Introduction to Cancer in Animals
Humans aren't the only ones affected by cancer. This section contains information detailing cancer in animals. If the animal/cancer type in which you are interested is not yet presented, please check back because we will be adding new information regularly!
Cancer in Pets
According to the American Veterinary Medical Association (AVMA), cancer causes almost 50% of deaths in pets over the age of 10. Some common types of cancers in pets include: skin, breast, head and neck, lymphoma, leukemia, testicular, abdominal, and bone. Of the two most common household pets, dogs tend to get cancer at a higher rate than cats.(1)
Cancer in Wild Animals
Cancer also occurs in wild animals. Some well documented examples of cancer in wild animals include facial tumors in Tasmanian devils,(2)fibropapillomatosis in sea turtles,(3) and genital carcinoma in sea lions.(4)
Introduction to Cancer in Dogs
According to the Texas A&M University College of Veterinary Medicine, "Dogs have 35 times as much skin cancer as do humans, 4 times as many breast tumors, 8 times as much bone cancer, and twice as high an incidence ofleukemia."(1)
Other common types of cancer found in dogs include cancer of the mouth, lymphoma, testicular and abdominal tumors.(2)
Canine Transmissible Venereal Tumor
Overview and Transmission
Cancer in humans is practically never transmitted from one person to another. Only very rare documented exceptions exist, usually involving surgical mishaps. Sticker's Sarcoma or Canine Transmissible Venereal Tumor (CTVT) is a very unusual form of cancer affecting canines (i.e. dogs). CTVT is transmitted by mating, licking, or other direct contact.(1) The tumor affects the genitalia and forms cauliflower- like mounds. In some cases the urethra becomes blocked making it difficult for the affected animal to urinate.(2) If the cancer is located at the mouth and nose, nosebleeds, facial swelling, and nostril discharge are common symptoms.
Cancer in humans is practically never transmitted from one person to another. Only very rare documented exceptions exist, usually involving surgical mishaps. Sticker's Sarcoma or Canine Transmissible Venereal Tumor (CTVT) is a very unusual form of cancer affecting canines (i.e. dogs). CTVT is transmitted by mating, licking, or other direct contact.(1) The tumor affects the genitalia and forms cauliflower- like mounds. In some cases the urethra becomes blocked making it difficult for the affected animal to urinate.(2) If the cancer is located at the mouth and nose, nosebleeds, facial swelling, and nostril discharge are common symptoms.
Many human cancers are caused by viruses, including the human papilloma virus (HPV), a major cause of cervical cancer. Infection with viruses can lead to changes in normal cells within the infected person and lead to the development of cancer. CTVT is different! In this case, the cancer cells themselves are transmitted from animal to animal. Once in the new animal, the tumor can grow and eventually be spread to additional animals.(3) Experiments have shown that CTVT is not transmissible via killed tumor cells or by cell's liquids.(1) Virus is usually present in the cell's liquid contents; if the liquid contents do not transfer CTVT then this is evidence that viruses are not responsible for the cancer. Also, all tumor cells examined so far have a molecular "fingerprint" in their DNA that is absent in normal cells. Specifically, the cancer cells contain a DNA sequence called Line-1 inserted near the oncogenes c-myc.(4)
Researchers have compared tumor DNA and normal DNA within different breeds of dogs. The results showed the expected differences between the normal cells' DNA, but all tumor DNA samples were very similar despite being from very different dogs. These results support that it is the tumor cells themselves that transfer CTVT between animals.(3) In most cases, the immune system detects and eliminates cells of other types that are introduced into an animal (or human). This does not happen with CTVT. Upon the initial infection, CTVT begins a state of rapid and intense growth that lasts anywhere from three to nine months and possible longer in old or weakened dogs.(1) This is usually followed by a variable regression phase.
