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Original research
Peer reviewed
Management factors associated with seropositivity to Lawsonia
intracellularis in US swine herds
Mark Bronsvoort, BVSc, MPVM; Bo Norby, DVM, MPVM; David P.
Bane, DVM, PhD; Ian A. Gardner, BVSc, PhD
MB: University of Liverpool, Department of Clinical Science
and Animal Husbandry, Leahurst, Nr Neston L64 7TE, UK; BN: Michigan
State University, Department of Large Animal Clinical Sciences,
College of Veterinary Medicine, East Lansing, MI 48824; DB: Elanco
Animal Health, Indianapolis, IN 46285; IG: University of California,
Davis, Department of Medicine and Epidemiology, School of Veterinary
Medicine, Davis, CA 95616. Tel: 530-752-6992;
Fax: 530-752-0414; E-mail: iagardner@ucdavis.edu
Bronsvoort M, Norby B, Bane DP, et al. Management factors
associated with seropositivity to Lawsonia intracellularis
in US swine herds. J Swine Health Prod. 2001;9(6):285-290.
Also available as a PDF.
Summary
Objective: To determine risk factors for Lawsonia
intracellularis seropositivity in the breeding and grower-finisher
units of US farrow-to-finish swine herds.
Methods: Serum was collected from 15 breeding females
and 15 grower-finisher pigs per herd in 184 farrow-to-finish herds,
a subset of 405 herds in the National Animal Health Monitoring
System (NAHMS) Swine '95 Study that examined management, health,
and productivity in herds with at least 300 finisher pigs. Sera
were tested by indirect fluorescent antibody test for L intracellularis.
Test results were linked with NAHMS questionnaire data, and a
logistic regression model of management factors associated with
L intracellularis serological status was developed. Separate
models were used for breeding and grower-finisher units.
Results: Risk factors for seropositive breeding units
were L intracellularis-seropositive status of the grower-finisher
unit, use of a continuous system of management for the farrowing
unit, and a young parity structure (<75% multiparous sows).
Risk factors for seropositive grower-finisher units were L
intracellularis-seropositive status of the breeding unit,
the number of pigs entering the grower-finisher stage, raising
pigs on concrete slats, and intensive management compared with
raising pigs on outdoor lots.
Implications: Use of all in-all out management in the
farrowing house and an older parity structure in the sow herd
were associated with a lower risk of L intracellularis
seropositivity in the breeding unit, and slatted concrete flooring
in grower-finisher houses was associated with a greater risk.
Alteration of these management factors might improve control of
L intracellularis infection in farrow-to-finish herds.
Keywords: swine,
risk factors, porcine proliferative enteropathy, Lawsonia
intracellularis, indirect fluorescent antibody test
Received: October 4, 2000
Accepted: February 15, 2001
Lawsonia intracellularis is recognized
as the primary cause of porcine proliferative enteropathy (PPE),
which is characterized by proliferation of crypt enterocytes and
thickening of the intestinal mucosa.1-4 Infection with
L intracellularis may cause chronic enteritis, manifested
clinically as reduced growth rate and diarrhea in weaned pigs
approximately 6 to 20 weeks of age. An acute syndrome characterized
by intestinal hemorrhage and sudden death occurs in pigs more
than 5 months of age. Porcine proliferative enteropathy is estimated
to result in direct losses of $3 to $11 per pig ($US), attributable
to increased feeding costs and time to reach slaughter weight.5
Porcine proliferative enteropathy is recognized worldwide as
a significant cause of enteritis in pigs. The National Animal
Health Monitoring System (NAHMS) Swine '95 survey reported that
clinical PPE occurred in 7% of US finisher herds, on the basis
of a confirmed laboratory result or the diagnostic opinion of
the herd veterinarian.6 Herd prevalence in other
countries ranges from 20 to 94%,5,7-9 but this variation
might be attributable in part to differences in the sensitivities
of the diagnostic methods used and in the numbers of pigs and
age groups sampled in different studies. Until recently, investigation
of herd-level risk factors has been hampered by lack of an accurate
ante mortem test. Development of a polymerase chain reaction (PCR)
assay10 has facilitated ante mortem diagnosis and prevalence
surveys, but the PCR is less sensitive than the indirect fluorescent
antibody test (IFAT) that detects serum antibodies to L intracellularis.11
The objective of this cross-sectional study was to assess management
risk factors associated with the L intracellularis serological
status of breeding and grower-finisher units in 184 farrow-to-finish
herds surveyed as part of the NAHMS Swine '95 study.
