HERD HEALTH PIH-38
PURDUE UNIVERSITY. COOPERATIVE EXTENSION SERVICE.
WEST LAFAYETTE, INDIANA
Pseudorabies (Aujeszky's Disease)
Authors
David G. Thawley, University of Minnesota
Donald P. Gustafson, Purdue University
Robert R. Ormiston, USDA, Washington, D.C.
Reviewers
John M. Cunningham, Lincoln, Nebraska
Willard Korsmeyer, Beardstown, Illinois
Gale and Lois Pohlman, Plymouth, Nebraska
Hilman Schroeder, Sauk City, Wisconsin
Introduction and History
Pseudorabies is an acute, frequently fatal disease affecting
most species of domestic and wild animals; however, man and cer-
tain apes are resistant to it. The disease is caused by a her-
pesvirus and is characterized by a variety of clinical signs; the
most prominent involve the nervous and respiratory systems.
Severe itching and self-mutilation are seen in most species, but
rarely in swine.
Aujeszky first recognized pseudorabies as a disease of cat-
tle and dogs in Hungary in 1902. It soon became evident, however,
that swine were the natural hosts of the virus, and pigs could
die as a result of the disease. For years in Europe, pseudora-
bies has been recognized as an important cause of death in swine
of all ages and as a cause of abortion. Until the late 1960's
and the early 1970's, the disease in the United States was con-
sidered important only as a cause of death in baby pigs and occa-
sionally in cattle, sheep, dogs and cats. However, pseudorabies
is more prevalent in the U.S. than was formerly believed, and the
present viruses are capable of causing a variety of clinical man-
ifestations, including death in newborn and adult swine and fetal
death with abortion in pregnant swine. The disease is widespread
and of considerable economic importance in several midwestern
states. A slaughter serum survey conducted in 1983 revealed a
nationwide prevalence of 18.8% in breeding swine with state rates
ranging from 0% to 34.3%.
Clinical Signs
Pigs less than 3 weeks old. In baby pigs, the disease may be
characterized by sudden death with few, if any, clinical signs.
Frequently death is preceded by fever which may exceed 105o F,
dullness, loss of appetite, vomiting, weakness, incoordination
and convulsions. If vomiting and diarrhea occur, the disease in
baby pigs closely resembles transmissible gastroenteritis (TGE).
In pigs less than 2 weeks old, death losses frequently approach
100%. Baby pigs may have become infected before birth and die
within 2 days after birth, occasionally after showing violent
shaking and shivering. Piglets infected immediately after birth
may show clinical signs within the first 2 days of life and usu-
ally die before they are 5 days old.
Pigs 3 weeks to 5 months old. After 3 weeks of age, pigs
usually develop a degree of resistance to the disease, and death
losses may decrease from 50% in pigs exposed when 3 weeks old to
less than 5% in pigs exposed when 5 months old. Death losses vary
with different strains of the virus, and even in grown pigs
severe death losses occasionally occur.
Fever is a prominent clinical sign in these growing pigs and
is followed by loss of appetite, listlessness, labored breathing,
excessive salivation, vomiting, trembling and eventually marked
incoordination, especially of the hind legs. Normally death is
preceded by convulsions. Involvement of the respiratory tract
with sneezing, rubbing of the nose and coughing may occur. Clear
to yellowish nasal discharges may be seen. Infected pigs that
recover have lost condition and will be slow to reach market
weight.
Mature pigs. The disease in adult pigs often is not severe,
but with some strains of pseudorabies virus, deaths may occur.
The disease in adult pigs is characterized by fever and respira-
tory signs which may include nasal discharges, sneezing, nose
rubbing and coughing. Pseudorabies is often found in operations
with other respiratory diseases such as Pasteurella and Actinoba-
cillus (Hemophilus) pleuropneumonia. Nervous signs such as trem-
bling, incoordination and itching occasionally occur, and blind-
ness may follow pseudorabies infection. Vomiting and diarrhea or
constipation may be seen. Since 1980, an acute, often fatal pneu-
monia caused by pseudorabies virus has increased in prevalence.
This condition is most often seen in herds having a prolonged
history of pseudorabies infection. Animals often die from a
fatal secondary bacterial pneumonia.
Sows infected in the early stages of pregnancy may return to
heat because of death and resorption of their fetuses. Sows
infected in middle pregnancy may eventually abort mummified
fetuses, whereas sows infected late in pregnancy often abort or
give birth to weak, shaker or stillborn pigs.
Postmortem Lesions
No gross lesions characteristic of pseudorabies are consis-
tently found. Small greyish-white spots of focal necrosis may
occur in the livers and spleens of pseudorabies-infected young
pigs. Congested pneumonic lungs are commonly seen. Virus isola-
tion and fluorescent antibody examination of these and other tis-
sues will reveal if the lesions are related to the disease.
