Title page for ETD etd-08082012-102301


Document Type Doctoral Thesis
Author Pini, Attilio
URN etd-08082012-102301
Document Title Strains of African swine fever virus isolated from domestic pigs and from the tick Ornithodoros Moubata in South Africa
Degree DVSc
Department Companion Animal Clinical Studies
Supervisor
Advisor Name Title
Prof P G Howell Supervisor
Keywords
  • viruses
  • African swine fever
  • domestic pigs
Date 1977-08-01
Availability unrestricted
Abstract
Between 1973 and 1975, 21 outbreaks of ASF were confirmed in the endemic area of the northern Transvaal after an interval of 10 years in which the disease was silent. The new series of outbreaks coincided with the isolation, for the first time in South-Africa, of two HAd- strains of ASF virus. The first of these virus isolates, Lillie-148, was obtained from swine which, judging from circumstantial evidence, had been infected by a warthog carrier of virus. The pigs on the farm were affected by a form of disease with a lower pathogenicity than that observed in previous epidemics. The second HAd- strain 24823 was obtained from a case from which neither clinical nor pathological observations were available. From the results of the biological tests carried out at the laboratory, however, it was deduced that the disease in the field may have had a chronic course.

When the carrier status of populations of the argasid tick Ornithodoros moubata collected from warthog burrows was investigated, it was found that the situation in South Africa is analogous to that in East Africa. Twenty five per cent of burrows were found to be infected; the mean infective titres of the tick suspensions varied between 104,5 and 105,2 BCHAd50 and the mean percentage of infected argasids varied between 1,62 and 3,45. Infected ticks were also found in the Marico district, which is adjacent to the endemic area, but ASF has never been recorded there. From tick suspension TS237, showing both delayed and reduced haemadsorbing effect in huffy coat cell cultures, a HAd population of ASF virus was segregated.

From these observations it was inferred that ASF virus may mutate from the HAd+ to the HAd- form in the primary virus reservoir. Furthermore, the virus appears to be evolving towards less pathogenic forms irrespective of prior adaptation of the infectious agent to domestic stock under the epidemiological conditions prevailing in South Africa.

During this investigation it was found that LLC-MK2 cell cultures were susceptible to ASF virus. Cytopathic effects were observed in primary isolation and peak infectivity coincided with complete destruction of the cell monolayers, attained after three to four serial passages. The sensitivity of LLC-MK2 cells for estimating the virus content of porcine tissues was in two instances comparable to that of huffy coat cells, but in another three cases it was 100 to 1000 times lower. It was concluded that LLC-MK2 cells were a suitable complement to huffy coat cultures for the cultivation of ASF virus, particularly for HAd- isolates.

After 35 to 45 serial passages in LLC-MK2 cells the HAd+ strains of ASF virus lost their haemadsorbing characteristics. A similar mutation, but more gradual, was also observed in huffy coat cell cultures.

The feasibility of plaque production was studied in LLC-MK2 cell monolayers. Plaques were obtained with all the strains studied, irrespective of their adaptation to LLC-MK2 or buffy coat cells when 0,4% Agarose was used as a solidifying agent. The diameter of plaques ranged from 0,3 to 3,0 mm and this characteristic was unrelated to the haemadsorbing properties of the strains used. Plaque technique was successfully used to detect the presence of HAd- virus particles in HAd+ populations by subculturing selected virus-plaques into buffy coat cultures.

The results of biological tests suggested that HAd strains have a reduced virulence which can vary within broad limits. The experience with strain Lillie-148 and 24823 showed that either acute or chronic or subclinical disease can follow infection of pigs with these isolates of virus. The results obtained with the two virus populations of strain TS237 emphasized the different degree of patho-genicity between HAd+ and HAd- virus. While the former was responsible for a peracute or acute form of disease, the latter produced chronic or subclinical infections. In pigs mild forms of ASF also developed following the inoculation of HAd+ strains obtained after serial passages in cell cultures. It was concluded that haemadsorption and pathogenicity are two characteristics that are not linked and can be modified independantly.

AFRIKAANS : Gedurende die tydperk 1973-1975 het Afrikaansevarkpes (AVP), na 'n afwesigheid van 10 jaar, weer sy verskyning gemaak in die endemiese gebied van Noord Transvaal en altesaam 21 bevestigde geval1e is aangemeld. Die nuwe reeks uitbrake het sa.amgeval met die eerste isolasie in Suid-Afrika van twee HAd- stamme van AVP. Die eerste virusstam wat geisoleer is, was Lillie-148. Hierdie virusstam is geisoleer van 'n vark wat volgens omstandigheidsgetuienis deur 'n v1akvark besmet is. Die virus waarmee die varke op die plaas besmet is, het 'n laer patogenisiteit gehad as virusse van vorige uitbrake. Die tweede HAd- stam nl. 24823 is verkry van 'n geval waar geen kliniese of patologiese waarnemings beskikbaar was nieo Uit die resultate van laboratoriurntoetse is die gevolgtrekking gemaak dat die siekte wel moont1ik 'n kroniese verloop kon gehad het.

