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Histopathology in Pets

Date Published: 01/25/2021
Photo courtesy of Depositphotos

What is Histopathology?

Your veterinarian wants to take a biopsy from your pet and send it out for histopathologic examination? Or maybe you’re waiting for histopathology results for your pet and want to know a little more about what that means? Histopathology, often shortened to histopath, involves looking at stained tissues on a slide under a microscope in order to gain clues about what disease process is going on. Staining is adding dyes to the slide to highlight cells and make them easier to see. A trained professional, known as a veterinary pathologist, is the one who examines your pet's tissue. The goal of pathologists is to provide a description of what they see happening in your pet’s tissue and provide a reason for the changes; in other words, a diagnosis. Histopathology can be a powerful tool for helping identify what is causing your pet’s problem. It can also help determine the best treatment plan for your pet as well as the likely outcome.

How is it Performed?

To produce histopath results, pathologists look at tissue samples under a microscope. These samples are called biopsies. A biopsy is obtained from your pet by cutting out a piece of tissue from their body. How that tissue is cut out depends upon a number of variables, including the type of tissue and where that tissue is located. For example, a biopsy can be a piece of skin from your pet’s inner front leg or it can be a piece of intestine from a specific region of the gut.

Depending on these variables, your veterinarian has several options for obtaining a biopsy. Some methods are simpler and do not require invasive surgery. In these cases, your pet may only require local anesthesia and sedation. For example, skin biopsies on regions such as the back of your pet are often good candidates for a simple skin punch biopsy. Other methods to get a biopsy may require general anesthesia, which carries associated risks. For instance, biopsies of the intestines may involve use of an endoscope or exploratory surgery of the stomach. In either case, your pet will need to be put under general anesthesia. Your veterinarian will explain how they will get a biopsy sample and assess whether or not your pet is stable enough to undergo general anesthesia. Depending on the procedure, your veterinarian may refer you a veterinary specialist.

Once a biopsy is procured and prepared, it is sent out to a pathologist. If your veterinary clinic is part of a large hospital or veterinary school, they may have a diagnostic laboratory and pathologists close by. In most cases though, it will take some time for the biopsy sample to ship to a laboratory and be evaluated. Your veterinarian will give you an estimate of when to expect results.  

As mentioned above, a biopsy involves cutting tissue from your pet. This means your pet will have a wound where tissue was removed and may have stitches, especially if there was surgery. Your veterinarian should provide you with post-surgical instructions and may provide pain relief medications depending on the procedure and your pet’s individual health. In general, the biopsy site needs to be kept clean and dry; your pet should not have access to it in case they irritate the wound further. Your veterinarian may recommend an Elizabethan collar or cone to help prevent your pet from touching it.  

Why Have Histopathology?

Histopathology can be a powerful diagnostic tool. There are many diseases that cause vague and overlapping signs: histopath is one way to tell them apart. The results can allow your veterinarian to know the specific disease your pet is suffering from and thus tailor their treatment. In general, the better treatment is tailored to your pet, the better their prognosis will be.

In some cases, histopathology is the only way to get a definitive diagnosis. There is a long list of diseases that can affect pets that require histopathology to be confirmed. Among these are various inflammatory, congenital, and cancerous diseases as well as many other causes. Congenital refers to a disease existing from birth and an example includes ductal plate malformations (DPM), which are abnormal structures found in the liver. Examples of inflammatory diseases that require histopath to be diagnosed include inflammatory bowel disease, glomerulonephritis (inflammation of the glomeruli, which are part of the kidneys ) and hepatitis (inflammation of the liver).

Cancer is another type of disease that requires histopathology for diagnosis. Seeing a mass on an X-ray, ultrasound or directly on your pet can raise the suspicion of cancer but it does not confirm it. Other factors such as the age and breed of your pet, the location of the mass, and your pet’s signs may provide further clues. However, only histopathology can correctly identify the mass as cancerous or not. This identification is important not only for detecting cancer early on in your pet but there are many different types of cancers and each may have a different treatment plan as well as prognosis. In terms of cancer in your furry companion, histopathology can also provide a grading of the cancer. Interpreted along with other factors, grading helps determine prognosis.

There are many diseases that do not require histopathology for diagnosis, but it can still be helpful and worth performing. For example, some infectious diseases may have other easier and less expensive diagnostic tests that can be performed but for one reason or another, these tests may come back as inconclusive. In these cases, a diagnosis can be made through the use of histopathology. Some examples include gastrointestinal infections with fungal, oomycetal and algal organisms. Moreover, histopath does not only provide a diagnosis in these cases: it can also direct treatment plans and prognosis.

When is Histopathology Recommended?

In general, histopathology is not the first line diagnostic test you turn to. It tends to be more expensive, invasive and complicated than other more basic diagnostics. Also, obtaining a biopsy is not without risks. Your veterinarian is aware of this and when they recommend a biopsy, they likely have good reason to believe it would be more helpful than harmful. This good reason may be true for your pet if other tests were inconclusive, your pet has had little to no response to empiric therapy (treatment based on your veterinarian’s experience), or your pet’s condition is worsening. In these instances, your veterinarian has already evaluated your pet and begun addressing their issue but your pet is not improving the way they should. As a result, histopath offers a different and reasonable next step in their approach.

There are instances when your veterinarian may more quickly recommend histopathology. Examples include unusual or severe skin abnormalities. There are also times where histopathology can be useful in a pet who isn’t displaying obvious signs of sickness. For example, some veterinarians who specialize in cancer (oncologists) recommend taking biopsies of masses once they are larger than a certain size or are persistent. This timing allows for early cancer detection and early treatment. Another example might be if your pet has unexplained increases in their liver enzymes on bloodwork. Your veterinarian can help you decide when histopathology is appropriate.

While histopath has many benefits, it is not always required or even recommended. In general, it should only be pursued if your pet is stable enough to undergo the procedure required to obtain the biopsy and if results will affect the treatment plan, and your willingness to pursue therapy and/or prognosis. If your veterinarian has a high suspicion for a specific disease, they may recommend treatment without a definitive diagnosis. This can resolve your pet’s problem without need for biopsy, although it may be needed further down the line if your pet does not respond well. Not all lumps and bumps require histopathology; some may be able to be diagnosed with a less invasive test called cytology, which involves merely sucking some cells out of a mass or affected organ with a needle. If there is low suspicion for disease, your pet is unbothered by what’s happening, and it is less than a certain size then your veterinarian may decide to hold off on histopath or cytology and instead cautiously monitor it.


While histopathology has the potential to provide valuable information, it has limitations. Error rates among veterinary pathologists are estimated to be around 1-10%, meaning there is a possibility of receiving an incorrect diagnosis. Reasons for these errors include inadequate tissue collection (e.g. due to missing the affected area, the tissue being “tough” and hard to biopsy), poor preparation of samples, and lack of clinical history. Your veterinarian can prevent many of these errors by preparing the tissue appropriately and working closely with the pathologist. In addition, they can always request a second opinion on the biopsy if the results come back as inconclusive or are unexpected. It may even be necessary to repeat a biopsy if there were issues with the collecting the first one. In some situations, additional special stains may be required to determine the nature of the problem more accurately, e.g., to look for scarring with inflammation of the liver or to determine the exact tumor type (some tumors can look so abnormal, it’s hard to tell what they are).

Getting Results

Your veterinarian will notify you when your pet’s histopathology results come back and discuss what they mean in regard to future steps. While the results may not always be good news, knowing exactly what is affecting your pet is one step towards ensuring they receive the best treatment and care possible.

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