Abstract
The aim of this study was to describe early infections with porcine circovirus type 2 (PCV2) in naturally infected piglets and the piglets’ serologic profiles. A total of 20 sows (15 PCV2-vaccinated and 5 unvaccinated) and 100 newborn piglets were studied. Colostrum and serum of the sows and serum of the presuckling piglets were obtained on the day of parturition. Milk samples were collected on day 20 postpartum. Blood samples were taken and the piglets weighed on days 1, 20, 42, 63, and 84 postpartum. Colostrum and milk were evaluated for infectious PCV2 and for PCV2 total antibody (TA), neutralizing antibody (NA), and IgA. Serum samples were evaluated for PCV2 TA, NA, IgA, IgM, and DNA. The sows had high levels of TA and NA in serum and colostrum; however, 11 and 5, respectively, of the 20 colostrum and milk samples contained infectious PCV2. In the serum, PCV2 DNA and IgM were detected in 17 and 5, respectively, of the 20 sows. Nine piglets were born with PCV2 antibodies, which indicates in utero transmission of PCV2 after the period of immunocompetence (> 70 d of gestation). On day 1 postpartum, PCV2 DNA was detected in 29 of the 100 serum samples from the piglets. There was no difference between the weights of viremic and nonviremic piglets throughout the study. In conclusion, even on farms with sows that have high PCV2 antibody titers, vertical transmission of PCV2 may occur, resulting in piglet infection.
Résumé
La présente étude visait à décrire les débuts d’infection par le circovirus porcin de type 2 (PCV2) chez des porcelets infectés naturellement ainsi que les profils sérologiques de ces porcelets. Au total, 20 truies (15 vaccinées avec PCV-2 et 5 non-vaccinées) et 100 porcelets nouveau-nés ont été étudiés. Du colostrum et du sérum chez les truies, et du sérum chez les porcelets ont été prélevés le jour de la mise-bas. Des échantillons de lait furent également prélevés au jour 20 post-partum. Des échantillons sanguins ont été prélevés et les porcelets pesés aux jours 1, 20, 42, 63 et 84 post-partum. Le colostrum et le lait ont été évalués pour la présence de PCV2 infectieux, ainsi que pour les anticorps anti-PCV2 totaux (TA), les anticorps neutralisants (NA), et les IgA. Les échantillons de sérum ont été évalués pour PCV2 TA, NA, IgA, IgM et l’ADN. Les truies avaient des niveaux élevés de TA et NA dans le sérum et le colostrum; toutefois, respectivement 11 et 5 des 20 échantillons de colostrum et de lait contenaient du PCV2 infectieux. Dans le sérum, l’ADN de PCV2 et des IgM ont été détectés chez, respectivement, 17 et 5 des 20 truies. Neuf porcelets sont nés avec des anticorps contre PCV2, ce qui indique une transmission in utero de PCV2 après la période d’immunocompétence (> 70 j de gestation). Au jour 1 post-partum, l’ADN de PCV2 a été détecté dans 29 des 100 échantillons de sérum provenant des porcelets. Tout au long de l’étude aucune différence n’a été notée dans les poids des porcelets virémiques et non-virémiques. En conclusion, la transmission verticale de PCV2 peut survenir même dans des élevages où les truies ont des titres élevés d’anticorps contre PCV2, entraînant des infections chez les porcelets.
(Traduit par les auteurs)
Introduction
Porcine circovirus type 2 (PCV2) is the etiologic agent of several syndromes collectively known as porcine circovirus-associated diseases (PCVAD), including postweaning multisystemic wasting syndrome (PMWS), enteritis, respiratory disease, and reproductive failure (1). Today, many PCV2 vaccines are available and can be used in piglets and sows. Owing to the combination of vaccine use and the ubiquitous nature of PCV2, most females in breeding herds have been exposed to the virus, and their piglets have varied levels of passively acquired PCV2 antibodies (1).
