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More Effective Disease Diagnostics

Recent widespread outbreaks of porcine circovirus-associated disease (PCVAD) in North America have served as a dramatic reminder that infectious diseases can be devastating to pigs and profits. And, it's not as if porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus, ileitis, or any of the diseases, such as Mycoplasmal pneumonia, have gone away. Fortunately, there are

Recent widespread outbreaks of porcine circovirus-associated disease (PCVAD) in North America have served as a dramatic reminder that infectious diseases can be devastating to pigs and profits.

And, it's not as if porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus, ileitis, or any of the “old” diseases, such as Mycoplasmal pneumonia, have gone away.

Fortunately, there are effective approaches for dealing with most of these challenges. Putting a plan together to prevent, control or eliminate specific diseases starts with an accurate understanding of the health status of swine populations.

Perhaps “health status” is the wrong choice of words, since health and disease tend to be quite dynamic in many pig populations.

To keep track of these health dynamics, the producer, veterinarian and diagnostic laboratory must work together in a partnership to develop diagnostic approaches that are effective and worthwhile. Each party brings key expertise that is essential for accurate and timely disease diagnosis.

Understanding the normal production and health dynamics within a herd is the foundation for early detection of disease. Early detection is the key for effectively managing endemic, emerging and exotic disease problems.

The Partners

The producer, veterinarian and diagnostic laboratory all have important roles in keeping tabs on the health of pig populations.

The point person in tracking herd health dynamics is the producer/animal caretaker. The producer sees the pigs every day and is the first to observe changes in performance or clinical signs of disease. Therefore, the producer is the “expert” on a particular group of pigs. He/she knows, for example, how the pigs looked last week, or how the previous group of pigs looked or performed. These are important details for a complete understanding of why a group of pigs is performing as they are.

Next in the herd health partnership are the veterinarians with their specialized training and knowledge in disease diagnosis, treatment and control. They are trained to recognize the clinical signs associated with specific diseases, and the sampling and submission procedures necessary for effective diagnostic testing in the laboratory. Veterinarians know and understand the diagnostic tests available for different diseases, and how to interpret the various tests as they relate to the clinical history of the farm.

Veterinarians offer a different perspective by benchmarking the client herd against other similar herds, and through an awareness of pig health concerns in the geographic area. Working with the producer, the veterinarian is able to develop appropriate health monitoring programs as situations and producer goals dictate.

To be most effective, veterinarians need to understand the health status and goals of a herd, and then work closely with producers to achieve those goals.

The third member of the health monitoring team is the veterinary diagnostic laboratory. Many swine practitioners provide some degree of diagnostic service in their practice. However, veterinary diagnostic laboratories provide more specialized equipment, knowledge and service. These labs develop, validate and conduct diagnostic tests, and maintain a portfolio of diagnostic procedures for working up different types of cases.

In addition, diagnostic labs serve as a resource for veterinarians; advise on test selection and interpretation of results; and provide current information on disease trends regionally, nationally and even globally. Diagnostic laboratories also maintain key relationships with disease researchers, other diagnostic labs and animal health regulatory officials.

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Baseline Monitoring — Production Performance

A good understanding of the herd baseline performance facilitates more rapid diagnosis of new diseases, helps direct testing strategy and streamlines interpretation of diagnostic results. Only by knowing the “normal” performance or health of a herd, can the producer or veterinarian identify the “abnormal” situations or diseases that may arise.

Tracking baseline performance relative to disease status also allows cost-benefit analyses of health interventions for existing diseases, providing a yardstick for measuring response to treatment/control measures. Baseline production performance monitoring includes such measures as average daily feed intake and water disappearance, individual treatment, mortality (adjusted for season, gender, etc.), pulls or culls and average daily gain.

An obvious and very important monitoring tool is the daily observation of pigs for clinical signs of disease. A consistent method of observing pigs can detect changes earlier and increase the success of treatment interventions.

It is important to remember three characteristics of most disease outbreaks in pigs:

  • Only a small portion of the group might be affected at any given time.

  • Important clinical signs, such as coughing or pigs going off feed, generally start as very subtle changes.

  • Early detection usually results in improved response to treatment.

To recognize these early, subtle changes, it is important to have a consistent and structured approach to walking the pens and observing the pigs (see sidebar on page 10).

Changes from normal performance can also include unusual lesions in pigs that are identified during a postmortem examination. Some producers work with their veterinarians to develop the skills necessary to perform postmortem exams. These skills must not be taken lightly. Every precaution must be taken to avoid exposure to enzootic disease.

Routine examination of carcasses for common causes of death, such as gut torsions or gastric ulcers, helps a producer become more aware of what occurs routinely in their herd, allowing more rapid detection of abnormal findings.

Performing postmortem examinations keeps the mind, eye and knife sharp, and improves the odds for early detection of new health problems.

