Contents
About This Book
Dedication
Acknowledgements
Chapter 1 Introduction to Veterinary Pathology
What is pathology?
Who ‘does’ pathology?
Pathology as an academic subject
Chapter 2 Aetiology
Introduction
Aetiology – the study of the causes of disease
Chapter 3 Cell Injury
Lesions – structural and functional changes
Definitions of cell response or injury following harmful stimulus
Cellular degeneration
Cell death – necrosis
Extracellular changes
Chapter 4 Inflammation
What is inflammation?
Acute inflammation
Chronic inflammation
Chapter 5 Pathology and the Immune System
The normal immune system
Diseases of the immune system
Chapter 6 Tissue Repair
Definition of tissue repair or healing
Tissue repair – general and specific examples
Healing in various tissues
Healing regulation and control
What can impair, prevent or alter healing?
Chapter 7 Circulatory Disorders
The normal circulatory system
Oedema
Impaired blood supply to tissues
Clotting (coagulation) of blood
Shock
Chapter 8 Disorders of Cell/Tissue Growth
Atrophy, hypertrophy, hyperplasia and metaplasia
Neoplasia
Glossary
Answers to Test Yourself Questions
Further Reading
Index
This edition first published 2010
© 2010 by Harriet Brooks
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Library of Congress Cataloging-in-Publication Data
Brooks, Harriet W.
General pathology for veterinary nurses / Harriet W. Brooks.
p. ; cm.
Includes index.
ISBN 978-1-4051-5590-8 (pbk. : alk. paper)
1. Veterinary pathology. 2. Veterinary nursing. I. Title.
[DNLM: 1. Animal Diseases–physiopathology–Nurses’ Instruction.
2. Pathology, Veterinary–Nurses’ Instruction. SF 774.5 B873g 2010]
SF774.5.B76 2010
636.089’607–dc22
2009027991
A catalogue record for this book is available from the British Library.
1 2010
Don’t skip this part! This section will explain how to get the most from this book.
Welcome to general pathology!
What do we mean by general pathology? Before answering that, let us talk about pathology in its broadest sense.
You may think of pathologists as people in paper suits, who get involved with murder investigations on the television, very remote from your everyday life. If you are already studying for your nursing qualification, you may view pathology as a dreary subject to be passed on the way to the more exciting subjects of surgical or medical nursing. You may have completed your studies and feel you ‘got by’ without needing to know too much pathology anyway.
Well, I hope that, through this book, I can help you to view pathology as not so boring after all, and relevant to every one of us involved in veterinary medicine.
Pathology underpins all you do as a veterinary nurse; it is central to veterinary science. Apart from some elective surgical procedures, all that exciting medicine and surgery hinge on pathological processes occurring within the body. Understanding these pathological processes will help you to understand the clinical presentations of your patients, to follow the rationale of the treatments used in the clinics, and to explain the conditions and procedures to anxious owners. A good understanding of pathology will turn a competent veterinary nurse into an outstanding one.
This question will be answered more fully in the first two chapters, but briefly, pathology is the study of the effects of disease on the body. It is a broad subject, encompassing a number of sub-divisions such as general pathology, systematic (or special) pathology and clinical pathology.
This book concentrates on general pathology, which is the study of the basic pathological processes that are not specific to particular organs or tissues. For instance, processes such as cell degeneration, inflammation and tumour formation are pretty much the same in all parts of the body, and so are considered under the heading of general pathology.
Other textbooks and references will cover systematic pathology, discussing specific organ systems. If you refer to these texts you will be able to apply the knowledge of general pathology that you gain from this book.
Clinical (or chemical) pathology is the pathology used in the laboratory, whether the practice laboratory or a diagnostic laboratory – many general nursing texts cover basic clinical pathology, e.g. urinalysis or blood biochemistry, so I have not covered these aspects in this book.
Each chapter starts with two boxes, one giving an outline of the chapter and the other explaining the aims of the chapter. Read the aims box carefully. Understanding the aims of each chapter will help to guide your reading of the topic.
