This study revealed circulation of both type 1 and 2 PRRSv genotypes in northern Uganda. However, PRRSv type 1 was found to be the more predominant genotype detected. These findings also indicate that PRRSv-1 likely to be prevalent elsewhere in Uganda, where its occurrence has not yet been investigated properly. This situation could have adverse implications for swine productivity in the country, herd economic performance and consequently livelihoods, if the virus becomes established in commercial breeding herds. Information about the predominant virus is important for implementing successful interventions for controlling the spread of the virus given the potential economic impacts of PRRSv on swine productivity.
These results showed the likelihood of PRRSv-1 detection decreased with pig age. While this was statistically non significant, it suggested a trend that needs further exploration with a larger sample size. This finding is consistent with the observation that the immune system of swine is able to completely eliminate PRRSv infection over prolonged periods of time, leading to sterilizing immunity . Pigs exposed to PRRSv become resistant to reinfection with a homologous strain, although the level of protection was incomplete . This was also corroborated by a study which found age-dependent resistance to infection, shown by reduced viremia and viral load in the blood of adult pigs compared to younger pigs . In contrast, other studies revealed that PRRSv tends to persist in infected herds [25, 26], suggesting increased likelihood of detection in older pigs. However, this finding was specific for larger herds and where there were increased re-introductions of infected gilts . In the smallholder pig production systems in northern Uganda, most farms were generally small in size (1–3 sows) and the replacements were infrequent, as highlighted in a related study (Oba et al. forthcoming).
The increase in PRRSv detection rates associated with gross pathologic lesions conforms to previous studies. The ability of PRRSv to induce clinical and macroscopic pneumonia, often as a co-infection with other pathogens such as M. hyo has been documented . No differences in detection rates between male and female pigs were observed in this study.
Our results are comparable to other studies which reported simultaneous circulation of both PRRSv type 1 and type 2 genotypes in various regions and show increased circulation of PRRSv type 1. In Europe, both types circulate but there is a predominance of type 1, with marked genetic variation among the genotypes . In Asia, studies report the predominance of PRRSv type 1 in China, although the American type 2 has also been documented . In the Republic of Korea, it was found that both type 1 and type 2 strains circulated in pig farms during the period between 2013–2016. However, type 1 PRRSv was reportedly predominant .
The information on PRRSv in African countries is limited but there are official reports submitted to OIE by a few countries in Africa (DR Congo, Benin, Burkina Faso, Egypt, Ivory Coast, Nigeria) that document occurrence of PRRSv, although none of these studies reported its genetic diversity or molecular identity . The current situation regarding the PRRSv genotypes circulating on the continent is largely unknown, as the few studies undertaken were based on serologic assays. In southwest Nigeria, a study reported a high seroprevalence of PRRSv of 53.8%, suggesting widespread exposure of pigs to the virus . However, the genotype of the virus was not determined.
In South Africa, the PRRSv strain responsible for the 2004 outbreaks was identified by RT-PCR as type 2 . Our results are contrary to expected since a large number of pigs are imported from South Africa and suggest a different source of the virus in Uganda, since PRRSv type 1 has not been reported in South Africa. The lack of reliable data on pig imports into Uganda limits our understanding of the likely sources of PRRSv introduction into the country. Further studies to understand the introduction and maintenance of PRRSv into Uganda are required. Knowledge gaps remain on the potential distribution of PRRSv strains in other regions of Uganda especially in high pig dense areas, which justify further studies.
The method used to detect PRRSv in this study utilised primers that were designed to simultaneously detect both PRRSv-1 and PRRSv-2. This approach is reported to have high specificity and sensitivity, at differentiating PRRSv-1 from PRRSv-2 isolates [22, 33]. This method is reportedly efficient and rapid for large scale detection and differentiation of PRRSv strains. However, this study was limited by the small sample size used and by the fact that the study was undertaken in only one region, implying that results cannot be extrapolated to other regions of the country. Because we sampled only pigs that presented with gross lung lesions, the true prevalence of PRRSv and the distribution of strains in all slaughtered pigs and in the general pig population still remains unknown and possibly is higher to what has been reported here.