Tracking baseline performance allows producers and veterinarians to establish triggers for action or thresholds for intervention when a particular factor exceeds a set value or changes from the normal, background level. This becomes part of a set of formal or informal farm standard operating procedures developed by the producer and herd veterinarian, by which performance is continuously monitored and compared to the baseline.

Baseline Monitoring — Disease Detection

Actively monitoring a herd for specific disease agents is another important tool for understanding the baseline performance in a herd. There are several different approaches for monitoring health status in pigs, but a brief overview of the basic diagnostic methods may help clarify how the different methods are applied.

Generally, when searching for specific disease agents, we use two approaches — searching for the agent directly or searching for evidence or clues left by the disease agent indirectly.

Finding the agent directly may be possible by visual detection. For example, some internal parasites of pigs are visible without magnification and eggs can be seen with microscopic examination of fecal samples.

At the other end of the spectrum, an electron microscope can magnify thousands of times to see the specific size and shape of Transmissible gastroenteritis (TGE) or rotavirus particles in a fecal sample. This confirms the presence of the virus in the sample.

A common method for directly detecting bacteria and viruses is to amplify the number of organisms by culturing them. With bacteria, the colonies are visible and can be further tested to identify the specific bacteria isolated. Viruses, on the other hand, cause changes in the cell cultures in which they are grown. Further testing provides positive identification.

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The advantage of growing the organisms is that they can be available for further testing or for use in autogenous vaccines, when appropriate. The disadvantage is that the procedure can be slower (in the case of virus isolation), and not as sensitive (less likely to find all positive cases) than some other methods.

Another method commonly used to directly detect bacteria and viruses is the polymerase chain reaction (PCR) test. This method is used to amplify the genome (DNA or RNA) of specific disease agents up to levels that can be detected by various means. PCR tests for several viruses (PRRS virus, Type 2 circovirus, swine influenza virus, TGE, classical swine fever) and bacteria (Lawsonia intracellularis for ileitis, Haemophilus parasuis, Actinobacillus pleuropneumonia, Actinobacillus suis, Erysipilothrix rhusiopathiae for erysipelas, and Leptospira sp.) have been developed for routine use at diagnostic laboratories.

PCR tests are generally faster and more sensitive than other methods for detecting disease agents directly. Because they are so sensitive, great care must be taken in handling tissues to avoid cross-contamination and false positive results.

In addition to finding disease agents directly, we also look for evidence of the disease agents by various methods.

A common tool used for monitoring swine health is serology, in which the serum component of blood samples is tested for antibodies that are produced against specific disease agents. Sometimes several tests are available for the same disease, but the basic principle of finding the antibodies is the same.

The advantage of using serology is that antibodies against the disease agents tend to last longer than the actual agent, so there is more time for detecting the agent. The disadvantage is that it can be days to weeks after infection before the antibodies appear in the serum, so it is less useful in the early stages of a disease outbreak.

Additional indirect evidence that a disease agent is present is the change it can cause in the pig, either outwardly (clinical signs) or by changes to tissues that are evident grossly or microscopically.

Observation for clinical signs was discussed earlier. More detail on postmortem examination is covered elsewhere in this issue. However, it is important to note that when evaluating gross lesions in diseased pigs, it is only possible to make a presumptive diagnosis — an educated guess — as to the specific disease causing the lesions. This is because the body has a limited range of possible reactions to disease, so the lesions caused by one disease may overlap with others. This is the reason for laboratory confirmation of a presumptive diagnosis.

The microscopic lesions caused by specific disease agents are central to pulling the diagnostic story together. Here, too, the lesions are sometimes nonspecific, especially when pigs are chronically affected with multiple infections. Still, the microscopic lesions are critically important, for example, in diagnosing PCVAD in pigs, since many pigs examined will test positive for the virus whether they have the disease or not. In these cases, the diagnosis requires a combination of typical clinical signs, specific microscopic lesions and the presence of the virus within the affected tissues. In other words, effective diagnosis requires input from all of the diagnostic team.

Infectious agents do not cause all health challenges. Many chemical compounds are associated with poor performance and even mortality in pigs when deficiencies or toxicities are allowed to develop. Testing directly for these compounds in tissues, feed or water is an important diagnostic capability performed by specialized laboratories.

Monitoring Refinements

Very often the diagnostic question goes beyond simply whether or not a disease is present. For example, it may be important to understand the level of activity of diseases at specific ages, such as the presence of PRRS virus or Mycoplasmal pneumonia at weaning, or the presence of antibodies against swine influenza in nursery pigs. This information is used directly in making health and production management decisions such as use and timing of vaccines and medications.

There are also instances when more information about the specific strain or type of a virus or bacteria can be useful. For example, keeping track of genomic sequence information on PRRS viruses isolated from a farm can be used to determine whether an “old” virus circulating in the herd caused a new outbreak or if a new virus has been introduced.

