Jaagsiekte

D W VERWOERD, R C TUSTIN, C V HALLWIRTH AND D F YORK

GENERAL INTRODUCTION: RETROVIRIDAE

Introduction

Jaagsiekte (JS) is a contagious bronchiolo-alveolar carcinoma of the lungs of sheep caused by a retrovirus. It was first recognized in South Africa more than a century ago,^{84, 89} when a detailed description of its clinical signs and macroscopic pathology was published by Hutcheon in 1891, along with evidence of its infectivity and an account of attempts to control it.³² This was followed by reports from England,³⁴ Germany²² and France.¹ The disease is now known to have an almost worldwide distribution and to have considerable economic impact in countries with substantial sheep populations. In South Africa, a survey over a 40-year period indicated that JS is by far the most common neoplasm reported in sheep (64 per cent).⁷

In the early literature, the disease was often confused with other lung conditions of sheep. The most important of these is maedi, which is caused by a lentivirus and characterized by chronic, non-neoplastic inflammatory lesions.⁵⁸ Today it is known that the two diseases are closely associated in many countries and are often present as co-infections in the same animal, which adds to the existing confusion.^{16, 32, 39, 64, 79} A list of the diseases of livestock caused by retroviruses is given in Table 59.1.

Table 59.1 Diseases of livestock caused by members of the family Retroviridae

*the animal species given in the brackets are uncommon hosts

Aetiology

Even though JS was known to be contagious long before its neoplastic nature had been elucidated, many years of research in various countries failed to reveal its causal agent, although filtration experiments suggested a viral aetiology. The first successful experimental transmission of the disease by co-habitation was reported by De Kock,¹³ yielding results that were to be confirmed in Iceland nine years later.²⁰ An ovine herpesvirus was the first candidate virus isolated,^{12, 17, 35, 37} but transmission and molecular hybridization experiments proved that this virus is a passenger and not directly involved in the aetiology of the disease.¹⁸

The possibility of a retroviral aetiology was first suggested by the observation in Israel of typical retrovirus particles in adenomatous lesions.⁶⁸ Retroviruses were also found in cell cultures established from JS-affected lungs³⁷ and reverse transcriptase (RT) activity in extracts of tumour tissue.⁷⁰ However, no transmission attempts were made in these studies and the results were difficult to interpret in view of the possible presence of maedi-visna lentivirus in the material studied.

Several lines of evidence cumulatively implicated a retrovirus called Jaagsiekte Sheep Retrovirus (JSRV) in the aetiology of JS:^{14, 25, 56, 88}

• Transmission studies showed that the disease could be reproduced by inoculation with cell-free extracts of tumours or cultured tumour cells, or with concentrated lung fluid containing RT activity characteristic of retroviruses. ^{27, 41, 65, 75, 90}
• Serial transmission in new-born lambs with semi-purified lung-rinse material containing RT activity with a magnesium ion preference resulted in incubation periods inversely proportional to the RT level in the inocula.⁹³
• Serological studies showed that both lung fluid and tumour homogenate contained proteins that cross-react with antisera to capsids and nucleocapsids of type D and type B retroviruses.⁷⁷
• Retrovirus-like particles were observed by electron microscopy in lung tumours and lung fluid of affected sheep.^{62, 68}
• The full genome of a JSRV isolated from the lungs of a JS affected sheep was cloned and sequenced, and genomic organization typical of typeDand B retroviruses shown.^{97, 98}
• JSRV was found to be consistently, specifically and absolutely associated with JS.⁵³

The above evidence did not prove unequivocally that infection with JSRV is sufficient to induce JS, and the possibility existed that JSRV was acting as a helper virus, complementing a hitherto undiscovered acutely transforming retrovirus. Further definition of the role played by JSRV in JS was hampered by the lack of an in vitro culture system for the virus. The precise nature of the association of JSRV with the disease was not known until Palmarini and co-workers constructed an infectious molecular clone derived from an integrated proviral exogenous JSRV sequence (called JSRV₂₁) isolated from a spontaneous case of JS.⁵⁵ The JSRV₂₁ genome was placed under the transcriptional control of the human cytomegalovirus (CMV) immediate early promoter. The promoter was positioned in such a way that the resulting RNA transcript would be very similar to wild-type exogenous JSRV RNA, giving rise to the plasmid pCMV2JS_21. The latter was transferred into the highly transfectable human 293T cell line, from which substantial amounts of JSRV₂₁ virus were released into the supernatant. Concentrated stocks of JSRV₂₁, obtained from transfected 293T cells, were used for intratracheal inoculation of new-born lambs. Two of the lambs developed JS (confirmed at the histological...