Prevalence of porcine reproductive and respiratory syndrome virus and porcine parvovirus antibodies in commercial pigs, southwest Nigeria

Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine parvovirus (PPV) infections cause significant economic losses to the pig industry and are considered the most economically important viral diseases of intensive swine production. Despite numerous reports on both diseases in several countries worldwide, the status of PRRSV and PPV in Nigeria remains largely unknown. Thus, a serological survey was conducted to determine the prevalence of PRRSV and PPV infections in pigs in Southwest Nigeria. Using commercial ELISA kits, 368 pig sera were screened for antibodies against PRRSV (types I and II) and PPV. Significantly higher antibody prevalence was obtained for PRRSV (53.8%) compared to PPV (36.1%). Since there is no vaccination against both diseases in the country, the findings of this study suggest that PRRSV and PPV are present in the pig population in southwest Nigeria. There should be continuous monitoring of pigs for these diseases in Nigeria since both viruses are associated with major economic losses in the swine industry, affecting all stages of production. This will help to ascertain the actual burden and increase awareness of both diseases to facilitate early detection in order to institute appropriate control measures in the country.

Diverse viruses have been shown to affect porcine reproductive performance, and it is likely that any virus capable of causing clinical illness in adult swine, or crossing the placental barrier to infect the porcine conceptus, is a potential reproductive pathogen (Mengeling et al., 2000). Hence, effective animal production depends partly on preventing infectious diseases that affect reproductive performance. Worldwide, porcine reproductive and respiratory syndrome virus (PRRSV) and porcine parvovirus (PPV) have been documented as the most common viral causes of porcine reproductive failure (Mengeling, 1978; Wensvoort et al., 1991). Porcine reproductive and respiratory syndrome (PRRS) virus is a small, enveloped RNA virus (Wensvoort, 1993) that is classified as a member of the genus Arterivirus of the Family Arteriviridae, Order Nidovirales (Cavanagh, 1997). PRRSV was first isolated almost simultaneously in Europe (Wensvoort et al., 1991) and North America (Collins et al., 1992). This virus is characterised by its ample genetic and antigenic variability, as well as by its immune suppressor properties and capacity to induce persistent clinical infections that complicate the diagnosis and control of the disease (Goldberg et al., 2000). There are broadly two genotypes of this virus, the European (type I) and the North American (type II) (Ellingson et al., 2010). This classification is significant in that vaccines made for one serotype will not completely protect against the other (FAO, 2007). However, the symptoms of these two genotypes are similar (Cavanagh, 1997). Although PRRS is generally characterised by reproductive failure of sows and respiratory distress of piglets and growing pigs, its clinical signs vary greatly between herds. The virus is excreted in various body secretions including saliva, nasal secretions, urine, semen, milk, and colostrum (Yoon et al., 1993; Wills et al., 1997). Mechanical transport and transmission via contaminated needles, fomites (boots and coveralls), farm personnel (hands), transport vehicles (contaminated trailers), and insects (houseflies and mosquitoes) have also been reported (Otake et al., 2002; OIE, 2015).

On the other hand, porcine parvovirus (PPV) is a non-enveloped DNA virus classified in the genus Parvovirus and Family Parvoviridae. It is considered an important cause of reproductive failure in pigs characterised by embryonic and foetal infection and death, usually in the absence of outward maternal clinical signs (Mengeling, 1999). All isolates of PPV that have been compared to date have been found to be antigenically identical, and so it appears that there is a single serotype (Mengeling et al., 2000). The spread of PPV occurs by oronasal contact with excretions and contaminated fluids like semen, infected foetal fluids and foetuses (Mengeling et al., 2000; Guerin and Pozzi, 2005). Contaminated premises are probably major reservoirs of PPV and may remain infectious for months in secretions and excretions from acutely infected pigs (Mengeling, 1999). There have been several reports on the prevalence of PRRSV and PPV in different countries of Central and South America, Europe and Asia (Melendez et al., 2008; Duinhof et al., 2011; Streck et al., 2011; Mahesh et al., 2015). However, in Africa, there are minimal reports on the disease situation (OIE, 2004). For instance, in Nigeria where the pig population is estimated to be about seven million (FAO, 2014) and pig production is quite popular, there is a need to investigate infectious diseases that affect swine reproductive performance. Apart from a recent report on PRRSV in a commercial piggery unit in Lagos State (Meseko and Oluwayelu, 2014), there is a dearth of information on PRRSV and PPV in pigs in Nigeria. Therefore, this study was carried out to determine the status of PRRSV and PPV infections in pigs from farms and abattoirs in Oyo and Lagos States, southwest Nigeria.

