Studies on epidemiology and socio-economic impact associated with African Swine Fever 2015 – 2017 outbreaks in Tanzania

Abstract

African swine fever (ASF) is a highly fatal hemorrhagic disease of domestic pigs caused by ASF virus (ASFV) that can cause mortalities reaching up to 100%, depending on the virus strain. There have been sporadic ASF outbreaks in Tanzania that have affected food security and the livelihoods of pig farmers. Previous studies have reported the genetic nature viral strains that caused ASF outbreaks in Tanzania. The present study was conducted to investigate whether new or already described ASFV strains were involved during the November 2015 to June 2017 ASF outbreaks in different parts of Tanzania. In addition, the socio-economic impact and risk factors that are responsible for the occurrence and spread of ASF outbreaks in Tanzania were investigated. The study involved visits to slaughter facilities, pig farms and pig markets. Clinical signs were observed in pigs suspected with ASF prior to sample collection and postmortem examination was undertaken to the dead pigs. Tissue samples including spleen, lymph nodes and kidney were collected from a total of 124 dead pigs during reported outbreaks. A semi-structured questionnaire was used to investigate the ASF risk factors and its socio-economic impact to the farmers. The presence of ASFV in collected samples was detected by polymerase chain reaction (PCR) by partial amplification of the p72 (B646L) gene using peste porcina Africana (PPA1/2) primers. Genetic characterization was conducted in samples that were positive for ASFV by amplification, by nucleotide sequencing of the variable 3’-end of p72 (B646L) gene using primers p72U/D. During the survey, suspected outbreaks of ASF were reported in Kalambo, Ileje, Mbarali, Rungwe, Mbeya Municipality, Mbozi, Kongwa, Dodoma Municipality, Mpwapwa, Gairo, Temeke, Mvomero, Morogoro Municipality, Kibaha, Bukoba, Magu, Ngara, Babati, Mwanza, Manyoni and Kigoma districts. The clinical signs observed in affected pigs included sternal recumbency, cutaneous congestion on the outer side of the pinna, abdomen and limbs, inappetence, staggering gait, shivering, hyperthermia and abortion in pregnant sows. Upon nucleotide sequencing and phylogenetic analysis, genotype II of ASFV was found in domestic pigs from Mbarali, Rungwe, Mbeya Municipality, Kalambo, Ileje, Mbozi, Kongwa, Dodoma Municipality, Mpwapwa, Gairo, Temeke, Mvomero, Morogoro Municipality and Kibaha districts, genotype IX in domestic pigs of Bukoba, Magu and Ngara districts while genotype X was found in samples collected from Babati, Mwanza, Manyoni and Kigoma districts. The spread of genotype II ASFV into Central and Eastern Tanzania, from southern highlands of Tanzania seems to be along the Tunduma-Dar es Salaam and Morogoro-Dodoma roads. This signifies a risk of further spread of genotype II of ASFV northwards along Dodoma Mwanza road and to the neighboring countries. Sharing of farm equipment was found to be significantly associated with the spread of ASFV (OR=2.47, CI95%=1.4-99, P=0.023). The possible occurrence and spread of ASFV in Tanzania is within the domestic cycle rather than sylvatic cycle. The presence of ASF was found to lead to the financial losses, loss of income, unemployment, mental disturbance and poor livelihoods. Also respondents failed to meet medical expenses, farm labor expenses and school fees for their children. The disease poses a great threat to the pig industry and food insecurity. Further studies are recommended in order to fully sequence ASFV isolates obtained from the present study in so that to fully understand the genetic relatedness, evolution and epidemiology of ASFV in the country.