The Risks of Misdiagnosis

The first step in dealing with any herd health challenge is diagnosing the problem. Without this critical step, launching into therapeutic or preventative responses is foolhardy and the chances of success will be severely limited. But what happens when the initial diagnosis is wrong? A misguided diagnosis could happen any number of ways: The presenting clinical signs might be confusing or misleading

The first step in dealing with any herd health challenge is diagnosing the problem. Without this critical step, launching into therapeutic or preventative responses is foolhardy and the chances of success will be severely limited.

But what happens when the initial diagnosis is wrong?

A misguided diagnosis could happen any number of ways:

  • The presenting clinical signs might be confusing or misleading to the untrained eye;

  • The presenting problem may be just one part of a disease complex;

  • There may be several disease processes occurring within the population at the same time, and only one (perhaps the most innocuous) rises to the degree of clinical disease;

  • The experience of the person making the diagnosis may be too limited when a “new” or different disease process occurs;

  • The problem may be over-simplified;

  • The diagnosis could be based on faulty information; or

  • Lab tests are either not specific or sensitive enough for a particular disease.

No Simple Answers

The days of simple answers to challenging health problems, making a “drive-by diagnosis” or expecting an accurate diagnosis over the phone are gone forever. Even a veterinarian with a lifetime of experience diagnosing diseases will submit lab samples to confirm or deny their preliminary diagnosis.

Recently, I participated in a large mortality surveillance study that involved performing necropsies (postmortem exams) on a large number of grow-finish animals. The pigs were necropsied and the findings were compared to the “barn” diagnosis.

The barn diagnosis is typically made by the caretakers and is based on the outward appearance of the carcass and where it was found within the barn. As is commonly done, the caretakers were instructed to choose the diagnosis from an abbreviated list of mortality reasons, which for the most part was outdated and incomplete.

The results of this survey were quite disturbing. Of more than 1,000 pigs that were necropsied, the barn diagnosis was correct on only 40% of the cases.

The technicians who performed the necropsies checked their accuracy by submitting tissues to a state diagnostic laboratory. Results showed they were nearly 100% correct in their determination of reasons for death based on gross lesions.

The lesson learned from this survey was that “eyeballing” carcasses and “educated guessing” should be avoided.

What is the most concerning about the outcome of this study was that we had been making significant herd health-related decisions based on severely flawed information.

In response to the monthly death loss summaries, vaccination protocols and antibiotic therapy were instituted, but they were doomed to fail because they were attempting to attack the wrong enemy!

Why Are These Pigs Dying?

Finishing pig health has been defined many different ways, but often the first noticeable sign of problems in the finishing phase of production is a rise in the mortality rate.

Across the industry, mortality rates vary widely between systems and even between sites or groups of pigs within a system.

Two decades ago, it was uncommon for a producer to report more than 3% mortality in finishing; 1-2% was the target.

When mortality occurs, eventually someone asks: “Why?”

Perhaps the question doesn't come with the first few mortalities, but when the death rate for the group exceeds the interference level for that particular group or system, it is imperative that we try to understand what is going on.

Morbidity is the term used to describe the illness rate or the frequency of animals in the population that are sick. Mortality rate describes the number of deaths that occur in a given group or from a given cause.

You may hear veterinarians refer to a disease as having high morbidity, but low mortality. This means that many in the group got sick, but few died from the disease. This would describe how Mycoplasmal pneumonia.

On the other hand, a syndrome such as hemorrhagic bowel syndrome (HBS) may affect only a few pigs in the population (low morbidity), but it kills nearly every pig it affects (high mortality).

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Steps to an Effective Diagnosis

Veterinarians and physicians have been trained to use a systematic, stepwise approach to making a diagnosis when confronted with a health issue or problem.

Step 1: Gather the History

The first step is to gather a thorough history of the patient, or in this case, a group of pigs. This generally requires asking a lot of questions. Here are some examples that may be asked about a group of feeder pigs (in no particular order):

Where did the pigs originate?

When did the pigs arrive?

What was the health status of the source herd?

Have there been problems with pigs from this source before?

What was the nature of the problems?

Were the problems resolved?

Is the current problem similar to past health issues?

Are there predictable spikes or peaks of mortality?

What did these pigs weigh when they arrived?

Are they gilts, barrows or mixed sex?

Were they vaccinated? With what?

Have they been treated? With what?

Was the treatment successful?

How many are or have been sick?

How many have died?

Have there been any issues with ventilation? Temperature? Feed? Water?

Are the pigs eating? Drinking?

What has been the feed disappearance rate and water usage for the site?

What feed ration is being offered?

Is the feed medicated? With what?

Who is the caretaker? Experience level?

Have any diagnostic tests or post-mortem exams been done?

Do production records indicate a level of performance that is within industry standards?

What biosecurity protocols are in place? Are they enforced?

Step 2: Physical Examination

The next step towards making a diagnosis is to physically examine the pigs and their environment. This step occasionally requires the use of all of your senses: sight, sound, smell, touch and at times, taste. A sixth sense — “pig sense” — may also prove valuable for the physical exam.

