ENDEMIC PRRS CASES: VIRUS ISOLATION
Nursery and grower pigs
PRRS virus can be isolated from the serum of live nursery
age pigs for approximately 4-7 weeks following natural
or experimental inoculation. ' This unusually long period
of viremia improves the chances of selecting a viremic
pig from an endemically infected nursery. Bleed 10 clinically
ill pigs that have been in the nursery for at least 2
weeks and 10 pigs that have been in the nursery for 5
weeks. Several samples in each age group can be pooled
to reduce laboratory costs.
From recently euthanatized nursery pigs, collect lung,
spleen, and serum and quickly chill or freeze them. Additional
tissue may be fixed in 10% formalin for diagnostic histopathology.
For best results, select clinically ill pigs, preferably
with respiratory difficulty, that have been in the nursery
or have commingled with older pigs for at least 2 weeks.
Because the virus is quickly inactivated in dead pigs
at nursery-room temperatures, do not select pigs that
have died naturally.
For antemortem and postmortem diagnosis in grower pigs
the samples are the same as for nursery pigs. However,
on endemically infected farms, grower pigs may be at
the end of the viremic stage andisolation would then be
less reliable.3 Clinically affected pigs with
respiratory distress should be selected.
In endemically infected herds, sows and gilts may not
be viremic (unless there is concurrent reproductive failure).
Also, only a small percentage of baby pigs may be viremic.
Sampling breeding stock and baby pigs on endemically
infected farms without reproductive failure is not very
efficient for confirming PRRS by virus isolation.
ENDEMIC PRRS CASES:
NURSERY-PIG SEROLOGY
A definitive diagnosis of PRRS requires virus isolation.
Nursery-pig serology can be an adjunct to diagnosis.
The goal of nursery-pig serology is to detect an increasing
seroprevalence over time. This may be confounded by passively
acquired maternal antibody that usually lasts for 6-8
weeks but can last up to 16 weeks of age. Eartag and
bleed 10-20 pigs at 3-4 weeks of age and bleed the same
pigs at 7-8 weeks of age. A greater percentage of seropositive
pigs at the second bleeding would indicate that virus
is actively spreading among pigs on the farm.19
ENDEMIC PRRS CASES:
HERD SEROLOGY
It may be desirable to know whether a swine herd is sero-negative
for PRRS virus. The most reliable way to determine
whether a herd is seronegativefor any given disease is
to bleed and test every animal. Obviously, this is impractical.
In reality, practical herd ser-ology depends on the prevalence
of disease on the farm. The serostatus of a herd is predicted
by testing a representative group of animals in the herd.
The number and ages of animals to be tested is determined
by the assumed seropreva-lence in each age group. Sero-prevalence
to PRRS virus in the breeding herd can vary widely depending
on how long the herd has been infected.
Herds with a recent history of reproductive failure have
often been infected less than 1 year and tend to have
a high seroprevalence (approximately 50%) in breeding
animals. To detect seropositive animals with 95% confidence
from a group of 30-10,000 animals (assume 50% seroprevalence
in the breeding animals) you would need to bleed at least
seven sows or gilts. We recommend taking 10-20 samples.
In endemically infected herds, which have probably been
infected more than 1 year, the seroprevalence is usually
low in the breeding herd (20% or less) and high in the
finishing hogs (50% or greater).3'20-22
In a breeding herd of 100 animals, to detect seropositive
animals with 95% confidence (assume 10% seroprevalence
in breeders and 50% seroprevalence in finishers) you would
need to bleed at least 25 breeders and seven finishing
pigs.3 Because passive antibody is usually
gone at 6-8 weeks of age and is always gone by 16 weeks
of age. Seropositive finishing pigs that are more than
16 weeks old were probably infected with the virus within
the last 4 months.
Remember, herd serology only demonstrates previous exposure
to the virus and does not provide a definitive diagnosis
of PRRS virus as a cause for the clinical problems on
the farm.
Because of the industry-wide interest in PRRS and the
many researchers investigating this disease, our knowledge
of PRRS will grow rapidly. These recommendations for
diagnosis may become quickly outdated as more is learned
about PRRS.
This article is the final installment taken from an
article, under the same title, published in Swine Health
and Production - July 1993.
The authors: William G. Van Alstine,DVM, PhD, Gregory
W. Stevenson, DVM, PhD and Charles L.Kanitz, DVM, PhD
are actively working on PRRS at the Animal Disease Diagnostic
Laboratory.
References available by writing Rebecca Gwin, Animal
Disease Diagnostic Laboratory, 1175 ADDL,Purdue University
West Lafayette, IN 47907-1175
