Veterinary Pathology - What is it all about?

What is pathology?

The word pathology comes from the Greek words Pathos, meaning feeling, pain, suffering and Logos, meaning study.

So it is the study of the science of the causes and effects of diseases, especially the branch of medicine that deals with samples of body tissue for diagnostic or forensic purposes examined in a lab.

Basically, we look at changes to cells, tissues, and processes in the biological systems to understand the body's irregularities, dysfunction, and disease.

What do we want to get out of pathology?

Studying pathology means studying the following 4 parts of a disease so that you can:

  1. Find out what causes the disease, also know as the aetiology (the cause, set of causes, or manner of causation of a disease or condition)

  2. Find out about the way that the aetiological factors actually cause the disease, also known as pathogenesis (the manner of development of a disease)

  3. Find out about the structural changes in the cells, tissues and organs, also known as the morphological changes (changes relating to the form or structure of things)

  4. Find out what the consequences of the morphological changes are, otherwise referred to as the clinical significance. You can read more about clinical significance here.

The language used in pathology

Pathology has its own terms and meanings about how diseases are described so that all terms have one meaning, making things easier to describe and understand for scientists.

Important terms to remember are:

  1. Lesions

  2. Aetiology

  3. Aetiological Diagnosis

  4. Pathogenesis

  5. Pathophysiology

  6. Sequela

  7. Remote effects

Each one is explained below.

What is a lesion?

In pathology, a lesion is any clear structural change or abnormality in an animal's body tissues or fluid.

What does aetiology mean?

As already mentioned above, aetiology (or, if you are in the US, etiology) refers to the thing causing the disease, for example, bacteria. Aetiology is the study of causes.

What is an aetiological diagnosis?

This is a combination of the definitive diagnosis (A diagnosis reached by an extensive workup on which therapy is based) and a morphological diagnosis (A diagnosis based on the predominant lesion(s) in the tissue(s). It may be macroscopic (gross) or microscopic and describes the severity, duration, distribution, nature of the lesion and location (e.g. severe, acute, locally-extensive, fibrinous bronchopneumonia). Also known as a lesion diagnosis).

To put it simply, an aetiological diagnosis is what the disease is and what form it takes now.

An example of this is nephritis in a dog caused by Leptospira canicola. The aetiological diagnosis would be leptospiral (cause) nephritis (morphology - form, shape or structure).

What does Pathogenesis mean?

This refers to the development of the disease inside the body.

It is the way that pathogenic bacteria cause infection.

An example is in BVD, the intestines' lesions develop because of the death of the Peyer's Patches. If you're interested in Bovine Viral Diarrhoea's pathogenesis, you can find a paper on this here.

What does Pathophysiology mean?

Pathophysiology is the functional changes in diseased tissue. An example of this is in bovine streptococcal mastitis; the udder stops producing normal milk.

What is a sequela?

A sequela is the consequence of disease in the body. The plural of sequela is sequelae.

What does remote effect mean?

A remote effect is the effect of the disease in tissue.


  • Aetiology = cause

  • Lesions = abnormalities that can be observed

  • Pathogenesis = what is causing the lesions or the development of disease and what has happened because of this, the chain of events leading to the disease

  • Pathophysiology = what changes of function has happened because of the disease

  • Sequelae and remote effects = what has happened in the body or the tissues because of the disease

There are three factors involved in the development of disease

  1. The animal

  2. The agent

  3. The environment

You need to understand the role of animal, agent and the environment in the development of the disease.

Also, know that not every animal will respond to an infection, lesion or injury the same way.

Why do a Post-Mortem in Veterinary?

Doing a post-mortem

  1. helps to show you what pathological changes happened in the animal's body when it was alive

  2. can give a lot more information about the cause (aetiology) and how the disease developed in the body (pathogenesis) than blood sample or biopsies. Remember that a biopsy is when tissue is examined from a living animal.

In a post-mortem, you'd need good knowledge of normal structure and function. A good book concerning the techniques used in post-mortems (Slauson & Cooper, 20020) can be found here.

As discussed in the above-mentioned book, steps when performing a post-mortem are as follows:

  • Good visual examination to look for gross changes in the tissues of fluids

  • Examine the cells in smears, fluids and aspirates (cytology)

  • Look at selected tissues under the microscope (histopathology)

  • Look at the activity of enzymes or antibodies of tissues under the microscope (histochemistry)

  • Electron microscopy to get high-resolution images of the specimen to identify the structure of tissues, cells, organelles etc.

  • Identify the pathogen or agent that is causing the disease (bacteriology, virology, toxicology, parasitology)

For all the other posts on pathology, click here.