium botulinum. Primarily seen in waterbirds, botulism has been documented in wading birds, shore birds, waterfowl, and diving birds. Some species, such as vultures, are extremely resistant to intoxication. Outbreaks rarely involve only one bird and may affect thousands of birds. There are several strains of botulism toxin (types A through F); types C and E are most commonly responsible for outbreaks in wildlife. This bacterium is very hardy because it is normally in an inactive spore form that allows it to survive extreme conditions, and it only grows in areas without oxygen. When conditions are favorable, the spore is activated, grows, and produces toxin. The bacteria producing type C botulism does not have the genetic material for toxin production and must first be infected by a bacteriophage (a virus that infects bacteria) in order to receive the genetic material necessary to produce the toxin. The toxin is only produced by active Clostridium (i.e., in the growth or vegetative stage), but the toxin remains functional even if the bacterium is destroyed. Maggots feeding on a carcass containing either types C or E toxin may accumulate the toxin, resulting in acute toxicity when ingested by another animal feeding on the maggots or carcass; therefore, carcass collection is key to controlling any botulism outbreak. Bottom-feeding mussels may concentrate type E toxin and perpetuate the outbreak when the birds feed on the mussels.

How Does an Animal Get Exposed to Botulism?

There are three ways to become poisoned by botulism:

• Most commonly, animals ingest pre-formed toxin.
• Wounds can become infected by C. botulinum, which produces toxin as it reproduces.
• Animals can also ingest clostridial spores which activate, grow in the gut and create new toxin. This usually only happens in very young animals; adults have normal bacteria in the gut that prevent clostridium from producing toxin.

What Happens When an Animal is Exposed to Botulism Toxin?

The toxin prevents signals from the brain from reaching the muscles by binding to motor endplates (the sites on nerves that release signals) and blocking the release of acetylcholine (the signal). Without these signals, voluntary muscles stop functioning correctly, resulting in signs of peripheral neuropathy (paralysis that moves from the outside inward). This neuropathy is dose related: the more toxin ingested, the fewer signals reach the muscles and the weaker the animal becomes. This reaction is irreversible as long as the toxin is present.

What are the Signs of Botulism?

Because the toxin acts very quickly, birds are characteristically in good body condition, and animals with varying degrees of incapacitation will be found together. Mildly affected waterfowl (Stage 1) cannot fly but may “wing-walk”. They are alert but cannot escape predators, so their level of stress is high. They are generally self-feeding and will recover with minimal supportive care. More seriously affected birds (Stage 2) cannot fly or walk and are not self-feeding. They can hold their heads up weakly, and possess a slow nictitans response (blink response). Critically ill birds (Stage 3) are almost completely paralyzed. They exhibit the classic sign of botulism, ‘limberneck’, which results from paralysis of the cervical musculature. They cannot hold up their heads; consequently, death frequently results from drowning. Paralysis of respiratory muscles may result in suffocation in birds in stage three. Inconsistent clinical signs include swelling of the eyelids and nictitans (third eyelid), ocular discharge, and hypersalivation.

Tri-State Bird Rescue & Research, Inc. Botulism Treatment Protocol

• Obtain body weight and PCV (packed cell volume or hematocrit)
• Give Pedialyte® or lactated Ringer’s® solution (LRS) PO followed by appropriate dose of activated charcoal (ToxiBan™)
• Flush eyes, apply TAO (triple antibiotic ophthalmic ointment); full fundic exam if indicated
• IV fluids for dehydrated birds or Stage 2 and 3 birds only

Although severely affected birds will not be able to hold up their heads, they must receive oral fluids to flush the toxin from their GIT. Rolled towels should be placed under their chins. ORAL VOLUMES SHOULD BE CONSERVATIVE! THESE BIRDS WILL HAVE DIFFICULTY SWALLOWING AND CAN ASPIRATE VERY EASILY.

DAILY CARE:

Stage 1 birds: LRS PO QID for the first day; Osmolite® or equivalent PO QID the second day; Osmolite®/Cereal diet
(see recipe below)PO QID the third day. After the third day, the bird’s condition should be re-evaluated. If the bird is selffeeding, active, and waterproof, it can be moved to an outdoor cage. Monitor the bird’s weight every three days.

Stage 2 birds: LRS PO Q 2 hours for the first two days; Osmolite® or equivalent PO Q 2 hours for the third day; Osmolite
® PO Q 3 hours the fourth day. The bird’s condition should be re-evaluated on the third day as it may be possible to decrease gavaging to Q 3 hours. The bird may be started on cereal diet on the fourth day as its condition permits. Baby cereal
gruel and water can be offered once normal head carriage has returned. Eyes should be flushed/TAO applied as needed, usually QID the first few days, decreasing to TID-BID by day 5. Vents must be washed at least BID to remove accumulated feces and to keep the feathers in good condition until the birds are standing.

Stage 3 birds: LRS PO Q 2 hours and D2.5LRS IV BID for the first two days; Osmolite® or equivalent PO Q 2 hours on the third day; Osmolite®/equivalent PO Q 3hours the fourth day. The bird’s condition should be re-evaluated on the fourth day; dilute cereal diet can be introduced on the fifth day. These birds should be weighed every 3 days. Flush eyes/apply TAO QID until day three, then decrease to TID. These birds often need a follow-up eye exam around day 4 to check for corneal lesions.

Stage 2 and 3 birds will need supplemental heat (brooder lamps) and rolled towels placed under their heads to keep them elevated. Position their heads so that their beaks point downward; any nasal discharge must be able to drain feely. They will initially need draped, padded playpens or similar restricted, soft-sided housing. Waterfowl that are deprived of bathing facilities for any length of time will often accumulate a build-up of dried mucus in the nares. Check their nares daily and remove any hardened debris.

Consideration for moving outside: Once the birds are self-feeding, active and waterproof, they can be moved to an outdoor enclosure with free access to pools.

Considerations for release: Botulism cases are often brought in by Fish and Wildlife personnel. They must be contacted when the birds are ready for release as they are often the people who decide where the birds will be released. Birds that are at or above incoming weight, active, self-feeding, waterproof, and flighted may be released.