Treatment for CTVT Infection
Treatment choices for CTVT include surgery, chemotherapy, or radiation therapy. Chemotherapy is often very effective, usually resulting in complete remission. Surgery does not have a similarly high success rate.(5)Because the cancer is transmitted between animals and across generations, the cancer itself is thought to be very old! CTVT is thought to be anywhere from 200-2500 years old and represents the longest line of unbroken replications of a mammal's body cell. Aside from domesticated dogs, it is also transmissible to coyotes, foxes, jackals, wolves and immuno-suppressed mice.(1) (6)
Treatment choices for CTVT include surgery, chemotherapy, or radiation therapy. Chemotherapy is often very effective, usually resulting in complete remission. Surgery does not have a similarly high success rate.(5)Because the cancer is transmitted between animals and across generations, the cancer itself is thought to be very old! CTVT is thought to be anywhere from 200-2500 years old and represents the longest line of unbroken replications of a mammal's body cell. Aside from domesticated dogs, it is also transmissible to coyotes, foxes, jackals, wolves and immuno-suppressed mice.(1) (6)
Canine Osteosarcoma
Introduction
Osteosarcoma (OSA) is a cancer that develops within bones. It is often invasive and metastatic.(1) OSA is relatively rare in small domesticated animals, accounting for only 5-6% of canine malignancies.(2) However, it is by far the most common primary bone tumor in dogs, accounting for 80% of cases.(2) (3)
The diagram above highlights some of the bones affected by canine osteosarcoma. This type of cancer occurs most commonly in the appendicular skeleton, including the radius and ulna, femur, tibia, scapila, humerus, and paw bones (phalanges). The front limbs (forelimbs) are more likely to be affected than the rear (hind) limbs. OSA also occurs in the axial skeleton, including the spinal bones, skull, jaw bone (mandible), and the vertebrae at the base of the spine (sacrum). In rare cases, it can affect extraskeletal tissues, such as muscle.(2) (3) Because of its ability to metastasize, OSA can spread to other parts of the body. Studies have shown that OSA invades the lungs in approximately 17% of cases.(3)
Risk Factors/Detection/Staging
Osteosarcoma is more common in adult dogs. Large animals, weighing from 44 to 88 lbs (20 to 40 kg), are at a higher risk than smaller animals.(3)
Signs of OSA in dogs include impaired movement due to pain and stiffness. Affected dogs will often limp to avoid putting weight on the tumor-afflicted bone. They also present with abnormal bone growth and swelling.
Large tumor size is associated with poor prognosis.(4) The location of the tumor can also affect the outcome of the disease. Axial skeletal OSA and appendicular OSA tend to have similar prognoses.(5) Tumors in the humerus bone tend to have a poor prognosis while tumors in the jaw tend to have better outcomes.(4)(5)
Treatment
Limb amputation is the standard treatment for OSA tumors located in the limbs. Analgesics such as morphine are used to relieve pain, and most dogs can walk within 12-24 hours after the surgery. Many dogs are able to adapt to the amputation within one month. Interestingly, studies have shown that an owner's positive attitude can speed the adaptation process.(6) Possible post-surgical complications include infection and recurrence of OSA in the stump.
Certain conditions can make dogs bad candidates for amputation, including arthritis, obesity, neurological problems and large breed. For this reason, vets will perform limb-sparing surgeries, in which they remove the tumor and leave the surrounding tissue and bone intact. This surgery has a higher rate of post-operation infection and OSA recurrence than amputation. Studies reveal that the most common reason vets are asked to perform limb-sparing surgery is not based on the animals condition. Rather, it is that owners are reluctant to proceed with amputation.(7) (8)
Vets can use radiation as a curative treatment or to reduce pain and make the animal more comfortable (palliative care). This type of therapy can reduce inflammation and heal micro-fractures. Studies show that 70% of dogs respond positively to radiation therapy and experience pain relief.(2)
Chemotherapy drugs such as doxorubicin, carboplatin, cisplatin, or a combination of these drugs may also be used. Chemotherapy alone has not been shown to extend survival time. OSA-affected dogs treated with surgery and chemotherapy tend to survive longer.(9)
Unfortunately, despite the treatment options available, the prognosis for canine osteosarcoma is poor. The estimated survival time is 6 to 12 months from the time of diagnosis, regardless of the treatment used.(1) The most common cause of death (or euthanasia) is disease spread (or metastasis).(7)
Canine Hemangiosarcoma
Introduction
Hemangiosarcoma (HSA) is a cancer that originates in cells that form blood vessels. HSA is responsible for approximately 7% of canine cancer cases. In fact, it is more common in dogs than any other species of animal examined!(1)
Hemangiosarcoma (HSA) is a cancer that originates in cells that form blood vessels. HSA is responsible for approximately 7% of canine cancer cases. In fact, it is more common in dogs than any other species of animal examined!(1)
HSA occurs most commonly in the spleen, skin, and the right atrium of the heart.(2) Primary tumors have also been reported in the lung, aorta, kidney, oral cavity, muscle, bone, urinary bladder, prostate gland, vagina, lining of the abdominal cavity (peritoneum), intestine, tongue, and the membrane that covers the white part of the eye (conjunctiva).(3) HSA is highly metastatic, meaning that it frequently spreads. When HSA is confined to the skin, it is less apt to spread. Veterinarians estimate that over 80% of all other cases have already metastasized at the time of diagnosis.(3)
HSA cells may release proteins that stimulate the growth of new blood vessels. This process is known as angiogenesis, and is critical for tumor formation. The presence of numerous blood vessels helps supply growing tumors with nutrients and oxygen and may serve as a "highway" for the cancer cells to metastasize to distant parts of the body.
