**1. Introduction**

Classical swine fever (CSF) caused by the CSF virus (CSFV) is one of the most important swine diseases, resulting in huge economic losses to the pig industry worldwide, and it is a World Organization for Animal Health (OIE)-listed disease. CSFV (previously called hog cholera virus) belongs to the genus *Pestivirus* within the family *Flaviviridae* together with bovine viral diarrhea virus 1, bovine viral diarrhea virus 2 and border disease virus [1]. Recently, CSFV has been redesignated as *Pestivirus C* [1].

CSF is an immunosuppressive disease in which several immune escape mechanisms of CSFV have been reported, such as apoptosis, autophagy and pyroptosis in bone marrow hematopoietic cells, lymphocytes and lymphoid organs [2]. A low CD4/CD8 ratio has been observed in the peripheral blood mononuclear cells of infected fetuses and piglets challenged with either high- or low-virulence CSFV strains. A low CD4/CD8 ratio indicates dysregulation of the immune response [3]. During CSFV infection, the clinical signs mainly depend on the ages of pigs and the virulence of the viral strains. The clinical forms of CSFV can show acute, chronic and persistent courses. The persistent course usually requires infection of sows at approximately 50–70 days of pregnancy [4–6]. In general, the acute form of CSF leads to clinical and pathological features that are very similar to those of African swine fever [5,6]. In addition, CSF must also be considered in the differential diagnosis of erysipelas, porcine circovirus type 2 (PCV2)-associated diseases (PCVAD), salmonellosis and porcine reproductive and respiratory syndrome (PRRS) [6]. The overlapping of the clinical presentations may lead to a misdiagnosis of CSF as PRRS virus (PRRSV) infection. PRRSV infection also causes reproductive symptoms in gestational sows and respiratory problems in young pigs [7,8]. PRRSV infection can induce several immunosuppressive responses [9], such as: i) dysregulation of NK cell cytotoxic activity [10]; ii) poor production of IFN-alpha [11]; and iii) promotion of the secretion of immunosuppressive cytokines such as interleukin-10 (IL-10) and transforming growth factor-beta [10,12,13].

Systematic vaccination and non-vaccination stamping-out are the two main strategies to control CSF [6,14]. Due to the enormous costs of stamping-out, systematic vaccination is a more effective strategy for CSF control in CSF endemic areas [6]. Two major kinds of CSFV vaccines, the modified live vaccine (MLV) and the subunit vaccine, are widely used in many countries [6,14]. The MLV vaccine can induce not only humoral immune responses but also cell-mediated immune responses against virulent CSFV. Subunit vaccines, such as E2 vaccines, usually only induce antibody responses [14]. However, the disadvantages of CSFV MLV vaccines are that their efficacy is inhibited by maternal-derived antibody (MDA) [14–19] and they lack differentiation with infection from vaccinated animals (DIVA) according to serological assays [16,20]. The CSFV subunit vaccines based on the E2 protein allow DIVA by Erns enzyme-linked immunosorbent assays and provide good protection [21–23]. The drawbacks of E2 subunit marker vaccines are that they induce protection later than MLV vaccines, and their efficacy also interferes with maternal antibodies [21,24].

Transplacental transmission of CSFV occurred before the onset of the antibody response when sows were challenged with either high- or low-virulence CSFV strains. Therefore, rapid and solid immunity after sow vaccination is required for the prevention of congenital viral persistence [3]. In Taiwan, CSFV MLV has been used since the 1950s and proven to be sufficiently protective. CSFV MLV vaccination is only recommended in sows at 4 weeks post-farrowing (nonpregnancy stage) to overcome persistent infection. To avoid MDA, which interferes with the efficacy of CSFV MLV in the clinic, piglets should be vaccinated at 6 and 9 weeks old when sows are vaccinated in the nonpregnancy stage. However, PRRSV is still a major problem and difficult to control in the nursery stage, which overlaps with the CSFV MLV vaccination period in Taiwan. Certain severe PRRS cases in the nursery were observed just after CSFV MLV vaccination (data from the Animal Diseases Diagnostic Center of National Pingtung University of Science and Technology, not shown). The most reasonable explanation for PRRS and porcine respiratory disease complex induction is stress, which would be caused not only by vaccination but also by the side effects of CSFV MLV, pathogens spread by needles, the synergistic effects of bacterial pathogens such as *Glaesserella parasuis* (*G. parasuis*, previously called *Hemophilus parasuis*) and other factors [25]. In contrast, the E2 vaccine is recommended for application in sows at 4–5 weeks pre-farrowing and elicits a high level of neutralizing antibody [26–28], whereas the vaccination of offspring can be delayed until they are 10–12 weeks old [28], which is when most piglets have recovered from PRRS. Therefore, the level of MDA is very important for CSFV vaccination programs in piglets. Previous research findings showed that PRRSV infection prior to CSFV vaccination significantly suppressed the antibody response [29,30]. In addition, CSFV immunization during the

acute phase of PRRSV infection could result in vaccination failure [31]. However, the correlation between the CSFV MDA levels produced in response to different types of CSFV vaccines and the PRRSV load in the field remains to be investigated. Herein, this retrospective study aimed to elucidate the sero-dynamics of the CSFV and PRRSV loads in piglets born from sows immunized with different types of CSFV vaccines to further the understanding of the interactions between the CSFV vaccine and the PRRSV, which is still prevalent in most areas of intense pork production in the field.

## **2. Results**
