Biosecurity programs, designed to prevent new disease entry and control diseases that already exist, have two parts:
Procedures used every day to keep new swine disease pathogens from entering a swine herd, and
Managing the diseases that are already present in the herd.
Routine diagnostic sampling is needed to accomplish both tasks.
Diseases in today's swine herds have become very complex, often involving both bacteria and viruses. Consequently, diagnosing disease is less straightforward. In order to completely understand disease entry and movement within a herd, it is necessary to routinely collect samples from pigs and interpret diagnostic results.
Historically, diagnostics have been used to help guide producers in making treatment decisions. Today, however, population testing is influencing so much more of our treatment decisions.
For example, routine herd monitoring helps guide decisions on gilt entry and semen into breeding herds and the movement of weaned pigs into nurseries. Sampling populations helps narrow the age at which specific pathogen exposure is occurring, which helps guide vaccination recommendations.
Increasingly, routine sampling is also being used to measure our success with disease eradication programs.
Pork producers have certainly taken a much more active role in the investigation of disease within their herds. This makes sense because they are in the barn every day and often are the first to recognize abnormalities. Once trained how to perform necropsies and identify lesions, timely samples can easily be collected.
At Swine Vet Center, conducting posting clinics and training sessions have been valuable in helping producers learn these important techniques.
Quality Sample Collection
We have also developed detailed posters to showcase lesions that occur with common diseases. Displaying these posters in hog barns has helped producers narrow down a preliminary diagnosis.
Available technology, such as digital cameras and videos, has also expedited our clinic's ability to get a preliminary diagnosis of a swine disease. Producers can send images of what they are seeing, which, with a good herd health history, helps the veterinarian formulate initial treatment recommendations.
When deciding to take any type of sample, it is important to remember that poor-quality samples will almost always yield poor-quality diagnostic results.
When selecting pigs for sample collection, make sure that the pigs are representative of the clinical or visual abnormalities you are observing within the group. Chronically infected pigs are often immune-suppressed and can be infected by several insignificant pathogens that will make diagnostic interpretation difficult.
On the other hand, pigs that died acutely or rapidly for reasons unrelated to the current condition, or that are visibly healthy, may not elicit any meaningful results.
Collecting Tissue Samples
When collecting tissue samples for disease diagnosis, we recommend sampling from at least two pigs that have recently begun showing clinical signs, and preferably, have not been treated.
Although treatment doesn't necessarily influence viral diagnostics, in a high percentage of cases there is often a secondary bacterial component that may be masked by previous antibiotic treatment.
As long as time and effort is being taken to identify, necropsy and collect samples for a disease diagnosis, we recommend that a complete set of tissues be collected.
Although clinical signs or lesions may suggest a specific organ or system is involved, submitting a complete set of tissues has helped identify other unsuspected pathogens.
Always use a clean knife and take a systematic approach to tissue collection. Begin sampling at the pig's head and neck, taking tonsil and lymph node samples, before moving through the thoracic cavity to collect lung and heart samples. If other lesions are visible, samples of those should also be taken at this time.
Once in the abdominal cavity, collect liver, kidney and spleen samples. Finish by taking samples of the intestinal system, collecting colon and small intestine samples.
Remember, when dealing with respiratory disease cases, it is important to take multiple pieces of lung. Similarly, when dealing with an enteric case, multiple pieces of small intestine and colon are warranted. Separate small intestine and colon samples when packaging tissues and keep them separate from all other tissues collected.
For clients who perform their own sampling, Swine Vet Center has found that a necropsy kit, including bags labeled with each tissue type, has been helpful.
For every tissue sample taken, be sure to cut a small piece (⅓ to ⅝ in. thick) to fix in 10% formalin. These tissues can be combined and placed in a ratio of at least five parts formalin to one part tissue. If clinical signs in the group include neurologic or joint problems, samples that include those tissues should also be taken.
Next Page: Live Animal Submission
Live Animal Submission
Even with the best tissue collection techniques, we sometimes are unable to diagnose exactly what is affecting a pig population. Often, we will widen our sample size to increase the odds of finding the pathogen that is suspected.
Depending on the severity of the problem and the desire of the producer or veterinarian to determine the exact cause, if multiple tissue submissions are unsuccessful, bringing live animals to the diagnostic laboratory should be discussed.
With live pig submission, tissues are collected immediately after death, so decomposition of highly perishable tissues (such as intestine) is minimized. Additionally, live animal submissions allow for thorough examination of the whole pig by a trained pathologist who may identify something that may have been missed in the field.
Several options for sampling live pigs also allow clinicians to increase their sample size without sacrificing several animals for necropsy.