The physical exam in pig barns may also be called a “barn walk.” You must observe the pigs' behavior; assess their environment, feed and water availability; and check for the presence of clinical symptoms, such as coughing, thumping, diarrhea, lameness or other physical or medical condition that would indicate a problem.

While you are examining the population, you must focus on individuals and assess every pig for clues as to the problem or problems affecting the population.

It is very important to “listen” to the pigs. In this case, listening is more than using your ears. It also includes carefully observing behavior, appearance, condition and activity levels. The pigs will “tell” you what is bothering them. The better “listener” you become, the fewer misdiagnoses or mistakes you will make.

The physical exam is the subjective step in the diagnostic process, and thus will force you to draw on your experience and impressions of how normal pigs should look and act. This is where “pig sense” is so invaluable. You must know and understand “normal” behavior and appearance before you can make an initial clinical diagnosis of “abnormal” or “sick.”

The best caregivers perform a physical exam every day. The only difference is they tend to call the process “chores.” Swine veterinarians know that good chores are dependent on the ability of the caregiver to observe pigs carefully and know the difference between normal and abnormal behaviors. Good chores generally lead to good health and performance.

Necropsy is a valuable and necessary tool that should be used as part of the physical exam. An experienced necropsy technician can differentiate normal organ systems from diseased systems, which allows the search for a diagnosis to become more focused.

Necropsy should be performed on every pig that dies. In some cases, sick pigs need to be euthanized and necropsied so as to best ascertain the lesions causing the clinical picture.

Without necropsy, the cause of death cannot always be accurately determined. Again, in the study referenced earlier, the technique of assigning death reasons based on outward appearance of a carcass was only correct about 40% of the cases, when compared to actual necropsy examination.

I often tell producers that the most valuable pig in the barn is the first one that dies. But that is true only if the animal can aid in the diagnosis of potential problems by revealing what caused its death.

Without performing a postmortem examination, it is often difficult to determine the cause of death. Swine veterinarians are the experts at conducting necropsies or autopsies, not only because of their training, but most importantly, because their experience enables them to recognize what is normal and what is abnormal.

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Veterinarians also know which tissues to collect, how to preserve them and which diagnostic laboratory to submit them to.

Producers should never pass up an opportunity to observe lesions when their veterinarian necropsies a pig, and should be ready to ask questions. Curiosity is the best learning tool we possess.

Obtain the tools necessary (primarily a sharp knife) and begin by examining pig mortalities. A digital camera is a valuable tool to preserve images that can later be e-mailed and shared with your veterinarian if he/she cannot visit the production site.

Remember, to understand what “abnormal” looks like, you must first know what “normal” looks like. The primary goal of a necropsy is to determine the cause of death, but always consider a necropsy as a chance to gain valuable knowledge or clues about what is going on in your pigs.

Step 3: Laboratory Confirmation

The final step in the process of diagnosing a problem requires the use of one or more of a myriad of lab tests that have been developed over the years. This confirmatory testing can be done by a veterinary diagnostic laboratory such as a state-supported lab or in some cases by the attending veterinarian at his or her clinic or lab.

The list of suspected causes uncovered in the history and physical exam will determine what samples or specimens should be collected and submitted to the laboratory.

Samples may include pieces of tissue such as lung, intestine, tonsils or lymph node, just to name a few. Or, the test desired may require only blood or serum from the affected or recovered pigs.

It is very important to work with your veterinarian when submitting tissues to ensure the right samples can be collected, prepared and transported in a manner that will give the greatest chance of securing a lab confirmation. If you submit wrong or poorly prepared samples, you will likely not receive accurate or meaningful results.

When submitting tissues for laboratory confirmation, it is very important to choose samples from a pig that died early in the course of disease and represents the problem at hand. Avoid the temptation to send tissues from only chronic, poor-doing pigs because they may not be representative of the disease you are trying to confirm.

It is important to include as much information and history as possible when submitting samples to the lab, since the pathologists will need a list of the differential diagnosis as well as a thorough history and description of the main clinical signs. Your veterinarian will supply this information along with a submission form in most cases.

The laboratory results are the objective portion of the diagnostic process. The results of a lab test tend to be positive or negative, and thus help to eliminate other possibilities that were on the list.

The Cost of Misdiagnosis

I was recently asked: “Which is worse — a wrong diagnosis or no diagnosis?”

In nearly every case, a wrong diagnosis is a bigger issue than having no diagnosis at all. While not detecting a problem early in its course can result in substantial loss, misdiagnosing the problem and setting off on the wrong course of therapy or prevention will delay making the correct diagnosis, plus inflict the cost of the wrong therapy or intervention.

This is particularly true when a diagnosis is made based solely on the physical appearance of the mortality, and the assumption is made that every pig that is found dead, and presents itself in a particular way, can be attributed to a single disease or syndrome.

Hemorrhagic bowel syndrome (HBS) is one such problem that is often misdiagnosed or over-diagnosed. If a postmortem exam or necropsy is not performed on these pigs, the tendency is to diagnose HBS on any unexpected or acute death of an apparently healthy pig. There are many potential causes of death in such pigs, and without a necropsy, the correct diagnosis will never be known.

