Hedgehogs are famous for curling into tight, prickly balls. But a rare condition can cause their bodies to swell to several times their normal size, leaving them unable to move or defend themselves. So what causes this syndrome and can affected animals survive?
What is balloon syndrome?
Balloon syndrome is a potentially life-threatening condition that causes hedgehogs to become abnormally swollen as air or gas accumulates beneath their skin.
Known medically as subcutaneous emphysema, the condition exploits a feature of hedgehog anatomy that normally helps protect them from predators.
“Hedgehogs have relatively loose skin allowing them to curl up into a ball for defence. But there is also a large subcutaneous space where air, if it enters, can spread widely. The animal then becomes visibly swollen – hence the name - and the spines may be raised due to the stretched skin,” explains Dr Katharina Seilern-Macpherson, Garden Wildlife Health Vet at ZSL's Institute of Zoology.
What causes hedgehogs to balloon?
The most commonly recognised cause is physical trauma, such as being hit by a car or attacked by another animal. An injury can allow air to escape from the lungs or airways and become trapped beneath the skin, where it spreads around the body.
But the exact cause often remains a mystery. “‘Ballooning’ is a clinical sign rather than a specific disease, and the underlying cause can often not be identified,” says Dr Seilern-Macpherson.
Lung disease and, more rarely, infections involving gas-producing bacteria have also been suggested as possible causes, although scientists have yet to establish how important these mechanisms are.
How quickly does it happen?
A hedgehog can transform from its usual shape into a swollen, rounded animal within hours.
Affected animals may struggle to walk or curl up, become lethargic and, if their respiratory tract has been injured, experience breathing difficulties.
However, scientific research into the condition is limited. Dr Seilern-Macpherson says reports from wildlife rehabilitators suggest swelling can develop rapidly, over anything from a few hours to several days.
Can balloon syndrome kill a hedgehog?
Although the trapped air itself is not necessarily fatal, the consequences can be devastating.
A severely swollen hedgehog may be unable to forage for food, move normally or roll into its characteristic defensive ball. This leaves it exposed to predators, cold temperatures and starvation.
If the underlying injury involves the lungs or airways, the animal may also have difficulty breathing.
Dr Seilern-Macpherson explains that deaths are generally more likely to result from the original injury or illness, and the animal's weakened condition, than from the trapped air alone.
Can hedgehogs recover?
Fortunately, some do recover with veterinary treatment. Depending on the circumstances, vets may remove trapped air, provide pain relief and treat any underlying injuries or illnesses.
But recovery depends on what caused the swelling, how severe it is and whether the animal has developed complications.
It is not known how many wild hedgehogs develop balloon syndrome. Although wildlife rescue centres encounter cases, there are no published figures establishing its prevalence.
What should you do if you find a swollen hedgehog?
A hedgehog that looks unusually round or inflated needs urgent professional attention.
Contact a local vet or wildlife rescue centre promptly rather than attempting to release the trapped air yourself.
If handling is necessary, wear gloves and wash your hands thoroughly afterwards, as sick hedgehogs can carry infections that may affect people or pets.
It is also important to record exactly where the hedgehog was found, so it can be returned to its home territory if it recovers. Dr Seilern-Macpherson adds: "It is important to be aware that there are several possible causes of ill health in hedgehogs, and whilst the risks are low, some may have the potential to cause ill health in humans and pets. Therefore, we always recommend following strict hygiene precautions."