Materials and methods
Study population and data collection
A total of 1477 swine operations from 16 major swine-producing
states (representing 91% of the US swine inventory) participated
in Phase 1 of the NAHMS survey. In Phase 2, questionnaires were
used to collect data about various management practices and health
issues in a sub-sample of 405 herds that had at least 300 finisher
pigs. A detailed description of the design of the NAHMS Swine
'95 survey is published elsewhere.12
Of the 405 operations that provided questionnaire data, 285
participated in serologic sampling of 15 breeding animals and
(or) 15 grower-finisher pigs per herd. Samples from females were
distributed among parities (mean 2.7, range zero to 13) and wherever
possible, finisher samples were obtained from pigs within 30 days
of slaughter (mean 159 days of age, range 90 to 260 days). Blood
samples were collected by herd veterinarians and shipped to the
National Veterinary Services Laboratory of the USDA. Serum was
stored in 0.4-ml aliquots at -40 degrees C until tested.
Serological testing and herd classification
Sera from the 184 farrow-to-finish herds that had provided
breeding and grower-finisher samples were tested for L intracellularis
by IFAT, as described by Knittel et al.11 Briefly,
an anti-porcine IgG-fluorescein-isothiocyanate conjugate (diluted
1:10 in PBS) was bound to porcine IgG (diluted 1:30 in PBS) that
was bound to L intracellularis-infected cell cultures in
the wells of 72-well microtitration plates. Plates were examined
by fluorescent microscopy, and wells with fluorescing bacteria
were interpreted as positive.
On the basis of a study by Knittel et al,11
the sensitivity of the IFAT was estimated to be 0.90 and the specificity
was estimated to be 0.99. These estimates were used in combination
with our best guess of prevalence of infection in each herd to
guide choice of an appropriate number of test-positive pigs to
designate the unit status for L intracellularis. Units
were classified as positive if three or more of 15 tested pigs
were IFAT-positive; inconclusive if one or two pigs were IFAT-positive;
and negative if no pigs were IFAT-positive. Breeding and grower-finisher
units in each herd were classified separately.
Statistical analysis
Questionnaire data and IFAT results were matched using the
swine herd's identification number in an Access database (Microsoft
Corporation, Redmond, Washington). We selected for risk factor
analysis a subset of questions that we considered were relevant
a priori to infectious enteric diseases such as PPE. Statistical
analyses were performed using BMDP 2D, 4F and LR (BMDP Statistical
Software, Inc, Los Angeles, California). Descriptive statistics
were calculated for each variable, and continuous variables were
categorized for use in analyses on the basis of the median and
quartiles. For each potential risk factor, associations with the
L intracellularis status of the breeder and grower-finisher
units were determined. Variables with P<.2 (chi-square)
were considered for inclusion in multivariable logistic regression
models.
Separate logistic regression models were developed for the
L intracellularis serological status of breeder and grower-finisher
units. Data for breeder and grower-finisher units of inconclusive
L intracellularis status were excluded from analysis to
increase the herd-level specificity of our classification without
compromising herd-level sensitivity.13,14 Variables
were added to the model when the chi-square P value for
the variable was <.10. Interactions between factors in the
final best-fitting models were also assessed. For the final models,
adjusted odds ratio (OR) and 95% confidence intervals were obtained
to quantify the strength of association with the different risk
factors. Overall model fit was assessed using the Hosmer-Lemeshow
(H-L) goodness-of-fit statistic.15
Results
Indirect fluorescent antibody testing
The median number of L intracellularis-seropositive
samples was five (range zero to 14) for breeding units and two
(range zero to 15) for grower-finisher units. Of 184 breeding
units, 90 (48.9%) were positive, 38 (20.7%) were negative, and
56 (30.4%) were inconclusive for L intracellularis using
our interpretive thresholds. Of the 184 growing units, 123 (66.9%)
were positive, 26 (14.1%) were negative, and 35 (19.0%) were inconclusive.
A total of 141 herds (76.6%) were classed as positive because
their breeding unit, grower-finisher unit, or both were positive
(Table 1).
Risk factors for seropositive breeding units
Eleven variables passed the initial screening criterion for
risk factors (P<.2) for L intracellularis serologic
status of breeding units (Table 2). Management variables related
to the grower-finisher unit were excluded from analysis except
for grower-finisher serological status. This variable was considered
to represent the combined effect of all grower-finisher risk factors
on the L intracellularis status of the breeding unit. The
total number of pigs in the herd was considered a potential confounder
on the basis of a prior study16 and was also included
in the initial modeling. Though a statistically significant variable,
the age when pigs left the nursery could not be biologically justified
as a risk factor for breeding unit serologic status and was not
considered in the final model.