There are sub-headings dividing up each chapter into main topics and at the end of each chapter is another text box summarising the main points covered in the chapter.
Dotted through the text there are diagrams and tables and a few photographs, all included as numbered boxes. These numbered boxes aim to summarise information or revise it, so that many important topics will be encountered three times – in the text, in the diagram and as a caption to the diagram. The diagrams and tables can also be used as a revision aid if you are unlucky enough to be facing examinations. A glossary of terms used in the book, or that you may come across in your reading, is included towards the back of the book.
You might find that some areas do not have summary boxes when you feel you could do with them or a useful term is omitted from the glossary; in this case, draw up your own boxes to help your study and add missing words to the glossary – do not just accept what I have given you! Remember this book is just a starting point for you – use it to gain the basics and the confidence to start to read around the subject. Who knows where you could end up?
Each chapter finishes with up to six test yourself questions. The questions vary slightly in their format and some require short snappy answers, whilst others could almost require short essay answers. These are to help you make sure you have understood what you have read; use them in whatever way helps you most. You may like to actually sit and write your answers from memory, you may like to jot notes or key words or you may find just reading through the questions helps you. At the end of the book are some suggested answers; however, you choose to use the questions, try not to cheat! It will help you most if you have a go at answering the questions yourself before you read my suggestions.
The book is designed to be a workbook. If this is your own book, do not be afraid to personalise it – jot notes or reminders in the margin, cross reference to published articles or to other parts of the book, or to cases you have seen in the clinics. Annotate the diagrams; draw moustaches on the cartoons. Make this book totally yours. (Note – do not do this if it is a library copy, if you are reading this in a bookshop or if it belongs to someone else.)
You will need to be an active reader to get the most from this book! Many sections within this book build on one another. Some prior knowledge of histology, anatomy and physiology is expected, so you may need to refer to other texts or study notes to supplement some areas if you are uncertain what is being discussed.
Get in touch – the author would like to hear from you if you have comments or complaints, or suggestions for future editions.
Most of all, I hope you will enjoy reading and using this book and that it might change your mind about pathology (unless, of course you liked it all along!).
This book is dedicated to my parents, with fondest love
I am indebted first and foremost to Wiley-Blackwell for inviting me to write this book, and thanks to many people there, but especially Katy Loftus, for encouragement, advice and extreme patience during its protracted production.
Many thanks also to all of my family, especially my mother and husband, for their support, love and a good deal of humour; I might have given up many times had you not all kept me going. Thanks also to Joe, my husband, for comments on Chapter 5, and for the use of his photograph in that chapter, and to Helen Wakeham for allowing us to photograph her sow and piglets.
My beloved canine companions – Daisy, Dilly and Edie – deserve a mention for keeping my spirits high and for taking me out for many a good walk to clear my mind.
This book is based on my pathology teaching material for trainee nurses at the Royal Veterinary College. So finally, but certainly not least, I acknowledge all past and future trainee veterinary nurses who have inspired me to write the book; I truly hope it helps to underpin your chosen careers, and I wish you every happiness and success in the future.
Harriet Brooks
The word pathology comes from two Greek words, Pathos – which literally means ‘experience’ or ‘something which one suffers’, but which in this context is used in terms of suffering from a disease, and -logy meaning ‘word’, ‘speech’ or ‘reason’. The suffix -logy is used in compound words (when it is added to another word such as in biology, physiology and entomology), and then it infers ‘study of’ or ‘science of’.
So, pathology is the branch of medical science that involves study of the causes of diseases, how they develop and their effects on the body. It encompasses any deviation from a healthy or normal condition in any living creature. There is even a branch of horticulture that involves study of pathology in plants.
In pathology, the effects of diseases can be studied at various levels: the whole body, the organs or tissues, cells and even within cells (at sub-cellular level) (see Box 1.1).
You might answer that question by saying pathologists do, and that is certainly correct. We will discuss pathologists in a moment, but the ‘pathologically trained’ professionals are not the only ones actively engaged in pathology. If you are working in general practice, you are too.
