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Diagnostic notes
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Non refereed
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Update on epidemiology
and diagnosis of porcine
proliferative enteropathy
Roberto Guedes,
DVM, MS, PhD
Department of Veterinary
Clinic and Surgery, Veterinary School, Universidade Federal de Minas Gerais,
Belo Horizonte, MG 31270-901, PO Box 567, Brazil; E-mail:
guedes@vet.ufmg.br.
Cite as: Guedes
R. Update on epidemiology and diagnosis of porcine proliferative enteropathy. J
Swine Health Prod. 2004;12(3):134-138.
 Also
available as a PDF.
 Proliferative enteropathy (PE) is an
enteric disease that occurs in pigs1 and
a number of other species.2 The
etiologic agent of PE in swine is the obligatory
intracellular bacterium Lawsonia
intracellularis.3-6 The disease in pigs,
commonly referred to as ileitis, occurs in two major clinical forms. Acute
hemorrhagic diarrhea and sudden death of
replacement animals and finishing pigs close to
market age is known as proliferative hemorrhagic enteropathy (PHE); and chronic,
mild diarrhea with poor growth performance in grower-finisher pigs (Figure
1) is known
as porcine intestinal adenomatosis (PIA).1,7
Proliferative enteropathy is widespread among swine herds (30 to 50% are
infected) in different types of production systems and in all parts of the
world.8-13 It was the most common disease problem in
grower-finisher pigs reported in the 2000 National Animal Health Monitoring System
survey, occurring on more than a third of all
sites and reported on 75% of large sites (10,000 or more total
inventory).14 Serologic studies have shown that the prevalence of
PE-positive herds ranges from 60 to 90% in
different countries.7,15-19 The economic impact
of PE on the swine industry is estimated to be very high. It was estimated to cost the
industry US$20 per sow annually in
Australia,20 and US$20 million annually in the
United States.21
The main impact of the disease has been due to increased use of antibiotics
and mortality related to the acute form of the disease (PHE). The chronic form (PIA)
is seldom detected and diagnosed. In this article, we will discuss some aspects of
the epidemiology of the disease, importance of the subclinical-chronic form, and
diagnosis of PE.
Transmission
Feces from infected pigs are the main source of new infections in susceptible
animals.1 A serologic study performed in
184 herds showed that seropositivity in the breeding herd is an important risk
factor for new PIA cases in grower-finisher
pigs.22 In addition, seropositivity in
grower-finisher animals was a risk factor for PHE
in replacement animals.22 In a study of
the prevalence of L intracellularis detected
in fecal samples by polymerase chain reaction (PCR) in an endemically infected herd
in Europe, the highest proportion of positives occurred 10 to 24 days after
weaning.23 Only 12.9% of the grower-finisher
animals and 0.9% of the mature animals were positive. In this study, the possible
epidemiological importance of transmission from breeding sows to young suckling
piglets was proposed. Fecal shedding has been reported in pigs as young as 3 weeks of
age24 and in pigs 25 and 42 days of
age.25 However, despite evidence suggestive of
sow-to-piglet transmission, this has yet to be
studied and proven.
On the other hand, pig-to-pig contact is an important route of transmission.
Rowland and Rowntree26 found an association
between a PHE outbreak in young breeding stock and the onset of chronic diarrhea
in in-contact weaned pigs a few weeks later. Lawsonia
intracellularis infection was transmitted between breeding stock and
young adult pigs in a natural PHE outbreak, where movement of sows and boars
between units was permitted.27 In an
experimental trial, sentinel pigs became infected when housed in contact with pigs
experimentally inoculated with a pure culture of
L intracellularis.28
Results from a questionnaire survey among British farm owners indicated that
slatted and meshed flooring were important risk factors for PE. They suggested that
such floors, commonly found in postweaning facilities, are often insufficiently
cleaned.29 However, the findings in this report
were based on the owners' opinions, and no diagnostic testing was performed to
support the results of the survey. Another study, which included a questionnaire
survey, production records, and fecal PCR
analysis, reported that the use of new buildings
and recent mixing of pigs were associated with
PE.30 These findings support a
hypothesis that subclinically infected pigs shed
L intracellularis in the feces, particularly
after stress.
Due to the difficulty in isolating, culturing, and maintaining
L intracellularis in vitro, the isolation or re-isolation of viable
organisms from diseased intestines is very difficult. Therefore, information about
survival and resistance of L
intracellularis in the environment is scarce. A unique
investigation into this area31 found that intestinal
colonization of pigs by L intracellularis was
detected after they had been orally inoculated with feces from positive animals.