Determining Sample Size
Although there are some situations where we test all animals in a population, economic and time constraints often limit us to testing a statistical sample size (Table 1).
In those cases, the correct number of animals needed to determine the presence of a pathogen or evidence of previous exposure to a specific disease organism is based on three factors:
The estimated infection rate (prevalence) of disease within the herd in question. In our experience, clinical signs often are observed about the time a herd reaches 5-10% infection rate.
The level of confidence desired to find a positive result — if one truly exists. Seldom do we drop below a 95% confidence level for disease monitoring and surveillance.
Population size — needed to determine the statistical number to sample.
Additional factors that should affect sampling decisions include:
Herd value — A high-value herd (genetic nucleus, multiplier or boar stud) should be sampled at a higher rate, including sample size and confidence level, because the consequences of not detecting a new infection are greater.
Return on diagnostic investment. Given a set budget for diagnostic testing, sampling fewer animals more often is better than sampling more animals less often.
For example, if sampling 10 piglets every week allows you to find a disease like porcine reproductive and respiratory syndrome or PRRS virus and divert pig flow, this would have a much greater return on diagnostic investment compared to sampling 40 piglets once every four weeks and missing those three weeks of opportunity.
To increase the chance of detecting a positive sample, we will often “stack the deck” by focusing our sampling selection on animals that are showing symptoms of the disease in question. Although this is a selective approach, it offers the best chance of having a positive result, and in theory, allows us to reduce the number of animals to be tested.
The number tested should still meet the determined statistical sample size, however. Don't stop short just because you have run out of visibly sick animals, especially early in the course of a disease when the incidence of infection is often low.
For example, if you determine that you need to test 30 pigs, but only have 10 displaying clinical signs, test those 10, plus 20 selected at random to achieve the required statistical sample size.
When designing testing protocols, especially for boar studs, sow farms and gilt developers, it is important to design an action plan that addresses what to do if there is an unexpected diagnosis. This may include redirecting pig flow, closing the herd, depopulation, test-and-remove, etc. If there is no plan of action when an unforeseen test result comes back, one must certainly question the role of testing in the first place.
One of the most common means of health monitoring a population of pigs is through blood testing. Serum, the fluid part of blood, contains the antibodies that are generated when pigs are exposed to viruses or bacteria. Viruses, including PRRS and porcine circovirus, also circulate within the bloodstream and are present in serum samples.
Several diagnostic tests have been developed to measure the presence of either antibodies or disease agents present in serum. So blood testing remains one of the best diagnostic sampling methods for today's swine herds.
An alternative to blood collection is blood swabbing, which has been used as a monitoring tool to test for the presence of PRRS virus within herds. Used initially in boar studs for sampling boars during semen collection, some producers now apply this sampling method in their breeding herds, sampling piglets by ear-notching as part of their PRRS virus surveillance program.
Next Page: Nasal Swabbing
Nasal swabs, like serum samples or blood swabbing, can be used for disease diagnosis as well as surveillance within a swine population.
Used primarily for respiratory disease agents, such as swine influenza virus and Mycoplasmal pneumonia, the approach to nasal swabbing should be similar to blood testing in a group of pigs in terms of the number of samples taken and the pigs targeted to sample. Pick pigs showing clinical signs of the disease that you are searching for.
With influenza, pigs that are coughing, feverish and have a clear nasal discharge are good candidates. For Mycoplasmal pneumonia, pigs that are coughing are often good pigs to select for sampling.
Oral Fluid Sampling
Oral fluid sampling has gained a lot of attention as another tool for disease surveillance in swine herds.
Monitoring using oral fluids is based on the principle that if present in a herd, the bacteria and viruses we commonly monitor can be found in oral secretions.
Antibodies elicited against several swine pathogens can also be found in oral fluids, so these can be measured as well. Oral fluids have been used primarily for PRRS virus, porcine circovirus-associated disease, Mycoplasmal pneumonia and swine influenza virus diagnosis and surveillance.
An advantage to oral fluid testing is that it is an easy way to sample numerous pigs within a population in a short amount of time. This is beneficial when working with a diagnostic budget because the total number of animals that can be sampled for a given cost far exceeds other surveillance sampling methods. Therefore, more frequent sampling can often be justified using oral fluid collection, which allows for more rapid disease detection and the potential to make subsequent decisions sooner. Keep in mind that there is a potential for reduced sensitivity because of the dilution of samples by multiple pigs. This can lead to an increase in false-positive results.
The goals of biosecurity of swine herds involve the prevention of new disease entry and the control of diseases that already exist. Routine diagnostic sampling and herd surveillance are critical to achieving both goals.
