Several outbreaks of a highly fatal hemorrhagic disease affecting domestic pigs, suspected to be ASF based on clinical signs and postmortem findings, were reported in different parts of Tanzania between 2015 and 2017. ASF remains a major constraints to the pig industry in Tanzania with reported outbreaks throughout the year. There is neither a cure nor vaccine to prevent ASF infection. The control and eradication measures of ASF are based on surveillance, epidemiological investigation, animal movement control, quarantine and zoosanitary measures. Early detection of the disease and its spread is important for a successful surveillance and accurate diagnostic procedures are important for effective quarantine and control measures (24). In this study, molecular methods were used to identify and characterize ASFV from domestic pigs that died of hemorrhagic disease outbreaks in Tanzania.
The results obtained in the 2015-2017 outbreaks confirm ASF outbreaks in domestic pigs in the studied areas. In the present study, these ASF outbreaks were confirmed to be caused by ASFV belonging to genotypes II, IX and X (Fig. 2 and 3). Prior to 2015, the ASFV that caused ASF outbreaks in Tanzania clustered into genotypes II, IX, X, XV and XVI (Table 2) (19,21–23). The recent ASF outbreaks were caused by ASFV that were 100% genetically identical to previously reported viruses, for each of the genotype II, IX and X (Fig. 2). The identity of ASFV between previous and recent outbreaks and the pattern of disease spread strongly indicate domestic pig-to-pig transmission.
Prior to 2015, genotype II ASFV were reported during outbreaks in Southwestern and Eastern parts of Tanzania (Fig. 2 and 4) (16). Genotype II ASFV is thought to have been introduced into Tanzania in Kyela, a town in Southwestern Tanzania at the Tanzania - Malawi border following an outbreak in Karonga in 2010 (16). Since the introduction of genotype II ASFV, the virus spread within Southwestern parts of the country with occasional incursion into Eastern Tanzania (16). In the present study, we found that genotype II ASFV continued to circulate in previously reported areas and it spread into new areas of Central Tanzania (Fig. 4). Previously, ASF outbreaks in Eastern Tanzania were linked to outbreaks in Southwestern Tanzania due to transportation of live pigs for sale in the main commercial city of Dar es Salaam (16). The Southwestern part of the country is linked to Dar es Salaam by a major highway from Sumbawanga via Tunduma, Mbeya, Iringa and Morogoro (Fig. 4). Furthermore, the different ASF outbreaks between 2015 and 2017 due to genotype II ASFV occurred in various locations along and in the vicinity of the Morogoro - Dodoma highway, which branches off in Morogoro from the Tunduma - Dar es Salaam highway. The spread of the virus along these highways could be due to illegal transportation of infected domestic pigs from areas under quarantine, as described in previous reports (16,19).
Genotype II ASFV is highly virulent and has been reported to spread beyond its traditional geographical boundaries of Malawi, Mozambique and Zambia into Madagascar, Mauritius, Zimbabwe, Tanzania, the Caucuses region, Russia, Europe and Asia (13–15,25–27). The 2015-2017 Tanzanian ASFV p72 genotype II isolates clustered with ASFV p72 genotype II isolates that have been reported to cause outbreaks in Europe (Belgium (2018), Georgia (2007), Russia (2012), Belarus (2013), Lithuania (2014) and Poland (2014)), and China (2018) (12,14,28) (Fig. 3). If appropriate control measures of these genotype II viruses are not strictly enforced, we predict that this virus could possibly spread northwards and ultimately into bordering countries of Rwanda and Uganda, as these two countries are connected with Tanzania by major highways (Fig. 4). We recommend that stakeholders involved with ASF control be vigilant in order to prevent further spread of genotype II ASFV beyond Dodoma city, where it has reached.
In the present study, we found that ASFV genotype X circulated in Northeastern Tanzania, similar to other ASFV genotypes that have been previously described in the area (16,21). In addition, we found that genotype X ASFV has spread into new areas within Central and Western Tanzania (Fig. 4). The similarity of current ASFV to previously documented ASFV in Northeastern Tanzania indicates the continuous circulation of the virus with its maintainance in the domestic cycle. Additionally, phylogenetic analysis clustered current ASFV genotype X isolates with previously characterized ASFV isolates from Burundi (1999) and Kenya (2008) (10,23). The alignment of 404 nucleotide long sequence of the variable 3′-end of the B646L (p72) gene of ASFV Northeastern Tanzanian with 2008 ASFV outbreak in Kenya show only three nucleotide substitution (A→T, C→T, A→G) (18). The Northern Tanzania is characterized with presence of wildlife protected areas. In East and Southern Africa, the ancient sylvatic cycle have been reported to play part in the epidemiology of the disease (4,18,23). The isolation of ASFV from domestic pigs reports the circulation of these strains in the domestic pigs in Tanzania. However, this study point up for further isolation and epidemiological investigation to fully understand the variations, extent and potential sources of current ASFV in the region.
The ASFV genotype IX was confirmed to cause ASF outbreaks in Northwestern Tanzania. It was observed that ASFV genotype IX is restricted to Northwestern Tanzania, as it was 100% similar to ASFV isolates that caused ASF outbreaks in 2005 in Mwanza, Tanzania. Additionally, phylogenetic analysis revealed that the ASFV IX isolates were closely related to ASFV characterized in Uganda (2003), Kenya (2008) and Democratic Republic of Congo (2009) (18,27) . ASFV sporadic outbreaks in Northwestern Tanzania is highly likely due to uncontrolled movement of pigs and pig products from affected areas to unaffected areas. However, studies that focus on ASF outbreaks investigation between neighboring countries should be encouraged for understanding the potential source of such viruses, variation and extent.
The occurrence and spread of ASF between different parts of Tanzania is likely due to breach of quarantine imposed in areas affected with ASF. It is mostly likely that pig traders smuggle and transport pigs or pig meat from areas affected with ASF, where the prices are lower, into unaffected areas. Poor biosecurity measures in affected farms and slaughter slabs and swill feeding increase the likelihood of ASFV spread at a given locality, as has been previously described (16,21). Transportation of pig and pig products for regional market should be controlled to prevent ASFV spreading to other states of the East African Community, as ASFV genotype II has previously known to spread beyond its geographical range.