Reproductive failure associated with PCV2 is characterized by late-term abortions, decreased numbers of viable piglets, and increased numbers of stillborn and mummified fetuses (2). Inoculation of sows with PCV2 3 wk before parturition can result in lethargy, abortion, and delivery of stillborn piglets as early as 7 d after inoculation (3). Although PCV2-associated reproductive failure has been characterized in experimental studies (2), some reports suggest that it is rare under natural conditions, whereas others have found that 13% of aborted fetuses and stillborns are infected (4,5). Reports of naturally occurring PCV2-associated reproductive failure have mainly concerned newly established production facilities with gilts that do not have PCV2 antibodies (6). Under field conditions, sow vaccination reduces PCV2-associated reproductive failure and improves sow performance (7). However, data from a recent experimental study indicate that vertical transmission of PCV2 can occur even in vaccinated gilts (8).
Field studies have shown great variability in the frequency of sow PCV2 viremia at parturition, from 8/105 (7.6%) to 1/130 (0.8%) (9,10). Sow PCV2 viremia was significantly related to pig mortality (10). At the moment, little is known about sow PCV2 viremia at parturition, its relation to PCV2 vertical transmission, and the onset in the postnatal period of PMWS in piglets infected naturally. The objective of this study was to describe the frequency of PCV2 infection in peripartum sows and the serologic profile of antibodies in naturally infected pigs.
Materials and methods
Animals and sample collection
The experimental protocol (089/04) was approved by the Ethics Committee in Animal Experimentation of the Universidade Federal de Minas Gerais, Minas Gerais, Brazil. Twenty sows with a parity range of 1 to 10 [mean ± standard deviation (s) 4.21 ± 3.39] from 4 farrow-to-finish herds (H1 to H4; 5 sows per herd) were studied. The sows in H1 were not vaccinated against PCV2. Those in H2, H3, and H4 were vaccinated twice with 2 mL of commercial PCV2 vaccine licensed for breeding animals (Circovac; Merial, Lyon, France), at 30 d and 15 d before parturition. On the day of parturition, samples of the sows’ blood and colostrum were collected. A total of 100 newborn piglets (5 piglets per sow) were ear-tagged, and presuckling blood samples were collected from the umbilical cord. Milk samples were collected on the 20th day postpartum. Further blood samples from the piglets were taken by venipuncture and the piglets weighed on days 1, 20, 42, 63, and 84 postpartum. Fat-free colostrum and milk were prepared by centrifugation at 900 × g at 5°C for 10 min; the skim colostrum or milk was pipetted carefully from underneath the fat layer and stored at −20°C until used.
Single radial immunodiffusion (SRD)
Total IgG antibodies in sow colostrum and piglet serum on the day of parturition and day 1 postpartum were detected by SRD performed according to a previously described protocol (11). In brief, IgG antibodies against porcine antigen were prepared in rabbits, purified, titrated, and mixed with an equal volume of 3% noble agar. The mixture was poured between glass slides 5 × 5 cm separated by a plastic frame, to a gel thickness of 1 mm. Wells 2 mm in diameter were punched, and 2-μL aliquots of the purified IgG or samples were added to each well. The development of zones was analyzed after 72 h of incubation of the plates in a moist chamber. Total IgG was quantified by a regression equation obtained with different dilutions of purified pig IgG as described previously (12). The sensitivity of the assay was 5 μg/mL. The concentration of total IgG was expressed in milligrams per milliliter.
Immunoperoxidase monolayer assay (IPMA)
A previously described IPMA protocol (13) was used, with modifications, to detect PCV2-specific IgG in serum and colostrum. Serial 4-fold dilutions from 1:20 to 1:5120 were pipetted into 96-well plates containing fixed PCV2-infected PK15 cells free of porcine circovirus (PCV). The plates were incubated for 1 h at 37°C and then washed with phosphate-buffered saline and Tween 20 (PBS-T). Then 50 μL of protein G conjugated with peroxidase (Molecular Probes, Eugene, Oregon, USA) at 0.25 μg/mL in PBS-T was added to the wells, which were incubated for another hour at 37°C. Finally, the plates were washed, and a substrate solution of 3-amino-9-diethylcarbazole in 0.1 M acetate buffer with 0.05% hydrogen peroxide was added to reveal the reaction. The TA concentration was expressed as the average log2 titer, with dilutions corresponding as follows: 20 = 4.32; 80 = 6.32; 320 = 8.32; 1280 = 10.32; and 5120 = 12.32. The TA titers were classified as follows: < 4.32 log2 = negative; 4.32 to 6.32 log2 = low; 8.32 to 10.32 log2 = moderate; and > 12.32 log2 = high. An age-group seroconversion was indicated when at least 50% of the sampled animals presented a 4-fold or greater rise in TA concentration between successive samplings.
