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Original research
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November and December, 1996 |
Relationships among seroconversion to Mycoplasma hyopneumoniae, lung
lesions, and production parameters in pigs
Maria Sitjar, DVM; Elizabeth P. Noyes, DVM, PhD; Xavier Simon, DVM;
Carlos Pijoan, DVM, PhD
MS, XS: Upjohn Farmoquímica SA, Avda. Prat de la Riba 171, 08780
Palleja, Barcelona, Spain; EPN, CP: Department of Clinical and Population
Sciences, College of Veterinary Medicine, University of Minnesota, St. Paul,
Minnesota. Reprint requests to: Dr. Carlos Pijoan, 385 Animal Science/Veterinary
Medicine Building, 1988 Fitch Ave, St. Paul, Minnesota 55108
Copyright (C) 1996, American Association of Swine Practitioners.
Also available in PDF format.
Summary
Objective--To assess the value of observing lung lesions at slaughter
as a measure of lifetime pneumonia, to investigate the role of Mycoplasma
hyopneumoniae in lifetime pneumonia, and to investigate the relationship
between lifetime pneumonia and growth performance.
Materials and methods--Forty-eight pigs from a 150-sow farrow-to-finish
farm with a history of respiratory problems were weighed and bled every
2 weeks until slaughter. Serum samples were processed for serology to measure
antibodies against M. hyopneumoniae, Actinobacillus pleuropneumoniae,
and pseudorabies virus (Aujeszky's disease). In addition, the thorax of
each pig was radiographed every 2 weeks until slaughter. Lung parenchymal
densities were determined from the radiographs and an average lifetime pneumonia
(ALP) score was calculated. For each pig, radiographs were evaluated over
time and used to determine when the pneumonia involved the highest percentage
of the lung (pneumonic peak). Lungs were collected at slaughter and superficial
lung lesions were measured. At the same time, pneumonic tissue was dissected
and its relative volume was determined for each lung. For each weekly group
of pigs, average daily gain (ADG) was calculated and correlated with pneumonic
lesion measures (average lifetime pneumonia, volumetric pneumonia, and superficial
pneumonia) for each pig as well as for each group. Onset of seroconversion
to M. hyopneumoniae and A. pleuropneumoniae was tested for
correlation with onset, extent, and severity of radiographic pneumonic lesions
(ALP) and lung lesions seen at slaughter.
Results--A significant correlation between peak average pneumonia
and seroconversion to M. hyopneumoniae was found. Groups showing
early onset of infection had higher ALP and higher pneumonia peaks. Lesions
seen at slaughter were a poor indicator of both average lifetime pneumonia
and peak pneumonia.
Implications--M. hyopneumoniae appears to play an important
role in the pneumonic peak of pigs. The growth of pigs infected later in
the finishing phase with M. hyopneumoniae is less compromised than
that of pigs infected earlier in the finishing phase.
Keywords: swine, pneumonia, Mycoplasma hyopneumoniae, Actinobacillus
pleuropneumoniae, radiography
Received: May 27, 1996
Accepted: September 26, 1996
Porcine lower respiratory tract disease (pneumonia), is routinely evaluated
at slaughter by observing gross lesions in lungs.1-3 The effects
of pneumonia on production have been studied by comparing average daily
gain (ADG) and feed:gain ratio (F:G) between pigs that had pneumonia at
slaughter to those values of pigs that did not.4 Much debate
has occurred regarding whether lung lesions found at slaughter are associated
with production parameters.5-7 Noyes, et al., found that pneumonia
observed at slaughter is not a good estimator of lifetime pneumonia, while
pneumonia observed during the life of the pig has a pronounced effect on
production.8 They used radiography to determine lifetime pneumonia
in pigs, which cannot be determined by conventional slaughter methods.
However, their study was not able to determine the microbiological cause
of the pneumonia because appropriate serological techniques for Mycoplasma
hyopneumoniae and Actinobacillus pleuropneumoniae were not available
at that time.
