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What is Chemotherapy?

Chemotherapy is becoming a more frequently utilized therapy for treatment of cancer in horses. Chemotherapy involves the administration of cytotoxic chemicals. Cytotoxic medications impair cell reproduction, resulting in the prevention of cell growth. Unfortunately, this affects both normal healthy cells and cancerous cells, and chemotherapy is associated with many serious side effects. However, because cytotoxic substances have the most effect on fast-growing cells, such as cancer cells, they are an effective treatment for halting the spread of cancerous cells while having less impact on healthy tissues in most cases. Successful chemotherapy aims to maximize the destruction of cancerous cells and tumors while minimizing toxicity to the horse and their normal tissues. Chemotherapy may be used in conjunction with surgical removal of cancerous masses and/or radiation therapy for the treatment of cancerous growths. In horses, due to gastrointestinal side effects, cytotoxic drugs tend to be administered locally, (intralesionally), or topically to cancerous lesions. Systemic administration by intravenous can be used but is more limited in the treatment of horses. Chemotherapy must be administered by a veterinarian. Administration of chemotherapy drugs presents risks to humans administering the medication and application in a hospital setting with support by trained veterinary staff is recommended.

Chemotherapy Procedure in Horses

A variety of chemotherapy drugs are available for use in horses. Cisplatin in one of the most effective and commonly used in horses, although others are available. Chemotherapy or cytotoxic drugs can be administered in horses in a variety of ways. For administration of chemotherapy, your horse may be sedated to prevent them moving around during administration of a chemotherapy medication which would present a risk to personnel administering them, and mishaps that could result in contamination. Personnel administering chemotherapy drugs should take safety precautions which may include the use of gloves, safety glasses and protective clothing.

Skin lesions may be treated topically. Due to the risk of contamination, run off of materials, and poor absorption this method has several disadvantages.

Intralesional administration of tumors on or near the surface of the skin presents less of a risk of contamination and prevents run off and absorption issues. Intralesional administration may involve the use of emulsions or biodegradable beads with cytotoxic drugs. Cytotoxic substances are combined with other substances to make their delivery more effective. For intralesional administration, your horse is sedated to prevent moving around during the procedure and the lesion is injected with the medication, which may be combined with other substances such as water and sesame oil, in an emulsion form or in the form of slow release beads. Beads and emulsions allow the drugs to disseminate slowly, increasing the treatment exposure time. The area to receive the chemotherapy drug should correspond to the area that would be surgically treated if chemotherapy alone is being conducted. Multiple injections may be required at different sites along the growth to adequately provide coverage with the chemotherapy drug.

For lymphomas, an alkylating agent/chemotherapy drug and an antimetabolite are administered by intravenous every one to two weeks for the determined duration of treatment. A corticosteroid is also administered daily. Sedation may be administered to immobilize your horse during the intravenous administration.

In all cases where sedation is administered your horse should be supervised until they recover from sedation.

Surgery or radiation therapy may also be conducted in conjunction with chemotherapy. If surgery is being performed it is usually performed first and healing allowed to occur before chemotherapy, due to chemotherapy interfering with the healing process. 

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Efficacy of Chemotherapy in Horses

Chemotherapy is becoming increasingly used in the treatment of cancer in horses as it meets with successful reduction in cancerous growths and increased understanding of treatment protocols. It is especially useful for treatment of skin tumors that can be treated intralesionally. In cases where surgical removal of a tumor would result in functional or cosmetic impairment such as where tumors occur around the eyes, lips, nose, ears or genitals, intralesional chemotherapy may be a more suitable and effective option.

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Chemotherapy Recovery in Horses

Side effects occurring from the administration of chemotherapy need to be addressed and treated with supportive therapy. Side effects of intralesional application include fluid build-up, tenderness, and skin irritation. Protecting the site of treatment from rubbing or interference is necessary. 

Handlers will need to protect themselves by taking precautions and wearing protective equipment while handling a recovering horse if treated topically, or if a treated lesion could become compromised. Your horse may need to be confined and quarantined during recovery to prevent contamination with handlers and other horses.

If treated systemically, side effects such as hair loss, immune system compromise, and gastrointestinal disorder are possible. Your horse should be closely monitored and side effects treated as required. Residence in a hospital setting during treatment and recovery may be recommended to address issues and complications as they present. 

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Cost of Chemotherapy in Horses

The cost of chemotherapy treatment in your horse will include sedation, administration of chemotherapy medication, and hospitalization. Multiple treatments will be necessary depending on the individual. The total cost of treatment can be expensive and ranges from $2,000 to $5,000.

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Horse Chemotherapy Considerations

Occupational exposure for handlers and medical personnel associated with chemotherapy medications require safety protocols and protective clothing to mitigate risk of contamination. Administration topically is associated with high contamination risks. Intravenous, systemic administration usage can be limited due to concerns regarding side effects, especially gastrointestinal side effects, as horses are unable to regurgitate and are prone to colic conditions.

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Chemotherapy Prevention in Horses

Prevention of cancer in horses may not be possible in most cases. However, skin cancers are often attributable to sun exposure. Providing shade for turned out horses, and application of sunscreens, if sun exposure is a concern, may reduce skin cancer incidence. In addition, protective bonnets for the face and eyes can be used to protect sensitive areas from sun exposure, where skin cancers commonly manifest. When abnormal growths occur, prompt attention by a veterinarian to address before the growth will reduce the likelihood of requiring more invasive treatments with chemotherapy.

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