Infected feces had been stored for up to 2 weeks
at temperatures between 5 and 15°C. In this same study, pure cultures of
L intracellularis were fully susceptible to a quaternary
ammonium disinfectant (3% cetrimide), less so to 1% povidone-iodine, but not
susceptible to 1% potassium peroxymonosulfate or a 0.33% phenolic mixture.
Other possible mechanisms of transmission that need to be considered in future
studies are transmission by mechanical vectors
(eg, rubber boots) and biological vectors (eg, mice, small birds, and insects). As a
broad range of animal species may be affected by
PE,32 interspecies transmission is a
real possibility. Proliferative enteropathy was reproduced in
hamsters33 and mice34 using homogenized mucosa from
PE-affected pigs, and in mice using pure L
intracellularis culture extracted from
pigs.35 Recently, a natural outbreak of PE was reported in
a colony of conventional mice in a University of Missouri research
unit.36 Natural PE cases have been reported in ratite birds
(eg, emu37 and
ostrich32,38,39), but there are no similar reports concerning other bird
species.
It seems reasonable to conclude that pig-to-pig contact is probably the main
mechanism of transmission, with subclinically infected animals being key elements of
this transmission. Future efforts are necessary to obtain further information about
sow-to-piglet transmission, resistance of the organism in the environment, and
possible mechanical and biological vectors, which will help to explain the high prevalence
of the disease among herds worldwide.
 Importance of subclinical proliferative enteropathy
In a field study40 and in a controlled
experiment,41 intermittent fecal shedding
of L intracellularis, as assessed by PCR,
has been detected for a period of up to 12 weeks. In the field study, no clinical
disease was observed among either PCR-negative or PCR-positive pigs. In the
experimental study,41 diarrhea occurred in
challenged pigs only from the second week to the
fifth week postinoculation. Growth performance was not evaluated in these studies.
In a recent study,42 clinical,
morphological, and microbial findings in animals from
good and poor performance herds were compared. The authors concluded that
clinically healthy animals from infected herds
were often infected with L intracellularis
(detected by PCR in fecal samples), and that growth performance in these animals
was poor compared to that in uninfected herds (Table 1). Three pigs with diarrhea
(case pigs) and three pigs with no signs of clinical disease (control pigs) were selected
from each of nine poor performance herds and compared to three control pigs from
each of four good performance herds, with pigs matched by age in each case. The
average age at which pigs reached a body weight of 25 kg was 85.6
+/- 3.6 days in poor performance herds, and 64.7
+/- 2.6 days in good performance herds. In most case
pigs, gross and microscopic lesions were identified that were consistent with PE,
colonic spirochaetosis, or both. The most frequently diagnosed enteropathogenic
agent was L intracellularis, followed
by Brachyspira pilosicoli. An interesting
result from this study was the high percentage of control pigs from poor performance
herds that were PCR-positive for L
intracellularis (41%).
These studies show that growth performance of pigs subclinically infected with
a pathogenic isolate of L
intracellularis is poor, and that they shed the organism
into the environment, resulting in infection of susceptible penmates. It appears that
subclinical infection with L
intracellularis may result in poor growth
performance, unthriftiness, and lost homogeneity in
a batch of grower-finisher pigs. These problems have been associated with
Mycoplasma hyopneumoniae or circovirus infection,
but now an additional possible culprit, L
intracellularis, should be considered.
Diagnosis
Diagnostic tools for detecting exposure or infection to
L intracellularis (eg, serology, fecal PCR, and immunohistochemistry
in tissue samples) have become more available in the last few years. Each
diagnostic method evaluates a different
epidemiological aspect of PE. Serology, for
instance, provides historical information on
exposure to the bacteria, while fecal PCR and immunohistochemistry are measures of
current infection.