As we said at the beginning of this chapter, pathology involves not just study of what causes diseases, but also how diseases develop and their effects on the body. Every time you record the temperature, pulse and respiration of a patient, use a dipstick to test an animal’s urine, run an automated blood analyser, change the dressing on an infected wound, or advise an owner about flea control to help their cat’s red itchy skin, or diet to control diarrhoea in a sensitive golden retriever, you are assessing the deviation from a healthy or normal condition in the animal; you are assessing pathological changes. So, you and the vets, physiotherapists and others with whom you work use their knowledge of pathology. But there will be times when you require help from a pathology diagnostic laboratory to make the diagnosis; perhaps you need tests beyond the scope of your practice laboratory.
Pathology laboratories may be independent businesses, or they may be based at a university veterinary school, or they may be government agencies. The pathologists who work at these laboratories are often classified according to the type of diagnostic work they do, though a few will be all-rounders and will do everything!
Anatomic pathologists study disease by looking at tissue and organs. This may be by performing post-mortem examinations (also called necropsies) and writing a post-mortem report, or by looking at tissues from live animals (called biopsies). Anatomic pathologists will look at the tissues or organs by eye (gross examination) to identify abnormalities, but also use histologic sections, mounted on glass slides to look at the tissue under the microscope (see Box 1.2).
Clinical pathologists assess disease in an animal by studying body fluids (such as blood, urine, joint fluid, abdominal tap fluid, cerebrospinal fluid and so on). They may look at the chemical composition (clinical biochemistry) or the types of cells in the fluid or in an FNA, using a microscope to study a stained smear of the sample on a glass microscope slide (this is called cytology). Clinical pathologists might spot bacteria or other infectious organisms in a cytology preparation.
Haematology is specifically the study of cell types in blood, and this can indicate an increase in white blood cells (leucocytes) in an animal fighting an infection or a decrease in red blood cells (erythrocytes) in an animal with anaemia.
An anatomic or clinical pathologist may suspect that infectious organisms are involved in the disease and may suggest a fresh (unfixed) sample should be sent for microbiology (see below) if the practice has not already done this.
Microbiologists study infectious organisms that may be associated with diseases, more specifically, bacteriologists study bacteria, virologists study viruses, and mycologists study fungi and yeasts. Clinical samples, such as urine, pus, mucus or even tissue may be sent to microbiology laboratories where they have the equipment, skills and expertise to grow (culture) and identify infectious organisms. In the case of bacterial infection, they may also be able to assess which antibiotics the organism is likely to be killed by (the sensitivity of the organism) which gives the vet an indication of what treatment to use.
Although the very small creatures studied by microbiologists could be described as being parasitic, parasitologists tend to be associated with the study of slightly larger organisms which live on or in other animals. So, parasitology encompasses the study of, for instance, parasitic worms in the gut, fleas living on the skin or demodex mites living in hair follicles.
Sometimes an infectious organism is suspected of causing disease in an animal, but that organism itself cannot be cultured in the microbiology laboratory or seen in samples under the microscope. In this case, the immunology laboratory may be able to tell whether the animal has been infected by the suspected organism by looking for antibodies. Antibodies are produced by the body’s immune system to help fight disease (this is part of what is known as an immune response); specific antibodies are produced for specific infectious agents, so finding certain antibodies will indicate that an animal has come into contact with a certain infectious agent (or has responded to a vaccine).
Infectious organisms have proteins on their surfaces, called antigens. These antigens are a sort of ‘fingerprint’ which the immune system can usually recognise as being ‘foreign’ and this stimulates the immune response. Sometimes specific antigens can be detected in samples by immunological tests.