Recently, workers in Sweden9 reported a relationship between
M. hyopneumoniae seroconversion and pneumonia. They found that
animals that seroconverted earlier in life tended to have less pneumonia
or regressing lesions, whereas late seroconverters had extensive pneumonic
lesions. This study was conducted in an off-site, all-in-all-out (AIAO)
finisher. These units commonly show delayed seroconversion as compared to
conventional farrow-to-finish units, which seroconvert soon after weaning.
Because of this, estimations of pneumonia in farrow-to-finish farms by slaughter
examination tend to underestimate the extent of the problem and may explain
the lack of correlation between pneumonia and growth.
This study aimed to determine whether there was a correlation between
onset of pneumonia, extent and duration of lesions with seroconversion to
M. hyopneumoniae, ADG, and weight at 180 days. An attempt was also
made to assess the efficacy of several methods for determining lung lesions
at slaughter by comparing them to radiologic measurements.
Materials and methods
Farm
A 150-sow, farrow-to-finish conventional herd with a history of cough
and high prevalence of pneumonia at slaughter was selected. This farm was
located in northeast Spain, where pseudorabies virus (Aujeszky's disease)
is endemic.
The starter feed for the piglets was medicated with oxytetracycline and
neomycin. Other than the respiratory problems described above, no other
chronic diseases had been reported during the 2 years previous to the start
of the trial. All animals including the experimental pigs were maintained
under identical management protocols.
Animals
Forty eight Landrace ¥ Large White pigs from different weekly weaning
groups were selected. At 3 weeks of age, pigs were weighed, sexed, tagged,
and then systematically assigned to one of four weekly groups of 12 animals
each. From 4 weeks of age until slaughter, pigs were weighed, bled, and
radiographed every 2 weeks. All pigs that died during the study were examined
by necropsy within 12 hours of death. At 180 days of age, regardless of
their body weight, all pigs were slaughtered and their lungs examined.
To ensure that manipulation and radiography of the animals did not affect
growth parameters, another 48 pigs from the same farrowing groups were used
as a control group. These animals were weighed and tagged at 3 weeks of
age and then weighed again when sent to slaughter. These animals received
no other manipulation beyond the normal management practices of the farm.
Radiographic technique
Every 2 weeks, beginning at 4 weeks of age, pigs were weighed and then
anesthetized by IM administration of 2 mg of azaperone (Stresnil(R),
Pitman-Moore Co., 36201, Vigo. Spain) per kg bodyweight followed by anesthetic
induction with isofluorane (Abbott Co., 08037, Barcelona, Spain)8,10,11
given by mask. When muscular relaxation was achieved, anesthesia was discontinued.
A blood sample was taken and ventrodorsal and lateral radiographic views
of the thorax were obtained using a portable x-ray machine (Min-X-Ray(R),
X803G 30-20 mA 50-80 Kv). All radiographs were taken during expiration.
Each pig was returned to the appropriate pen after recovery from anesthesia,
usually within 10-15 minutes after anesthesia was induced.
At the end of the study, radiographs were blind-read with assessment
by a radiologist12 (D.A. Feeney, University of Minnesota). Lung
parenchymal densities were classified as interstitial or alveolar on the
basis of apparent opacity, relative coalescence, and presence of air bronchograms.13,14
All densities were considered to be evidence of pneumonia. Percentage of
the lung with lesions was determined by the method described by Noyes, et
al.8 Radiographically defined lung lesion percentages observed
over the life of each pig were used to determine ALP by adding the individual
radiographic pneumonia scores and dividing by the number of observations.
Lung analysis
Lungs from experimental pigs were collected at slaughter and the pneumonic
lesion was evaluated for each individual pig. Slaughter superficial pneumonia
(SSP) lesion was measured with the corrected method described by Morrison,
et al.15 Briefly, each lung lobe was assigned a percentage based
on the lung area and the percentage of pneumonia was calculated on the basis
of the amount of total lung affected, as determined by observation and palpation
of lung tissue. Slaughter volumetric pneumonia (SVP) was determined from
the amount of water displacement by the affected area as described by Hill,
et al.16
Serological analysis
Blood was collected every 2 weeks with 5-mL vacutainer tubes from either
the vena cava or jugular veins of the animals from 4 weeks of age until
the end of the experiment. Samples were marked with the animal ID and allowed
to settle until a clot was formed. Blood samples were kept refrigerated
in a polyurethane box with ice packs and on the day of collection were delivered
to the Diagnostic Laboratory of the University of Veterinary Medicine of
Barcelona for processing. Samples were then centrifuged to separate the
serum and frozen at -20°C until processed for serology at the end of
the experiment.