Risk Factors
Older animals are at higher risk for developing hemangiosarcoma. Affected dogs are usually diagnosed between 9 and 12 years of age.(4) HSA tends to affect large breed animals more often than smaller ones. Breeds that are more prone to this type of cancer include German Shepherds, Golden Retrievers, Labrador Retrievers, Pointers, Boxers, English Settlers, Great Danes, Poodles, and Siberian Huskies. Breeds with short hair, light hair, light skin, and/or less pigmentation (i.e. Whippets) are more prone to hemangiosarcoma of the skin(5) (3) (6)
Older animals are at higher risk for developing hemangiosarcoma. Affected dogs are usually diagnosed between 9 and 12 years of age.(4) HSA tends to affect large breed animals more often than smaller ones. Breeds that are more prone to this type of cancer include German Shepherds, Golden Retrievers, Labrador Retrievers, Pointers, Boxers, English Settlers, Great Danes, Poodles, and Siberian Huskies. Breeds with short hair, light hair, light skin, and/or less pigmentation (i.e. Whippets) are more prone to hemangiosarcoma of the skin(5) (3) (6)
Detection/Symptoms
Symptoms of HSA vary depending on the location and stage of the cancer, but may include weakness, weight loss, increased heart rate (tachycardia), abnormally fast breathing (tachypnea), abdominal swelling, lethargy, and paleness of mucous membranes. Episodes of acute weakness and/or collapse may indicate that a tumor has ruptured, causing uncontrolled blood loss. If internal bleeding occurs and the blood is reabsorbed from the body cavity back into blood vessels, the animal may gradually recover. In more severe cases, tumor rupture can cause sudden death.(3)
Symptoms of HSA vary depending on the location and stage of the cancer, but may include weakness, weight loss, increased heart rate (tachycardia), abnormally fast breathing (tachypnea), abdominal swelling, lethargy, and paleness of mucous membranes. Episodes of acute weakness and/or collapse may indicate that a tumor has ruptured, causing uncontrolled blood loss. If internal bleeding occurs and the blood is reabsorbed from the body cavity back into blood vessels, the animal may gradually recover. In more severe cases, tumor rupture can cause sudden death.(3)
Veterinarians often detect HSA during physical examinations. They also use blood tests, urine analysis, chest x-rays, abdominal ultrasound, ultrasound of the heart (echocardiogram), tissue samples (biopsy), and more. One type of blood test, the complete blood count (CBC), can detect anemia, misshapen or fragmented red blood cells, and other signs of HSA. Some tests can even indicate the organ(s) being affected by the disease. A blood smear that shows an increased number of immature red blood cells (normoblasts) may indicate cancer of the spleen.(3)
Treatment
Surgery is the standard treatment for dogs with HSA. In one study, researchers from Colorado State University attempted surgery on 76 animals with HSA (66 dogs and 10 cats). In 42 cases, surgeons were able to remove the tumor(s) completely, leaving no visible cancer behind. Following surgery, all 42 of these animals remained healthy for at least 12 months. In the remaining 34 cases, the animals experienced incomplete tumor removal and/or other complications, resulting in worse clinical outcomes. These findings confirm that complete tumor removal is the most significant prognostic factor for this type of cancer. Researchers also found that certain tumors (i.e. eye or skin) were much easier to remove than others (i.e. bone or muscle).(7)
Surgery is the standard treatment for dogs with HSA. In one study, researchers from Colorado State University attempted surgery on 76 animals with HSA (66 dogs and 10 cats). In 42 cases, surgeons were able to remove the tumor(s) completely, leaving no visible cancer behind. Following surgery, all 42 of these animals remained healthy for at least 12 months. In the remaining 34 cases, the animals experienced incomplete tumor removal and/or other complications, resulting in worse clinical outcomes. These findings confirm that complete tumor removal is the most significant prognostic factor for this type of cancer. Researchers also found that certain tumors (i.e. eye or skin) were much easier to remove than others (i.e. bone or muscle).(7)
Veterinarians can use other methods to treat and manage hemangiosarcoma, but their effectiveness is limited. Chemotherapy can be used in addition to surgery. Research has shown doxorubicin to be the most effective chemotherapy drug for this type of cancer. Despite treatment, the survival time for HSA-affected dogs typically does not exceed 6 months. Death often results from metastasis to the lungs and other parts of the body.(1)
Canine Mast Cell Tumors
Introduction
Mast cell tumors, also called mastocytomas, are the most common skin cancer in dogs.(1) Mastocytomas develop most often in dogs seven and a half to nine years of age, but can occasionally be found in dogs as young as four to six months.(2) Different breeds also have different rates of mast cell tumors. Boxers and Boston terriers have the highest rates.(2)