Lawsonia intracellularis is an obligate
intracellular organism that infects intestinal epithelial cells. Therefore serum, IgG is
not likely to be protective against infection, while secretory IgA and cell-mediated
immune responses may play more important
roles.41 Nevertheless, detection of
serum IgG is a useful tool to evaluate exposure to
L intracellularis. Optimization and validation studies of serologic tests for PE
have been carried out in recent years, creating new opportunities for a better
understanding of the immune response induced by
L intracellularis
infection.43-45 Indirect immunofluorescent antibody
(IFA)44,46 and immunoperoxidase monolayer
assay (IPMA)45 are serologic tests that detect
L intracellularis-specific serum IgG. Each
has a sensitivity of approximately 90% and a specificity of approximately 100%,
determined in controlled experimental infection studies. No cross-reactivity was
observed when these serologic tests were used on convalescent sera from pigs infected
with several Campylobacter species,
Salmonella serovar choleraesuis, Salmonella
serovar typhimurium, Escherichia coli
K88, Brachyspira hyodysenteriae, B
pilosicoli, or porcine reproductive and respiratory
syndrome virus.45 Serum IgG is first
detected in the second week postinfection and persists for 3 to 13 weeks, depending on
the form of the disease (PHE or PIA) and its
severity.40 Serum IgG was detectable for
up to 12 weeks in gilts after a natural outbreak of the acute form of PE, and in
5-week-old pigs infected with high doses of
pathogenic L intracellularis. Conversely,
seropositivity in grower-finisher pigs in field
conditions usually persisted for only 2 to 3 weeks
and was detected mainly in 18- to 26-week-old
pigs.40 However, age at seroconversion
in grower-finisher pigs may vary depending on the feed medication program, pig
flow, and type of flooring.30 Although we
were unable to demonstrate a statistically significant association between severity of
gross lesions and serum titers in pigs 3 weeks after experimental
infection,43 we believe that the level of infection correlates
with serum titers. Serum IgG titer decays gradually after reaching its peak; therefore,
the higher the peak serum titer, the longer detectable serum IgG persists.
Although the specificity of PCR for detection of
L intracellularis DNA in fecal samples is virtually
100%,47 the sensitivity of the technique ranges between 39
and 72% in experimentally infected
pigs.43,46 In field and controlled experimental
studies, animals became positive by fecal PCR 1 to 2 weeks before they
seroconverted.41,43 When animals are PCR-positive and
seronegative, either they are in the early stage of infection and have not yet had time
to seroconvert, or the level of infection is not sufficient to induce a systemic
humoral immune response detectable by the serologic test. When animals are
PCR-negative and seropositive, either they have been
previously exposed to L intracellularis
and are no longer shedding the organism, or
detection of fecal shedding was limited by the low sensitivity of the PCR technique
in fecal samples. Nonetheless, stage of infection with
L intracellularis, based on the percentage of seropositive pigs or the
percentage of PCR-positive pigs (representing fecal shedding), or both, and
observation of a clinical problem represented by
diarrhea or poor growth performance, must be evaluated as a whole.
 Immunohistochemistry in histologic sections of ileum, using antibodies specific
for L intracellularis
antigens,48,49 has a sensitivity of 87%, compared to histological
examination using hematoxylin and eosin staining (sensitivity 37%) and or
Warthin-Starry silver stain (sensitivity
50%).43 Lawsonia
intracellularis can be detected by immunohistochemistry in just a few
intestinal crypts early in infection and in the cytoplasm of macrophages in the
lamina propria late in the course of the disease (Figure 2). Those two stages of the
disease cannot be differentiated in sections
stained with hematoxylin and eosin or Warthin-Starry silver stains.
The PE status of a herd should be evaluated in two common field
situations.50 The first one is observation of poor
performance, diarrhea, or both in a group of grower-finisher pigs. It is
recommended that two or three gaunt pigs with
diarrhea be selected from the most severely
affected pens for euthanasia, necropsy, and submission of samples of large and small
intestine to a veterinary diagnostic laboratory.
In addition to the standard bacteriologic and histologic tests, specifically request
immunohistochemistry for L intracellularis
and bacteriology for Brachyspira (species
identification). Collect fecal samples (at least a pea-sized amount of feces) from 20
pigs with loose stools and submit refrigerated samples for PCR testing. Although
pooling of fecal samples from two to three animals is acceptable to reduce costs of testing,
this does inherently reduce the sensitivity of the test.
The second field situation is the necessity for knowing the probable time of
infection in order to determine the optimum time for strategic medication of
grower-finisher pigs with growth performance
problems, which might be related to L
intracellularis infection. Serological testing is
recommended in this situation, as it costs less than PCR testing, is suitable for
testing large numbers of samples, and provides an estimate of time of exposure.
Serum samples from several groups of at least 20 grower-finisher pigs with 3-week age
differences should be submitted for serologic testing. Strategic medication,
vaccination, or both, are usually recommended 2 to
3 weeks before the age of seroconversion.
Further research focusing on the epidemiology of
L intracellularis infection and transmission is imperative in order to
design eradication protocols for PE. The presence of subclinically-chronically
infected pigs seems to be the major factor contributing to the economic impact of
poor growth performance and dissemination of the disease in the herd. Careful
interpretation of appropriate diagnostic tests,
which may be used to determine the time for treatment or vaccination to be most
effective, will help to minimize losses due to PE.
Acknowledgment
I thank Dr Connie Gebhart for the critical review of the manuscript.
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