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Original research
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November and December, 1996 |
Streptococcus suis colonization of piglets during parturition
Sandra F. Amass, DVM, MS; L. Kirk Clark, DVM, PhD; Kay Knox; Ching Ching
Wu, DVM, PhD; Michael A. Hill, MS, PhD, MRCVS
SFA, LKC, KK, MAH: Swine Production Medicine, Purdue University, 1248
Lynn, West Lafayette, Indiana, 479071248; CCW: Animal Disease Diagnostic
Laboratory, West Lafayette, Indiana, 47907
Copyright (C) 1996, American Association of Swine Practitioners.
Also available in PDF format.
Summary:
Objective--To determine whether piglets were colonized with Streptococcus
suis of sow origin during parturition
Materials and methods--Multiple samples were collected from 43
piglets of eight dams. Oral and vaginal swab samples were collected from
each sow prior to farrowing. Piglets were removed from the vagina using
individual sterile obstetrical sleeves into which they were immediately
placed. Swab samples of the oropharynx and dorsal surface of each piglet
were collected. Umbilical blood from each piglet was collected into tubes
of culture media. Room air was sampled to estimate the concentration of
airborne S. suis. All collected samples were culturally examined
for S. suis. Streptococcus suis isolates were serotyped using antisera
to S. suis serotypes 1/2-34.
Results--Fifty-four isolates of S. suis were identified.
Multiple serotypes of S. suis were isolated from samples collected
from a single animal in 11 of the 51 sows and piglets sampled. Streptococcus
suis was isolated from the oropharyngeal samples of all of eight sows
and from the vaginal swab samples from three of eight sows. Streptococcus
suis was isolated from the oropharyngeal swab samples from nine of 43
piglets, the surface swab samples from 13 of 43 piglets, and the blood samples
from two of 43 piglets. In three of eight dams, S. suis isolated
from samples collected from the dam was of the same serotype as the S.
suis isolated from oropharyngeal or surface swab samples of that dam's
piglet. In two of these cases, the sow and her piglets matched on two different
serotypes of S. suis. Streptococcus suis was not isolated from air
samples.
Implications--In these studies, we concluded that the source of
S. suis was the sow, and that S. suis was transferred to the
dorsal surface and oral cavity of the piglet during parturition when the
piglet came into contact with S. suis from sow vaginal secretions.
Keywords: swine, Streptococcus suis, serotypes, vertical
transmission
Received: June 15, 1996
Accepted: August 9, 1996
Streptococcus suis infection in swine is a problem encountered
by producers and veterinarians worldwide.1 Streptococcosis primarily
affects nursery pigs, and cannot be prevented by either segregated or medicated
early weaning (SEW, MEW) protocols.2-5 Many serotypes of S.
suis have been associated with a septicemia resulting in meningitis,
arthritis, endocarditis, polyserositis, and pneumonia, primarily of nursery
pigs.1 Streptococcus suis serotype 2 is widely considered
the most common cause of clinical streptococcosis in swine, and is the focus
of much research; however, in the herds we sampled in Indiana, other serotypes
of S. suis are more commonly isolated from cases of meningitis. The
production of a naive subpopulation of SEW pigs may render streptococcal
virulence factors more important than serotype.
The age at which a piglet becomes infected with S. suis has been
investigated for many years. Clifton-Hadley, et al.,6 concluded
that S. suis type 2 may infect suckling piglets, but that the spread
of the organism among weaned pigs in intensive production units was what
was of prime importance. The importance of S. suis colonization of
suckling piglets, however, is gaining increasing recognition as early weaning
procedures become widely implemented in the swine industry. Piglets colonized
at an early age carry S. suis into the nursery, where it may be transmitted
among other weaned pigs and cause clinical disease as maternal immunity
declines. From the results of previous transmission studies,4
we concluded that the sow sheds multiple serotypes of S. suis in
bodily secretions and excretions. Piglets can become colonized with S.
suis shortly after birth when they come into contact with these sources.
Cesarean section is one method to produce piglets free from S. suis.
Cesarean-derived piglets from S. suis-infected dams have been reported
to be free of all serotypes of S. suis.7,8 However, cesarean
section is not likely to be adopted as a method for obtaining piglets for
commercial pork producers. Decluzeau, et al.,9 have produced
piglets free from certain organisms by surface decontamination of the piglets
immediately after birth. Thus, surface contamination by S. suis of
piglets during birth might be negated, if it occurs, and allow the production
of nonsurgically derived piglets free of S. suis infection.
The purpose of this study was to determine whether S. suis colonized
the oropharynx, surface, or blood of piglets during birth.
