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Head tilt in rabbits is seen with some frequency and can be caused by a variety of diseases. Another common name for head tilt is "wry neck". The correct medical term is vestibular disease (which can include other signs besides a head tilt). Another term that is often used is "torticollis" which means that the neck muscles are contracted and if this occurs, it develops as a consequence of the disease which keeps the head in a tilted position where over time the muscles may contract. This does not occur in all rabbits with vestibular disease, therefore the term "torticollis" is not the best term to use for the cause of the disease because in reality it is a potential consequence of the disease. Rabbits with vestibular disease can have a head position that ranges from a few degrees to 180 degrees off the normal position. They can fall over, circle, have difficulties standing and develop eye injuries because the prominent eye globe (especially of the "down" eye) is prone to trauma. The cardinal signs of true vestibular disease in the rabbit are a persistent head tilt and a loss of balance. In this article I will discuss the causes and treatments of the diseases that can cause head tilt in the rabbit. Causes of head tilt can be divided into peripheral (not involving the brain) and central (involving the brain). Being able to make this distinction is critical to determining a plan of treatment and prognosis.

ANATOMY

First let's look at the anatomy and function of some vital areas in order to understand what does and does not contribute to head tilt in the rabbit.

External Ear - The external ear canal is easily visible at the base of the ear flap called the pinna. The opening to this canal (which is very long and ends at the ear drum) and should be visually inspected at least weekly by the caregiver. Occasionally, external ear disease can extend to the middle or inner ear. Disease in this area alone can cause head shaking, drooping ear and pain but does NOT cause a persistent head tilt or loss of balance. It is important to note that the rabbit's head might be tilted temporarily due to discomfort but the head can be easily righted and there is no loss of balance.

Middle Ear - This area includes the tympanic membrane (ear drum), Eustachian tube, three auditory ossicles (tiny bones), and the tympanic nerve (a branch of the facial nerve CN VII). All of this area is protected by the tympanic bulla (a bony shell attached to the skull). Infectious organisms can enter this area one of three ways: (1) through the external ear, (2) from the pharynx up through the Eustachian tube, and (3) through the blood. Disease of the middle ear can cause head shaking, drooping ear and pain as well as deafness but does NOT cause a persistent head tilt. As with external ear disease there may be a temporary head tilt due to discomfort but no loss of balance.

If the branch of the facial nerve that passes through the middle ear is damaged it can cause a drooping lip and ear, drooling and a decreased or no eye blink reflex. If the cranial nerves CN III, IV or VI are damaged, Horner's Syndrome results. Horner's Syndrome is characterized by a drooping eyelid, sunken eye and relaxed third eyelid (where it stays "up" over the corner of the eye and can't retract) and a pupil that is more constricted than in the other eye. In some cases the eye becomes very dry because of loss of the ability to blink and replenish the eye with tears or because of damage to the nerves affecting the tear duct. Occasionally, the drooping eye and ear from facial nerve damage might mimic a mild head tilt. However there is NO loss of balance.

Inner Ear - The inner ear includes the cochlea, vestibule and semicircular canals and is innervated by the vestibulocochlear nerve (which originates in the brain and controls balance and normal head posture through sensors in the semicircular canals). The inner ear controls balance and hearing. Disease can enter this area through the same three pathways as in the middle ear. Signs of disease of the inner ear include deafness, HEAD TILT, LOSS OF BALANCE and horizontal or rotary nystagmus (eye "flipping" or "rolling") which is spontaneous and unrelated to the position of the head. Proprioception (the ability of the rabbit to sense where the feet and legs are) and postural reactions (the ability of the rabbit to try to return to a normal standing position) are normal. The rabbit will not act weak in other parts of the body and continues to try to maintain normal body position even if it is difficult and the head is tilted. It is unknown if rabbits have a feeling of nausea accompanying the loss of balance. Rabbits cannot vomit so nausea is difficult to detect. Nystagmus is not a definitive sign of nausea.

Brain - I will not discuss the entire anatomy of this very complex organ. However, a specific area of the brain stem contains the vestibular nuclei, the origin of the vestibular nerve in the inner ear. The vestibular nuclei serve as the body's central balance control. Signs of disease to this tiny area of the brain stem include HEAD TILT, LOSS OF BALANCE, circling toward the affected side, rolling, vertical nystagmus, positional nystagmus (occurs only when the rabbit's head position is changed), delayed or absent proprioception (animal has no sense of where the feet or legs are) and loss of postural reactions (the animal cannot properly right itself).

