Mycoplasma pneumonia (MPS) is still present in swine after all these years. Change seems to be the operative word in the swine industry, but it would seem that MPS may be one disease risk that steadfastly lurks as a profit robber.
Since it was recognized in the 1950s, enzootic pneumonia or MPS associated with Mycoplasma hyopneumoniae (Mh) has been a fairly reliable contributor to respiratory disease in grow-finish operations. The major reasons for this include:
• Mh infections are widespread in pig populations;
• Mh infections are easily transmitted between pig populations;
• Mh infections can be present for long periods of time without causing substantial clinical signs; and
• A potpourri of stressors and co-infections can exacerbate disease occurrence.
Infection of pigs with Mh is a potential disease threat that is simply waiting for an opportunity to be expressed. Any deficiency in management, environmental conditions, or an infection with another respiratory agent will likely exacerbate an MPS outbreak.
Although Mh infection is common, MPS is variably expressed. The clinical picture is often confounded by a potpourri of other respiratory pathogens that are found in pig populations and the widespread use of mycoplasma vaccines. Current mycoplasma vaccines aren’t expected to eliminate disease or infection, but they are expected to decrease the magnitude and duration of disease.
Pinpointing Actual Disease
To determine if actual disease is associated with Mh, histopathology (study of tissue changes characteristic of disease) is performed on lung tissues, and the lesions are evaluated in the context of other agents (e.g. porcine reproductive and respiratory syndrome virus, swine influenza virus, porcine circovirus type 2) that may be present. When Mh is detected and lesions typical for Mh disease are present, a diagnosis of MPS is warranted. Of course, the herd veterinarian must balance the impact of MPS with the other agents that may be contributing to the pneumonia in the herd.
Disease Trends
Using the diagnostic criteria of “Mh detected and associated with typical lesions in lung tissue,” several long-term trends can be illustrated by case data from the Iowa State University Veterinary Diagnostic Laboratory (ISU VDL). Since 2003, the seasonal trend suggests MPS to be more commonly diagnosed in the fall (Figure 1), which is not at all that surprising, considering the seasonal nature of swine respiratory diseases generally.
One might suspect that overall cases of MPS would decrease over the years but this does not appear to be the situation. Despite fewer submissions of tissues to the VDL over the past three years, the number of MPS diagnoses has stayed fairly constant (Figure 2).
A decade or so ago, the industry was talking about the “16- week wall,” roughly defined as the age where growth and performance seemed to stall out on many farms due to the porcine respiratory disease complex. Of late, a seemingly similar trend is suspected with many of the MPS diagnoses occurring in later finishing stages between 16-24 weeks of age (data not shown). The author leaves it to the reader to speculate on reasons for this, if indeed it is true.
Issues Related to Diagnostics
Please remember that it is very difficult to grow or isolate Mh from clinical specimens. The use of bacterial isolation of Mh as a diagnostic method is not practical. No diagnostic laboratory routinely isolates Mh as a method of diagnosis. We are then left with other diagnostic tools to be applied to situations at hand.
Diagnosis in “Negative” Populations
Increasingly, there are herds or populations that are allegedly not infected with Mh. It is impossible to prove all animals negative at all times with antemortem testing. Since Mh resides on cilia of airways, it is very hard to detect by polymerase chain reaction (PCR) in routine samples (e.g. nasal swabs or oral fluids), although a positive PCR is sufficient to call the population positive. Because Mh can be a mild, superficial infection, its presence does not consistently induce detectable serum antibody in infected individuals.
Despite that constraint, serologic testing may remain the most sensitive method to detect infected populations. There is no definitive test strategy for establishing a population as negative. It seems the best approach, thus far, is to use combinations of test types and monitoring protocols to add confidence that the population is indeed negative, but that confidence can never be 100%. False-positive test results occur but should be thoroughly investigated before being discounted as falsely positive. A useful tip: To monitor the infection status of a non-vaccinated, negative sow herd, perform serology on suckling piglets. Colostrum concentrates antibody which can then be detected as antibody titers in suckling pigs.
Diagnosis in “Positive” Populations
For herds that are known or suspected positive for Mh infection, the diagnostic approach is different. Simply detecting infection of an animal or herd with Mh does not necessarily predict disease severity or define the economic impact of disease within the individual pig or the herd. The assessment of the magnitude of the economic impact of MPS will not be made by laboratory testing.
There are two basic questions that can be answered by laboratory testing to confirm a role for MPS: 1) Is Mh present? 2) Is the Mh present actually contributing to disease as MPS?
Although these questions sound simple, the reality is each situation is slightly different such that a different testing strategy will be required to reliably answer them.
Is Mycoplasma Present?
The most direct method of answering both questions, at least partially, is thorough clinical examination and selection of appropriate pigs for diagnostic work-up. Euthanasia and necropsy of a typical, clinically affected animal and submission of samples for laboratory testing are quite useful. Infection in lung tissue is confirmed by laboratory testing (e.g. polymerase chain reaction [PCR], fluorescent antibody test, immunohistochemistry). The role for Mh as a cause of disease and lesions is confirmed by histopathology.
Additional laboratory testing can be used to rule out other coinfections. When only MPS is found, herd assessment using clinical signs, production records, variation in growth, medication use, and vaccination status should offer insight into the herd impact of the disease and provide economical alternatives for its control in future groups. When other infectious agents confound MPS, the analysis becomes more daunting and a systematic approach to diagnosis may be warranted.
Population tools:
1. Cross-sectional or longitudinal necropsy and laboratory testing can determine if Mh is present and associated with lesions, as well as when disease is detectable. Herd impact would still require some measure of herd performance. Systematic necropsy and laboratory testing is probably the most reliable method to confirm a role for MPS in swine herds.
2. Serology (cross-sectional or longitudinal) can determine if Mh infection is present, but must be interpreted in context of vaccination. With experience and knowledge of nuances of various vaccines and vaccination timing, this is doable. Advantages are that infection can be confirmed and approximate timing of infections can be estimated. The disadvantage is that serology offers no information as to whether infection is actually causing disease or the extent or magnitude of impact that Mh infection may have on herd performance.
3. The use of oral fluids to detect infection (via PCR) has not been validated. It is likely that this will be a tool for monitoring populations of growing pigs. Of perhaps greater economic utility is the use of oral fluids for detection of antibody. Evaluation and validation efforts are underway to use oral fluids for detection of antibody.
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Kent Schwartz, DVM
Iowa State University Veterinary Diagnostic Laboratory
[email protected]