Materials and methods
Experimental design
The herd of origin was a 1200-sow, farrow-to-feeder-pig unit. Streptococcus
suis serotypes 2, 3, 4, 5, 6, 7, 8, 9, 21, and 22 were known to exist
in the herd prior to the study.4,5 Clinical streptococcosis was
not a significant problem in this herd. The study sample consisted of eight
parity-two to parity-five crossbred sows from one weekly farrowing group,
and their 43 newborn piglets, so that:
- six piglets were sampled from each of five sows (sows 1,2,3,5, and
6),
- five piglets were sampled from one sow (sow 8), and
- four piglets were sampled from each of two sows (sows 4,7).
A prevalence of at least 25% of S. suis was assumed among the
population of 60 sows from one weekly farrowing group. Thus, the organism
should have been detected in at least one sow with 90%-95% confidence with
a sample size of eight sows. A 30% prevalence of S. suis was assumed
among the population of piglets in a single litter (born alive = 10 piglets).
Thus, the organism should have been detected in at least one piglet per
litter with 90%-99% confidence if four to six piglets per litter were sampled
(Epi Info version 5.01-b, 1991. Centers for Disease Control, Atlanta, Georgia).
The adequacy of these sample sizes was confirmed in previous trials, in
which we found S. suis colonization in all of seven sows and 65 of
70 2-week-old piglets.4,5
Sample collection
Farrowing was induced with 263 µg cloprostenol sodium injected
intramuscularly (Estrumate(R), Miles, Shawnee, Kansas) 30 hours
before the start of sample collection. Prior to farrowing, oral and vaginal
swab samples were collected from each sow using sterile swabs (S/P(R)
Brand culturette system, Baxter Diagnostics Inc., Deerfield, Illinois).
Using a sterile obstetrical sleeve, the piglet was removed from the vagina,
and the sleeve immediately was inverted over the piglet and sealed. Sterile
gauze was used to remove any fetal membranes covering each piglet's rostral
surface. The oropharyngeal region and dorsal surface of each piglet (before
it was cleaned) were swabbed with sterile swabs (S/P(R) Brand).
The umbilical cord of each piglet was exposed and the distal 1/4 to 1/3
was removed using a sterile scissors. Approximately 1 mL of umbilical blood
was collected by milking the blood into tubes of Todd-Hewitt broth (THB).
Room air was sampled at a rate of 1 cubic foot per minute onto sheep
blood agar plates using an impactor (Gast Model 0523-101Q-G5820X, Benton
Harbor, Michigan) for 5, 10, 15, and 30 seconds.
Cultural examination
Sow oral swab samples were plated onto sheep blood agar and incubated
within 24 hours of collection.
Sow vaginal and piglet oral and surface swab samples were plated onto
sheep blood agar within 11 hours of collection. The sow vaginal, piglet
oral, and piglet surface swab samples were then enriched in THB for 12 hours,
and a sample of the THB plated onto phenylethyl alcohol (PEA) agar with
5% sheep blood, and incubated for 18 hours.
Piglet umbilical blood samples were incubated within 11 hours of being
collected and plated onto PEA agar after 12 hours of incubation in THB.
After 18-24 hours of incubation at 37°C with 5% CO2, we
selected up to three 0.5- to 1-mm flat, a-hemolytic colonies per site for
each sow and piglet sampled. These isolates were then Gram stained and biochemically
tested.
Of the Gram-positive diplobacilli that did not grow in 6.5% NaCl solution,
a culture had to meet all of the following criteria to be considered a S.
suis suspect:
- catalase-negative,
- acetoin-negative, and
- amylase-positive.
Serotyping
All S. suis suspects were serotyped using a polyvalent coagglutination
method and then a monovalent coagglutination method10 with antisera
to S. suis types 1/2-34 (antisera provided by R. Higgins, University
of Montreal, Saint-Hyacinthe, Quebec, Canada). Suspect isolates
were considered to be S. suis if they agglutinated the antiserum.
No conclusions were drawn from suspects that did not agglutinate the antisera
to S. suis 1/2-34, even though these isolates could potentially have
been S. suis of other serotypes.
Evaluation of results
The number of viable bacteria in the air samples was calculated using
previously published methods.11 Identifying S. suis from
any samples of piglet origin was considered to be the result of colonization
of that piglet during birth.
Results
Eleven serotypes of S. suis were identified in the fifty-four
isolates:
- serotype 5 (50% prevalence),
- serotype 12 (14.8%),
- serotypes 10 and 34 (7.4% each),
- serotype 9 (5.6%),
- serotypes 8 and 27 (3.7% each), and
- serotypes 4, 11, 13, and 30 (1.85% each).