Spinal cord - The spinal cord contains the base of nerves that branch out to control all body functions. The spinal cord carries both sensory (sensation-receiving) and motor (muscle triggering) nerves. Damage to the spinal cord can result in signs that are either unilateral (one sided) or bilateral (both sides). A wide range of signs that can result from spinal cord disease including weakness or paralysis of any or all extremities, compromised bladder and bowel control, compromised ability to breathe, and so on. HEAD TILT is not a sign of primary spinal cord disease. However, a unilateral (one-sided) weakness might, in some cases, mimic the circling or rolling seen with vestibular disease due to the rabbit's inability to move the muscles properly on the affected side. There will not be a persistent head tilt, although there will be noticeable deficits or abnormalities in all the reflexes on the affected side.

The following is a chart that summarizes how to tell the difference between head tilts caused by disease of the brain stem and those that are caused by inner ear disease, which are the two main areas that generate a persistent head tilt.

DIFFERENTIATING CENTRAL (BRAIN STEM) FROM PERIPHERAL (INNER EAR DISEASE)*

DISEASES RESULTING IN HEAD TILT

A major differentiation that has to be made when diagnosing the cause of head tilts is whether it is peripheral (affecting areas other than the brain) or central (involving the brain and most specifically the vestibular nuclei). Once this differentiation is made, the veterinarian can then proceed along a further course of diagnostics and treatment. In general, the prognosis for full recovery is better for peripheral disease then for central disease, but each rabbit is unique and treatment and prognosis must be determined on an individual basis by the veterinarian handling the case.

OTITIS INTERNA (Inflammation of the inner ear) - This is the most common cause of head tilt in the house rabbit. Causes of inflammation of the inner ear can include the following:

• Infectious disease - Microorganisms, including a number of bacteria, can invade the inner ear from the respiratory tract via the Eustachian tube (the most common route) or from the external ear or middle ear downward. Occasionally, microorganisms could also be introduced through the blood stream. Organisms include a wide variety of bacteria, both aerobic (oxygen-loving) and anaerobic (living without oxygen), and fungi. (At the time of this writing, there is no credible scientific evidence that Encephalitozoon cuniculi affects the inner ear; therefore, it is not listed as a cause of otitis interna.)
• Foreign bodies - Foreign materials can occasionally enter the ear canal and penetrate the inner ear
• Trauma - Trauma can occasionally affect the inner ear, including overly aggressive ear flushes when the ear drum has been ruptured, or fractures to the tympanic bulla or petrosal bone.
• Neoplasia - Cancer is rare, but could occur in the inner ear or surrounding area resulting in signs of vestibular disease
• Toxins - Toxins can enter either through the blood stream (a toxin that was eaten or inhaled) or through the ear canal (if the ear drum was ruptured). Certain ear medications or disinfectants applied in the presence of a ruptured ear drum can cause damage to the inner ear including; iodine products, chloramphenicol, gentamicin, neomycin and chlorhexidine (Nolvasan; Chlorhexiderm).

The most accurate way to diagnose otitis interna is via a CT scan or MRI. External ear exam, X-rays and blood tests are frequently negative for any significant findings. Remember that a negative finding on an x-ray does NOT rule out otitis interna. If the middle ear is also affected it may be possible to obtain a bacterial culture via a surgical procedure, which is helpful in determining the treatment to be used.

Treatment for otitis interna depends on the primary cause, but since the majority of head tilts in rabbits are likely caused by bacterial otitis interna, it is advantageous to use a long term course of antibiotics (3 to 6 weeks up to several months). Often it is not possible to find the exact organism involved, therefore it is best to use a broad spectrum antibiotic or combination of antibiotics such as (but not limited to); oral enrofloxacin, trimethoprim sulfa, chloramphenicol, azithromycin, metronidazole or an injectable form of penicillin. It is currently recommended to avoid the use of corticosteroids in rabbits if at all possible, and if absolutely necessary to use them only for a very short period of time (24 to 48 hours). Rabbits may be more sensitive than other animals to developing immunosuppression in the presence of corticosteroids, either topically, orally or parenterally. (See the end of the article for additional nursing care suggestions.)