Such immunological tests may be done on blood serum (this is called serology). Some immunological tests can also be carried out on tissues mounted on microscope slides, and this is then known as immunostaining. All types of cells of the body have their own ‘fingerprint’, though in a healthy individual the immune system recognises these and doesn’t start to react against them. Sometimes we can use this property of cells to confirm the diagnosis, for instance, if a pathologist is having trouble identifying a particular skin tumour under the microscope immunostains for specific cell types can be applied to the tissue and can help to reveal the identity of the tumour.
In some cases, toxicologists may be asked to analyse samples for toxins or poisons, for instance, you or the pathologist might send stomach contents, urine or even fresh tissue from a necropsy in the case of an animal suspected of being poisoned. The laboratory may need some guidance as to which toxic substance is suspected, such as a reliable history of known or likely contact of an animal with that particular substance. Note also that very often the toxins or poisons break down or are metabolised after having their damaging effects, and may not be detected in biological samples. In these cases, the animal presents with clinical signs that require diagnosis and treatment, such as a severely anaemic animal that has eaten anticoagulant rodent poison. The priority is to treat the anaemia, and toxicology may not be helpful, though it could be argued that confirming the cause of the animal’s signs may help to prevent poisoning in other animals.
There are a small number of veterinary pathologists who deal with forensic cases; that is, cases where there may be suggestions of cruelty or malicious harm to men or animals, or police involvement due to suspected illegal activity of one sort or another. This subject is beyond the scope of this book, and it is usually best for general practices to seek advice if they get drawn into such a case unless they are experienced in dealing with them. As a rule of thumb all those involved with the case, including veterinary nurses, may be asked to give evidence at a later stage, and should always keep notes, photographs, logged telephone calls or case records securely and safely stored, in case they need to submit them to the authorities as part of the investigation. Any biological material, including bodies of deceased animals, should be logged and labelled, and stored securely until removal by an authorised person.
Some pathologists are employed as veterinary investigation officers, and work for government agency laboratories. These laboratories principally investigate diseases in farm or production animals. As well as investigating disease in individual or small groups of animals, these pathologists are important in helping to maintain herd or flock health on farms and nationally. This helps to prevent widespread infectious diseases and to protect our food quality (and safety) and human health.
Veterinary pathologists work at pharmaceutical laboratories too. Here they will help to investigate diseases and to develop drugs to treat men and animals. They will also take an interest in apparent unexpected drug reactions.
This chapter has so far discussed some of the ways pathology is carried out in practice and who undertakes it. Pathology is a broad academic subject, and when we study it, we often divide it into general pathology and systematic (or special) pathology. What does this mean?
From now on this book focuses on general pathology, but we will use examples of specific organs or body systems to help you understand the processes we are discussing, and perhaps to relate the topic to diseases you may have encountered in general veterinary practice.
Let us start with a few terms you may come across in your reading. Aetiology is pronounced eet-ee-ology. In American textbooks you may see it spelt without the first ‘a’ – etiology, but it is still pronounced as above.
You would have spotted that aetiology is another compound word (like the word pathology, discussed in Chapter 1) in which ‘-logy’ denotes ‘study of’ or ‘science of’. The first part comes from the Greek word ‘aitia’ meaning ‘cause’. So, aetiology means the study or science of the causes of disease.
The word pathogenesis is associated with aetiology. Pathogenesis involves ‘pathos’ again, as introduced in Chapter 1, but in this case it is linked with genesis, which comes from the Greek verb for ‘to become’ or ‘to produce, to bring forth’. Thus, pathogenesis relates to things which produce disease, and tends to be used when discussing how factors lead to disease, or the mechanisms of disease development. It describes the chain of events from the initial stimulus to the manifestation of the disease or the lesion produced.
There are some other terms we use which relate to pathogenesis. A factor which is capable of producing disease may be referred to as pathogenic, and an infectious agent (bacteria, virus and fungus) capable of causing disease may often be referred to in non-specific terms as a pathogen. The term aetiological agent is also used for a factor capable of causing disease.
Do not worry too much about these terms at this stage; you will become more familiar with their usage as you read through this book and other texts.
So, now we have got some definitions out of the way; let us start to discuss aetiology.
