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Control and Eradication of PRRS
It has now been approximately three years since we were informed of the etiology of "Mystery Swine Disease." Since that time, the name of the syndrome has changed twice and is now known as Porcine Reproductive and Respiratory Syndrome (PRRS). Furthermore, there has been an explosion of new information concerning the characteristics of the virus, its epidemiology, its effect on the immune system of the pig, as well as diagnostic and control methods. While PRRS has indeed been a devastating disease, in my opinion it has been helpful in some aspects because it has forced veterinarians to try and control a disease without the use of a vaccine. Until recently it has not been possible to vaccinate against PRRS virus. Now that the option is available, I feel it is imperative that we do not forget what PRRS has taught us over the last few years. As I attend conferences and listen to producers and practitioners, I am concerned that vaccination is being viewed as a "silver bullet" which can solve all PRRS related disease problems. In reality, it is a very new product which has not been thoroughly studied in the field and is frequently being used in ways which are not yet approved, i.e. vaccination of pregnant sows. Before we go too far with mass immunization programs, I think we need to remember the importance of solving PRRS problems by formulating plans using a combination of accurate diagnostics followed by cost effective control strategies that emphasize management, and maybe vaccinaiton, implemented at the proper time in the life of the pig. The purpose of this paper is to review how PRRS virus is maintained on a farm, how virus transmission within specific populations of pigs may increase the risk of infection or reinfection and how the spread of virus can be monitored using currently available diagnostic tests. It is the authors hope that once these concepts are understood, control measures can be implemented with a higher level of success. 1. Proper isolation of incoming breeding stock is critical for control of PRRS Over the years, perhaps no disease control strategy has been handled poorly as isolation. A proper isolation facility consists of a building located on a separate site. Here incoming stock can be held for a period of time and tested for the presence or absence of antibodies to certain diseases. Another purpose of the facility is to properly acclimatize the new stock to the microflora of the recipient farm. In the past, isolation periods were recommended to be approximately 30 days in length. This was based on published data on incubation periods of well known swine viral diseases such as PRV and TGE. However, due to the prolonged period of viremia following infection with PRRS virus, and the fact that the incubation period of PRRS has still not been defined, I feel it imperative that isolation periods be lengthened to 45 - 60 days. New animals should be tested for both the American and the European strains of PRRS on arrival and prior to introduction to the breeding herd. Our work at the University of Minnesota shows that the primary means of viral entry into a farm is through the addition of infected breeding stock. Not only will this protocol provide better protection against introducing viremic pigs into the breeding herd, it will also allow new stock another month to mature. It is also helpful to house PRRS negative sentinel pigs in the isolation facility. Testing of the sentinels should coincide with that of new stock and can be another aid towards detecting infection. 2. The replacement gilt is critical towards maintaining stability in the breeding herd1 As we know with Parvovirus, exposure of naive gilts prior to breeding is critical to build natural immunity. Such is the case with PRRS virus. Frequently I encounter recurrent reproductive problems in previously infected farms. More often than not, a parity analysis will indicate that gilts are the primary parity affected. Serologic follow up usually reveals high titers with positive isolation of virus from gilts exhibiting signs of reproductive failure, and negative results from new replacement stock. Therefore, the need for proper exposure of naive gilts prior to breeding is essential. This procedure can begin during the isolation/acclimatization period and appears to be an excellent opportunity to use vaccine. On the other hand, I have heard practitioners recommending the purchase of IFA positive gilts with high titers because the high titers equate to protection. This is not true! These animals may be the source of further viral introduction into the population and the predisposing factor for recurrent reproductive problems. If the source of replacement stock is infected, the ideal animal to enter into a PRRS positive herd is not the animal with the high titer, it is the animal which has been previously exposed, is no longer viremic and has demonstrated a reduction in titer. These animals are usually protected and while can become reinfected, do not display episodes of PRRS related diseases. 3. PRRS serology is a valuable tool for assessing the spread of virus on a farm2,3 The indirect flourescent antibody test (IFA) for detection of antibodies to PRRS virus is an accurate, effective test if one understands how to use it properly. It must be remembered that the detection of an antibody by any serologic test indicates that the animal has only been exposed to an antigen. It does not mean that the animal is immune. Serology must also be used on a population basis, over a period of time. A single bleeding can provide a quick assessment of seroprevalence, but in order to properly assess the situation, the profile needs to be repeated. To assess the PRRS status of a farm, I recommend an initial testing of ten sows, ten four week old pigs, and ten five - six month pigs. If more information is required, a larger sample can be drawn. I usually find that bleeding ten animals from each stage is adequate, however at times, I may need to repeat my sampling and collect thirty samples from the stage in question, i.e. the breeding herd. Titers are also important to assess whether viral shedding may be occurring in the population tested. Animals with high IFA titers (1:256 - 1:1024) have recently been exposed to virus and may be viremic. The development of new serologic tests for detection of IgM antibodies to PRRS should help to confirm the presence or absence of viremia as well as detect acute infection. Unfortunately, titers from vaccination mimic those of naturally infected pigs and can make interpretation of serology confusing. 