OTITIS MEDIA (Inflammation of the middle ear) - This is also a common disease of rabbits and may occur along with or even be the cause of otitis interna. However, disease in this area alone does not cause a persistent head tilt. The rabbit's head may tilt temporarily to one side due to pain but there is no loss of balance and the tilt can easily be corrected when the rabbbit is alert. The causes of otitis media are the same as for otitis interna, with the exception of parasitic disease in the form of ear mites. In a very severe ear mite infection it is possible to have a ruptured ear drum and parasites invade the middle ear.

Signs of otitis media include periodic head tilting and shaking on the affected side (due to the pain of the disease), head rubbing, and ear scratching. If both sides are affected (as with most ear mite infections) the rabbit may alternate these signs from side to side. If there is damage to the cranial nerves that run through the middle ear, additional signs that may be seen are drooling, reduced or no blink reflex, dry eye, drooping ear, deafness or Horner's Syndrome (drooping eyelid, sunken eye, relaxed third eyelid, and a constricted pupil). The rabbit's appetite usually remains normal.

Diagnosis and treatment are generally the same as those listed for otitis interna. On occasion, an external ear infection will be present or a ruptured eardrum will be seen. Extreme caution must be taken when flushing or cleaning ears with a ruptured ear drum. Some cases of otitis media require surgery to remove excessive debris from the tympanic bulla surrounding the middle ear. If the middle ear is affected, then significant pain may be present and may interfere with appetite. A pain medication with anti-inflammatory properties, such as meloxicam, should be used.

BRAIN STEM DISEASE - Disease at the brain stem, specifically the vestibular nuclei, can cause similar signs as seen with inner ear disease. Because the vestibular nuclei are deep in the brain, it is likely that disease affecting this area will also affect surrounding brain tissue. Therefore, additional neurologic signs may be present such as loss of appetite, mental dullness, paralysis and sudden death. If the disease is also affecting the cerebellum additional signs such as seizures can be seen.

