An 18 year old Stockhorse mare presented for investigation of a lump on the left mandible and a foul smell emanating from the mouth. Four years earlier the mare had sustained an injury to the mandible and since then she had always had a small lump at the same site. Over the last few months the mare had been agisting at another property. The owners of the property contacted the owner of the horse to report that the lump on the mandible was much larger than before. The owner brought the mare back to his property. He agreed that the lump was indeed much larger than it had been. There was a scab on the surface, suggesting discharge had escaped from the mass, and when he opened the mare’s mouth, there was a foul odour. The mare had a good appetite and showed no difficulty eating.
On physical examination the mare’s vital signs were within normal limits, in particular, her temperature was normal. The lump on the left mandible was in the region of the first cheek teeth. It measured approximately 15cm long by 10cm wide. It was very hard. The mare responded painfully when the mass was palpated. There was a scab on the inside of the mass, but no discharge was visible. The decision was made to place the mare on a course of penicillin, as it was possible that the mass was an abscess, and bring her to the clinic for a thorough evaluation of her mouth and radiographs (x-rays) of the area.
At this point in time the major differential diagnoses were considered to be an abscess associated with the root of the cheek tooth in that area, or a tumour. As the mare had injured the area a few years before, and no radiographs were done at that time, it is difficult to associate that injury with the current problem. However, it is possible that the mare fractured her mandible at the time, and may have fractured her tooth roots as well. When this occurs, it is not uncommon for an associated problem to arise, and this can be several months to years later. Sometimes it is necessary to remove any teeth that may be involved, if they are loose or unhealthy.
A few days later the mare came to the clinic at VEVS. After the course of penicillin the mass had reduced slightly in size and was softer than previously. The mare was sedated and placed in the crush. Her mouth was examined; this revealed a discharging sinus lateral to the first and second cheek teeth of the mandible on the left hand side. What this means is that when the horse’s mouth was opened, a tract could be seen on the outside (towards the cheek not tongue) of the first and second cheek teeth (the big teeth used for grinding and chewing) on the left hand side of the mandible. The presence of this sinus tract was highly suggestive of an abscess, and would account for the foul smell coming from the horse’s mouth. The tooth was deemed to be solid and secure, as manipulation of it did not suggest it was loose.
Radiographs were taken of the affected area to gain a better understanding of the structures involved and the nature of the mass. These revealed a large destructive bone lesion adjacent to and below the second cheek tooth, but with possible involvement of the first cheek tooth roots. Within this bony lesion was a large cavity, with evidence of a sinus tract running ventrally (downwards) towards the site of the scab on the skin.
The radiographic appearance of the mass was consistent with an abscess, but a tumour could not be ruled out. Antibiotics alone would be unlikely to resolve the abscess. The best treatment was surgery. The horse was anaesthetised and the cavity was opened up and curetted to remove all the diseased tissue, including unhealthy bone. The wall of the cavity was thoroughly examined to look for exposure of the tooth roots; the tooth roots were not exposed. Another examination of the relevant teeth indicated they were healthy and did not require removal. Samples of the diseased tissue were submitted for bacterial culture, to identify any organisms present and determine which antibiotics they were sensitive to, and also for histopathology, to determine if the diseased tissue was just an abscess or if it was a tumour.
A Foley catheter was placed into the now open cavity. This enabled the owner to flush the cavity with sterile saline containing iodine, to remove any pus and bacteria and enhance healing. The mare was placed on an initial course of antibiotics (Sulprim powder), whilst awaiting the results of culture and sensitivity. These results indicated that the antibiotics needed to be changed, so the mare was placed on a prolonged course of an appropriate antibiotic.
Although bacteria were cultured, the cause of the mass was not an abscess; the infection was secondary to a tumour. The type of tumour present was a squamous cell carcinoma (SCC), a type of soft tissue tumour. SCC is the second most common type of tumour to affect the horse and is the most common type of neoplasia to affect the mouth. The most common sites in the mouth for SCC to occur are the lips, hard palate, tongue and oral mucosa (lining of the mouth). Other places in the body where SCCs commonly occur are the eye, external genitalia and other mucocutaneous junctions (where the skin meets the mucous membranes {lining of the body cavities}).
In the mouth, excluding the tongue, the types if neoplasia (cancer) that can occur can be grouped into three classes: odontogenic (meaning derived from dental tissue), osteogenic (meaning derived from bone) and soft tissue tumours (associated with the gums, lips or muscle). The location of a tumour gives a hint as to the type of tumour it will be. The cells that form a tumour are an abnormal version of a normal cell type. Thus, a tumour of a particular type originates within the type of tissue that it is related to. For example, lymphoma is the most common neoplasia in the horse and originates from lymphoid tissue, such as lymph nodes (‘glands’). In the current case, the mass was identified as originating within the mandible, which is bone. There are tooth roots within the bone. Therefore, it was anticipated that if the mass was cancerous, it would be odontogenic or osteogenic in origin, so it was quite a surprise when the result came back as squamous cell carcinoma, which is derived from the soft tissues. As this result was so surprising, the laboratory was contacted to discuss the result. The pathologist who made the diagnosis understood our surprise and reviewed the samples a second time, along with two other expert pathologists. They all agreed that the tumour was definitely SCC.
Squamous cell carcinomas are derived from the types of cells that form skin and mucous membranes. This type of cell is present in very small numbers within the mandible, associated with the tooth root, and is very rarely involved in neoplasia at this location. This case appears to be one of those occasions and may have occurred because of the injury sustained several years earlier.
The prognosis for this horse is poor. The neoplasm is in a location that is difficult to remove fully. If the tumour was further forward along the mandible, close to the incisors, there is a surgery available to remove the affected part of the jaw. This is major surgery and careful consideration is required by the owner, to ensure they fully understand the process and the outcome. Despite having part of their jaw missing, affected horses seem to cope well and be able to eat normally. If this surgery is not performed or is not possible, the horse can live for several months and even several years with the tumour. Although the tumour is very locally invasive, into the surrounding tissues, it does not tend to metastasize to other parts of the body. In rare cases it can metastasize to the local lymph nodes, but does not tend to go any further than this. The owner of this horse is hoping to breed from the mare, and there is no reason not to as long as she is comfortable, eating well and maintaining her body weight. If the mare starts to suffer at any time, such as having difficulty eating or losing weight, that is the time to consider whether to carry on and try to keep her comfortable, or whether to put her to sleep.