When the level of deaths attributed to HBS reaches a level that is perceived to require intervention, such as feedgrade antibiotics or the addition of mitigating feed ingredients, the end result will simply be a waste of resources.

In some scenarios, a misdiagnosis that leads to the wrong intervention could allow the true problem to progress or worsen, potentially resulting in even more loss that would have been prevented with the correct intervention.

Interventions are not always therapeutic. Preventive tools such as vaccines are also at risk of being misapplied in cases of misdiagnoses. Certainly, a vaccine intended for ileitis will not reduce mortality due to HBS, and the cost of the vaccine (if not offset by controlling ileitis) would have been wasted.

Avoiding Misdiagnosis

To avoid making a misdiagnosis, first avoid taking preconceived ideas into the process. This is called an “anchoring mistake.” It leads to not considering other options or possibilities when your mind is made up early in the process.

In many cases, the wrong diagnosis results from errors in thinking or judgment. Often, simply looking at or listening to the pigs would have resulted in the correct diagnosis. If the initial attempts at remedying or treating the problem do not work, you must reassess and try again.

It is important to ask your veterinarian and yourself the right questions:

“What else could have caused this?”

“Could there be more than one thing going on?”

Rarely do diseases act entirely alone. Most are multi-factorial and will require a systems approach to resolve them.

Lastly, never stop listening to the pigs! In most cases, the pigs will tell you what is wrong and help you make the right diagnosis.

‘Why Do We Need to Bleed So Many Pigs, Doc?'

Researchers have made great strides in developing and improving the serological tests available for swine. Although the perfect serology test has yet to be discovered, it is important to know both the strengths and weaknesses of any test you plan to use. When used correctly, serology can be a very valuable diagnostic tool.

Serological tests are assessed based on two criteria — sensitivity and specificity. Sensitivity is the measure of how good the test is at identifying animals that are truly infected as “positive.”

Specificity is the measure of how good a test is at identifying truly non-infected animals as “negative.”

If a test tends to mistakenly identify true positives as “negative,” we say it has poor sensitivity. Poor sensitivity tests are prone to producing “false negative” results.

This type of error would be disastrous if, for instance, you were testing boars for porcine reproductive and respiratory syndrome (PRRS) virus prior to using their semen in naïve sows and the test failed to identify an infected boar.

If the test wrongly identifies true negative animals as “positive,” it is said to have poor specificity. A test with poor specificity is prone to producing “false positive” results.

If you are working with a population expected to be negative for PRRS, for example, “false positive” results will likely force you to retest the animals in question to confirm that the test was truly a “false positive.” Too many “false positive” results will add to expense and frustration.

Before you sample the first pig, there are several issues you need to consider. First, why are you testing and how do you plan to use the results? Do you simply want surveillance for the presence (or absence) of a particular disease in your herd, or do you have a need to know the prevalence of a disease that already exists in the population?

You can also use some serological tests to roughly determine when a disease entered and infected a group of animals. This may be valuable in determining the best time to vaccinate ahead of the exposure.

To gain the most benefit from the tests, a basic understanding of what the test is detecting is important. Today's serology tests generally are looking for the presence of the bug itself (antigen) or for the pig's immune system response to the infectious agent (antibody). Not all pigs in a population respond the same, and there are certainly differences between infectious agents as to when and to what degree they express themselves to a given test.

Three statistical factors determine how many samples are needed:

  1. The number of animals in the group or population being tested,

  2. The expected prevalence of the disease being tested among the population, and

  3. The confidence level you wish to achieve with your testing.

For example, in a 1,000-head finishing barn, to achieve a 90% confidence level of detecting a 10% subset of the pigs that are positive for a disease, statisticians have calculated that you must sample 22 animals in order to detect at least one positive in a given time frame.

If you raise the desired confidence level to 95% and lower the suspected prevalence to 5% in the same 1,000-head barn, you must increase your testing to 57 pigs. It really is a numbers game where statisticians make the rules.

Interestingly, as population size goes above 1,000, the number needed for the same confidence and prevalence values does not go much higher. There are published statistical sample size charts that are a handy reference for determining the number of samples required for various confidence and expected prevalence levels. Figure 1 offers an abbreviated version as an example.

Risk aversion may be the determining factor as we decide how many animals to bleed. If you are bringing a small number of gilts (100) into a known PRRS-naive herd, you may want to be nearly 100% confident that a single animal will be detected. If so, the statistics tell us we must test every animal in the group. The problem with this approach is that it often exceeds the specificity of the test, and then you will have to deal with one or more false positive results. False positive results will likely lead to retesting some of the animals using a confirmatory test.

Remember, the perfect test has yet to be discovered. Until it is, we will just have to go on asking: “Why do we have to bleed so many pigs, Doc?”
John T. Waddell

Figure 1
Number of Samples
Expected prevalence 10% 5%
Confidence level 90% 95% 90% 95%
Population size
50 18 22 30 35
100 20 25 37 45
500 22 28 43 56
1000 22 29 44 57