• Infectious disease - Bacteria, fungi and viruses can affect the brain stem and can enter through the inner ear, through the blood stream or through upper molar root infections. Individual abscesses can form, or a more generalized brain infection (encephalitis) might be present. A variety of bacteria have been found to affect the rabbit brain including Pasteurella, Bordetella, Staphlococcus, Listeria monocytogenes, and anerobic bacteria. In addition, there has been at least one case of, Herpes simplex 1 virus, causing encephalitis in a domestic rabbit. This disease may have been transmitted from humans to the rabbit.
• Protozoal disease - Encephalitozoan cuniculi is a one-celled organism called a microsporidian that can infect rabbits. Adult rabbits are infected by either ingesting or inhaling the spores (the reproductive organisms) passed in the urine of animals with active disease. However, the majority of rabbits become infected from their mother through the placenta during pregnancy. After entering the body of the rabbit, the infective spores pass through the intestine and then move throughout the body to the heart, lungs, liver and spleen. At this point in the disease most rabbits show no outward signs of illness. In some rabbits, however, E. cuniculi will eventually spread onward to the kidneys, eye and brain. Even at this stage, many rabbits will show no signs of disease and live normal lives. Others however can show serious consequences, including death if the infection overwhelms the ability of their immune system to contain it. For the purposes of our discussion we are most concerned about the infection that takes place in the vestibular nuclei in the brain resulting in head tilt. There is an ongoing controversy over the prevalence of E. cuniculi as a cause of primary head tilt (where there are no other signs of central disease as described previously) in the rabbit. It has been extremely difficult to demonstrate a definitive correlation between head tilt and active E. cuniculi infection. Serological testing for E. cuniculi has some value but is not definitive and, if not interpreted appropriately in the light of its shortcomings, may be misleading (see comments under Serology for E. cuniculi). The only way to diagnose E. cuniculi as the definitive cause of a head tilt is to take brain tissue samples from the rabbit while there are clinical signs and find the organism and its damage in the microscopic samples. No one has yet proven this correlation because a brain biopsy is dangerous for the rabbit and the E. cuniculi organism can be difficult to find in brain tissue. There are very few, if any, case reports or studies definitely proving that E. cuniculi is a significant pathogen in the nervous system. A presumptive diagnosis can be made based on ruling out other disease and looking for additional signs of disease coming from the brain. It is important to note that it is unlikely that E. cuniculi would affect only the vestibular nuclei as it invades the brain and therefore rabbits with head tilt caused by E. cuniculi will likely have at least one other sign of central disease such as weakness, paralysis, blindness, altered appetite, tremors or seizures. Having said all this, if a rabbit shows signs compatible with central vestibular disease, has a positive test for E. cuniculi, and all other diseases have been ruled out, some veterinarians will choose to treat for E. cuniculi empirically. Proper and effective treatment for E. cuniculi is controversial because in an illness where the definitive diagnosis is almost always made post-mortem, properly evaluating therapeutic protocols for effectiveness and comparing them against other treatment protocols is almost impossible. Additionally, there are very few science-based data showing a clear correlation between treatment protocols and alleviation of signs. It is fairly well accepted that at this time there is no drug that will clear an animal completely of E. cuniculi once an infection is established, but treatment may stop progression of disease and reduce clinical signs. Further complicating research into treatment, some rabbits may be able to recover spontaneously (without the benefit of treatment) from E. cuniculi infection. This makes it even more difficult to explain clinical improvement by the use of medication. Some cases improve because the rabbit's immune system effectively cured it. Some of the medications that have had been used to treat infection with E. cuniculi include albendazole, fenbendazole and oxibendazole. In rare cases, another protozoan, Toxoplasma gondii, may cause encephalitis in rabbits causing a wide range of signs including a head tilt. Because of its very low incidence rate, I will not discuss it further in this article.
• Parasitic disease - The most common parasite associated with head tilt in a rabbit is the Common Raccoon Roundworm, Baylisascaris procyonis. Raccoons that are infected with the worm pass the eggs in the feces. It takes about 3 to 4 weeks for the larva inside the egg to become infective, or be able to create disease in an animal. When raccoons ingest the infective egg the adult worm develops and stays in the intestine where is causes minimal damage. However, when species other then the raccoon ingest the infective egg of Baylisascaris, the larva that hatch and migrate throughout various areas of the body including the brain. Once they reach their destination they become encapsulated where they can remain alive for some time. The large size of the larva and its aggressive migration coupled with the small size of the rabbit brain creates a significant inflammatory reaction causing damage to the surrounding tissues. The infection rate with Baylisascaris in raccoons ranges as high as 82% depending on the area of country, therefore it is considered to be a parasite that is very widespread. At least 57 species of mammals, including humans and 43 species of birds have been diagnosed with this disease. Raccoons have a habit of defecating in "latrines" in areas such as on top of hay or straw in hay lofts, on house or boat decks, fallen trees or at the base of tress. The eggs can remain infective for years depending on environmental conditions. Rabbits can eat the eggs when they are allowed to graze in contaminated areas, when fed hay contaminated with raccoon feces or when contaminated straw is used as bedding. It is important to buy hay or straw from sources that store it properly in raccoon-proof enclosed areas. In addition, make sure any area in which a rabbit is allowed to roam outside is thoroughly inspected for signs of raccoon activity or fecal contamination. Removing and disposing of raccoon feces shortly after they are deposited helps to prevent infective eggs from developing because it takes 3 to 4 weeks for this to happen. However, remember also that eggs are microscopic and can be left in soil even after you have cleaned up. Signs observed in rabbits with Baylisascaris may include head tilt, tremors, weakness, blindness, seizures or sudden death. Definitive diagnosis of Baylisascaris infection in the live rabbit can be difficult because it can mimic other central vestibular (head tilt) disorders. Knowledge of a history of exposure to raccoon feces is helpful in allowing the veterinarian to make a diagnosis. There are no commercially available serologic tests to diagnose this disease. A spinal tap may be helpful, but is not definitive. Discovery of larvae in tissues after death of the host is often the way definitive diagnosis is made. Antiparasitic medications have not proven effective against Baylisascaris unless given before or very early (within days) after ingestion of the egg. Pyrantel drugs in the feed are known to prevent infection and subsequent disease. For early treatment, albendazole would be the drug of choice, but may not curative. Anecdotally some people have reported that oxibendazole may have a similar efficacy. The main problem is that once the signs of disease are present, there has already been significant damage done to the brain, which is irreversible even if there were medications effective at killing the parasites. In addition, there is concern that killing the larvae in the face of serious CNS disease may create an even greater inflammatory reaction due to the release of larval antigens from the dead parasite. Any antiparasitic treatment performed in a rabbit with signs of disease should be coupled with short-term corticosteroids to prevent inflammation (48 hours). Prevention of exposure to the parasite eggs is clearly the best way to counteract this disease. The only way to reliably prevent exposure is to keep your rabbit indoors, let your rabbit outdoors only in areas where raccoons have not defecated in many years, or to be sure to keep your rabbits off the ground in a raised hutch when they are outdoors. The raised hutch needs to be cleaned periodically and finding any fecal material that did not come from your rabbit should instigate a thorough cleaning.

