Motion sickness is characterized by signs referable to stimulation of the vestibular and autonomic nervous systems, including excessive salivation and vomiting. Affected animals may also yawn, whine, or show signs of uneasiness and apprehension; severely affected animals may also develop diarrhea. Motion sickness is seen during travel by land, sea, or air, and signs usually disappear when vehicular motion ceases.
The principal causative mechanism involves stimulation of the vestibular apparatus in the inner ear, which has connections to the emetic center in the brain stem. The chemoreceptor trigger zone (CRTZ) and H1-histaminergic receptors are involved in this pathway in dogs but apparently are less important in cats. Recent evidence has revealed that the neurokinin 1 substance P receptors (NK1) in the emetic center play a major role in motion sickness in both dogs and cats and are more important than the receptors in the CRTZ. Fear of the vehicle may also become a contributory factor in dogs and cats that develop a conditioned response to the event; signs may be seen even in a stationary vehicle. In this situation, behavioral modification may be needed to eliminate this fear, or the use of drugs that provide a sedative effect may be needed.
In some cases, motion sickness can be overcome by conditioning the animal to travel. In others, anxiolytic and antinausea drugs can be used with good results. Antihistamines (such as diphenhydramine hydrochloride, dimenhydrinate, meclizine, and promethazine hydrochloride) prevent motion sickness, provide sedation, and inhibit drooling. The centrally acting phenothiazine derivatives (such as chlorpromazine, prochlorperazine, and acepromazine maleate) have antiemetic as well as sedative effects. Cats have no histamine receptors in the CRTZ; therefore, antihistamines are ineffective in treating motion sickness in this species. Motion sickness in cats probably is best treated with an alpha-adrenergic antagonist (eg, chlorpromazine) instead of a pure H1-histaminergic antagonist.
Maropitant, an NK1 receptor antagonist, is effective in treating motion sickness in dogs. NK1 receptors are located in the emetic center of the brain stem, which is the source most responsible for the vomiting and nausea of motion sickness. Blocking these receptors is more effective than inhibiting the CRTZ. Therefore, maropitant is probably the drug of choice to treat motion sickness in dogs. Another benefit is that treatment is given once daily. The drug is approved and available in tablet and injectable forms for use in dogs, while only the injectable form is approved for use in cats. However, cats can exhibit severe aversive and pain behaviors to subcutaneous injection. Cats should be 16 weeks old and dogs 8 weeks old before being administered maropitant.
Phenobarbital and diazepam can be used to produce a general sedative effect if anxiety is a problem. Oral administration of one of these drugs several hours before departure should reduce or eliminate the signs of motion sickness. (Also see Drugs to Control or Stimulate Vomiting (Monogastric).)