Introduction

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) in swine has caused significant economic losses to the swine industry worldwide¹. Losses range from 75,000 euros, in a 1,000-sow farm with a "light" infection, to 698,000 euros² and $664 million USD per year in the United States³. The disease is caused by an arterovirus that emerged in the late 1980s in the USA⁴ and later in Europe, spread rapidly and became enzootic in the pig population worldwide⁵. The disease exhibits a wide variety of signs that reflect the virulence of the strain and are related to the physiological stage of the animals, their immune status and the presence of other diseases^5,6. The first phase lasts approximately 2 wk and is characterized by acute viremia causing anorexia and lethargy, as well as pyrexia, tachypnea and dyspnea and cutaneous hyperemia with cyanotic extremities. The second phase, which may begin before the first phase is completed and can last up to 4 mo, is characterized by reproductive failure, mainly in sows that were infected during their third of gestation⁵, and also it generates a respiratory condition in growing pigs⁷.

The disease is caused by a RNA virus of which there are two varieties: classical strains (C-PRRSV) and highly virulent strains (HP-PRRSV)⁸. It is also classified according to its genetic variations and antigenic differences, into two types: PRRSV-1, the European type and PRRSV-2, the North American type⁹. Animal health can be complicated when the virus is associated with other pathogens such as type II porcine circovirus (PCVII), Pasteurella multocida, Haemophilus parasuis, Bordetella bronchiseptic, and Mycoplasmas^10,11,12.

After the outbreak, farm production tends to improve gradually (4 to 6 mo), not reaching preoutbreak production levels. On the other hand, when PRRSV remains circulating, the farm is exposed to disease outbreaks and to the persistence of the virus in the herd¹².

The results of interventions to reduce the cost of the disease, mainly in the fattening stages, have been positive, but they allow and cause greater losses in breeding. In 2005, losses in the U.S. breeding herd were 12 % of the total cost of PRRS, while in 2011, the cost in the breeding herd amounted to 45 %³. During this period, different intervention protocols have been implemented, including vaccination, depopulation, and biosecurity protocols¹³, among others.

The negative impact of PRRS on the economic margin per pig produced has stimulated efforts to control and eventually eradicate the disease. PRRS virus control relies on aspects such as early diagnosis and monitoring, biosecurity, herd management and immunization¹⁴. However, these standard control methods have not been effective as vaccines do not reduce disease prevalence and many producers have to depopulate after an outbreak¹⁵.

PRRS is a host/virus model in which disease is determined by virus pathogenicity, breeding herd susceptibility and phenotype, bacterial co-infection pressure, and environmental conditions¹⁶.

Therefore, the objective of this study was to evaluate the productive performance and production costs per weaned piglet and finished pig in swine farms with two PRRS virus vaccination protocols to determine their effectiveness and which one offers the producer better productive and economic indicators.

Material and methods

The study was carried out in two farms, which, according to the general technological classification of SAGARPA, correspond to semi-technified farms. The farms are located in the Central Highlands of Mexico. One is in the state of Hidalgo and has a dry temperate climate, an average annual temperature of 14 ºC, and an annual rainfall of about 610 mm. The other is in the State of Mexico with a semi-dry temperate climate, an average annual temperature of 16 to 17 ºC, and an annual rainfall of about 600 mm. The number of breeding sows was 480 and 180 respectively. The data analyzed were taken from individual sows’ records. The period was from their first birth to the last recorded birth, with cutoff as of September 2017, in a lapse of their productive life, the latest data recorded being the births of the first half of 2017.

The technical indicators analyzed were total piglets born, piglets born alive, piglets born dead, mummies, piglets weaned, piglets’ weight at birth, piglets’ weight at weaning, age of the pigs at slaughter, weight of the pigs at slaughter, weight of the pigs at slaughter. The farms have been PRRS-positive since 2003 and a vaccination schedule is being implemented on both farms. Vaccination protocol 1 (P1) considers the farm that vaccinates breeding sows and piglets at approximately 21 days of age. Vaccination protocol 2 (P2) considers the farm that vaccinates only the breeding sows. Mass/blanket vaccination of breeding sows in both farms occurs every four months. Both protocols use a modified live PRRS vaccine with a dose of 2 ml per animal.

Both farms have similar biosecurity, genetics and feeding schemes.

The cost analyses were performed using the modification to the general cost formula of Muñoz and Rouco¹⁷.

TC=F+V, where TC= cost per weaned piglet, F= fixed costs and V=variable costs.

Fixed costs were formed by F=L=L+S+Co+R+A+A+Fi+OC+Oth<, where: L=labor costs, S=supply costs, Co=energy and fuel costs, R= repair and maintenance, A=depreciation of fixed assets, OC=opportunity costs, and Oth=other minor costs.

