WE JUST MIGHT BE ABLE TO ERADICATE
PSEUDORABIES IN NORTH CAROLINA AFTER ALL
Monte B. McCaw DVM PhD
Department of Food Animal & Equine Medicine
College of Veterinary Medicine
North Carolina State University
Raleigh, NC 27606
Three years ago most people involved with pseudorabies virus (PRV) infection of pig herds felt hard pressed to control the spread of PRV from herd to herd let alone eliminating it from "large herds" ~ 500 sows) which was 80% of the North Carolina problem. The number of new infections was still rising rapidly in Sampson, Duplin, and Green counties and only a few herds were eliminating the disease, usually by depopulation. In 1990 we found 63 % of offsite finishers were sero-positive to PRV ELISA indicating that they were infected with the virus. At that time vaccination in the nursery of many of the pigs going to offsite finishers was begun. New vaccination schedules for incoming gilts were applied and better segregation of PRY "positive" vs "negative" pig flow were adopted.
Current
Status
Today with the aid of genetically engineered PRY vaccines and basic management practices PRY seems to be under control and on the way out. Only 26 % of the offsite finishers tested in 1991 (N= 54 sites) were positive (only 16.7% had more than 1/60 animals tested positive). While the total number of quarantined herds in the state has decreased only slightly in the past 2 years, this figure hides the true progress that most people thought was not possible. In the fall of 1991 we began testing in infected sow farms to determine whether the vaccination / management programs were helping to slow the spread of PRY from old sows to gilt additions. When differential PRY ELISA (can differentiate vaccinated versus infected pig antibody responses) results were collected and analyzed by parity it was found that in the 8 herds tested 99.6% of parity 1 and 2 animals (N =240) were negative and 87.5% of parity 3 and 4 animals (N= 120) tested were negative. Even 65% of 5th, 6th, and 7th parity animals (N = 120) were found to be negative indicating that virus circulation had been stopped for up to 3 years in these herds. If no major changes occur in this PRY control success such as PRRS infection or a severe winter it is likely that MOST large herds in Sampson, Duplin, and Green counties will be essentially free of PRY within the year by normal culling alone. However producers in areas with many PRY quarantined herds should continue to vaccinate all sows and incoming gilts for at least two years after the AREA has eliminated PRY. Currently PRY is a problem in areas where there is not as many infected herds but most producers with infected herds have chosen to not vaccinate their finishing pigs as well as the sow herd. Therefore PRY infection continues to actively spread in these herds and serve as a source of infection to unvaccinated herds in the region.
The biggest challenge to the PRV elimination program today is the ability to rapidly diagnose and contain new outbreaks of disease by immediate vaccination of the herd and proper management of replacement breeding stock / finishing pigs to stop virus circulation as quickly as possible. The following are suggested procedures to follow if you suspect your herd has become infected with PRV (sows off feed, large numbers of piglets dying, unresponsive pneumonia outbreaks).
To minimize losses to the producer and risk of transmission to neighbors quick confirmation of PRY infection and immediate vaccination of the herd is imperative. Test serum from sows which are off-feed or have aborted by latex agglutination or, if available, fluorescent antibody testing of brain from piglets with clinical signs gives same- day results. Confirmatory virus isolation takes 2 to 4 days and pigs may not sero-convert to PRY screen ELISA for 6 days after infection. Monitor or cease all pig movements into or off the site until the diagnosis has been confirmed. Once the diagnosis is confirmed all neighbors should be notified that PRY is in the area and informed of vaccination options that may be prudent preventive management for protection of their investments and reduction of the opportunity for further area spread.