A monoclonal-antibody-blocking ELISA (Mycoplasma ELISA kit, DAKO A/S,
DK-2600, Glostrup, Denmark)4,17,18 was used to measure antibodies
against M. hyopneumoniae. To measure antibodies against A. pleuropneumoniae,
a blood-agar hemolysin assay developed by Utrera and Pijoan19
was used. Briefly, this test is based on detecting A. pleuropneumoniae
hemolysin I (APX I), which appears to be important in the pathogenesis
of the disease and in inducing protective immunity against infection. The
antigen was prepared from an overnight culture of the reference strain of
A. pleuropneumoniae serotype 1. Bacterial cells were then harvested,
washed with PBS, and resuspended in 0.15% NaCl solution to an A650 of 0.2.
For the test, 50 µL of the bacterial suspension were mixed 1:1 with
50 µL of the serum to be tested and incubated at room temperature
for 30 minutes. After incubation, 20 µL of the mixture was placed
in triplicate onto the surface of 1% sheep blood agar plates and incubated
overnight at 37°C. A positive reaction was characterized by a complete
lack of hemolysis in the area of bacterial growth. A negative result was
characterized by a complete halo of hemolysis, similar to that observed
with the negative control sample.
For pseudorabies virus, a commercial kit (Ingelvac Aujeszky Diagnostic
Kit(R), Boehringer-Ingelheim, 08017, Barcelona, Spain) to detect
GI and GII antibodies was used.
Statistical analysis
Average lifetime pneumonia (ALP), SSP, and SVP were calculated as described
above for each animal. Correlation analysis (r values) was performed:
- comparing ALP, SSP, and SVP;
- between these values and ADG, and
- between these values and weight at slaughter.
Linear regression between ALP or slaughter pneumonia and weight was performed
and compared to previous results.8 Animals were also analyzed
when divided into initial lightweight pigs or initial heavyweights, depending
on whether their weight was above or below average. Seroconversion to M.
hyopneumoniae and A. pleuropneumoniae was established for each
animal. Time of seroconversion was correlated against pneumonia peak and
ALP.
Results
In general, the lungs of the study pigs had a very low percentage of
lesions, with ALP < 10%. Pneumonic scores at slaughter varied widely,
but were usually lower than industry standards which commonly average approximately
>15% lung lesion.
Animals assigned to treatment groups and handled every 15 days had significantly
(P = .0001) lower weights (mean of 7 kg lower) than nonhandled controls.
Effect of ALP on growth
Average lifetime pneumoina was negatively correlated with ADG (r = .39),
(P = .080) in the lightweight animals, which had more lesions than
the heavier weight pigs. ALP was not significantly correlated (although
there was a positive trend) to ADG in initially heavyweight animals.
Effect of slaughter pneumonia on gain
Both superficial (SSP) and volumetric (SVP) slaughter pneumonia showed
a marginally significant positive correlation (r = .26, P < .1)
with weights at 180 days, but not ADG. This was probably due to variability
in the initial weights of the pigs. Animals with higher percentage of lung
slaughter lesions had heavier weights compared to animals without lesions.
Relationship between pneumonia measurements
Slaughter volumetric pneumonia and SSP showed a strong significant correlation
(r = .83, P < .01), indicating that they are essentially the same
measurements. On the other hand, ALP was not significantly correlated to
SSP (r = .23, P < .2) or SVP (r = .17, P < .1).
Slaughter lesions were better correlated to age of onset of pneumonia
than to ALP, being more extensive in animals that were infected late in
life. These animals also tended to have higher 180-day weights.
Seroconversion to different microorganisms versus ALP
There were small differences in ALP among the four weekly groups of pigs
(Figure 1). For example, group
1 pigs showed seroconversion to A. pleuropneumoniae during
the finishing period, while animals in group 2 did not.