Immunity
Recovery by swine from pseudorabies confers some resistance
for at least 12 months. Re-exposure may result in reinfection,
but it is usually asymptomatic. The passive immunity passed on
from an immune sow to her offspring through the colostrum may
protect the piglets for 5 to 10 weeks, after which they gradually
become fully susceptible. However, the passive immunity may be
too low to protect the piglets so the offspring of immune sows
also may die of pseudorabies.
Vaccines have been used in Europe for years, and in the
United States since 1977. The research consensus is that vaccines
reduce swine losses and spread of the disease but do not totally
prevent infection and the establishment of a carrier state in
recovered swine. Vaccines have been reported to enhance the con-
trol and eradication of pseudorabies. Newer ``differentiable''
vaccines combined with their appropriate serological tests permit
vaccinated animals to be distinguished from those infected with
``field'' strains of virus. Differentiable vaccines permit the
monitoring of herd infection status in vaccinated herds.
Spread of Infection
Pseudorabies is spread mainly by direct contact between
swine; the nose and mouth are the main entry points for the
virus. Nasal discharges and saliva contain the virus; therefore,
drinking water, bedding and other objects such as clothing and
instruments may become contaminated. The virus can be spread
without movement of pigs. When entering swine premises, clean
clothes should be worn, and boots should be disinfected upon
entering and leaving the premises. Virus also may spread by the
movement of air within buildings and for short distances outside
depending upon climatic conditions. Air spread in late winter and
early spring is suspected to be over greater distances than pre-
viously thought.
Recovered pigs may remain carriers of the virus and later
can infect susceptible pigs or cattle with which they come into
contact. Severe cattle losses from pseudorabies have occurred as
a result of contact infection from apparently normal carrier
swine. The disease also has been introduced to swine farms by
introduction of carrier pigs.
Dogs and cats are very susceptible to pseudorabies and usu-
ally become infected through contact with infected pigs. Wild
animals such as raccoons, skunks and mice are also susceptible to
the disease. Dogs, cats and wild aniimals are potential spreaders
of the disease within an endemic area, but are not considered a
factor in the spread outside the area.
Diagnosis
The clinical signs of pseudorabies are variable so clinical
diagnosis should always be confirmed by laboratory tests. Several
tests-the Serum-Virus Neutralization Test (SN), Virus Isolation
(VI), Fluorescent Antibody Tissue Section Test (FATS), the Enzyme
Linked Immuno-Sorbent Assay (ELISA) and the Latex Agglutination
Test (LAT)-have been approved for the diagnosis of pseudorabies.
Other tests are being developed.
The SN, LAT and ELISA tests detect pseudorabies antibodies
in serum of pigs that have been infected with the virus. These
antibodies appear in the serum about day seven of infection and
may persist for years. The presence of pseudorabies antibodies is
evidence that the pig has been infected with the virus in the
past or has been vaccinated. Absence of antibodies indicates that
the animal has probably not been infected or that it may be in
the early stages of the disease. Diagnosis of a pseudorabies out-
break can be made by conducting SN tests on paired serum samples,
one taken from the pig early in the disease, and the second 3 to
4 weeks later. A significant rise in antibodies between the first
and second bleeding indicates active pseudorabies infection has
been present.
The SN, LAT and ELISA are extremely reliable tests. While
these tests accurately detect antibodies to pseudorabies, they do
not differentiate between antibodies resulting from natural
disease and those resulting from vaccination. Only the differen-
tial tests will permit such a distinction.
Serum submitted for SN examination must be collected in
clean, sterile tubes (not Brucellosis tubes) and submitted packed
in ice. If serum is badly hemolyzed or contaminated with bac-
teria, the SN test is unreliable.
Control of Infection
The chances for introduction of the disease can be minimized
if the owner strictly controls movement of people, animals and
objects into swine premises. Clean clothes and boots should be
decontaminated with a good disinfectant before introduction.
Cats, dogs and other animals should be kept away from pigs. Add
breeding stock from a herd known to be pseudorabies-free, and all
additions should be tested and found free, isolated for at least
30 days, and then retested. Untested feeder pigs should never be
brought onto premises where farrowing operations exist.
When pseudorabies occurs on a farm, quarantine the premises;
all movement of people and animals should be strictly controlled.
If possible, separate healthy pigs from the sick. Control move-
ment between them. Dispose of dead pigs by deep burial or
incineration. Recovered pigs should be sold only for slaughter to
prevent spreading the infection to other farms by carrier swine.