The final multivariable model, which excluded 56 units with
inconclusive serologic test results, is given in Table 3 with
the OR for each variable. Records for three breeding units tested
and recorded in Table 1 were not used in the final model because
of missing risk factor data. The odds of the breeding unit being
positive were 3.5 times greater when the grower-finisher unit
was positive. Use of an all in-all out farrowing policy and having
75% multiparous sows each reduced the odds of being seropositive
about fourfold. Two-way interactions between risk factors were
not significant (P>.05). Overall fit of the model was
good (H-L goodness-of-fit c2=2.27, 8 df, P=.97).
Risk factors for seropositive grower-finisher units
Twelve variables passed the initial screening criterion for
risk factors (P<.2) for L intracellularis serologic
status (Table 4). Sow serologic status was used as a surrogate
variable to represent the combined effects of risk factors in
the sow herd that might have influenced the risk of transmission
of L intracellularis to the grower-finisher herd. The L
intracellularis serological status of sows, percentage of
animals on concrete slats, rearing some pigs on outdoor lots,
and the number of pigs entering the unit were included in the
multivariable modeling. Waste management in the grower-finisher,
though potentially a risk factor, was not evaluated further because
of sparse data that created instability in the model. In addition,
when categories were combined, the association with seropositivity
was no longer evident. Five health indices for growers (number
of deaths, diarrhea, total disease problems, cull rate, and culled
because of diarrhea) were not considered further because it appeared
that these were outcomes of L intracellularis infection
rather than risk factors for it. The variable 'treat sick pig
and remove to separate pen' improved model fit but was not included
because of difficulty interpreting the categories.
The final logistic model, which excluded 35 units with inconclusive
serologic results, included four variables (Table 5). Records
for an additional four herds tested and reported in Table 4 were
not used in the final model because of missing risk factor data.
The serologic status of the breeding unit was the most influential
variable, with the odds of the grower-finisher herds being positive
almost eight times greater when the breeding unit was also positive.
Grower-finisher seropositivity was also associated with the percentage
of pigs housed on concrete slats, with the odds of being positive
7.5 times higher for units where more than 65% of pigs were on
concrete slats compared to all other floor types. Finally, there
was a strong association between grower-finisher status and whether
the pigs were on outdoor lots, with the odds of being seropositive
substantially lower for pigs on outdoor lots compared with all
other surfaces. The overall fit of this model was adequate, with
the H-L goodness-of-fit c2=7.68 with 8 df (P=.465).
Discussion
In this study, we identified three factors (L intracellularis
serological status of grower-finisher unit, farrowing house management,
and percentage of multiparous sows) that best explained the variation
in L intracellularis serologic status of breeding units,
and four factors (L intracellularis status of breeding
unit, number of pigs entering the unit in the previous 6 months,
percentage of pigs housed on concrete slats, and rearing some
pigs on outdoor lots) that best explained the variation in L
intracellularis serologic status in grower-finisher units.
We found that the L intracellularis serologic status
of the grower-finisher herd was strongly associated with the serological
status of the breeding herd and vice versa. This finding is consistent
with evidence that L intracellularis is primarily transmitted
by contact with infective fecal material and that the minimal
infectious dose for pigs is low.17 The sow herd probably
acts as an important source of infection for unweaned pigs, although
the potential for transmission of L intracellularis among
pigs might be greater in nursery and grower-finisher units because
of increased pig-to-pig contact and larger population sizes. However,
questionnaires did not have sufficient detail to document the
proximity and management relationship (including specific replacement
stock practices) between the units in individual herds. For many
herds with single-site production, it is possible that replacement
gilts and humans or other fomites might have contributed to spread
of the agent between the grower-finisher unit and the breeding
unit.
The 'protective effect' of a higher proportion of multiparous
sows suggests that either there may be a greater resistance to
L intracellularis with age, or that there is a lower turnover
of breeding animals in these herds and thus less contact between
susceptible and infected animals and (or) a lower risk of introduction
of the pathogen. The association of an all in-all out farrowing
policy with a reduced risk of being seropositive suggests that
transmission of L intracellularis within breeding units
is reduced by minimizing the direct and indirect contact between
infected and susceptible animals. Alternatively, it may reflect
a generally higher level of management, with better cleaning and
disinfection of the farrowing room.
Our results are broadly consistent with those reported by Smith
et al,16 who studied risk factors for owner-reported
occurrence of PPE in 319 British pig herds. These authors found
that use of all in-all out policies and breeding herd sizes of
<500 sows had a protective effect. The association between
small herd size and clinical PPE was attributed to management
differences between small and large herds. In the British study,
it is also possible that owners of smaller herds (<500 sows)
might have been less familiar with PPE and did not identify occurrences
of mild and subclinical disease. In contrast, we did not find
an association with herd size, whether herd size was measured
as the total number of pigs or the total number of breeding pigs.