Similarly, subtyping bacteria, such as Haemophilus parasuis, has been used to determine whether clinical disease is the result of bacteria that are already present in commercial or autogenous vaccines.

Develop a Plan

The final step is pulling the diagnostic story together so that an action plan can be implemented. This step requires the input of the producer regarding the pig population (ongoing clinical signs, response to treatments, etc.); the interpretation of the diagnostic results by the veterinarian; and, perhaps, the diagnostic laboratory.

It is important to emphasize that diagnostic testing should not be initiated unless you know how you will apply the results. Diagnostics are an integral part of an overall herd health and biosecurity plan. The key word is “plan.” The return on investment from applying diagnostic tests comes from using them as a part of an overall herd health strategy.

There are many case studies demonstrating successful partnering by producers, veterinarians and diagnostic laboratories.

A dramatic example includes a case in which PRRS PCR testing was performed on a large scale to eliminate PRRS-infected breeding stock, shortly after moving into a herd from an isolation facility that was infected in the late stages of quarantine. In this instance, the producer and the veterinarian had set herd health goals and established a plan of action in the event of a PRRS virus infection.

The infection was detected through active monitoring of the incoming replacement breeding stock for PRRS, according to the plan. Because of the veterinarian's relationship with the diagnostic laboratory, the testing capabilities of the lab were well understood and were immediately set into motion.

In the end, the technical expertise of the laboratory and the timely testing was accomplished and reported. This allowed for a successful test and removal of infected pigs, which prevented a costly infection of the breeding herd.

All of these elements — effective partnering, established herd health goals, plans for diagnostic testing and the subsequent response — are essential for efficient and effective management of the dynamic health environment of pig populations.

Circle the Barn, Circle the Pen, Circle the Pig

To be confident that subtle changes truly reflect a change in pig health, it is important for producers to develop a consistent observation technique that can be applied every time you check a barn.

Daily observation of pigs for clinical signs of disease is a very important herd health-monitoring tool. A consistent method of observing pigs will help detect changes earlier and increase the odds that a successful treatment or response will be found.

If your approach to checking pigs in a facility varies from day to day, the small changes you see might simply be the result of viewing the pigs differently.

Often, early disease is accompanied by changes in feeding behavior. But we also know that pig activity and feeding behavior changes during the day. If you observe pigs during their peak feeding activity — generally in the morning during summer months — but check the pigs in the afternoon the next day, the differences in feeding behavior may not be significant.

Likewise, if an incomplete or random approach is used, the most obvious clinical signs (i.e., coughing) and less severe signs (i.e., mild lameness) that might help understand and diagnose the problem, may go unnoticed.

There are many ways to standardize observations. The key to developing a standard method is to find one that is thorough, consistent and comfortable.

Here's an example of consistent observation in a finishing barn with small pens and a central alleyway:

Start at the entrance to the barn. Think in terms of three successively smaller circles — circle the barn, circle the pen, circle the pig.

Begin at the first pen on your right side and move pen by pen down that side of the barn, ignoring anything on the opposite side of the barn.

At each pen, evaluate the entire pen thoroughly — starting at the front left corner and looping your visual pattern around the pen in a circle. Make sure to look directly at each pig, the feeder, the water source, the floor, the pen walls and the gates.

The goal of this first pass is to identify any pigs that appear to be abnormal in activity, posture or mobility, or have obvious signs of illness. Pen walls and floors should be viewed for evidence of diarrhea or blood from wounds. Be sure water is available — the floor below the water source should not be dry. Confirm that the feeder is providing clean, dry feed. Repeat this process with each pen.

Any pig that appears to be different from its penmates in terms of activity, posture, mobility or obvious signs of illness, such as respiratory difficulty, should then be observed using a thorough, systematic circling procedure focused on the pig.

Begin at the head, checking the eyes for excessive discharge or redness and the nose for discharge. Follow along the top and back of the pig, checking its physical condition. If you can see the pig's spine, it's likely the pig has not eaten well for several days.

At the rear of the pig, check for signs of tail or vulva biting and diarrhea. Severe scours with watery diarrhea might not be as obvious as staining of the legs. However, you might observe a reddened, irritated area around the anus. Excessive tail movement with the tail curled down may be a sign of irritation.

Moving forward along the pig's underline, observe both rear legs for abnormal posture, redness or swelling of the joints and a willingness to put full weight on both feet. Follow the flank forward to assess fullness of the belly — an indication of feed intake. Abnormal flank movements may indicate respiratory problems. In severe cases, you may observe “thumping” motions. Evaluate any signs of wounds or injuries. Repeat this evaluation on the front legs.

When you have completed the right side of the barn, turn and check all of the pens on the other side.

The three-circle approach, applied systematically, improves your chances of seeing the important clinical signs of health problems and leads to faster, more accurate diagnosis and intervention.
Locke Karriker, DVM