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Background: African swine fever (ASF) is an infectious transboundary animal disease which causes high mortality, approaching 100% in domestic pigs and it is currently considered as the most serious constraint to domestic pig industry and food security globally. Despite regular ASF outbreaks within Malawi, few studies have genetically characterized the causative ASF virus (ASFV). This study aimed at genetic characterization of ASFV responsible for the 2019 outbreak in northern Malawi. The disease confirmation was done by polymerase chain reaction (PCR) followed by molecular characterization of the causative ASFV by partial genome sequencing and phylogenetic reconstruction of the B646L (p72) gene, nucleotide alignment of the intergenic region (IGR) between I73R and I329L genes and translation of the central variable region (CVR) coded by B602L gene., Results: All thirteen samples collected during this study in Karonga district in September 2019 were ASFV-positive and after partial genome sequencing and phylogenetic reconstruction of the B646L (p72) gene, the viruses clustered into ASFV p72 genotype II. The viruses characterized in this study lacked a GAATATATAG fragment between the I173R and the I329L genes and were classified as IGR I variants. Furthermore, the tetrameric amino acid repeats within the CVR of the B602L gene of the 2019 Malawian ASFV reported in this study had the signature BNDBNDBNAA, 100% similar to ASFV responsible for the 2013 and 2017 ASF outbreaks in Zambia and Tanzania, respectively., Conclusions: The results of this study confirm an ASF outbreak in Karonga district in northern Malawi in September 2019. The virus was closely related to other p72 genotype II ASFV that caused outbreaks in neighboring eastern and southern African countries, emphasizing the possible regional transboundary transmission of this ASFV genotype. These findings call for a concerted regional and international effort to control the spread of ASF in order to improve nutritional and food security.

The presence of virus replication centers in neutrophils from pigs inoculated with a highly virulent strain of African swine fever virus is described for the first time in vivo. Virus antigens were observed from 3 days post-inoculation (dpi) onwards by means of an immunohistochemical technique. At this time (3 dpi), transmission electron microscopy studies revealed the presence of large amounts of neutrophils in the vascular lumens. At 5 and 7 dpi, neutrophils with phagosomes frequently contained virus particles. In addition, within the cytoplasm of some mature and immature neutrophils, both viral particles and virus replication centers were observed at 5 and 7 dpi.

This paper reports the presence of mature viral particles within the lymphocytes of samples taken from pigs inoculated with a highly virulent African swine fever (ASF) virus isolate (Malawi 83), and the adhesion of the lymphocytes to macrophages containing the virus replication sites. Virus replication in lymph-node medullar tissue macrophages was observed from 3 days post inoculation (pi). At 3 days pi, transmission electron microscopy revealed hemadsorption in some infected macrophages. At 5 and 7 days pi, a number of macrophages with virus replication were surrounded by a ring of lymphocytes. In such cases, mature viral particles were observed in membrane evaginations of the infected cell that corresponded to invaginations of the lymphocyte membrane. Also at 5 and 7 days pi, mature virions were observed within the cytoplasm of some lymphocytes. However, incomplete virions and African swine fever virus replication sites were not observed within the lymphocytes.

A serological survey was conducted in July 1991 on domestic pigs in two areas of southern Malawi which were severely affected by the African swine fever (ASF) epizootic in 1989-1991. Sixty-six of the 216 owners questioned reported having witnessed ASF in their pigs. Forty-seven owners had pigs with antibodies against ASF virus, and the overall prevalence of pigs with anti-ASF virus antibodies was found to be 12.4%, in 445 pigs sampled in 35 villages. Spread of ASF was thought to occur principally through the slaughter and sale of infected animals, and due to the free-ranging of pigs. Permanent penning of pigs significantly reduced the attack rate (chi 2 = 7.59, P < 0.01, 1df) in pig pens in Thyolo, an area where permanent penning of pigs was widely practised. Feeding of kitchen scraps did not appear to have been an important means of virus spread. Ornithodoros ticks were found in only 1 of the 35 villages. Although virus was not isolated from 203 pooled sera from pigs in the Mulanje district or from collected ticks, the seroconversion of a small proportion of pigs born after the last reported date of ASF occurrence suggests that the virus had continued to circulate to a limited extent in this area.

Sequence analysis of the SalI g, h, i and j restriction fragments of the African swine fever virus (ASFV) genome from the virulent isolate Malawi (LIL20/1) identified three open reading frames (ORFs) encoding predicted proteins of 125.0K (g10L), 80.4K (j10L) and 58.0K (j11L) which showed homology to members of the DNA and RNA helicase superfamily. ORFj10L was related to protein 4 of the Kluyveromyces lactis killer plasmid pKG12 and to two putative helicases, D6R and D11L, of vaccinia virus. ORF g10L was most closely related to ASFVj10L and to vaccinia virus D11L. ORFj11L was homologous to A18R, a third putative helicase of vaccinia virus. The possible functions of these genes in the replication of ASFV are discussed and the evolutionary implications are considered.

A detailed study was made in 1983-5 in three villages in Mchinji district in the African swine fever (ASF) enzootic area of Malawi, following an outbreak of ASF which affected all three villages. Ticks of the Ornithodoros moubata complex were collected from both pig sties and houses shortly after the outbreak, and approximately 24% contained ASF virus. The proportion of ticks infected did not differ significantly in the three villages, or more surprisingly in different types of premises, and was equivalent in all stages of ticks. The proportion infected decreased with the passage of time, but infected ticks were still present in all three villages 8 months after the outbreak, some with high titres of virus. The proportion of seropositive pigs in the three villages approached 100% following the outbreak, with many apparently healthy pigs being seropositive. It is suggested that Malawian isolates of ASF virus may be less virulent in African than European breeds of domestic pig.

DNA prepared from a field isolate of African swine fever virus, which causes high mortality and severe disease in domestic pigs, was cloned in bacteriophage lambda and plasmid vectors. Clones containing DNA inserts overlapping with each other and together covering the complete genome, apart from short fragments close to the cross-linked termini of the genome, were obtained. A complete restriction enzyme site map of the genome for three enzymes was deduced.

Ticks of the Ornithodoros moubata complex were collected from domestic pig sties and dwelling houses, and from a warthog habitat, and tested for the presence of African swine fever (ASF) virus. Collections were made in 9 of the 24 districts of Malawi, these being primarily the districts in which O. moubata is most numerous. ASF virus was isolated from ticks collected in both domestic pig sties and houses in certain villages in Mchinji district where ASF outbreaks had recently occurred. Mchinji district is in the centre of a large ASF enzootic area which stretches into other districts of Malawi and also into Zambia and Mozambique. The high titre of virus in some of the ticks demonstrates that O. moubata can act as a virus reservoir and potential vector of disease in the field situation in Malawi.