Heat stress, exhaustion, prostration, hyperthermia, overheating or heatstroke in rabbits is a life-threatening condition earmarked by a rise in the rabbit’s core body temperature and dysfunction of the central nerves systems that may induce delirium, convulsions, and coma.
A rectal temperature of more than 40° C will indicate a heatstroke, and if it goes beyond 42° C, it will be fatal. Without first aid, this condition can make “your rabbit's organs to shut down and result in death.“ . Dehydration, excessive exercise, cardiovascular and neuromuscular disease may worsen it.
While in the wild, bunnies will tend to spend time inside their burrows if the environmental temperature changes. There is also the adaptational changes in fur density during colder and warmer seasons with warmer ones having thinner and scanty fur.
The optimum temperature that rabbits require usually ranges from 60 to 70 degrees Fahrenheit. These pets do not thrive well in temperatures above 90°F and if the humidity is high, temperatures above 80° F may be harmful. This can be triggered by direct sunlight, a hot car (while traveling with them), placing your rabbit next to a heat radiator or weather changes.
The “aged, overweight and pregnant rabbits are most susceptible to heat prostration.” . This problem may also be common in breeds with a dense coat such as the angora breeds as well as young ones. However, it is not significant since rabbits do not primarily cool their bodies by perspiration.
Hyperthermia is also a common problem during hot summers or in regions that experience higher temperatures. Expect other problems such as flystrike during summer too.
How cooling occurs in rabbits
To appreciate how a heat stroke occurs in these pets, you deserve to know how they cool their body temperatures. Rabbit’s cool their bodies primarily through respiration (breathing), their nasal mucosa area and ears.
Over 80% of the cooling occurs through breathing. For the case of the nasal mucosa (the lamellae), it occurs as air passes through this membrane. Finally, the ears help in cooling by allowing blood to flow to their coolest point, far away from the body core. The pinna has a dense and complex blood vessel network as well as arteriovenous anastomoses that dilate.
It is common for rabbits to stretch their body as this helps heat loss by convection and radiation and you will note the stretching when temperatures rise.
Since they do not have many sweat glands, only a very small amount of heat is lost through perspiration. This cooling is further restricted by their coat. Therefore, only a small amount of evaporation that occurs.
When the temperature goes beyond 39°C/102.2°F, the cooling system may not help much, and your pet will start suffering from hyperthermia.
Some of the indicators that your rabbit is suffering from heat stress include the following:
- Panting - short, quick breathes to help humify air in their lungs
- Salivating, moisture or waterdrops on the philtrum and nostril periphery accompanied by serious nasal secretion.
- Reddening of their ears due to enlarged blood vessels
- Slow movements, drooling and general weakness
- Bloody discharge from nose or mouth.
Hyperthermia treatment and recovery
A heat stroke requires emergency medical attention. Call your vet immediately. Some of the ways to help cool its temperature includes the following.
- Move the rabbit to a calm airy, cool room out of the sun but not a draught.
- Humidify their ears using lukewarm water.
- Wrap an iced water bottle inside a towel and place it next to the affected bunny’s body. You can also place a damp piece of cloth on it.
- In case of dehydration, consider fluid profusion such as saline sodium chloride 0.9%. However, avoid hyperosmolar solution Ringer's lactate as they have been found to worsen brain swelling. 
- Do not immerse the rabbit in cold water. Although it can help lower their body temperatures, it has side effects, some of them being cardiac arrest or heart arrhythmia.
- Place them near a fan
During recovery, you need to continue monitoring it for several days.
Their housing should be in a place that allows free air circulation but not draughts.
- Provide an area with plenty of shade for both the indoor and outdoor ones. Avoid direct sunlight.
- Move your hutch to a colder part of your house or garden such as inside your basement if it is cooler.
- Provide plenty of water to your bunnies. Water is very important, and rabbits must be given an unlimited supply of fresh, clean water. You can put a few ice cubes inside a bowl for them to lick
- Always, groom your rabbits, especially if they have longer fur
- Freeze water inside a bottle, wrap it with a piece of cloth and place it inside the cage or hutch. Some may decide to snuggle near it if they feel the heat.
- Consider evaporative air coolers and air conditioning including a fun but do not place them directly on your rabbits and they must have bunny-proofed electric codes, or in areas where they cannot access them.
- Place a large marble or ceramic tile inside the hutch or cage as it often stays cooler than the room and it will be a great place for it to lie on.
- Ensure your car is well ventilated if you intend to travel with this pet. You can open windows
- Spray their ears with water mist to help them improve their cooling efficiency but do not wet them completely.
- Provide diets with leafy greens as they can help them stay hydrated.