Breast, ovarian, prostate and skin cancer is becoming prevalent in humans. Do they occur in horses too? Cancer isn’t diagnosed as frequently in horses as it is in humans but horses don’t show immunity to breast, ovarian, prostate and skin cancer. While such cancers are relatively peculiar in horses, given the amount of cancer we see in humans there’s probably more equine cancer than we realize. Horses are prey animals signs of weakness attract predators, therefore a horse’s survival instinct will cause it to mask symptoms and pain delaying diagnosis of illness and treatment. Horses are susceptible to cancer too; the number of horses with cancer is growing but often remains undiagnosed.

Cancer is a growth disorder of cells. Normal cells grow in an uncontrolled and invasive way developing into masses or tumours. They invade healthy tissues and can spread through the blood or lymph system to any part of the body. The abnormal growth of cells, either malignant or benign is called neoplasia. It is called carcinoma when the tumour involves epithelial tissue, such as the skin. Sarcoma when there is abnormal tissue growth involving the muscles. Lymphoma when abnormal tissue grow in the lymph nodes. It is called Melanoma when there is abnormal tissue growth in pigmented cells such as the skin. In humans it is frequently seen as skin cancer caused by over-exposure to the sun. Melanoma is more prevalent in gray horses because of the dark skin pigmentation. Melanoma and lymphoma are synonymous with malignancy. Metastasis happens when cells leave a cancerous mass and spread forming new tumours.

Here’s a list of some cancers found in horses:
Melanoma
Melanomas are tumours originating from cells that produce melanin or skin pigment. This type of skin cancer is more common in grey horses but can also occur in any dark-skinned horse. Grey horses over 6-years old are more prone to melanoma. Most grey horses over 15 years of age develop melanomas. The prevalence is estimated to be 80%. Most common sites for tumours are on the under-side of the tail near the base, around the mouth or in the skin over the parotid gland (near the base of the ear) and on the prepuce. Lumps may appear singly or they may multiply into multi-lobed masses.

Squamous Cell Carcinoma
This is the second most common cancer in horses accounting to 20% of all diagnosed cancer. SCC often appears in the eye, periorbital area and the penis. SCC is often slow growing tumours that cause extensive local tissue destruction through metastasis. This type of cancer often occurs in 8 to 15-year-old horses. It is most common in areas of unpigmented skin, especially those parts with little hair covering such as around the mouth, under the tail or on the sheath. SCC in the eyelid is fairly common in any horse with white faces, pink skin around the eye or with light skin.

Sarcoid Tumours
Sarcoids are one of the most common skin tumours on horses. They are believed to be caused by the bovine papilloma virus. Sarcoid tumours are uncontrolled proliferation of cells that don’t spread internally. They stay in their local area even if they get huge, they don’t travel to the bloodstream to other parts of the body. If the tumours are not removed early, they can become so large that it disables the horse. They can be removed surgically or treated with chemotherapy but they can return if not all of the effected tissue is removed.

Lymphosarcoma
The lymph tissue becomes cancerous starting with one lymph node spreading to others. The symptoms of Lymphosarcoma can vary depending on the location of the cancerous tissue. There are four general types of Lymphosarcoma:
1. Generalized (multi-centric) – is the most common among the four, it includes multiple peripheral and internal lymph nodes. The large masses are commonly found on the nodes around the throat latch, at the base of the neck, in between the jaw, in the superficial inguinal, mesenteric, and the pectoral region of the horse.
2. Intestinal – this lymphoma involves the diffuse tissue within the intestinal wall which causes malabsorption problems of the intestines. This causes diarrhoea leading to severe weight loss and sometimes colic.
3. Mediastinal – tumours that appear on the chest in between the lungs. It can cause coughing, increased heart rates, fluid on the chest or within the chest.
4. Cutaneous – this is the least deadly form, lymph nodes turn into tumours under the skin and don’t migrate throughout the body. Horses with cutaneous tumours typically do well and live fairly long with minor symptoms.

Breast Cancer
Breast cancer or mammary neoplasia in mares is uncommon compared to women. The reported incidence of breast cancer documented is barely two per cent. Necropsies results showed that horse mammary cancer is likely to be malignant and carries a poor prognosis for long term survival.

Ovarian Cancer
Mares have higher incidence of ovarian tumours (teratoma, cystadenoma, dysgerminoma, granulosa-theca cell) compared to other domestic animals. The occurance ranges from 2.5% to 6% of all equine cancers. Mares with ovarian tumours tend to have reproductive dysfunction and exhibiting stallion-like behaviour. The tumour occurs in cells that naturally produce sex hormone causing an overproduction of estrogen and testosterone. Ovarian cancer most frequently occur in mares ages 5 to 9 years.

Prostate Cancer
Geldings and stallions are susceptible to the same cancer condition as human. Prostate cancer in male horses in the horse racing industry is becoming more of a problem because horses are exposed to abnormal external and internal factors. Most cancer incidence occurs in the sheath, penis and glands in the form of cauliflower growths as a sarcoma, sarcoid or melanoma. The presence of prostate cancer can be insidious before it can be detected.

Testicular Cancer
Testicular cancer is uncommon in stallions and geldings as most male horses are castrated at an early age. Tumour of the testicles can either be germ cell tumours or non-germinal tumours. Seminoma and teratoma are forms of germ cell tumour that usually affects the aging stallion. These tumours are usually round or oval in shape that is usually found in the abdominal testicles. They are rarely malignant but can cause increased testosterone production.

Horses can contract cancer too. Diagnosis is often made from clinical signs such as masses, weight loss, anaemia, skin crusting, scaling, high heart rate, edema of belly and legs, fever, coughing, diarrhoea and a variety of other symptoms caused by the tumours that affect the tissue they are close to. Diagnosis can be difficult if tumours are not visible. A biopsy of the tumour can confirm the diagnosis, but it can also be tricky as the procedure can introduce tumour into the blood stream and spread the cancer further.

Horse cancer, any medical clues for us humans? The research continues in finding a solution to horse cancer, as in the case in human medicine. The same treatment approaches that have been successful with humans also work for horses, opening the doors to a variety of options for horse owners other than radical surgery. The treatment choices for horses with malignant or benign tumours include chemotherapy, radiation therapy, combination surgery with chemotherapy and alternative therapy. One effective strategy for treating cancer in human medicine is the use of DNA vaccines that target cancer cells. A new cancer vaccine for horses is in the works. The proponents are currently conducting field trials in horses with tumours. The DNA vaccine used in this study appears promising as the immune system is stimulated to attack foreign bodies.

Horse owners who have a horse diagnosed with cancer or suspect cancer in their horse should seek immediate medical advice. Endeavour to build a qualified team to help with the nutritional plan to aid the healing process. Healing cancer depends on the destruction of cancer cells and the correction of the underlying causes. To reduce the impact of cancer, the greatest horse ally is an educated owner. Horse owners can always draw hope as dedicated researchers and healers are continuing to study the different options to treat horse cancer in new ways.