2.1. Study area and animals

A total of 368 pigs of both sexes (162 males and 206 females) were used for this study which was carried out between May 2014 and July 2015. They comprised 206 pigs from major abattoirs in Oyo (n = 159) and Lagos (n = 47) States, and 162 from ten different farms in Oyo State. The sampled pigs were observed for presenting clinical symptoms and the farmers interviewed on herd health history. Also, the level of farm hygiene practice was observed. The pigs ranged in age from 12 weeks to 2 years and belonged to five breeds; including Large White (2 2 4), Large Black (38), Duroc (92) and Hampshire (12), which are common in Nigeria, and the Camboro breed (2) which was imported from neighbouring Republic of Benin.

2.2. Sample collection and serology

About 3 ml of blood was aseptically collected from the anterior aortic vein of each farm pig using sterile syringe and needle, while blood was collected at slaughter from abattoir pigs. Each blood sample was dispensed into sterile anticoagulant-free sample tube and allowed to clot at room temperature. Separated sera were stored at -20°C until analysed for PRRSV and PPV antibodies.

The sera were screened using indirect enzyme-linked immunosorbent assay (ELISA) kits for the detection of antibodies against European and American strains of PRRSV (Green Spring PRRSV-Ab ELISA kit, version 2014–01, Shenzhen, China) and PPV (Green Spring PPV-Ab ELISA kit, version 2013–01, Shenzhen, China). All the assay steps were carried out according to manufacturer’s instruction and optical density (OD) values were obtained using an ELISA reader (Optic Ivymen System 2100-C, Spain) at double wavelength of 430 nm and 630 nm.

2.3. Validation of ELISA results

For PRRSV, valid results were obtained when the average OD values of the PRRSV positive control and negative control sera were greater than 0.70 and less than 0.10, respectively. Samples with OD values > 0.33, 0.30 to 0.33, and < 0.30 were considered positive, doubtful and negative, respectively. For PPV, the readings were valid when the OD values were ≥ 0.6 and < 0.1 for positive (PC) and negative (NC) control sera, respectively. Results were expressed as Sample/Positive percentage (S/P%) using OD value for each serum sample: S/P% = (Sample _OD – N_C)/(P_C – N_C). Samples with S/P% ≥ 0.25 and < 0.25 were considered positive and negative, respectively.

2.4. Statistical analysis

Data obtained were analyzed with Graph Pad prism version 5.0 (Graph Pad software, San Diego, USA) using Chi-square (X2 ) test with the level of significance determined at an alpha level of 0.05.

Out of the 368 sera tested, 198 (53.8%), 32 (8.7%) and 138 (37.5%) were positive, doubtful and negative, respectively for PRRSV (types I and II) antibodies while 133 (36.1%) and 235 (63.9%) were positive and negative, respectively for PPV antibodies. There were statistically significant differences in the seroprevalence rates for PRRSV and PPV based on location, sex, breed and age group (Table 1). Dirty and unhygienic environments as well as large population of houseflies were common observations at the abattoirs and farms, while common clinical symptoms observed among the farm pigs include lethargy, rough hair coats, variation in litter size, and respiratory distress in piglets. Also, herd history obtained from interviews conducted on the 10 farms revealed that the above clinical symptoms, as well as piglet deaths shortly after birth, abortions and stillbirths, were common occurrences on all the farms.