Swine diseases will become increasingly complex, but with the diagnostic sampling methods and testing tools that are available in the pork industry, we are able to identify these diseases more quickly than ever.
Add this to an industry structure that allows us to isolate animals before their arrival into herds or divert the flow of pigs, and we can effectively prevent a lot of the health instability that can be so damaging to herd productivity.
|Estimated Prevalence of Disease|
|Total Number of Animals in Group||20%||10%||5%|
Next Page: Common Surveillance Practices for PRRS Virus Control
Common Surveillance Practices for PRRS Virus Control
Porcine reproductive and respiratory syndrome (PRRS) virus remains the swine disease that we spend the greatest amount of time, effort and money trying to control.
Several virus surveillance programs have been designed and implemented to accomplish the goal of eliminating virus introduction into breeding herds and boar studs through replacements and/or semen. Routine monitoring programs are also applied to breeding herds to track PRRS stability. This gives producers the ability to make more informed pig movement decisions, reducing the commingling of pigs with different PRRS virus status.
When we monitor PRRS virus status in the sow herd, we are also able to make vaccination recommendations for weaned pigs.
Efficient delivery of samples to the diagnostic laboratory, coupled with routine diagnostic tests, allows for quick turnaround of results. Internet-based access has also added a level of convenience for producers who rely on diagnostic results in their daily decision making.
Following are the primary areas of production in which we commonly perform routine PRRS surveillance. These three practices are important in limiting the chances of being infected with PRRS:
- Boar stud surveillance
PRRS can be transmitted through semen. Because semen from a boar stud will often be delivered to multiple sow farms, constant monitoring for PRRS virus at the stud level is of great importance. As a general preventative health practice, any sow herd using semen from a stud that does not routinely monitor for PRRS infection is at an increased risk for virus introduction. Most boar studs today routinely test a serum or blood swab sample from each boar that has been collected on a given day.
Breeding herds should require that semen cannot be released for use until PRRS-negative results of those boars is confirmed. The number of boars sampled each day at the stud normally follows a statistical 95% confidence with 5-10% prevalence testing level (normally 30-45 boars/day). High-value sow herds (nucleus or multiplication) normally require that all boars going into the farm be tested during semen collection.
- Monitoring replacement animals
An easy way to move PRRS virus into a herd is by allowing animals that are actively shedding virus to enter without testing. To avoid this, a PRRS monitoring program for replacement animals should be developed regardless of the herd's ongoing health status.
The pre-entry screening program depends on the herd's value, history and level of PRRS virus transmission risk compared to all other potential routes of virus entry. For high-value herds (nucleus, multiplication and boar studs), a protocol that tests every animal after entry into isolation and prior to entry into the main herd is required because the consequences of missing the virus are great.
Similarly, for herds with low risk of area spread, the replacement animals are probably the greatest risk the farm faces. In this case, all replacements should be tested after entry into isolation and prior to entry into the main farm.
Contrast this against herds in pig-dense areas that have a history of a new PRRS virus introduction every two years or less due to area spread, where replacements likely aren't the biggest risk for new virus entry. In this case, a surveillance protocol that spends less money and effort for replacements prior to entry into the main herd is reasonable.
Closed herds that have no animal introductions should be comfortable using routine piglet monitoring as an indicator of replacement status.
Regardless of the sow herd, a testing protocol should be put in place that takes into account the type of testing needed, the sample size required and the frequency in which testing should take place.
- Piglet monitoring
Several breeding herds that have a history of PRRS virus introduction or that commingle weaned pigs from other farms within a system have developed routine piglet PRRS virus testing protocols that serve as an ongoing indicator of the current status of the herd. This allows those farms to make pig flow decisions that prevents pigs from farms of different PRRS status from being brought together in the nursery. In most of those herds, piglet testing for PRRS has replaced routine blood testing of sows to monitor herd status.
The piglet monitoring process typically involves testing once every 1-4 weeks, depending on the herd. For herds with a history of frequent PRRS virus introduction, weekly sampling is recommended so the virus is quickly detected and pig flow can be redirected to preserve the negative status of the pigs from other sow herds. Every day sooner that virus is found and well informed decisions are made adds further value to a routine monitoring program and easily justifies once-a-week testing. Figure 1 shows the impact of health stability on pig survival.
When choosing piglets to test, select those between 5 days of age and weaning age. Poorer pigs should be selected to increase the chance of finding virus. Much like boar stud testing, an alternative to drawing blood samples is taking blood swabs. Ear-notching provides a simple way for sow farm staff to obtain blood swab samples from piglets for routine monitoring.
— Brad Leuwerke, DVM