The results of the British study may have differed from our results
in part because we evaluated risk factors for L intracellularis
seropositivity, while the British study recorded the occurrence
of clinical disease. In our study, serologic evidence of L
intracellularis infection was nearly ten times more likely
to occur than clinical disease, which was reported in about 7%
of grower units during the 12 months prior to serologic testing.6
Risk factors that we identified in our grower-finisher unit
model were similar to those reported by Smith et al,16
who found a greater risk of disease when pigs were kept on slats
or meshed flooring. The question in the NAHMS questionnaire was
worded differently to the one used in the survey by Smith et al,
and categorization of slatted flooring varied. However, in our
study, there was an association between housing on concrete slats
in >64% of the pens on the farm and an increased risk of the
grower-finisher herd being seropositive. Smith et al16 suggested
that retention of feces on the flooring (possibly between slats)
between batches of pigs might be the biological reason for the
relationship. Because our study was cross-sectional, we could
not exclude the possibility that farms had switched to slatted
flooring in response to a clinical PPE problem. We found that
use of outdoor lots had a strong protective effect, but this most
likely is a proxy for lower stocking density, reduced stress,
and a more natural digestive flora from rooting in the soil.
To our knowledge, there are no published studies of risk factors
for L intracellularis infection in US swine herds, and
only one study of risk factors for PPE outbreaks. Bane et al18
used a case-control study to evaluate risk factors for clinical
PPE in mid-western herds, and found that placement of pigs in
new facilities and recent mixing of pigs (£2 weeks) were
important risk factors for PPE. However, because the NAHMS questionnaires
did not address these factors, it was not possible to directly
compare findings from the two studies. Bane et al also reported
that management factors, such as flooring type, continuous pig
flow, lack of washing and disinfection, and lack of isolation
of breeding stock, were not associated with clinical disease.
Although these results appear to contradict ours and those of
Smith et al,16 Bane et al18 used a small
sample size and did not evaluate the serologic profiles of the
case or control herds. Consequently, they were unable to determine
unequivocally whether or not control herds were infected with
L intracellularis.
It was difficult to determine the optimal cut-off value (number
of test-positive pigs) for classification of L intracellularis
serologic status because of the lack of reliable estimates of
the sensitivity and specificity of the IFAT. Knittel et al11
suggested indirectly that the sensitivity of the IFAT for
an individual test was approximately 0.9 and the specificity was
0.99. However, test sensitivity in field studies is often lower
than in experimental studies, and there is evidence that sensitivity
varies with factors such as age, and stage and severity of disease,
which might also vary among herds. Moreover, in the field, there
might be an increased risk of exposure to organisms that induce
cross-reacting antibodies.19 Our solution to this potential
problem was to require a minimum of three test-positive pigs to
designate a positive unit compared with the threshold of one or
two seropositive pigs that is commonly used in epidemiologic studies.
We excluded units with one or two seropositive pigs from the logistic
models. Although it reduced the sample size for our analyses,
we believe that this approach reduced misclassification of a unit's
true status and resulted in less bias in estimated odds ratios.
The risk factors identified in this study were non-specific,
with the exception of the use of concrete slats. In part, this
was because the questionnaires were not designed specifically
to study risk factors for L intracellularis infection or
clinical PPE. Moreover, the cross-sectional design of the study
precluded us from differentiating factors associated with the
introduction of L intracellularis and factors associated
with transmission of L intracellularis once infection had
been established in the herd.
Implications
- Use of all in-all out management in the farrowing house and
an older parity structure in the sow herd were associated with
a lower risk of seropositivity to L intracellularis in
the breeding unit.
- Slatted concrete flooring in grower-finisher houses was associated
with an increased risk of seropositivity to L intracellularis,
but further studies are needed before making specific flooring
recommendations.
- Rearing pigs on outdoor lots might reduce the risk of transmission
of L intracellularis in grower-finisher herds, reflecting
a less intensive production system and less stressful environment
for the pigs.
- Alteration of these three management factors might improve
control of L intracellularis infection in farrow-to-finish
herds.
Acknowledgements
The Wellcome Trust (UK) generously supported the senior author
through a Research Training Fellowship in Tropical Clinical Epidemiology
at the University of Liverpool (UK). We thank Dr Eric Bush, Centers
for Epidemiology and Animal Health, USDA-APHIS, VS, for providing
the questionnaire data and for his helpful discussions, and Dr
Kenton Morgan, University of Liverpool, for technical guidance.
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