Diseases occur when a harmful trigger (of whatever type) causes loss of normal health or disrupts a tissue or organ. Many diseases tend to have distinct and recognisable cause(s) (aetiology), development processes (pathogenesis), lesions and clinical signs.
Let us consider a familiar example to illustrate this. Two cats have a fight, and a few days later one of them develops an abscess in the skin on its back. Now think about the features of diseases in turn and apply them to our cat:
Cause(s): bacteria from one cat’s mouth are introduced into another cat’s skin via the teeth.
Development process: the bacteria multiply and start up the process of inflammation in the skin of the bitten cat. Inflammatory cells and bacteria die and accumulate as a pool of pus1, with a rim of active inflammation around it (an abscess).
Lesion: surface skin wound and scab; abscess in the skin; heat (due to inflammation).
Clinical signs: pain, heat, swelling, pus, depression, loss of appetite, grumpiness etc.
In other cases it may be harder to clearly define diseases in this way. Many diseases involve more than one cause (aetiological agent), or they may be made more complex by other factors such as secondary infections. Development processes may be altered by other concurrent diseases. Finally, especially in veterinary patients, the lesions and clinical signs are often complicated by self-trauma – the animal licking, biting or scratching a diseased area, for instance.
We now go on to discuss some of the aetiological agents and some of the complicating factors that can affect disease development.
We said above that aetiological agents are factors capable of causing disease or tissue damage. Our knowledge of aetiological agents has altered as our scientific understanding has increased. In historical times, evil spirits, bad humors and foul smells were all considered to cause disease. Old rags were thought to cause bubonic plague (the Black Death) during medieval times – it was actually fleas on rats living in the rags which carried the plague bacterium.
The first microscopes were developed in the second half of the 1600s, allowing closer study of tissues and even description of bacteria, though the role of bacteria in disease was not recognised for another 200 years or so. Rudimentary, though successful, attempts at vaccination for diseases we now know to be caused by viruses, such as smallpox, were carried out from the 1770s.
Yeasts and fungi were first recognised for their roles in fermentation, and later some types of these organisms were found to be involved in disease especially in patients with weakened immune systems. The roles of nutrition and hygiene started to be taken seriously in the late 1800s and continued to gather momentum since then. By 1855, it was realised that a cholera outbreak in London was linked to a particular supply of drinking water in Lambeth, and since then our understanding of the infectious and environmental factors involved in disease development and tissue injury has grown enormously.
Sadly, the great wars have added to our understanding of physical trauma (but also have clearly illustrated the importance of emergency nursing for longer-term prognosis). More recently, molecular biology has increased our knowledge of the DNA in our genes; this has helped us to recognise the genetic basis of some diseases.
Throughout this book, we shall tend to consider aetiological agents in general terms, though we shall use a few specific examples to illustrate pathological processes. Other textbooks will be the sources of more specific information on causes of particular diseases (see Further Reading, page 238).
To help our understanding we can usefully classify aetiological agents in various ways. You may see other classifications in other textbooks, but a helpful start is to consider aetiological agents as internal or external factors, such as in Box 2.1.
Box 2.1 Classification of aetiological agents
You might wish to slightly reclassify some items, for instance, in external factors (the bottom half of the table), physical and chemical factors could be classified under environmental factors. Or temperature of the environment could be considered a physical factor. Should hormones be included in the list of internal factors, after all they have detrimental effects in disorders like hyperadrenocorticism (Cushing’s disease) and diabetes mellitus?
Do not be afraid to annotate or draw up your own version of this table as you read on through this book!
Diseases can broadly be considered as follows.
These are diseases which develop at some stage during life, as a result of the effects of one or more aetiological agent acting during life.
Examples of acquired diseases would be pneumonia or dermatitis due to fleas.
These are diseases which the animal or person is born with. Congenital diseases occur because the aetiological agent acts on the developing embryo or foetus, on the uterus or placenta, or on the mother, either before or during pregnancy. Note that clinical signs of a congenital disease may not be seen at birth, but may show up later in life. They are still called congenital and not acquired because the aetiological agent actually had its effect before birth.