Variable costs consisted of the following items: V=((ABS+FB+FP+M+T+OC/(TOTS*W))*z; where: ABS= amortization costs of breeding stock; FS= feeding of the sows; FB= feeding the boar; AB= amortization of the boar; FP= feeding of piglets; M= medications; T= transportation; OC= opportunity costs; TOTS= total number of sows on the farm; W= Weighting factor, since all variable costs will refer to the production unit of a commercial piglet, and z=number of weaned piglets.

Depreciation of breeding animals was calculated as follows:

ABS=(PPS-(SCP-(1-MORBS)))/(ANFS/FSY)-BR; where: PPS= purchase price of the sow, SCP= sow cull price, MORBS= mortality of breeders expressed as a percentage, ANFS= average number of farrowing sow, FSY= number of farrowing per sow per year, and BR= breeder replacement.

The average number of births per breeding herd can be calculated at any time during production, regardless of the physiological stage of the sows.

ANSF = ∑(NS * n)/TOTS; where: NS= the number of sows and n= farrowing number.

FSY=365/(114,5+LAC+INT)*(1-NM+NES/SER)), LAC= duration of lactation, INT= weaning-first fertile service interval, NM= total number of miscarriages, NES= number of empty sows, SER= services performed. In turn, INT is formed by the sum of the weaning-first service intervals (INT1), p. 100 first repetitions*21 (INT2), p. 100 second repetitions*42 (INT3), p. 100 third repetitions*63 (INT4) and p. 100 acyclic repetitions average days of onset.

REP=FSY/ANFS and the weighting factor is w = FSY * PBA * (1 - MOR)*(1-MORT); where: FSY= number of farrowing per sow per year, PBA= piglets born alive per farrowing, MOR= mortality in lactation, MORT= mortality in transition from weaning to commercial piglet expressed in percentage points.

For the calculation of fattened pig costs, the formulas are as follows:

F= F=L+S+Co+R+A+Fi+CO+Oth, applicable for the fattening process and V=((M+FP+OC)/w)*z, applicable to the fattening process, where z is the number of piglets fattened.

In order to determine differences in sow productivity by farrowing (parity) and by type of vaccination protocol, a time-repeated design was used. The best covariance structure was determined and an adjusted Tukey’s test was used to determine significance¹⁸. As for the fattening production data, the variables days to sale and slaughter weight were determined through an analysis of variance.

The differences in income were determined using as a measure 1 weaned piglet and 1 kilogram of fattened pig. The price for the calculation was $28.00 MXN. In the case of weaned piglets, an average value of $800.00 MXN was used.

Results

The two farms analyzed showed similar levels of technology, animal genetics, feeding, production and sanitary management, with the exception of the PRRS vaccination protocols. Table 1 summarizes the productive performance of breeding sows and their piglets between vaccination protocols. It is noteworthy that although Protocol 2 (P2) resulted in a higher number of piglets born (P<0.05), the number of piglets weaned was higher (P<0.05) with Protocol 1 (P1).

In this sense, the number of total piglets born per parity was only different in sows in their first farrowing (P<0.05) with values of 10.14 0.11 and 11.28 0.16 for protocols 1 and 2, respectively.

With respect to parities (Tables 2 and 3), differences were observed in the number of mummies in parity one (0.36 ( 0.03 and 0.66 ( 0.05) and parity five (0.36 ( 0.03 and 0.66 ( 0.05); the number of piglets weaned in parity two (9.19 ( 0.11 and 8.01 ( 0.14) and parity three (9.30 ( 0.13 and 7.89 ( 0.17) in favor of P1. In relation to litter weights at birth, the performance was better for P1 at parities one through five. Litter weight at weaning, however, differed in favor of P1 only, in parities of 1 to 3.

The number of total piglets born on the farm with P1, recorded an increase in parities 2, 3, 4 and 5 with respect to farrowing 1 (P<0.05). The values were 10.14 ( 0.11, 10.84 ( 0.16, 11.38 ( 0.15, 11.56 ( 0.17 and 11.23 ( 0.21, respectively. In the farm where P2 was applied, there was no difference in the indicator. The number of mummies in P1 exhibited no differences, while the number of mummies in P2 differed (P<0.05) in farrowings 1, 6 and 7. The values were 0.66 ( 0.05 (birth one), 0.43 ( 0.05, 0.36 ( 0.09 and 0.44 ( 0.11. The piglet weaning performance with P1 was different in farrowings 2 and 3 with respect to farrowing 1, while no differences in this indicator were recorded for pigs in P2. Birth weights with P1 were lower in farrowings 2, 3, 4 and 5 (P<0.05) and exhibited no differences with P2. Finally, litter weaning weights behaved in a regular manner, with differences in piglets weaned under P1 in farrowing 2 and 3, in relation to farrowing 1; in P2, the difference was observed in parity five.

