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Biosecurity, Control and Eradication Strategies for PRRS and
Aujesky’s Disease Steve Henry, DVM, Dipl ABVP, Abilene Animal Hospital, P.A., Abilene, KS
PRRS
and Aujesky’s viral diseases are prevalent and costly problems for
swine producers. Preventing the entry of new infections into herds
depends primarily on the discipline of biosecurity. For herds already
infected the first strategy is to control the infection and limit
losses. The second strategy, following the control phase, is to
eliminate the viral infection if possible. While biosecurity discipline
is similar for the two diseases, it is not the same. Understanding the
diseases and the controls will assist the veterinarians and producers
who design the specific biosecurity plans for each farm. Transmission
of the virus from one premise to another is primarily by way of live
animals or body fluids. Hygiene measures that consider animal movement,
equipment, transportation, and people movement are paramount for either
disease. Repeated experience has clearly shown that hygiene failures
have resulted in the introduction of new infections of PRRS and/or
Aujesky’s. Less clear is the actual level of hygiene necessary to
prevent mechanical transmission.1 The combination of
sanitation, disinfection and down time needed to prevent transmission of
these viruses is a focus of continuing research but scientifically
validated rules are not available. This report is the author’s opinion
based on clinical experience and the available literature. PRRS
and Aujesky’s – some comparisons Understanding
virus survival and propagation is important to long-term management and
elimination of infections. Neither virus survives particularly well
outside the host animal. This feature makes possible the potential to
eliminate them as pathogens from a particular facility, region or even
country. This is in contrast to environmentally hardy viruses such as
parvovirus for example. The
mechanisms by which PRRS and Aujesky’s persist in a population are
quite different. PRRS virus depends on a very long viremia, with
quantities of virus shed over time thus increasing the odds that
susceptible, new hosts will appear and sustain the virus population.
After the viremia, however, sterilizing immunity develops in the
recovered animal and that animal is no longer a potential source for
virus. In addition, recovered animals resist re-infection by the
homologous strain of PRRS. Cross-protection to other PRRS virus variants
is not complete, however, and multiple strain infection is known in many
herds. There remain some questions as to the length of time an animal
will harbor infectious PRRSv and there are certainly differences in the
response of younger and older animals to PRRS infection.2 While
the same term, ‘persistent’, is used when discussing the two viral
infections, there is a substantial difference in the actual mechanism
and the impact. Aujesky’s
infection results in a brief viremia but recovered animals may harbor
virus in tissue cells for an extended period of time. This latent
infection remains dormant until antibody levels fall and the
recrudescent infection develops, resulting in the animal once again
becoming a shedder of virus. Once infected, animals need to be
considered as potentially persistently infected for life, capable of
once again becoming actively viremic and thus a risk to non-immune
animals in the herd. By contrast, a repeated viremia due to homologous
strain PRRSv has not been demonstrated to occur in the same animal. PRRS
virus transmission PRRS
virus is highly infectious, requiring very few viral particles to
initiate infection. During the prolonged viremia, often lasting weeks,
viral particles are present in most body fluids and excretions. PRRS is
not particularly hardy in the environment outside the pig and does not
generally persist for a long period especially at warm temperatures. It
also dies out rapidly in decomposing tissue, thus autolyzed carcasses
pose little risk as a source of virus. Contact with fresh blood, feces,
urine, saliva, semen and nasal secretions containing virus will result
in infection of susceptible pigs. While
most infections of naïve herds occurs by either infected pig
introductions or through contaminated semen, investigations into other
mechanical means of transmission have provided important knowledge.
Hypodermic needles used on multiple pigs easily spread the virus.3
This is a very important consideration given the number of vaccinations
and treatments administered to pigs. Caretakers
and visitors could transmit virus and infection when clothing and boots
were not changed and when the personnel did not wash or shower when
going from infected to susceptible animals. However, every hygiene
protocol tested was effective in preventing transmission when clothing
was changed and people washed or showered. Merely changing coveralls and
boots as well as washing hands was effective in the studies reported. Aerosol
transmission has been very difficult to document with PRRSv
experimentally, and even then could be confirmed only at distances less
than 1 meter. Whether or not aerosol transmission occurs under field
conditions is much debated. It is clear from our experience that farms
in relatively isolated locations (>3km in separation) do not appear
to become infected even though surrounding farms are positive for virus.
