Nebraska Swine Report 2005 : PRRSV Negative Herds: A Survival Analysis
By Locke A. Karriker and Ruby Destajo, Veterinary Diagnostic and Production Animal Medicine Department, Iowa State University and published by the University of Nebraska - Despite a significant body of research, interventions for PRRSV infection remain elusive. Traditional approaches to managing the risk of diseases have not been successful in many cases of PRRSV infection.PRRSV Negative Herds: A Survival Analysis
Summary and Implications
While elimination of the virus from farms is possible, it is not without cost and re-infection is common. This survey sought to quantify the expected duration of PRRSV negative status on farms that were repopulated with PRRSV negative animals or had undergone a PRRSV elimination program.
Results of 96 cases reveals
a range of <1 to 312+ weeks duration
of negative status. A survival analysis
of 84 farms revealed a probability
of surviving with negative status for
two full years of 58.3% with a standard
error of 11.5%. The probability
of survival through 4 years was 42%
with a standard error of 16%. A greater
percentage of farms that were reinfected
shared resources such as
equipment, personnel, and/or vehicles
with known positive farms. Positive
farms also had a relatively shorter
average distance to known positive
farms than those remaining negative.
The results of this study indicate
that PRRSV-negative farms are
not very likely to remain negative for
a long duration given current technologies.
Longer survival of negative
status appears to be associated
with greater distance from known
positive farms and stricter biosecurity.
Sharing of equipment and other
resources as well as a closer distance
to other farms should be considered
risk factors that can lessen the probability of farms maintaining negative
status.
Introduction
Porcine Reproductive and Respiratory
Syndrome Virus (PRRSV)
remains a constraint to productivity
and profitability in swine
herds world-wide, costing about
$228 to $302 per female in the
breeding herd and about $6.25 to
$15.25 per pig in the growing herd
according to Iowa State University
research. While an extensive
body of research over the past
decade has characterized the
virus, the pathological lesions
associated with infection, interactions
with other disease etiologies,
and eventually led to the
development of diagnostic tests
for both the virus and antibodies
to it, control in and among swine
herds remains elusive.
Elimination of the virus from
swine herds has proven to be a
challenging task and has increasingly
been the focus of discussions
surrounding the topic of
PRRSV management and interventions
in the herd. Some of the reasons
for its persistence in the herd
pertain to what is known or commonly
accepted about the behavior
of the virus in individual
animals:
- Boars can shed virus in semen intermittently for extended periods of time with few or no clinical signs.
- Persistently infected animals exist and can shed to naïve contacts.
- The immune response in the pig is poorly understood and/or variable given different contexts.
- PRRSV subpopulations exist in endemic breeding herds.
- Multiple genetically diverse PRRSV strains can coexist on farms simultaneously.
- Vertical and horizontal transmission of PRRSV occurs by many known routes and potentially by additional unknown routes.
- Infection can occur in utero and produce piglets that are viremic at birth
- The introduction of negative gilts to positive farms, or gilts that have been exposed to a genetically diverse strain of PRRSV can lead to sustained PRRSV circulation on farms.
- Vaccine efficacy is highly variable depending on vaccine type (killed versus modified live virus), genetic relatedness of the vaccine virus and wild type, and timing of vaccination relative to exposure.
Because successful, profitable
production requires successful risk
management, increasingly formal
risk analysis methods are being
applied to swine health. One formal
definition of risk was presented
by Iowa State University
in 2003:
Risk = Pr(event)*consequence
Where:
Pr = probability
Event = the defined hazard
Therefore, to reduce risk, it is
necessary to reduce the probability
of disease and/or reduce its
consequences. Speaking of disease
in general, probability reducers
include: biosecurity, uni-directional
pig flow, shower-in /
shower-out, traffic and visitor
control, eliminating employee
contact with other swine herds
and rodent control. Historical
approaches to reducing the consequence
of disease include: adequate
nutrition, appropriate
ventilation, vaccination, acceptable
stocking densities, water quality
and availability, and reduction
of stressors.
In the context of “financial loss
(or cost) due to PRRSV” as the
hazard, reviewing what is already
known and generally accepted
about PRRSV yields few opportunities
to reduce the probability or
consequences of clinical PRRSV
infection on farms. Most of the
typical reducers of consequence
have been shown to have little or
no impact on the cost of PRRSV.
Additionally, farms that have
virus circulating would be considered
at high probability for clinical
signs in animals and therefore
have a high risk of cost due to
PRRSV. This has led to consideration
of PRRSV negative status as
an opportunity to greatly reduce
the probability of cost due to
PRRSV on farms.
Several PRRSV strategies have
been outlined to eliminate virus
from positive farms. The most common
methods are 1) total depopulation/
repopulation, 2) “rollover”
scenarios where positive farms
take advantage of a decline or stop
in circulation and switch to introduction
of negative replacements,
3) herd closure, and 4) test with
removal. These strategies have several
common elements including
the necessity to stabilize immunity
(through depopulation, natural
circulation over time, or forced
exposure/acclimation) and reduce
the risk of new virus exposure.
Farms that are populated with
naïve animals initially also share
the latter risk. It is unrealistic to
expect negative farms to remain
negative indefinitely because:
- There are costs associated with the various elimination strategies,
- Many anecdotal experiences have been described for farms that have been infected or re-infected despite significant biosecurity interventions,
- Biosecurity interventions incur costs,
- Existing facility location and design have been identified as risk factors and are not easily changed, especially in light of the political regulatory climate.