In the case of fattening, the days to sale recorded were 181.08 5.01 and 168.81 4.81 for P1 and P2, respectively (P<0.05); the final weight was 95.46 ( 3.27 and 92.28 ( 3.93 (P<0.05), respectively.

Production costs per weaned piglet in P1 were $389.55 pesos, and $424.25 in P2. Of the total costs, 93.62 % were variable costs in P1 while, in P2, variable costs were 96.27 %.

The fattening cost was $1,812.81 in P1 and $1,930.07 in P2. With these production indicators, the average cost per day of fattening was $10.01/day in P1, and $11.43/day in P2.

The income per weaned piglet was $410.45 M/N for P1, and $375.75 in P2. Likewise, for a finished pig, the income was $860.07 and 653.77 M/N, respectively.

Discussion

When breaking down the data by parameter, by farm and by parity and comparing it to the PIC¹⁹ production cluster for Mexico, 2.75 and 2.2 fewer total piglets were born in farms with P1 and P2, respectively. As for the number of piglets born alive, the difference was -2.18 and 2.23, and for piglets weaned, -1.95 and -2.58, respectively. These PIC data correspond to the top 10% in terms of production. However, the farms with P1 and P2 are the values are lower by 1.21 and 1.26, respectively, compared to the 10% with the worst productive performance²⁰. Similarly, weaning weights were lower in the analyzed farms: -470 and -360 g.

In general, the differences in these results suggest that PRRS affecting breeders can cross the placenta approximately at day 70 of gestation, causing premature deliveries, on the one hand, and a higher number of stillborn piglets and mummies, on the other. Likewise, it is possible to register an increase in pre-weaning mortality, meaning a lower number of weaned piglets^14,21. Although the literature mentions that an affected farm returns to "relative normality" within six months¹⁴, there are some farms where this disease is chronically found²¹. The PRRS virus creates synergy with other viruses or bacteria that may be the cause of increased mortality during lactation²². In an open cycle, the PRRS virus in unstable farms can be detected in all groups of pigs, including piglets¹⁶, which implies that adverse outcomes caused by the virus can occur, even including vaccination protocols.

In studies involving PRRS virus vaccination of multiparous sows and gilts, it has been shown that passive immune protection is conferred on piglets up to 84 days of age, regardless of whether or not the piglets are vaccinated before this period²³. Although studies on protection at older ages are scarce, these data suggest susceptibility in fattening animals, as long as they are not immunized.

In a study carried out in stable farms with modified live PRRS virus vaccination, it was found that there are some losses and negative changes in the sows' productive indicators²⁴. There was a slight increase in the preweaning mortality rate, and there were no significant changes in the rate of miscarriages, neonatal losses, pigs weaned per litter and wean-to-first-service interval.

Mass vaccination of the entire breeding herd has been reported as a favorable strategy for protection against the PRRS virus, increasing by 1 weaned piglet per sow per year²⁵. However, other studies suggest that the change is minimal²⁴ with no significant differences²⁶ and even adverse indicators have been reported²⁷.

Regarding fattening performance, taking as a reference the average PIC results for Mexico, the difference in age was 18 days more for P1 and 6 days more for P2 and -23.63 kg for P1 and -26.81 kg for P2.

PRRS affects fattening pigs by producing respiratory type diseases with pulmonary lesions, allowing other viral or bacterial diseases such as Influenza, Streptococcus suis, Mycoplasma hyopneumoniae, Salmonella cholerasuis, Haemophilus parasuis, Pasteurella multocida, Porcine circovirus, Porcine coronavirus, and Actinobacillus pleuroneumoniae^14,22 to be associated with it. These diseases reduce pig growth, decrease daily gain, and increase days to slaughter.

It is important to note that the farms used in this study are PRRS-stable farms according to the American Association of Swine Veterinarians, and these results are not applicable to PRRS-free farms or PRRS-positive unstable farms.

Despite differences between scientific reports, after 20 years, vaccination against the PRRS virus with modified live virus vaccines continues to provide protection, and the results have been confirmed in 35 million pigs that have been vaccinated²⁸.

Conclusions and implications

As mentioned, these data provide information to feed economic models that will assist swine production specialists and producers in making field-evidenced decisions regarding the use of PRRS vaccination as a preventive strategy. Although no significance was observed on farms vaccinating breeders, the cost per weaned piglet was lower with P1, at $389.55, than with P2 ($424.25). For the fattening or finishing stage, vaccinating both the breeding sows and the piglets (P1) had the best productive and economic results.