The rabbit in front of me looked fairly normal with the exception that she kept her head cocked to one side as if she were listening for voices that only a rabbit could hear emanating from the surface of the exam table. “She’s been otherwise acting pretty normal, eating and drinking and so on, she just doesn’t seem to want to move around as much and she always has her head tipped like that.” said her owner.
She had developed a problem that shows up somewhat frequently in rabbits as well as occasional rats and guinea pigs. The common term for this syndrome is wry neck, but the source of the problem usually lies within the inner ear.
At a young age most rabbits, rats, and other rodents are exposed to upper respiratory diseases that generally cause mild sneezing symptoms and then go away on their own. Their immune systems manage to suppress the disease enough to make the symptoms stop, but often remnants of that infection linger in the body silently for the remainder of the animal’s life. Occasionally the rabbit or rodent gets unlucky and a few of the lurking bacteria sneak up the Eustachian tube and make a nice little nest for themselves in the inner ear.
As the bacteria multiply they form an abscess, or a pocket of pus. As that pocket expands within the bony confines of the inner ear it starts affecting the normal structures that run through that part of the world. One thing housed in that area is the body’s balance center. It is a matched pair of sensors, one in each ear, that help the brain detect where the body is in space and whether it is tipping to one side or the other. When an abscess starts affecting one of these sensors the brain interprets those signals as a feeling of falling, even though the animal is not actually moving. By tipping the head to one side the animal can alleviate the sensation of falling by tricking the malfunctioning sensors into detecting what they want to in order to be satisfied that the body is not about to crash to the ground. As the sensation of falling becomes more severe the head tipping is not always enough to override it. Affected animals may start pushing themselves over on one side, and may even start rolling over again and again because their brain and their body are in conflict about what exactly is happening.
Another structure that runs through the inner ear is the facial nerve. Its most obvious function is imparting the ability to blink. Many affected animals lose their ability to blink on the side to which they are tipping their heads. Testing whether an animal can blink involves lightly tapping on the inside and outside corners of the eyelids. Animals that cannot blink will often use the muscles attached to the back of their eyes to pull the eyeball down into the socket as an alternative to closing the lids.
Unfortunately even though an infection is usually the problem it can be very resistant to treatment. Rabbits and rodents have a natural ability to wall off infections so that they can’t gain easy access to the rest of their body. This serves them well in the wild, allowing them to survive for long periods of time with serious infections. However the wall that keeps the infection contained also acts as an effective barrier to antibiotics that could otherwise get in and help fight the infection off. I find that antibiotics seem to bring some short term relief and can put off the onset of severely debilitating symptoms, but rarely do I get an animal to resolve completely. In the long run most rabbits and rodents that develop this problem progress to the point where they cannot eat or stand. I imagine that the constant vertigo becomes very unpleasant as well. Most eventually need to be euthanized, but every now and then I get a case that does remarkably well, so I usually recommend trying if the animal is not miserable.
In the case of rabbits in particular another cause of head tilt could be a one-celled parasite called encephalitazoan. It has a fondness for brain and kidney tissue and is implicated as a possible cause in head tilt cases as well as in the development of cataracts and drinking and urinating excessively. There is a blood test that can detect whether the rabbit has been exposed to this organism, but a positive test doesn’t prove that the symptoms are actually being caused by encephilitazoan, only that the rabbit has been exposed at some time. Although the organism is easy to kill with some inexpensive and safe medication the damage it has done to the brain is still left behind after the active infection is cleared. Because the treatment for encephalitazoan is inexpensive and safe and the test is quite expensive and not completely conclusive I usually just treat for the parasite as well as starting antibiotics.