Introduction
Most people involved in the swine industry have heard of the effect that PRRS (Porcine Reproductive and Respiratory Syndrome) can have on productivity and overall performances of sow herds. But what about its impact on growing-finishing performances? In the following lines we will briefly summarize what is known.
Cause
It is now well accepted that all PRRS virus strains do not have the same virulence. When infected experimentally, pigs may have no clinical signs at all, or severe respiratory symptoms that may last for 14 days or more, depending on the strain used. Furthermore, possibly because of its deleterious effects on alveolar macrophages, the problems associated with this virus are very frequently complicated by other bacterial and viral pathogens. It is thus difficult, sometimes, to attribute a relative role or importance to the different microorganisms involved.
Clinical Signs
Growing-finishing pigs may have high antibody titers against PRRS virus, indicating a recent contact with the organism, without having shown any clinical signs or deterioration in performances. This is, in fact, the most common situation.
In other cases however, fever, dyspnea (thumping) and a reduction in appetite can be observed on a significant proportion of the herd. In severe outbreaks, mortality and the number of poor doing pigs will be increased. Transient cyanosis (blue coloration) of the ears, lasting a few hours to a few days, is infrequently seen, but when present it constitutes a particularly suggestive clinical sign. Other diseases can produce cyanosis, but unless it is efficaciously treated this cyanosis tends to last and is usually not transient.
Polypnea (rapid breathing) is sometimes observed, particularly in younger pigs. A frequent comment made by producers is that if one jumps in a pen to inject a few pigs, the relatively small exercise imposed seem to induce dyspnea in a lot, if not most, of the other pigs. Finally, the clinical signs and overall severity will vary according to the secondary pathogens that are associated, or not, with the syndrome. Among the organisms that can complicate PRRS problems in growing-finishing units, let's mention Streptococcus suis, Haemophilus parasuis, Actinobacillus pleuropneumoniae (porcine pleuropneumonia), Salmonella choleraesuis (septicemic salmonellosis), PRCV (Porcine Respiratory Corona Virus), classical and atypical strains of SIV (Swine Influenza Virus), and Mycoplasma hyopneumoniae.
Economic Significance
The impact of PRRS on growing and finishing performances has been studied in different countries. In U.K. a group of 48 finishing pigs being individually evaluated on a feed trial got infected with PRRS virus. The infection was associated with recurrent, moderate, severe and acute periods of inappetence, with a consequential loss of performance despite prophylactic medication in the feed (Table 1).
| Inappetence score* | ||||
| Number of pigs | ||||
| Daily gain (gm) | ||||
| Feed efficiency | ||||
| * Inappetence when feed intake was reduced by more than 35% for at least 2 days. The more inappetent were the pigs, the higher is the score. | ||||
During the last 2 weeks of the study, the pigs also got infected with a new Influenza virus strain (195852/92) that went through a lot of herds in this country. Table 3 shows the dramatic effect observed on performances of affected pigs.
| Inappetence score* | ||||
| Number of pigs | ||||
| Daily gain (gm) | ||||
| Feed efficiency |
The sequential infection by these 2 viruses resulted in a severe loss, estimated at approximately $10 per pig.
In Holland, 3 companies reported the impact of PRRS on their finishing performances. In one case there was no change in the mortality, growth rate or feed efficiency of the pigs. In the second case mortality went from 1.9 to 3.5% and in the third from 1.8 to 2.6%. These increases were said to have lasted a long time.
In Germany, a study was done on 12 herds comparing the performance of finishing pigs before the supplying breeding herds had PRRS, and after. The overall mortality rate went from 2.2% before to 4.3% for the first 3 months after the PRRS outbreak, and had declined to 2.8% 5 months later. Table 3 shows how the pre-PRRS mortality rate in these herds was related to the post-PRRS death loss.
It was concluded that the more hygienic and well managed were the herds, the less severe were the consequences of PRRS infections.
Another source in Germany looked at the results of 700 fattening units over the years 1988 to 1993. Since the first outbreaks occurred in this country in 1990, mortality rates increased from 2.8 to 3.8% and average veterinary costs almost doubled (3.75 to 7.00 D.M.).
In Quebec, Canada a devastating disease called Proliferative and Necrotizing Pneumonia (PNP) was first identified in 1987. It was already, in 1990, the most frequently diagnosed swine disease in this province. While initial research demonstrated that PNP lesions could be reproduced with different (atypical) strains of Swine Influenza Virus (SIV), further work showed that in field cases, PRRS virus could be isolated or identified very frequently from the lungs of affected pigs. In fact, using an immuno-histochemical technique a group of researchers identified PRRS virus antigen in 74% (28/38) of lungs affected with PNP and SIV in only 3% (1/38). The PRRS virus could also be isolated from 12 of 13 lungs submitted by the author with PNP lesions, while in no case was any Influenza virus identified. Many veterinary practitioners now believe PRRS virus acts, to say the least, as an important triggering factor in PNP outbreaks. In these cases mortality rates usually vary between 3% and 8%, but went up to 50% in extreme situations. Losses, particularly when commingling pigs from many different sources, have been estimated to go as high as $12 per pig.
What the literature is telling us is that PRRS has the potential to be a costly disease in growing-finishing units and that this is especially true when virulent secondary pathogens are also present. At this time, it does not seem to be a very significant problem in North Carolina. It is presumed that the all-in/all-out system used almost everywhere, the limited number of sources (generally 1 or 2) and the relatively high health status of many of the farms does not favor the expression of this virus' pathogenic potential.
Prevention and Treatment
The negative impact of PRRS is likely to be worst if:
If pigs from different sources have to be mixed and if it is possible to know the serological status of the nursery from where they came, then pigs coming from positive nurseries should be kept together and not mixed with those coming from negative nurseries. Such a strategy, used in Quebec, has significantly reduced losses associated with the disease.
The value of the commercially available vaccine for growing-finishing pigs has not been thoroughly evaluated. Results from Quebec suggest that a better response is obtained when piglets are vaccinated a few weeks before introduction in the finishing units, rather than at entry.
As for treatment, since PRRS is caused by a virus no treatment will effectively and constantly control the problem. Large spectrum antimicrobials are usually used to reduce losses associated with the presence of secondary pathogens. The choice of these antimicrobials should be based on sensitivity results from the laboratory and, if available, on previous experience with each particular herd.
Take-Home Message
PRRS has been a significant problem in growing-finishing units of other countries but not, so far, in North Carolina. The economical losses appear to be related mainly with the number and virulence of the secondary pathogens present, as well as with the management level of the herd. Differences in the pathogenicity of the virus strains could also be involved.