Mast cell tumors, also called mastocytomas, are the most common skin cancer in dogs.(1) Mastocytomas develop most often in dogs seven and a half to nine years of age, but can occasionally be found in dogs as young as four to six months.(2) Different breeds also have different rates of mast cell tumors. Boxers and Boston terriers have the highest rates.(2)
Mast cell tumors originate from mast cells, immune system cells found in many tissues of the body.(3) Because mast cells can be found almost anywhere, mastocytomas have the potential to appear in all different regions of the body. They are most commonly located on the skin on hind legs, the region between the neck and abdomen (thorax), or genital regions.(4) Mast cells contain a variety of biologically active substances, including histamine, heparin, serotonin, and prostaglandins. These chemicals are released from the cells during an allergic reaction.(5) These are the substances that cause the symptoms associated with allergies: redness, itching, swelling, blood pressure drops, tearing, nausea, wheezing, ect. Normally these chemicals are highly regulated, but when mast cells become cancerous they no longer appropriately control the release of the chemicals.
Risk Factors/Detection/Staging
The unregulated release of chemicals by a mastocytoma can cause a variety of symptoms. The symptoms any particular dog shows is variable, but can include vomiting, ulcers, bloody excrement, abdominal pain, and blood-clotting difficulties; gastro-intestinal problems are the most common signs.(6) Mastocytomas appear as raised lumps that are often irritated or red in appearance. Unfortunately, mast cell yumors have no characteristic form or color and are impossible to identify without laboratory testing.(1) Often when a mastocytoma is touched the skin becomes red, itchy, and swollen.(7) (2) This response is also called Darier's Sign . It occurs because when the mast cells in the tumor are compressed they release the chemical histamine, and this causes irritation of the skin.
The unregulated release of chemicals by a mastocytoma can cause a variety of symptoms. The symptoms any particular dog shows is variable, but can include vomiting, ulcers, bloody excrement, abdominal pain, and blood-clotting difficulties; gastro-intestinal problems are the most common signs.(6) Mastocytomas appear as raised lumps that are often irritated or red in appearance. Unfortunately, mast cell yumors have no characteristic form or color and are impossible to identify without laboratory testing.(1) Often when a mastocytoma is touched the skin becomes red, itchy, and swollen.(7) (2) This response is also called Darier's Sign . It occurs because when the mast cells in the tumor are compressed they release the chemical histamine, and this causes irritation of the skin.
Although normal mast cells are typically fairly stationary, advanced mast cell tumors may spread (metastasize) to other places in the body. Veterinarians classify (stage) mast cell tumors using a six tiered system (stages 0 to 5) based on the extent that the cancer has spread in the body.(1) Staging takes into account the number and size of tumors, lymph node involvement, and recurrence rate.(1) A higher stage indicates greater body involvement with Stage 5 representing distant metastasis including bone marrow or blood involvement.(2) Another classification system, the histological grade (values from 1-3), is based on the physical appearance of the cells in the tumor. More abnormal mast cells/tissues are associated with a higher histological grade and have a higher possibility of being malignant.(2)
Treatment
Surgical removal of the tumor is the most common treatment for mastocytomas of intermediate and lower stages. Healthy tissue up to about one inch (2-3 centimeters) around the tumor must also be removed to prevent leaving any cancerous mast cells behind. For more advanced stages surgery is still recommended, but is often combined with radiation therapy or chemotherapy.(2) If surgery is not thorough enough to remove all cells, external beam radiation therapy can be used to kill the remaining cells. External beam radiation may also be used to reduce the size of a tumor before surgery.(2)
Surgical removal of the tumor is the most common treatment for mastocytomas of intermediate and lower stages. Healthy tissue up to about one inch (2-3 centimeters) around the tumor must also be removed to prevent leaving any cancerous mast cells behind. For more advanced stages surgery is still recommended, but is often combined with radiation therapy or chemotherapy.(2) If surgery is not thorough enough to remove all cells, external beam radiation therapy can be used to kill the remaining cells. External beam radiation may also be used to reduce the size of a tumor before surgery.(2)
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