Isotype-specific IPMA
The PCV2-specific IgM antibodies in piglet serum and IgA antibodies in milk, colostrum, and piglet serum on day 1 postpartum were measured by isotype-specific IPMA. First, 96-well plates containing PCV2-infected PK15 cells that were free of PCV1 were blocked with 2% casein for 1 h at 37°C. The plates were then washed with PBS-T, and serial 4-fold dilutions (1:20 to 1:5120) of samples in PBS-T that contained 0.2% casein were pipetted into the wells. The plates were incubated for 1 h at 37°C and then washed. Next, goat antibodies against porcine IgM or IgA (Immunology Consultants, Newberg, Oregon, USA) were added to the wells, and the plates were again incubated for 1 h. The plates were washed, and IgG against goat antigen conjugated with peroxidase was added (Sigma Chemical Company, St. Louis, Missouri, USA), and the plates were incubated for another hour. The cells were stained for PCV2 with the use of protein G visualization as previously described (13).
Detection of PCV2 NA
A previously described protocol (13), with modifications, was used to detect PCV2 NA in colostrum and serum. First, 50 μL of each sample was serially diluted 2-fold (1:16 to 1:2048) in complete Dulbecco’s Modified Eagle’s Medium (DMEM). The samples were added to 96-well plates, and 200 median tissue culture infective dose [(TCID50) of PCV2] was added to each well. After 1 h of incubation, freshly trypsinized PK15 cells free of PCV 50 000/cm2, were added to the samples, and the plates were incubated for 72 h at 37°C in 5% CO2. The plates were then fixed and incubated for 1 h at 37° C with an anti-PCV2 polyclonal serum (VMRD, Pullman, Washington, USA). Next, the plates were incubated for 1 h with protein G conjugated with peroxidase and stained for PCV2 as previously described (13). The NA titer was established as the reciprocal of the last dilution in which a given sample was able to reduce by 50% the number of PCV2-infected cells. The equivalent dilutions were as follows: 16 = 4; 32 = 5; 64 = 6; 128 = 7; 256 = 8; 512 = 9; 1024 = 10; and 2048 = 11. The titers were classified as follows: < 4 log2 = negative; 4 to 6 log2 = low; 7 to 8 log2 = moderate; and >9 log2 = high. An age-group seroconversion was indicated when at least 50% of the sampled animals presented a 4-fold or greater rise in NA concentration between successive samplings.
DNA extraction and amplification
The presence of PCV2 DNA was tested in serum from the sows and piglets obtained on day 1 postpartum. The DNA extraction was performed with a Wizard Genomic DNA Purification kit (Promega, Madison, Wisconsin, USA). Quantitative real-time polymerase chain reaction (qPCR) was performed as previously described (14), with modifications. The results were recorded as the number of PCV2 copies log10 transformed per milliliter of serum.
Virus isolation
Colostrum and milk samples were tested for the presence of PCV2 by virus isolation based on a previously described protocol (15), with modifications. Serial 10-fold dilutions (1:10 to 1:10 000) of the samples were added to 96-well plates containing 1-d-old PK15 cell monolayers free of PCV. After adsorption for 1 h, complete DMEM supplemented with 5% fetal bovine serum was added. After incubation for 72 h at 37° C in 5% CO2 the cells were fixed and stained for PCV2 as described for PCV2 NA. The PCV2 titers were expressed as TCID50 per 50 μL.