  [Many thanks to Dr. Kevin Kazacos, Department of Pathobiology, Purdue School of Veterinary medicine for verifying the information in this section on Baylisascaris procyonis.]

• Cerebrovascular accident (stroke) - Disease affecting the circulatory system of the brain can affect a number of different areas and cause a wide variety of signs.
• Neoplasia - Cancer can affect the brain in a variety of areas
• Trauma - Trauma to the head or neck may result in brain or upper spinal cord lesions
• Toxins - Toxins either ingested or absorbed through the skin or respiratory tract can lead to brain damage. The most common toxins are the heavy metals such as lead and zinc (paint, cages, costume jewelry, imported pottery, curtain weights), plant toxins, some insecticides or antiparasitic medications, and carbon monoxide.
• Metabolic Disease - This would be any disease, such as liver or kidney disease or gastrointestinal disease that leads to a buildup of toxins in the body due to malfunctioning of that particular organ. The toxins eventually affect brain function and may lead to some of the signs of weakness, loss of balance, etc. Fortunately, rabbits with these diseases should show signs of illness far in advance of the brain's becoming affected.
• Heat stroke - High temperatures and high humidity can cause heat stroke in the rabbit, and can affect all areas of the brain. The signs initially might be circling, staggering, weakness and loss of balance. Knowledge of a history of exposure to high temperatures is the key to diagnosis.

Diagnosis of brain stem disease is difficult, and depends heavily on the physical exam, neurological exam and detailed history. The presence of additional signs beyond the vestibular signs is an indication of central disease, rather than strictly peripheral. See the discussions under each diagnostic test for potential diagnostic tools to be used when central disease is suspected.

Treatment of brain stem disease varies, depending on the suspected cause. However, the same nursing care is required as for peripheral disease and additional more intensive and long term nutritional and fluid support may be necessary, as many rabbits so afflicted have a loss of appetite and dulled mental state.

DIAGNOSTIC APPROACH TO HEAD TILT

Here are some of the diagnostic tests that might be used to determine the cause of a head tilt in a rabbit.

HISTORY - a detailed history is of vital importance to determination of the cause of disease. This is where the caregiver needs to make careful observations and take notes to remember details.

• History of any prior illness (especially respiratory, ear, kidney and neurologic disease)
• History any prior bouts with head tilt, weakness or incontinence
• Possibility of exposure to environmental toxins or parasites - access to the outside yard, wildlife droppings, other unrelated rabbits, plants, household toxins
• Exposure to other rabbits that are or were ill (particularly with neurological disease)
• Possibility of trauma
• Possible contact with human with active herpes viral infection
• The current state of your rabbit. This includes what clinical signs you are noting, when you first noted them and if they have gotten better, worse or stayed the same since you have noted them. Your rabbit's appetite and diet (not only what you offer but what it eats). Any change or loss of appetite or loss of body weight.

• Mental attitude: Is the rabbit still alert and active, or dull and depressed?
• Head tilt: Look at persistence, side to which rabbit tilts, is circling involved?
• Balance: Does the rabbit try to right itself if given support?
• Gait: Any abnormalities in gait?
• Nystagmus: If present, is it spontaneous or positional, is it horizontal or vertical?
• Ear exam: Are there signs of external or tympanic membrane disease? Are there signs of fluid, blood or pus beyond the tympanic membrane?
• Respiratory: Are there signs of respiratory disease?
• Systemic: Any other neurological signs, weakness (particularly hind limb), paralysis, incontinence, behavioral changes, external signs of trauma, particularly around the head and neck?