Multiple serotypes of S. suis were isolated from samples collected
from a single animal in 11 of the 51 sows and piglets sampled. Streptococcus
suis was isolated from the oropharyngeal samples from eight of eight
sows and from the vaginal swab samples of three of eight sows (Table 1). In the litters of sows 4
and 5, no S. suis was isolated from any of the piglets. In the litter
of sow 7, S. suis was isolated from all the piglets. In the remaining
litters, S. suis was isolated from some but not all of the piglets.
A single serotype of S. suis was isolated from littermates in
two of eight litters (Table 1).
Multiple serotypes of S. suis were isolated from littermates in four
of eight litters. Streptococcus suis was isolated from the oropharyngeal
swab samples from nine of 43 piglets, the surface swab samples from 13 of
43 piglets, and the blood samples from two of 43 piglets. In three of eight
dams, S. suis of the same serotype isolated from samples collected
from the dam were detected in oropharyngeal or surface swab samples of that
dam's piglet. In two of these cases, the sow and her piglets matched on
two different serotypes. There were 8-16 colony-forming units of organisms
per liter of room air; however, S. suis was not isolated from air
samples.
Discussion
We previously reported that cesarean-derived piglets were free of S.
suis, but 1-day-old piglets were colonized by S. suis.4,8
Whether piglets were colonized with S. suis during parturition
was not known.
These results indicated that the oropharyngeal area and vaginal secretions
of sows were colonized by multiple serotypes of S. suis. However,
S. suis was not isolated from piglets in two of eight litters or
from the vaginal tracts of five of eight sows. Thus, either S.
suis was not present at these sites or cultural examination did not
detect the S. suis that was present. It is possible that the vaginal
tract of the sow was not colonized or colonized in such low numbers that
transmission did not occur from sow to piglet. Because we only chose three
suspect isolates from each sample we collected, however, our ability to
detect S. suis was limited. It is possible that S. suis was
present at the sampled site, but not cultured, or that S. suis was
cultured, but those isolates were not included in our sample of three suspects.
Additionally, S. suis might have been present, but of a serotype
other than 1/2-34, and consequently not detected. Serotype differences among
sows and their piglets may also be explained by the low sensitivity of the
culture procedure. Thus, piglets and sows may have been colonized
by multiple serotypes of S. suis, but we were unable to detect all
of them.
There are two possible explanations for the isolation of S. suis
from the umbilical blood of two piglets:
- the sample might have been contaminated during collection because the
blood was milked from the umbilical vessels.
- Streptococcus suis might have been drawn up the cord via the
blood during the birth of the piglet when the end of the umbilicus contacted
S. suis-infected sow vaginal secretions.
Our results provide strong evidence that in most cases, vaginal secretions
from the dam were the initial source of S. suis that colonized the
oropharynx and surface of neonatal piglets during parturition. The
surface of the piglet appeared to be colonized as the piglet passed through
the vaginal canal. The most likely explanation for the colonization
of the oropharynx of piglets with S. suis was that they swallowed
the colonized vaginal secretions of the sows during the birth process.
Our results are supported by Robertson and Blackmore,12 who
reported that piglets derived from dams whose vaginas were colonized with
S. suis type 2 became nasal carriers of S. suis type 2. We
are currently analyzing the genomic DNA from the S. suis isolates
of both sow and piglet origin in this study to further demonstrate vertical
transmission at the molecular level.
It is possible that the samples were contaminated by airborne S. suis.
This is unlikely, however, considering that S. suis was not isolated
from the collected air samples, but large numbers of S. suis were
isolated from pig and dam samples.
Surface disinfection of piglets at birth thus appears not to be a feasible
method to prevent S. suis colonization of swine because in our study
some piglets appear to have swallowed S. suis from vaginal secretions
of the sow during the birth process, and in some piglets S. suis
entered the navel. We hypothesize that the sow is colonized by multiple
serotypes of S. suis and provides passive immunity to the piglet
against these serotypes. The piglet is then colonized by some, none, or
all of these serotypes of sow origin. As passive immunity declines, the
pig becomes susceptible to those serotypes of S. suis that are not
its "normal" flora. The pig is most susceptible to clinical streptococcosis
at the time when it is in the SEW nursery and is exposed to multiple serotypes
of S. suis being shed by other nursery pigs. Currently, cesarean
derivation is the only method of obtaining S. suis-free piglets.
Implications
- Piglets are colonized with S. suis during the birth process
when they contact and/or swallow S. suis from vaginal secretions
of their dam.
- Surface decontamination cannot be relied upon to obtain S. suis-free
piglets.
- Cesarean derivation is the only method of obtaining S. suis
free piglets.
References
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