Worldwide, porcine reproductive and respiratory syndrome virus (PRRSV) and porcine parvovirus (PPV) are major threats to the swine industry and have been documented as the most common viral causes of porcine reproductive failure (Mengeling, 1978; Wensvoort et al., 1991). Their economic impact is due to the increase in repetition of heats, reduction of litter size and fertility (Collins et al., 1992). Hence, profitable pig production depends partly on preventing the occurrence of such infectious diseases that affect swine reproductive performance. In the present study, which is part of ongoing surveillance for porcine viruses of economic importance in Nigeria (Aiki-Raji et al., 2014, 2016), overall prevalence of antibodies against PRRSV and PPV was 53.8% and 36.1%, respectively. The seroprevalence rates of 48.6% and 89.4% for PRRSV, as well as 36.1% and 36.2% for PPV in Oyo and Lagos States, respectively indicate that PRRSV infection is more prevalent in Lagos than in Oyo State while PPV is of same prevalence in both states. In Nigeria where vaccination against PRRS and PPV infection is not practiced, detection of antibodies to both strains of PRRSV and PPV in the sera of pigs from farms and abattoirs suggests that they had been naturally exposed to these viruses. According to Evans et al. (2008), seropositivity of young stock is indicative of virus persistence within the population whereas in adults it could indicate past exposure. Thus, the detection of PRRSV and PPV seropositive animals among young and adult pigs in this study, coupled with the observation of variation in litter size, rough hair coats, respiratory distress of piglets and herd history of abortions, stillbirths and piglet deaths shortly after birth indicate circulation of both viruses in the study area and support the fact that PRRS and PPV infections are economically important viral diseases of intensive swine production. To our knowledge, apart from a recent report on PRRSV in a commercial pig farm complex in Lagos State (Meseko and Oluwayelu, 2014), there is no other report on

this disease in the country. This study also represents the first report of PPV antibody-positive pigs in Nigeria. The high seropositivity obtained for female and male pigs in this study portrays a potential risk of PRRS and PPV infection in the pig herds tested. Infected boars that shed these viruses via semen have been reported to constitute a potential source of transmission of the two diseases and, as such, the chances of both viruses being transmitted through semen is high (Robles et al., 2004; Guerin and Pozzi, 2005). In addition, boars used in natural mating commonly make contact with infected sows and during the process of mating they get infected (Benfield, 2004). Therefore, both female and male pigs may serve as carriers of PRRSV and PPV as they interact with infected and non-infected pigs. In nursery and finisher pigs, the economic impact attributable to infection with PRRSV and PPV could be due to increases in morbidity and mortality rates, reductions in feed efficiency and growth rates, and an increase in unmarketable pigs (Neumann et al., 2005). Thus, the detection of PRRSV and PPV antibodies in all the age groups and breeds of pigs tested in this study is of both veterinary and economic importance. It is noteworthy that the Camboro pigs imported from Republic of Benin also had antibodies against PRRSV and PPV. However, based on the scope of this study we could not ascertain whether the pigs acquired the infection locally or were already incubating the disease at the time of their importation into Nigeria. The prevention of PRRSV and PPV introduction into a herd is difficult considering the several means of transmission reported (Goldberg et al., 2000). Therefore, the observed presence of numerous insects, especially houseflies, and unhygienic condition of the abattoirs and some of the farms in this study is of epidemiological significance. It has been reported that mechanical transport and transmission via contaminated farm materials and environment as well as contaminated trailers/trucks which are capable of conveying the virus over significant distances contribute to spread of these diseases (Mengeling et al., 2000; Pitkin et al., 2009). Furthermore, there are indications that possible circulation of PRRSV and PPV in the study area may be maintained by purchase of gilts and boars from PRRSV- and PPV-positive herds, or movement of other types of infected animals between different stages of production (Mortensen et al., 2002; Larochelle et al., 2003). Consequently, as previously reported (Trincado et al., 2004; OIE, 2015), we advocate that mechanical vectors such as insects (houseflies and mosquitoes), waterfowls and perhaps wild and domestic animals, should be considered in developing prevention and control strategies for the diseases. In conclusion, the findings of this study reveal that PRRSV and PPV presently circulate among pigs in Oyo and Lagos States, southwest Nigeria. Since these viruses are reported to be responsible for major economic losses in the swine industry and affect all stages of production, our findings underscore the need for continuous monitoring of both diseases among pigs in Nigeria in order to establish the true status and increase awareness of these diseases. This will facilitate the development and institution of appropriate control measures in the country. Further studies aimed at detecting and characterizing PRRSV and PPV strains circulating in Nigeria are already being undertaken.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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