Mosquitoes
have been shown, in one experiment, capable of transmitting the virus
after feeding on infected pigs and then being placed on susceptible
animals. Work continues on other possible insect vectors as well. It
is perhaps dangerous to presume that the PRRS virus is always fragile,
once outside the pig, in all circumstances. Mixing virus with snow and
then sampling the snow and melted solutions under a variety of
conditions did show the potential for prolonged survival and
infectivity. Such studies point to the need for clean, dry and
disinfected materials that enter pig facilities whenever possible.4
Aujesky’s
virus transmission Most
of the items noted above for PRRSv relate to Aujesky’s as well,
certainly so in regard to mechanical movement of body fluids and
hygiene. There important differences to be recognized in biosecurity
planning, however. The insect vector is not an issue with Aujesky’s as
it potentially is with PRRSv. Aerosol or area spread of infection is
known to occur with Aujesky’s and infected premises nearer than 3 km
are considered a high risk to susceptible farms and pigs. Unlike PRRSv,
Aujesky’s infects other mammalian species which do not transmit the
disease directly. As infected carcasses do pose some risk, the use of
rendered meat meal is discouraged. Beyond these considerations, however,
the hygienic and animal flow procedures and controls in biosecurity
plans can be considered as one. Control
and elimination of Aujesky’s virus Modified
live marker vaccines have been the critical tool making possible
Aujesky’s elimination from most regions of the US. In addition to
strict movement control procedures, a systematic vaccination strategy
combined with testing has been very effective. Although
there are regional variations, the preferred vaccination protocol
follows this pattern:5 The breeding herd is
vaccinated with G1 deleted marker vaccine every 90 days as a blanket vaccination of all animals in the breeding herd. Because effective
immunity is short-lived, this repetition is critically important. Entering replacement
animals are vaccinated with 2 doses, the first at entry and the second
following 3 weeks later. Pigs are vaccinated at 9
weeks of age and again at 13 weeks of age. Maternal antibody
interference suggests that only 50% of pigs are effectively immunized at
9 weeks of age but that nearly all can be immunized by 13 weeks. An
alternative to this approach, in an effort to avoid maternal antibody
interference, is the administration of the vaccine by the intranasal
route while pigs are still nursing. In this case the second dose is
administered at 13 weeks of age by injection. A
committed and coordinated effort by all producers and veterinarians is
critical if an area or regional eradication of Aujesky’s is to be
successful. Vaccination must be done with care and by all people.
Fortunately, the gene deleted vaccine testing methods can be used to
monitor compliance and to assess progress. Control
and elimination of PRRSv Unlike
Aujesky’s virus, vaccine is not yet a predictable, safe and effective
tool in elimination programs. True marker vaccines are yet to be
developed against PRRSv. Sows
that develop immunity following field infection produce uninfected,
PRRSv negative offspring. Through a system of segregation and herd
closure, it has been possible to generate large populations of negative
animals from previously infected sows.6 This phenomenon has
been employed by seedstock producers most effectively in the past 5
years. Today there is a significant population of PRRS negative (naïve)
breeding animals available to the industry. With these animals as a
source of non-infected, clean animals, repopulation of positive
commercial herds is beginning to occur. In
contrast to Aujesky’s control and eradication which is highly
systematized, PRRS eradication efforts are still being explored in
individual herds and production systems. Given the immense cost due to
PRRS and the very limited benefits of immunization, it appears certain
that eradication strategies must be developed. Conclusion Advancement
of disease control and elimination strategies is critically important
for world pork production. Systematized and global production and
distribution demand ever greater biosecurity, both to protect the health
of animals and the safety of the food. These two viral diseases are
teaching us a great deal about effective biosecurity and the processes
necessary to protect our herds.
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