Statistical analysis
The data are reported as mean ± s unless otherwise indicated. The Shapiro-Wilk test was used to evaluate the normality of the data distribution of the examined variables. The nonparametric Kruskal-Wallis test was used to analyze the PCV2 TA, NA, IgM, and IgA levels and the difference in PCV2 titer between herds. Fisher’s exact test was applied to compare the proportions of positive and negative results of PCV2 antibody testing and viral titers between the herds. Spearman’s correlation was used to correlate the levels of the different immunoglobulins and to correlate immunoglobulin level with PCV2 titer. A linear regression model was built to correlate the TA and NA levels in piglet serum. The results were considered significant at P < 0.05.
Results
Total IgG in colostrum and piglet serum
Total IgG levels were measured in samples of sow colostrum and piglet serum on the day of parturition and day 1 postpartum to assess the transfer of passive immunity. The colostrum samples had high total IgG levels (93.82 ± 20.03 mg/mL), similar to those reported from other studies (95.60 ± 32.50 mg/mL) (16); there was no significant difference between the herds (Table I).
Table I.
Number of animals positive for IgG antibodies to porcine circovirus type 2 (PCV2) and concentrations of total IgG in colostrum and piglet serum at parturition and on day 1 postpartum in the 4 herds
| Number of positive animals/total number of animals; total IgG concentration (mean ± s), mg/mL | |||
|---|---|---|---|
| Piglet serum | |||
| Herd | Colostrum | Day of parturition | Day 1 postpartuma |
| 1 | 5/5 (107.84 ± 37.26) | 6/25 (0.04 ± 0.06) | 25/25 (46.59 ± 14.01)a |
| 2 | 5/5 (80.48 ± 45.07) | 5/25 (0.02 ± 0.04) | 25/25 (40.87 ± 13.32)a,b |
| 3 | 5/5 (97.48 ± 48.50) | 2/25 (0.00 ± 0.02) | 25/25 (45.78 ± 16.69)a |
| 4 | 5/5 (89.91 ± 38.68) | 5/25 (0.01 ± 0.03) | 25/25 (35.6 ± 7.95)b |
| Total | 20/20 (93.82 ± 20.03) | 18/100 (0.02 ± 0.04) | 100/100 (42.23 ± 13.89) |
s — standard deviation.
Different superscripts within the column indicate significant differences (P < 0.05) between the groups.
On the day of parturition 18 of the 100 piglets had serum levels of total IgG from 0.08 to 0.26 mg/mL, higher than previously described for natural fetal antibody (0.03 mg/mL) (17), which suggests antigen stimulation in utero after the period of immunocompetence (> 70 d of gestation). There was no significant difference in total serum IgG level between the herds (Table I). Of the 18 piglets demonstrating some total IgG on the day of parturition, 5 were positive for PCV2 DNA on day 1 postpartum (data not shown).
On day 1 postpartum the total IgG levels in the piglet serum ranged from 18.32 to 77.52 mg/mL, similar to the levels reported from other studies (18.7 ± 39.0 mg/mL) (18). The average concentration of total IgG was higher in H1 and H3 than in H4 (P < 0.05) (Table I).
Serologic profile and viremia in the sows
On the day of parturition all the sows had high titers of PCV2 TA and NA in the serum (11.63 ± 1.32 log2 and 9.51 ± 1.01 log2, respectively). There was no significant difference in mean PCV2 TA, NA, or IgM level between the herds (Figure 1). A single sow in H2, 3 sows in H3, and 1 sow in H4 (5/20, 25%) had moderate PCV2 IgM titers (6.32 to 8.32 log2). No correlation was found between the levels of PCV2 IgM, which is indicative of recent infection, and PCV2 shedding in colostrum and milk.
Figure 1.
Mean titer of porcine circovirus type 2 (PCV2) total antibodies (TA), neutralizing antibodies (NA), and IgM, and PCV2 load in sow serum from herds H1 to H4 at parturition. Standard deviation is indicated by error bars.
