The growth of a virulent strain of African swine fever virus in domestic pigs

Abstract

SUMMARYPigs were infected by the intranasal instillation of a large dose (ca. 107·0 ID 50) of a highly virulent strain of African swine fever virus (ASFV) and the progress of the infection was studied by the ‘routine titration approach’ (Mims, 1964) using pig bone marrow cultures.Virus growth was established within 16–24 hr. in the retropharyngeal but not in the alimentary or nasal mucosae or the tonsils. By 24–40 hr. the virus was consistently present in the retropharyngeal lymph nodes, almost invariably the medials; titres in these nodes exceeded those in the associated mucosa by 48–72 hr. Generalization, presumed to have occurred via the tracheal lymph ducts and the blood stream, was generally demonstrable after 72 hr., i.e. by the time of the onset of pyrexia or 24 hr. prior to this.On average 11% of the total infectivity in the blood was present in the plasma, with the rest assumed to be cell-associated. A mean of about 45% of the total infectivity was recovered in erythrocyte fractions in which the concentration of leucocytes had been materially reduced; fractions with increased leucocyte counts contained relatively little virus and it was concluded that the great majority of circulating virus was closely associated with the erythrocytes. Adsorption of ASFV to normal pig erythrocytes was demonstrated in vitro.The greatest concentrations of virus were recorded in the lymph nodes, especially those of the cephalic region, and in the spleen, where titres commonly attained 108·0 to 109·0 HAD 50/g. and exceeded those in the blood. They were, therefore, thought to be the source of much circulating virus, although there was some evidence that the liver, lungs and bone marrow may also have contributed, at least in some animals. There was no evidence that the mucosae of the alimentary and respiratory tracts or the kidney, myocardium and brain were a source of significant amounts of virus. The virus demonstrable in Peyer's patches did not exceed that in the intervening ileal mucosa.Although contact transmission of ASF does not normally occur during the first 12–24 hr. of fever, considerable amounts of virus were usually present in the nasal and intestinal mucosae at 72 hr. It was probable that this infectivity was due to the blood content and that excretion did not occur until the epithelium was breached.Three pigs, all of which had lesions of a portal cirrhosis, showed a delayed or restricted generalization of virus, in comparison with the other twenty-eight animals which behaved according to a regular pattern.We are grateful to Mrs M. O'Sullivan and Mrs E. Douglas for painstaking technical assistance and to Mr C. S. Rampton, A.I.M.L.T., for the preparation of the figures.

The excretion of two virulent strains of African swine fever virus by domestic pigs

“…Failure of the virus to spread to susceptible pigs in close contact with infected animals during their first 12-24 hr. of pyrexia must, therefore, be due to inadequate degrees of contamination of the environment, including food and water, rather than the absence of virus excretion, as suggested by Plowright et al (1968). The dose of ASFV necessary to infect pigs by natural routes has not been determined accurately, but Heuschele (1967) quoted Maurer et at.…”

“…Virus strains (a) The Tengani strain caused an epizootic of ASF in Malawi in 1962 (Cox & Hess, 1962) and was known to cause a rapid lethal disease in domestic swine, the mean death-time being 2-9 days (Plowright et al 1968). The virus was stored at -700 C. in the form of a 10 % (w/v) spleen suspension which, when titrated in pig bone marrow (PBM) cultures, had a titre of 107 HAD 50/ml.…”

“…Montgomery also showed that reacting pigs are not infectious to stallmates during the first 12-24 hr. of fever, an observation which has been repeated by Plowright, Parker & Staple (1968). Scott (1965) suggested that pigs are infected through nuzzling or ingestion of fomites, a hypothesis supported by recent work on the pathogenesis of the disease (Heuschele, 1967;Plowright et al 1968;Colgrove, Haelterman & Coggins, 1969).…”

“…Washed cell suspensions were prepared from the long bones of pigs by the method already described (Plowright et al 1968). The cells were suspended in medium to a concentration of 7-9 x 106/ml.…”

“…viraemia were used to prepare a reference curve for each group of pigs, indicating the time of generalization and reflecting the level of virus proliferation in the predilection tissues (Plowright et al 1968;Colgrove et al 1969). The earliest demonstrable viraemia, 2 days before the onset of fever, occurred in two pigs infected by contact with each strain; all animals, except one Tengani contact case, were viraemic with the onset of fever.…”