Many herds which are infected may be freed of infection by
using either ``test and removal'' procedures or offspring segre-
gation. Vaccinate exposed or offspring segregated gilts in the
first generation with differentiable vaccines if random sample
testing is used to determine herd status. Monitor until the
second generation turnover. Results using these procedures are
very encouraging except in herds undergoing an acute infection.
In highly concentrated herds, the virus appears to cycle continu-
ously. In these herds vaccination with a differentiable vaccine
may stop spread and permit effective removal of pseudorabies
virus (PRV) from the herd. In less concentrated operations, the
virus appears to cycle intermittently, and many offspring are
pseudorabies-free. In these herds, a testing program with isola-
tion and removal of infected animals appears to be an effective
herd clean-up strategy, and vaccines may also be used to advan-
tage.
State regulations and requirements on PRV control vary among
states. Consult the state veterinarian about individual state
requirements/regulations on PRV testing, control and eradication
procedures. More information on eradication can be found in the
publication "Plans for Elimination of PRV from a Swine Herd"
available from Livestock Conservation Institute, 6414 Copps Ave-
nue, Suite 204, Madison, Wisconsin 53716.
U.S. Eradication Plan
A national eradication program (with a starting date of
January 1) was approved by the National Pork Producers Council in
1989. This decision was made after many discussions, meetings and
debates that had been going on for about 15 years since the
disease first began to cause problems in swine herds. The plan,
endorsed by several other livestock groups, was written by a task
force created by Livestock Conservation Institute (LCI) made up
of representatives of all segments of the pork industry.
A series of pilot eradication projects in five states over a
three-year period were conducted to answer two questions: Are
tools available to eradicate the disease? What would eradication
cost and would it be cost effective? The technical advisory com-
mittee to the pilot projects concluded that pseudorabies area
control is feasible and can be accomplished by methods which are
acceptable to pork producers and to the program coordinators.
The eradication plan developed and supported by the industry
is flexible. It calls for establishment of state committees made
up of producers and other segments of the industry to determine
program activities and advancement from stage to stage in indivi-
dual states. The first stage of the program is preparation, dur-
ing which state committees are formed, prevalence of the disease
is measured and plans are made for future activities, and what-
ever changes in state laws and regulations and/or legislative
authorities needed are determined.
The second stage is the control stage, during which states
will implement surveillance programs to find infected herds;
quarantine such herds and if they choose, begin a voluntary pro-
gram of eliminating the virus from infected herds.
The third stage is the start of mandatory herd clean-up,
during which owners of infected herds will be required to develop
and implement individual plans to eliminate the virus from their
herds.
The fourth stage is for states which have completed the herd
clean-up phase and have no known infected herds but continue to
look for infection. The fifth and final stage is pseudorabies-
free status.
Summary
1. Pseudorabies is a disease of economic significance in
the United States.
2. Severe death losses, abortions and reproductive failure
may occur in pseudorabies-infected swine herds.
3. Cattle, sheep, dogs, cats and wild animals also die
from the disease.
4. Pseudorabies does not cause disease in humans and is
not related to rabies.
5. Recovered animals may be carriers of the virus but
rarely become ill upon re-exposure.
6. Many vaccines, both modified and killed, have been ap-
proved for use in the United States subject to the con-
trol of the State Animal Health Official. Vaccines re-
duce the severity of the disease, but do not totally
prevent natural infection or the establishment of the
carrier state. When PRV occurs on a farm, vaccination
should be encouraged immediately- subject, however, to
individual state laws and regulations.
7. Diagnostic methods are reliable if adequate specimens
are submitted.
8. Spread of the disease can be reduced by quarantine,
proper disposal of dead pigs, and strict application of
hygienic measures.
9. Additions to breeding herds should be PRV negative and
from PRV negative herds. They should be isolated for at
least 30 days and retested prior to release into the
herd.
10. Untested feeder pigs should not be introduced onto
premises where farrowing operations exist.
11. Infected herds often can be freed of infection by test-
ing and animal selection procedures.
12. ``Differentiable'' vaccines may be used effectively in
most herd ``clean-up'' programs.
13. The National PRV Eradication Program began January 1,
1989.
References
The Epidemiology of Pseudorabies. 1990. Livestock Conserva-
tion Institute, 6414 Copps Avenue, Suite 204, Madison, WI 53716
Plans for Elimination of PRV from a Swine Herd. 1990. Lives-
tock Conservation Institute, 6414 Copps Avenue, Suite 204,
Madison, WI 53716.
Pseudorabies Eradication, State-Federal-Industry Program
Standards. USDA, Animal and Plant Health Inspection Service,
Veterinary Services. U.S.G.P.O.:1989-617-013/04354.
REV 6/91 (7M)
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