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. In the present study, these outbreaks were confirmed to be ASF 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 1) (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 3) (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. 3). 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. 3). 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 (13–15,24–26). 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. 3). 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 of in, similar to viruses that have been previously described (16,21). In addition, we found that genotype X ASFV has spread into new areas within Central Tanzania (Fig. 3). However, genotype IX was found restricted to Northwestern Tanzania, similar to what is reported in previous studies.
The occurrence and spread of ASF between different parts of Tanzania is likely due to bleach of quarantine imposed in areas affected with ASF. It is mostly likely 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).