As discussed here and elsewhere, the costs of PRRSV (and therefore, the expected improvement in animal performance upon elimination) have been estimated. With the addition of information on the likely duration of negative status, the financial return can be estimated. Thus, a preliminary survey was conducted to quantify the expected duration of PRRSVnegative status on farms that were repopulated with PRRSV negative animals or had undergone a PRRSV elimination program.
Methods
The survey was conducted in October and November, 2003 among 45 selected swine veterinary practitioners who are American Association of Swine Veterinarians members. Selection criteria included their experience in handling PRRSV-negative farms and /or whether they have initiated elimination of the virus from positive herds.
Results
Responses were received from
39 veterinarians, an 86% response
rate. Forty-six percent of the veterinarians
who responded had
PRRSV elimination projects
occurring on 96 farms.
A majority of the farms serviced
by participating veterinarians
were farrow-to-wean (63%).
About half of those (31%) were
farrow-to-finish. Twenty-three
percent of the farms (n = 22)
inventoried 1,000 or fewer females,
about 37% of the farms (n = 36)
inventoried 1,000 to 2,000 females,
21% of the farms (n = 20) had
2,000 to 3,000 females, and 19% of
farms (n = 18) had more than 3,000
females.
Although the reasons that
motivate swine practitioners to
strive to eliminate PRRSV are
interrelated, most of the respondents
indicated a need to eliminate
the virus to be able to provide
negative replacements (81%) for
their current stock. Others were
primarily motivated to improve
the farm’s commercial (17%) and
genetic performance (10%). Some
did it for other purposes like
research and Actinobacillus pleuropneumonia
depopulation (8%).
About 17% of the respondents cited
multiple reasons for beginning an
elimination effort.
Of the methods of elimination
discussed above, 44% of the
respondents incorporated herd
closure as a technique, wherein
introduction of replacement stock
was suspended for a defined
period of time and subsequently
a new naïve batch was introduced.
Nearly 40% opted for complete
depopulation of the farm and subsequent
repopulation with naïve
animals after thorough cleaning.
Only 1% followed the test-andremoval
method in eliminating
the identified positives from the
herd. Some (27%) used a combination
of the known methods while
26% were not satisfied with these
methods and tested other means
not mentioned in the list.
The respondents were asked
about the week and year of recent
clinical PRRSV occurrence prior
to elimination to establish the timing
of original break and the week
and year of completion of the elimination
action to establish the timing
of the elimination action.
Completion of an elimination
activity was arbitrarily defined
as the date when the first PRRSV
naïve animal farrowed. If the farm
was subsequently infected, they
were asked about the week and
year of the first clinical PRRSV
infection after the completion of
the previous elimination action
to establish the timing of subsequent
reinfection. Initial PRRSV
infection, elimination and reinfection
had to be confirmed with
diagnostic testing.
Eighty-four of the farms serviced
by the respondents were
included in the survival analysis.
There were 48 farms that had remained
negative for PRRSV at the
time of the survey. The remaining
36 farms experienced subsequent
reinfection (rebreaks). The period
of survivorship was defined as
that period from the time of completed
elimination action to the
date of survey (for negative farms)
or to the time of rebreak (for positive
farms). Survivorship curves
and standard errors were
estimated for 1) all farms, 2)
farms that used depopulationrepopulation
method, and 3) farms
that used herd-closure technique.
No significant differences were
found between the curves. Survival
analysis of all farms resulted
in a decreasing probability of
remaining PRRSV-negative with
time (Figure 1).
Figure 1: Probability of survival through time period — all farms (N = 84).
When asked about the causes
for PRRSV rebreaks, 36% of the
veterinarians who responded
indicated they had no explanation.
About 31% believed area
spread, including lateral spread
by insects was believed to be the
culprit. Infected replacement
stocks including transmission
through semen and infected gilts
were reasons cited by 22% of the
respondents. Eleven percent
blamed the cause for reinfection
on the equipment being used in
the farm (11%). A greater percentage
of positive (40%) than
negative farms (12.5%) shared
resources with a known positive
farm. As expected, the average
distance of a positive farm to a
known PRRSV-positive farm was
shorter than that of a negative
farm (5.38 miles for positives; 15.15
miles for negatives).
Swine practitioners were also
encouraged to give their comments
about PRRSV. One of the problems
encountered with remaining
PRRSV negative is farm
location, according to some
respondents. The area around the
farm became highly populated
with swine sometime after the farm
was built. Another problem mentioned
was that the farm’s isolation
facility was located too close
to the sow farm to prevent infection
of incoming gilts. Some complained
of positive pigs located
within half a mile of the farm.
There were those who suspected
transport transmission but could
not verify it because no similar
PRRSV isolates were identified in
the nearby area. Others obtained
PRRSV viral isolates that were
more closely related to the
neighbor’s farm than historical
isolates from the home farm.
While a good starting point,
the results of this preliminary survey
are limited because of bias in
recall or in targeting farms, small
sample size, in the classification
of strategies, in definitions, and
in the setting of an arbitrary starting
point. To further understand
the issue, it is recommended a
prospective study and a thorough
epidemiologic investigation of
breaks be performed.
Further Information
To view the full list of Nebraska Swine Report 2005 articles, click here
Source: University of Nebraska, Lincoln - September 2005