Uit ondersoeke na die vektorstatus van populasies van die sagte bosluis Ornithodoros moubata, wat verkry is uit vlakvarkgate, is gevind dat die situasie in Suid-Afrika soortgelyk is aan die in Oos-Afrika. Daar is bevind dat 25 persent vlakvarkgate besmet is; dat die gemiddelde virus konsentrasies van bosluissuspensies varieer tussen 104,5 en 105,2 BCHAd50 en dat die gemiddelde persentasie van besmette bos1uise wissel tussen 1,62 en 3,45. Besmette bosluise is ook aangetref in die Marico-distrik wat aangrensend is aan die ensoötiese gebied en waar AVP nog nooit voorgekom het nie. 'n HAd- populasie van AVP virus is geisoleer van 'n bosluissuspensie, TS237, wat in wit selkulture 'n vertraagde en verminderde heem-adsorberende effek getoon het.

Uit hierdie waarnemings is die gevolgtrekking gemaak dat AVP virus in die aanvanklike virus reservoir instaat is om van HAd+ na HAd- te muteer. Hieruit blyk dit dat onder die huidige epidemiese toestande, wat tans in Suid-Afrika heers, die virus skynbaar verander na 'n vorm van laer patogenisiteit. Dit geskied ongeag vroeëre aanpassing van die infektiewe agens by die plaaslike varkpopulasie onder heersende epidemiologiese toestande in Suid-Afrika.

Gedurende hierdie ondersoek is dit aangetoon dat LLC-MK2 selkulture vatbaar is vir AVP virus. Primere virus isolasies toon sitopatogeniese effekte. Infektiwiteit bereik 'n piek na drie tot vier agtereenvolgende oorspuitings met algehele vernietiging van sellae. Die gevoelligheid van LLC-MK2 selle vir die bepaling van die virus inhoud van varkweefsel was in twee gevalle vergelykbaar met die van wit selle. In drie ander gevalle was dit 100 tot 1000 keer laer. Die gevolgtrekking is gemaak dat LLC-MK2 selle 'n geskikte aanvulling is vir wit selkulture vir die kweek van AVP virus, veral vir HAd- isolate.

Die heem-adsorberende eienskappe van die HAd+ stam van AVP virus het verlore gegaan na 35 - 45 agtereenvolgende oorspuitings in LLC-MK2 selle. In wit selkulture is 'n soortgelyke mutasie waargeneem, hoewel dit meer geleidelik plaasgevind het.

Die moontlikheid van plaket vorming in LLC-MK2 sellae is ondersoek. Wanneer 0,4% agarose as stollingsagens gebruik is, het alle stamme wat ondersoek is plakette opgelewer ongeag of hulle aangepas was vir LLC-MK2 selle of wit selle. Plakette se deursnee het gewissel tussen 0,3 en 3,0 mm. Hierdie eienskap is egter nie gekorreleerd met die betrokke stamme se heem-adsorberende eienskappe nie. Die teenwoordigheid van HAd- virus partikels in HAd+ populasies is aangetoon deur subkulture van geselekteerde plakette in wit selkulture te maak.

Uit die resultate van biologiese toetse is die gevolgtrekking gemaak dat die HAd- stamme 'n verlaagde virulensie het wat kan wissel tussen wye grense. Die ondervinding met stam Lillie-148 en stam 24823 het aangetoon dat varke wat met hierdie virus stamme besmet raak akute, kroniese of subkliniese siekte toestande ontwikkel. Die graad van verskil tussen die patogenisiteit tussen HAd+ en HAd- virus is beklemtoon deur die resultate wat verkry is met die twee virus populasies van starn TS237. Die HAd+ stam veroorsaak perakute of akute vorms van die siekte terwyl HAd- starn kroniese of subkliniese infeksie tot gevolg het. Matige vorme van AVP is ook verkry nadat varke geinokuleer is met 'n HAd starn wat 'n aantal oorspuitings in selkulture ondergaan het. Die afleiding is gemaak dat heem- adsorpsie en patogenisiteit twee eienskappe is wat nie verbonde is nie en dus onafhanklik van mekaar gemodifiseer kan word.

Copyright © 1977, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria

Please cite as follows:

Pini, A 1977, Strains of African swine fever virus isolated from domestic pigs and from the tick Ornithodoros Moubata in South Africa, DVSc thesis, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-08082012-102301 / >

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