All groups showed variable pneumonia at the beginning, steadily increasing
to a distinct pneumonic peak between 12-14 weeks of age, then tending to
decrease. However, pneumonia lesions were minimal, never exceeding an average
of 10% of the lung. All groups showed some seropositive animals to PRV at
the beginning of the study, but most of these became seronegative by 12
weeks, suggesting the presence of maternal antibodies. Maternal antibodies
were also detected against A. pleuropneumoniae for up to 8-10 weeks
of age, but not to M. hyopneumoniae, even as early as 3-4 weeks of
age. All groups showed seroconversion to M. hyopneumoniae between
18-20 weeks of age (about 6-8 weeks after onset of peak pneumonia). A trend
to earlier seroconversion when the pneumonic peak was higher was also observed,
although it was not significant (P = .05). A. pleuropneumoniae
seroconversion followed M. hyopneumoniae seroconversion closely in
group 1 pigs, but was absent in pigs in the other two groups. Pigs that
seroconverted to A. pleuropneumoniae had higher ALP scores
(r = .077, P < .01).
Discussion
An association between radiographically measured lifetime pneumonia (ALP)
and slaughter pneumonia (SSP and SVP) was not found. This result confirms
earlier observations that slaughter pneumonia is a poor indicator of lifetime
pneumonia.8
Hill, et al.,16 suggested that some of the inconsistency reported
between slaughter pneumonia and production could be due to inaccuracies
in measuring superficial pneumonia lesions. They suggested that a volumetric
measurement might be more precise, although they did not compare the two
techniques directly. We found these two measurement techniques (SSP and
SVP) to be essentially identical, as long as the proper percentage of SSP
was given to each lobe as described by Morrison, et al.15 However,
commonly used simplified techniques that assign arbitrary 10% values to
anteroventral lobes probably overestimate pneumonia lesions.
Slaughter lesions were poorly correlated with gain. Generally, animals
with the largest percentage of affected lung tissue had a tendency to have
the heaviest weights. This can be explained by the fact that slaughter lesions
were found to be highly correlated with the time in which pigs developed
pneumonia. Apparently, pigs developing pneumonia late in life reached market
weights with large unresolved lesions in the lungs. These animals tended
to grow better, probably because they spent most of their life without pneumonia
(Figure 1). Conversely, animals
that became affected at an early age may have been capable of resolving
the lesions before they reached market age, but suffered growth retardation
through most of their growth period, and may have tended to be more severely
affected with pneumonia.
The pneumonia dynamics on the farm were very striking, with a clear peak
at 12-14 weeks of age. This coincided with pigs being moved from the nursery
to the grow-finisher at about 10 weeks of age. There was also a clear (albeit
delayed) association between M. hyopneumoniae seroconversion
with this pneumonic peak. The delay of approximately 8 weeks has been reported
previously4,18,20,21 and is probably due to the fact that the
serological test used measures IgG. This IgG response is apparently a delayed
response in M. hyopneumoniae infections, because the organism
is poorly invasive. This delay in IgG response should be taken into account
when seroprofiling herds for M. hyopneumoniae for treatment and control
strategies. Time to seroconversion to A. pleuropneumoniae on this
farm was more variable, but appeared to coincide with M. hyopneumoniae
seroconversion. This variability may again be related to the serological
test used -- measuring Apx I antibody -- only found in pigs after infection
with A. pleuropneumoniae serotypes 1, 5, 9, and 10. These are not
the most prevalent serotypes in Spain.
Correlation between ALP and ADG was weaker than found previously.8
This discrepancy might be due to the fact that the farm studied had a less
severe respiratory problem than the one studied previously. Even so, there
was a strong correlation between ALP and decreased ADG in lightweight pigs,
which generally had more pneumonia than heavier weight animals.
Implications
- The use of slaughter lung lesions to assess the health status of the
population or its performance is limited, and should be accompanied by
seroprofiling of the herd.
- To accurately detect M. hyopneumoniae infection, seroprofiling
of the herd should be performed when pigs reach 12-14 weeks of age.
- Seroconversion to M. hyopneumoniae was strongly associated in
this study with the onset and peak of pulmonary lesions.
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