On the day of parturition 17 (85%) of the sows were qPCR-positive, with 3.20 to 3.69 log10 PCV2 copies/mL of serum. The mean viral load of the herds was 2.94 ± 1.28 log10 PCV2 copies/mL (Figure 1). There was no difference in the number of viremic sows between the herds (data not shown).
Antibodies and virus titers in colostrum and milk
On the day of parturition all the colostrum samples were positive for the highest PCV2 TA, NA, and IgA dilutions tested (TA and IgA, 12.32 log2; NA, 11.00 log2). In the milk samples the PCV2 IgA titers were variable but averaged 8.54 ± 3.05 log2 (Table II). There was no significant difference between the herds in the IgA titer in colostrum or milk. Regardless of the high titers of PCV2 antibodies, PCV2 was isolated from 11 (55%) of the colostrum samples (mean titer 101.00 ±1.03 TCID50/50 μL) and 5 (25%) of the milk samples (mean titer 100.30 ± 0.57 TCID50/50 μL). For colostrum the number of PCV2-positive samples and the virus titers were higher in H1 than in H3 and H4 (P < 0.05), although no difference was found for milk (Table II). There was no correlation found between infectious PCV2 shedding in colostrum and milk, between the infectious PCV2 titer and the TA and NA titers in colostrum and milk, or between the infectious PCV2 titer and the IgA titers in colostrum and milk.
Table II.
Number of animals positive for IgA to PCV2 or for infectious PCV2 and the corresponding titers in colostrum and milk from the 4 herds
| Number of positive animals/total number of animals | ||||
|---|---|---|---|---|
| Colostrum | Milk | |||
| Herd | PCV2 IgAa | PCV2 (TCID50/50 μL)b | PCV2 IgAa | PCV2 (TCID50/50 μL)b |
| 1 | 5/5 (11.00 ± 0.00) | 5/5 (2.00 ± 0.70)a | 5/5 (9.95 ± 2.16) | 1/5 (0.40 ± 0.89) |
| 2 | 5/5 (11.00 ± 0.00) | 4/5 (1.40 ± 0.89)a,b | 4/5 (6.26 ± 4.48) | 1/5 (0.20 ± 0.45) |
| 3 | 5/5 (11.00 ± 0.00) | 1/5 (0.40 ± 0.89)b | 5/5 (9.84 ± 0.98) | 3/5 (0.60 ± 0.54) |
| 4 | 5/5 (11.00 ± 0.00) | 1/5 (0.20 ± 0.44)b | 5/5 (9.15 ± 3.00) | 0/5 (0.00 ± 0.00) |
| Total | 20/20 (11.00 ± 0.00) | 11/20 (1.00 ± 1.03) | 19/20 (8.54 ± 3.05) | 5/20 (0.30 ± 0.57) |
s — standard deviation.
Log2 transformed titer (mean ± s).
Log10 transformed titer (mean ± s). TCID50 — median tissue culture infective dose. Different superscripts within the column indicate significant differences (P < 0.05) between the groups.
Serologic profile and viremia in the piglets
On the day of parturition 7 piglets had low levels of PCV2 TA (Table III), ranging from 4.32 to 8.32 log2, 1 piglet had an NA titer of 5.0 log2, and 2 other piglets had low PCV2 IgM titers in the serum (4.36 log2) in the absence of PCV2 TA. There was no significant difference between the herds in the number of seropositive piglets at parturition.
Table III.