For all new outbreaks of PRV herds should
be vaccinated with gene-altered or naturally attenuated vaccines that use the
gp I differential ELISA. This recommendation should be followed until problems
with gpX ELISAs are corrected. Immediate vaccination may help reduce the amount
of virus shed by the herd during the outbreak as well as clinical losses. All sows,
gilts, and boars should be vaccinated IM immediately. Sows due to farrow within
3 weeks could be vaccinated IN as well. Revaccinating all breeding stock again
3 -5 weeks after the outbreak should be considered. All piglets in the
farrowing house should be vaccinated IN and for the next 2 -4 weeks vaccinate
all piglets IN at birth (only with SELECI vaccines). Nursery pig vaccination
should be considered to minimize mortality and the amount of virus shed during
the outbreak. If signs of pneumonia and anorexia were not widespread in
grow-finish pigs (indicating most of them were naturally infected and therefore
immune) they should be vaccinated IM to minimize weight loss, secondary
pneumonia, and virus shedding.
Adjacent
herds should consider vaccination with gpI deleted vaccines and moving to
a sentinel herd status. Vaccination of nursery and finishing pigs (one time) should also be considered to limit the number of susceptible pigs in the area.
Post-Outbreak Pig Flow
Pigs from infected farms moving offsite should be vaccinated out of the nursery and placed in high prevalence areas if possible until virus circulation on the sow farm has been controlled and groups of finishing pigs have been produced that show no evidence of infection. If proper vaccination and pig flow management is being used in the infected herd nursery pig vaccination may only be needed for 16 weeks post-outbreak. Once virus circulation on the sow farm has been controlled pigs from these nurseries should pose little risk of spreading PRY.
Post-Outbreak
Vaccination to Control Virus Circulation
All sows should be revaccinated IM 3 -5
weeks prior to farrowing in herds which have accurate open sow culling
programs. If open sows are not consistently culled after the 2nd return to
estrus whole herd revaccination should be used 3 to 4 times a year. All gilts and boars must be purchased from
PRY free sources. Gilts and boars should be isolated and retested prior to
entry into the herd. Gilts and boars should be vaccinated immediately upon arrival
and again 3 -4 weeks later. If possible gilts should receive their first
vaccination prior to addition to the herd. All gilts should be revaccinated a
3rd time 3 -5 weeks pre-farrowing. If it is determined that PRV infected gilts
have been added to the vaccinated sow herd all suspect gilts should be tested
and positive animals remove immediately. Frequently these animals have not shed
virus to the rest of the sow herd if the above vaccination protocol is used.
The sow and gilt vaccination program should be continued after the herd tests
PRV free using differential ELISA until all area herds are tested PRV free.
Three months after the outbreak bleed some of the oldest pigs in the nursery to determine their PRY SN passive antibody titer. If their titers are £ 1:4 pigs can be vaccinated IM as they leave the nursery .If many pigs have titers > 1:8 as they leave the nursery the IM vaccine may be neutralized. These pigs should be vaccinated 2 to 4 weeks after placement into the grow -finish building.
Pigs should continue to be vaccinated in the nursery if the herd is a farrow to finish operation or if the pigs are being sent to a quarantined finishing floor where multiple age groups of pigs are being held in the same building or the same site. In farrow to finish herds nursery pigs should continue to be vaccinated until pigs from all grow-finish buildings test negative on a statistical bleeding. When virus circulation has stopped in grow-finish pigs consider eliminating nursery pig vaccination if it is determined that sow herd virus circulation has stopped. On farrow to feeder pig operations nursery pig vaccination can be stopped 3 months after the outbreak if pigs are being sent to finishers that are being operated all-in all-out by site where there is no evidence of virus circulation and there are no negative herds near the site. If pigs from the infected herd are being sent to finishing sites or buildings that are not run all-in all-out vaccination in the nursery should continue until virus circulation has been stopped on those sites. If virus circulation has not been stopped in grow -finish pigs reevaluate the vaccination program and husbandry practices.
If these practices are strictly followed then newly infected herds should be able to eliminate the infection within 3-4 years without disruption to normal pig production and minimal cost. Barring any unforeseen changes in the effectiveness of the current program PRY infection should be largely eliminated from North Carolina in the next 2-3 years with the steady support of the swine industry.