4. Viral shedding can be controlled in adult animals by closure of the herd4 Closure of the breeding herd to outside replacement stock has been shown to be an effective method to prevent spread of the virus among adult swine. Viremic periods are much shorter in these animals compared to nursery age pigs. While culling procedures and breeding herd inventory may be briefly interrupted, the speed at which a consistent level of exposure and subsequent natural immunity can be obtained is very beneficial. Temporarily, replacements can be selected from the finishing facility. To monitor shedding, specific sows can be tested every 30 days for a period of three - four months. Animals which are no longer viremic usually demonstrate a decline in IFA titers over time. Once viral shedding is controlled in the breeding herd, pig flow control measures can be implemented. This strategy may be limited in herds with large breeding herd inventories (> 1,000 sows). Based on preliminary serologic evidence, subpopulations of potentially naive animals may exist in previously infected populations. Such animals may be the source for future infections and viral shedding. It appears that insuring consistent natural exposure to virus is difficult. Therefore, the development of safe and efficacious vaccinations approved in the breeding herd appears to be crucial to insuring consistent exposure and the development of a stabilized population of immature animals. 5. Pig flow strategies can be useful to control PRRS5 We have found that the interruption of viral spread between groups of pigs can effectively take place following a calculated change in pig flow. We have demonstrated the value of Nursery Depopulation in over 30 farms in the US. This technology is now being used successfully in Europe and Korea. As mentioned earlier, it takes some planning, but results have been good. Once again, serologic profiling is very helpful to determine whether it is the proper time for implementing such control measures. The profile demonstrated in the first part of table 1 describes circulation of PRRS virus during the nursery stage. Notice that sows and weaned pigs are IFA negative or have low titers. This is indicative of the absence of recent exposure to virus in these areas. This is in contrast to 8 - 10 week old nursery pigs, all of which have been exposed. The second profile depicts recent exposure throughout all stages of the farm and it is likely that a high level of viral shedding is taking place. Depopulation of this nursery will more than likely fail, because weaned piglets may carry the virus into the nursery. If reinfection occurs, the depopulation procedure may need to be repeated but there appears to be little reduction in performance. Fortunately, it is inexpensive and does not require excessive labor or multi-site production. The Future6,7 The information reviewed in this paper is well known to all of us, it just took a new disease to reestablish its importance. So, with what we know, how can we use it? Obviously, there are a lot of PRRS problems waiting to be solved. But what about the possibility of a new disease? Surely something new will happen over the next 5 - 10 years to keep our jobs interesting! I have heard a well respected pathologist from England describing a syndrome he is encountering in his country. This problem involves a new strain of influenza virus, unlike any we have encountered in the US. A similar situation exists in Canada with proliferative and necrotizing pneumonia. We have also debated over the significance of Porcine Respiratory Coronavirus. What about coinfection with multiple viruses? We all know that if placed under enough pressure from the immune system, viruses will undergo antigenic drift or shift. This results in viruses, with antigenic variations foreign to previously well-adapted immune systems. Therefore, we must be aware of the potential for new diseases to affect pigs at all times. Let's examine a hypothetical situation involving the occurrence of irregular levels of mortality (>5 percent) in post weaning pigs. Respiratory signs are evident. Anorexia and fever (105 - 106° F) are present in the breeding herd. What do we do? Well, let's use what PRRS has taught us! 1. Conduct a proper diagnostic workup, including fixed and fresh tissue and a serological profile as previously described. Test for both strains of PRRS, as well as PRV, TGE, PRCV and different strains of influenza. Identify certain animals with an ear tag for resampling in the future. 2. Close the breeding herd to build a stable immune population. 3. Prevent introduction of new replacement stock from the offsite isolation facility until all testing is complete. Test the new stock for exposure to the previously described pathogens. If a diagnosis is obtained from the samples collected in step #1 and incoming stock are negative, proper acclimatization steps need to be taken. If specific vaccines are available, they may be indicated here. If new stock are highly positive, it may be important to extend the isolation period along with serological monitoring for changes in titers to prevent introduction of viremic animals. 4. Use serologic profiling to attempt to understand the pattern of virus movement on the affected farm. Is it spreading or is it localized to a specific stage of the farm? Can calculated changes in pig flow assist in interrupting the spread of the virus? In conclusion, there are many proven strategies for control of PRRS which may be applicable to other viral infections as well. It is important for the swine practitioner to implement such strategies in combination with a properly timed vaccination program. When used together, these strategies provide effective disease control with minimal investment. Suggested Reading 1. Dee SA, Joo HS: Clinical investigation of recurrent reproductive failure associated with PRRS virus infection in a swine herd. JAVMA, October, 1994. 2. Dee SA, Joo HS: Prevention of the spread of PRRS virus in edemically infected swine farms using Nursery Depopulation. Vet Rec, 1994: 6 - 9. 3. Dee SA, Joo HS, Pijoan C: Control of PRRS virus transmission: Handling infected seedstock. Comp Cont Ed, 1994: 927 - 933. 4. Dee SA, Joo HS, Pijoan C: Controlling the spread of PRRS virus in the breeding herd through management of the gilt pool. Swine Health & Production. March, 1995. 5. Dee SA, Joo HS: PRRS Eradication: The Science of Nursery Depopulation. Proc. Allen D Leman Swine Conference for Veterinarians. St. Paul, MN, September, 1994: 219 - 224. 6. Done SH, Brown IH: Pathogenesis of Swine Influenza. Proc. Allen D Leman Swine Conference for Veterinarians. St. Paul, MN, September, 1994: 154 - 158. 7. Carlton J: PRRS and Pig pneumonia. Swine Practitioner, January 1993: 4 - 7. Table 1: Two Example of Serologic Profiles for PRRS Antibodies Pattern 1* Stage
# Tested % Positive
Titer Range Pattern 2** Sows
10 50-100%
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