Number of piglets born PCV2-seropositive, as well as PCV2 IgA titers and PCV2 load in serum on day 1 postpartum
| Number of positive animals/total number of animals
|
||||
|---|---|---|---|---|
| Herd | PCV2 TAa | PCV2 IgM | PCV2 IgAa,b | PCV2 DNAc |
| 1 | 1/25 (0.17 ± 0.87) | 0/25 | 25/25 (11.00 ± 0.00)a | 7/25 (1.14 ± 1.90) |
| 2 | 4/25 (0.93 ± 2.29) | 2/25 (0.35 ± 1.21) | 25/25 (10.00 ± 2.21)b | 5/25 (0.64 ± 1.32) |
| 3 | 1/25 (0.17 ± 0.89) | 0/25 | 25/25 (10.16 ± 1.51)a,b | 9/25 (1.27 ± 1.75) |
| 4 | 1/25 (0.17 ± 0.87) | 0/25 | 25/25 (9.52 ± 2.08)a | 8/25 (1.32 ± 2.00) |
| Total | 7/100 (0.36 ± 1.39) | 2/100 (0.09 ± 0.62) | 100/25 (10.41 ± 1.93) | 29/100 (1.09 ± 1.75) |
s — standard deviation.
Log2 transformed titer (mean ± s). TA — total antibodies.
Different superscripts within the column indicate significant differences (P < 0.05) between the groups.
Log10 transformed titer (mean ± s).
On day 1 postpartum, PCV2 DNA was detected in the serum of 29 of the 100 piglets (3.34 to 5.05 log10 PCV2 copies/mL); there were 1 to 3 positive piglets per litter. Of these animals, 1 had PCV2 TA and another had PCV2 IgM on the day of parturition. There was no difference in the number of viremic piglets between the herds (Table III). There was no correlation between sow viremia or PCV2 infectious shedding in the colostrum and piglet viremia on day 1 postpartum. There was no difference between the weights of viremic and nonviremic piglets at any of the 5 times of weighing from day 1 to day 84 postpartum (data not shown).
The evolution of the piglet serologic profile from the day of parturition to day 84 postpartum is shown in Figure 2. The herds had similar PCV2 TA profiles up to day 63, with a gradual decline in the titers of maternally derived antibodies. The TA and NA titers were lowest on days 42 and 63 in herds H1, H2, and H4. The PCV2 IgM levels increased from day 42 in H1 and H4, peaking on day 63. In H1 and H4, PCV2 TA and NA seroconversion occurred on day 84, 6 wk after the IgM level started to rise. In H2, PCV2 IgM was detected 3 wk later than in the other herds, on day 63, and a trend of increasing TA and NA was noted 3 wk later. Although the IgM level started to increase on day 42 in H3, there was no seroconversion to other immunoglobulin types until the end of the study.
Figure 2.
Mean titer of PCV2 TA, NA, and IgM in the serum of piglets from H1 to H4 on days 0, 20, 42, 63, and 84 postpartum. Standard error is indicated by error bars.
The correlation between TA and NA levels in piglet serum was moderate to high (P < 0.000): on day 21, r = 0.61; on day 42, r = 0.75; on day 63, r = 0.53; and on day 84, r = 0.81.
Discussion
In the piglets in this study, the PCV2 seropositivity at birth and the high frequency of viremia on day 1 postpartum suggest that early infection can occur because of vertical transmission of PCV2 even from naturally infected sows with high PCV2 antibody titers. In addition to transplacental infection, piglets could be infected with PCV2 soon after birth via infected colostrum or environmental contamination. Although the occurrence of natural antibodies reacting with pathogens cannot be discarded (17), it seems that IgM and not IgG has a leading role in those specific reactions (19). Furthermore, the detection of NA in 1 newborn piglet reinforces the idea of in utero infection after the period of fetal immunocompetence.
Experimental and field studies have associated in utero transmission of PCV2 with reproductive failure resulting in stillborn, mummified, and nonviable piglets at birth (3). In utero transmission of PCV2 has typically been reported in newly established herds with a high proportion of gilts (6,20). However, in a recent study (8), reproductive failure was not observed in gilts infected experimentally at 56 d of gestation; regardless, PCV2 DNA was recovered in the serum of 4 of 80 neonates. Together with our findings, these data indicate that transplacental transmission of PCV2 may occur, even in the absence of reproductive failure and in sows with high levels of PCV2 antibodies, at a greater frequency than previously reported.