Developmental abnormalities, such as heart defects, and forms of muscular dystrophy in certain dog breeds, such as golden retrievers, would be examples of congenital diseases (see Box 2.2).
Some diseases, notably cancers, are a bit more difficult to divide up this way as they may involve both congenital and hereditary damage to genes and exposure to some factor(s) during life to start the cancer growing (see section on Neoplasia in Chapter 8). In diseases involving both acquired and congenital phases, we consider that the initial congenital gene mutation has made that individual susceptible to later diseases if he or she comes into contact with a suitable trigger factor during life.
There are some diseases of which we do not (yet) know the cause, though they may have recognisable development processes, lesions or clinical signs. These are diseases we refer to as idiopathic.
If a disease has an uncomplicated development that could be summarised as:
it would be known as a simple disease (see Box 2.3).
In reality, there are few diseases or disorders which are this uncomplicated, and in the vast majority of cases we need to consider what other factors act to make disease more likely to occur in the animal, or which modify the disease (for instance, factors which make the disease more severe or last longer in one animal compared with another). Factors associated with increasing likelihood of disease or which modify the course of a disease are shown in Box 2.4.
So, rather than being simple, as defined above, many diseases are more complicated and their course is affected or modified by lots of other factors and infectious agents. A good example of just such a multifactorial disease is canine infectious respiratory disease (CIRD) often called ‘kennel cough’ (see Box 2.5).
| • Age | Often very young or old patients are more vulnerable to diseases |
| • Immune system | Lack of immunity in young, unvaccinated or naïve* animals, or any cause of reduced immunity in a previous immune animal will increase vulnerability to diseases |
| • Genes | A fault or mutation in a gene may make an animal more susceptible to the disease-causing agent |
| • Other disease | Having another disease already can often increase vulnerability by reducing the animal’s ability to produce an immune response |
| • Environmental factors | Poor hygiene, extremes of temperature, mental stress, hunger, thirst, overcrowding etc. can increase vulnerability |
| • Some drugs | Certain drugs may affect the course of a disease, for instance, corticosteroids which may be used to reduce inflammation but can also limit immune responses and healing processes |
*Naaïve – pronounced ‘ny-eeve’. This means animals who have not encountered a particular infectious agent before and so have not developed immunity to it.
CIRD involves viruses and bacteria, but is also associated with various other factors, such as barking, housing in groups, age and so on; there is even a suggestion that certain breeds of dog are more vulnerable to contracting respiratory disease.
You can see that control, prevention and treatment of multifactorial diseases are somewhat more complicated than that for simple diseases, and veterinary professionals must consider many factors when giving advice or devising treatment and control plans.
So, this concludes a brief discussion of aetiology, or causal factors, of disease. In the next chapters, we look more at the responses, desired and harmful, of the body to these causal factors.
1 Pus = thick fluid sometimes formed as a result of inflammation, consisting of white blood cells (especially neutrophils, see Chapter 4), dead cells and often also containing living and dead bacteria. By the way, note the correct spelling. Examiners HATE to see it referred to as ‘puss’!
Lesions are pathological changes in tissues or organs. We think of lesions as abnormalities we can see, such as an area of abnormal colour on a liver or an ulcerated area on the skin; we call them gross lesions when we can see the abnormalities with the naked eye. But we need to remember that gross lesions are actually manifestations of abnormalities or changes in the cells which make up that tissue or organ (we can call these changes or abnormalities in cells cellular lesions). These changes in cells occur in response to injurious, that is harmful, stimuli (in other words, aetiological agents, see Chapter 2).
So, if abnormalities occur at the cellular level they are likely to lead to changes in tissue structure. The other effect they may have is to alter the function of the tissue, so for instance a harmful toxin that damages liver cells will, if enough liver cells are damaged, alter the function of the liver (as well as altering the look – colour, texture, size, shape – of the liver) (see Box 3.1).
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