Persistent domestic circulation of African swine fever virus in Tanzania, 2015–2017

Abstract

Background African swine fever (ASF) is a highly fatal viral hemorrhagic disease of domestic pigs that threatens livelihoods and food security. In Africa, ASF virus (ASFV) circulates in sylvatic (transmission between warthogs and soft argasid ticks) and domestic (transmission between domestic pigs) cycles, with outbreaks resulting from ASFV spill-over from sylvatic cycle. A number of outbreaks were reported in different parts of Tanzania between 2015 and 2017. The present study investigated ASFV transmission patterns through viral DNA sequencing and phylogenetic analysis. A total of 3120 tissue samples were collected from 2396 domestic pigs during outbreaks at different locations in Tanzania between 2015 and 2017. Partial sequencing of the B646L (p72) gene was conducted for diagnostic confirmation and molecular characterization of ASFV. Phylogenetic analysis to study the relatedness of current ASFV with those that caused previous outbreaks in Tanzania and representatives of all known 24 ASFV was performed using the Maximum Composite Likelihood model with 1000 bootstrap replications in MEGA 6.0. Results ASFV was confirmed to cause disease in sampled domestic pigs. ASFV genotypes II, IX, and X were detected from reported outbreaks in 2015–2017. The current ASFV isolates were similar to those recently documented in the previous studies in Tanzania. The similarities of these isolates suggests for continuous circulation of ASFV with virus maintenance within the domestic pigs. Conclusions Genetic analysis confirmed the circulation of ASFV genotypes II, IX, and X by partial B646L (p72) gene sequencing. The similarities of current isolates to previously isolated Tanzanian isolates and pattern of disease spread suggest for continuous circulation of ASF with virus’ maintenance in the domestic pigs. Although certain viral genotypes seem to be geographically restricted into certain zones within Tanzania, genotype II seems to expand its geographical range northwards with the likelihood of spreading to other states of the East African Community. The spread of ASFV is due to breach of quarantine and transportation of infected pigs via major highways. Appropriate control measures including zoosanitary measures and quarantine enforcement are recommended to prevent ASF domestic circulation in Tanzania.

African Swine Fever Virus Circulation between Tanzania and Neighboring Countries: A Systematic Review and Meta-Analysis

“…The genotype X was recovered from domestic pigs in 1950 in Kenya [ 15 ], since then, it was regularly reported to circulate and cause ASF outbreaks in Kenya [ 15 , 16 , 21 , 23 ], Uganda [ 21 , 23 ], Tanzania [ 22 , 23 , 61 , 62 ], Burundi [ 22 , 23 , 64 ], and DRC [ 51 ]. Recent molecular studies have reported high genetic similarity at three genomic regions of ASFV (p72, CVR and I73R-I329L ) between the ASFV p72 genotype X strains responsible for ASF outbreaks in 2016 in Kagera region in Tanzania [ 62 , 64 ], the 2018 ASF outbreak in Rutana region in Burundi [ 64 ], and the ASF outbreak that occurred during December 2018 to January 2019 in South Kivu province of the DRC [ 51 ]. Commercial traffic and cross-border movements of domestic pigs and pork products were speculated as the main drivers of the ASFV spread between DRC, Burundi, and Tanzania [ 51 , 53 , 64 ].…”

“…The ASFV p72 genotype IX was recovered from domestic pigs and warthogs. This genotype is described as the most predominant in Uganda and Kenya [ 42 ], but also, it was described in DRC [ 50 , 53 ] and Tanzania [ 56 , 62 ]. Although associated with lethal ASF outbreaks [ 21 , 47 , 48 ], an increasing number of studies reported the genotype IX in asymptomatic domestic pigs in Tanzania [ 56 ], DRC [ 50 ], Uganda [ 44 ], and Kenya [ 65 ].…”

“…In addition, homology between ASFV strains from ticks collected at pig shelters and those from warthog burrows was observed, suggesting the possibility of ASFV transmission at the wild and domestic pigs interface [ 20 ]. Among countries included in this review, the ASFV p72 genotype II was described in Zambia [ 17 , 40 ], Mozambique [ 20 , 22 , 49 ], Tanzania [ 56 , 59 , 62 ], and in Malawi [ 63 ]. In 2007, the ASFV p72 genotype II escaped from its African geographical distribution to Georgia and subsequently spread to countries of the European Union and Russia before it reached Asia in August 2018 [ 13 , 25 ].…”

“…In 2007, the ASFV p72 genotype II escaped from its African geographical distribution to Georgia and subsequently spread to countries of the European Union and Russia before it reached Asia in August 2018 [ 13 , 25 ]. Additionally, the transboundary spread of the highly virulent ASFV p72 genotype II similar to the Georgia 2007/1 strain is highlighted by the introduction of this genotype into Tanzania probably from Malawi [ 59 , 62 , 63 ] and the emergence of this ASFV genotype in Zimbabwe in 2015 after several years of ASF absence probably from the neighboring Mozambique [ 68 ]. Furthermore, the genotype VIII was found to circulate in Zambia, Mozambique and Malawi [ 21 , 22 , 23 ], genotype XII was described in Zambia and Malawi [ 22 ], whilst genotype XV seems to be confined to Tanzania where both domestic and sylvatic cycles have been described for this genotype [ 22 , 57 , 58 ].…”

“…High genotypic variability was reported by included studies with 15 different ASFV genotypes being reported to circulate in countries concerned by this review. Movement of domestic pigs and pork products were cited as the main factor of ASFV spreading within and between countries, for instance the ASFV spread pattern was linked to trade highways in Tanzania [ 62 ] and Uganda [ 47 ] highlighting the importance of the anthropogenic factors and the ASFV domestic pig cycle in the spread of ASF in concerned countries. In addition, the ASFV sylvatic cycle seems to play a role in the maintenance of ASFV in concerned countries as 13 among the 15 circulating genotypes were isolated either from warthogs or ticks.…”