Although in utero transmission of PCV2 and its association with reproductive failure has been repeatedly described, little is known about its asymptomatic occurrence and its implication in PCVAD. A study found that sow PCV2 viremia in the peripartum period was significantly related to piglet mortality (9). In another study, prenatal PCV2 infection did not induce PMWS, but prenatally infected animals had a significantly lower body weight compared with control pigs (21). These findings differ from our finding in this study that animals that were PCV2-seropositive at birth and viremic on day 1 postpartum were similar in weight to the other animals at the 5 times of weighing from day 1 to day 84 postpartum. We could not find an association between mortality, weight, and the occurrence of PMWS among piglets that were viremic on day 1 postpartum compared with nonviremic piglets. This may be related to the time of infection or to the quantity and virulence of PCV2. In addition, our study found that only 7 of the 18 piglets with some level of total IgG at birth were positive for PCV2, which suggests that these animals could have been infected in utero by other pathogens that affect the postnatal antibody profile.
The high number of sows positive for PCV2 by qPCR at parturition suggests that the vaccination protocol may not be sufficient to protect sows against PCV2 viremia. These results differ from the finding in a previous study that vaccinated sows were protected from viremia after experimental infection (8). This difference may be explained by the low sample size (n = 3) of that study, which investigated specific pathogen-free, PCV2-seronegative gilts, or by differences in vaccination protocol, time of infection, or virulence of the PCV2 isolate. In addition, persistence of PCV2 viremia has been demonstrated (22,23), and studies have suggested that an animal can be infected with different PCV2 isolates (24). This would help to explain why some sows had PCV2 IgM simultaneously with high levels of PCV2 TA at parturition, indicating recent reinfection. In this study, the occurrence of low PCV2 antibody titers in pigs that did not exhibit gross lesions at birth indicates that in utero infection occurred close to parturition (25).
A recent study suggested that the local IgA immune response is more important for controlling PCV2 replication during early infection than the local IgG and IgM responses or the IgG systemic response (26). Although no correlation between the PCV2 IgA titer in milk and piglet weight at weaning was found in this study, the presence of PCV2 IgA may protect against early mucosal infection in suckling piglets. Thus, PCV2 IgA may have a role in reducing piglet mortality that is similar to the protection provided by maternal PCV2 TA (9,26).
Studies have indicated that although maternal antibodies provide some protection against PCV2 infection through their neutralizing activity, these antibodies are unable to completely prevent infection (9,27). However, as suggested by the serologic profiles found in this study, high titers of maternal antibodies seem to control infection in a way that prevents an antigenic stimulus sufficient to elicit an active immune response in piglets. Interestingly, regardless of early piglet viremia, the PCV2 IgM titer increased only at day 42 postpartum, which suggests that PCV2 stimulates antibody production only after the titer of passive antibodies declines to a subprotective level. In fact, this study showed that the level of PCV2 IgM increased earlier in herds in which piglets had lower titers of NA. The PCV2 TA and NA increases coincided with declining levels of IgM. The response delay of NA compared with TA has been described previously (13). In this study no such delay occurred, possibly owing to the 3-wk interval between sample collections and to the difficulty of distinguishing the decline in passive immunity from the onset of active immunity. Previous studies found that some PMWS-affected pigs lacked or had low levels of NA to PCV2 even in the presence of high TA titers (13,28). However, there was a moderate to high correlation between TA and NA in pig serum in this and other studies (13).
In conclusion, high PCV2 antibody levels in sows at parturition did not prevent early PCV2 infection and viremia in piglets from the first day of life or peripartum maternal viremia and virus shedding into the lacteal secretions. Vertical transmission of PCV2 could generate PCV2 seropositivity in viable piglets even on farms with no symptoms of reproductive failure.
Acknowledgments
This work was funded by Fundação de Amparo à Pesquisa do Estado de Minas Gerais, Minas Gerais, Brazil, and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brasilia, Brazil. We thank veterinarians Flávia F. Pinto, Túlia M. Oliveira, and José E. Cavalcanti for their assistance with the herds. We also thank Grazielle Galinari for her excellent technical assistance. Dr. Lobato has a research fellowship from CNPq.
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