Authors:
Goetz TE. DVM; Ogilvie, G.K. DVM; Keegan, K.G. DVM, MS; Johnson, P.J. BVSc
Institution
Department of Veterinary Clinical Medicine, College of Veterinary
Medicine, University of Illinois, Urbana 61801.
Title
Cimetidine for treatment of melanomas in three horses
Source
JAVMA, Vol 196, No. 3, February 1, 1990
During a 2-year period, 3 pivately owned adult horses with multi-focal, progressive, cutaneous melanomas were identified and treated with cimetidine. The horses ranged in age from 9 to 17 years and included 1 stallion and 2 geldings. Melanomas were diagnosed on the bases of history and physical examination, and when owner compliance permitted, microscopic evaluation of biopsy specimins. All horses were treated with cimetidine for at least 2 months. Response to the treatment was quantified by the changes in tumor diameter and by changes in the quantity of tumors. Follow-up examinations were done as permitted by economics and logistics.
Case 1 - A 9 year old gray male Lippizaner was admitted to the teaching hospital of the University of Illinois College of Veterinary Medicine for examination of numerous small masses on the ventral aspect of the tail near its root. The horse had been in the possession of the current owner since it was one year old, and had always been healthy. The masses had only recently been observed. Physical examination revealed eight 2-mm diameter black masses. Microscopic examination of one mass that had been excised revealed it to be a benign melanoma. The horse was released from the clinic without treatment.
Follow-up examination 18 months later revealed little change in the masses on the tail, but now there was a multifocal accumulation of 10 masses, ranging in size from 0.5 to 2.0 cm in diameter, on the prepuce, penis, and surrounding ventral abdominal wall. Again, the horse was released without treatment.
The horse was admitted to the clinic a third time, 39 months after the initial examination. Previously described masses had changed little in appearance, but now there was a hairless, 2-cm diameter mass approximately 15 cm to the right of the dorsal spinous process of T14. The owner was unable to ride the horse because the saddle traumatized the mass, with resultant hemorrhage, swelling, and apparent pain. The owner was advised to cut a circular hole in the thick saddlee blanket in a location that corresponded to the position of the mass on the horse's back to prevent trauma to the mass.
Four years after the initial examination, the owner reported that the use of a modified saddle blanket had been of little benefit and that for the past year, she had been unable to ride the horse under saddle because of the mass on the horse's back. Since the previous examination, the number and size of the masses had not changed. At this time, treatment was begun with cimetidine (2.5 mg/kg po, q 8 h). Repeat examination 4 weeks later revealed regrowth of hair and a one-third reduction in the size of the mass on the horse's back. Masses on the ventral aspect of the tail had decreased in number to 5 and in size by one third; however the number and size of the masses in the region around the external genitalia and the ventral abdominal wall were unchanged. Three months after treatment was initiated, the mass on the horse's back could no longer be found; the site was replaced with what appeared to be normal-haired skin. The masses on the tail had decreased in number and size by one half, but the masses around the prepuce and ventral abdominal wall were still unchanged.
Treatment was terminated after one year, at which time the status of the masses was identical to that described after 3 months of treatment. The horse has been off treatment and has been ridden and used in a routine fashion for 41 months. During this time, the number and size of the masses have remained static.
Case 2 - A 17 year old gray Tennessee Walking Horse gelding was admitted to the teaching hospital of the University of Illinois College of Veterinary Medicine because numerous black masses has developed on the ventral aspect of the tail near the root within the preceeding 6 months. Within the week prior to admission, the masses under the tail were rapidly increasing in size, and several other masses had appeared on both sides of the horse's neck.
Physical examination revealed 6 black masses on the underside of the tail near its root that ranged in size from 0.4 to 2.0 cm in diameter. On both sides of the horse's neck (lateral surfaces of the neck from the throatlatch to just cranial to the scapulate) there were hundreds of diffusely distributed black pedunculated, 1.0-mm diameter masses. The masses on the horse's neck were not obvious and could only be seen when the hair was parted and the neck examined closely. The clinical diagnosis was diffuse melanomatosis. Treatment with cimetidine (2.5 mg/kg, po, q 8 h) was initiated, and the horse was released.
When the horse was reexamined 8 weeks later, the masses on the neck were no longer evident, the other masses had decreased in size by a mean of 19%, and new masses had not appeared. Treatment was continued for 4 months. At termination of treatment, the number and size of the masses had decreased by one half. This horse has been off of treatment for 31 months, duing which time no new masses have appeared; masses that remained at termination of treatment have not increased in size.
Case 3 - A 10 year old gray Quarter Horse gelding was admitted to the teaching hospital of the University of Illinois College of Veterinary Medicine for evaluation of masses on the medial aspect of the left stifle. The first mass was noticed 10 months prior to admission. Since that time, the original mass grew in size, and several more masses appeared.
Twelve black masses of various sizes were found on the medial aspect of the left hind limb at the level of the stifle. Four of the largest (approximately 3-cm diameter) lesions were excised, and the remaining masses were cryonecrosed twice to -20 degrees c. The histologic diagnosis was malignant melanoma.
Reexamination one year later revealed masses (most of which were 1-cm diameter) in the following locations: 32 in the region previously treated, 6 proximal to the left upper eyelid, 4 in various locations on each forelimb, 1 over the left scapulohumeral joint, and 1 on the dorsal midline over T16. All of the lesions were cryonecrosed as before, and the horse was released.
One and a half years later, the masses were again located in the same areas, in approximately the same number and size. At this time, the horse was treated with cimetidine (2.5 mg/kg, po, q 8 h). Within 2 weeks, the masses were beginning to decrease in size. Treatment was continued for 2 more months, during which time there was slow continual decrease in the size of all masses.
Three months after initiating treatment, the dosage was reduced (2.5 mg/kg, po, q 12 h) because of the inconvenience of treating the horse 3 times daily. The dosage was again reduced (2.5 mg/kg, po, q 24 h). The horse was maintained at this dosage for 7 months, at which time the masses were almost imperceptible. Because of the availability of economically priced ranitidine, the horse's medication was changed at that time to 0.6 mg of ranitidine/kg,po,q 24 h. Treatment with ranitidine was continued for 5 months, at which time the owners noticed that the melanomas were again increasing in number and size, the medication was changed back to cimetidine (1.6 mg/kg, po, q 24 h). After reinstituting treatment with cimetidine, the masses again began decreasing in number and size. Presently, the horse is still receiving 1.6 mg of cimetidine/kg, po, q 24 h.
