Feline Leukemia Virus

T he feline leu k em ia v iru s (FeLV) is the cau sa tiv e ag e n t o f th e m o st im p o rta n t fata l in fec tious-d isease com plex of A m erican d om estic cats today . It is a h o rizo n ta lly tra n sm itte d (i.e., con ta­ gious), ribonucleic-acid (RN A ) v iru s b e lo n g in g to th e fam ily R etrov iridae . R e trov iru ses cau sin g ly m p h o id tu ­ m o rs h a v e b ee n id en tified in a n u m ­ b e r of an im a l species, in c lu d in g cats, cattle, d om estic fow l, n o n h u m a n p r i­ m ates , a n d ro d en ts . A re tro v iru s k n o w n as th e human T-lymphotropic virus (HTLV) has b ee n iso la ted from p eo p le w ith ce rta in agg ressive ly m ­ p h o id m alignancies; d a ta ind ica te th a t th is v iru s is in d e ed in v o lv ed in th e d ev e lo p m e n t o f these tum ors. T he oncogen ic re tro v iru ses a re collectively re ferred to a s th e RNA tumor viruses, o r oncornaviruses (oncogenic RNA viruses). In ad d itio n , th e re a re o th e r m e m b ers o f the R e trov iridae fam ily, th e lentiviruses, th a t in d u c e n o n m a lig n a n t d isease p rocesses su c h as p ro g ress iv e p n e u ­ m o n ia of sheep , arth ritis-encephalitis o f goats, in fec tious an em ia of ho rses, a n d ac q u ired im m u n o d efic ien cy sy n ­ d ro m e of h u m a n b ein g s (AIDS). A n e w a n d u n iq u e len tiv iru s, th e feline immunodeficiency virus (FTV), w a s iso­ la ted a n d cha rac te rized in th e m id 1980s. FTV causes a n im m u n o ­ defic iency s y n d ro m e in ca ts sim ila r in m a n y respec ts to th a t p ro d u c e d by h u m a n im m u n o d efic ien cy v iru s (HIV) in peop le . R e trov iru ses n o t y e t associ­ a ted w ith a n y reco g n ized d isease process a lso exist, su c h as the syncytium-forming ("foam y") v iru ses th a t h a v e b ee n reco v ered from a n u m ­ b e r o f species, in c lu d in g cats. R ep lication -com peten t re tro v iru ses ca rry w ith th e m a n en z y m e called reverse transcriptase, w h ich is capab le of m a k in g a deoxyribonucleic-ac id (D N A ) copy o f th e re tro v ira l RN A . T he D N A co p y can b e in se rted in to th e ch ro m o so m al D N A o f th e in fected cell. T his a lien in tru d e r, k n o w n as a provirus, o r proviral DNA, is rep licated w h e n e v e r th e h o st cell d iv id e s a n d can se rv e a s a te m p la te fo r th e p ro d u c ­ tio n o f n e w v iru s particles. T he n ew partic les a re assem b led in th e cy to­ p la sm of th e cell a n d th e n a re re leased b y b u d d in g th ro u g h th e cell m e m ­ b ran e . D u rin g th is b u d d in g p rocess from th e cell, the v iru s partic les o r virions acq u ire a n o u te r lip id envelope a n d su rface p ro jec tions k n o w n as knobs, spikes, o r peplomers. T he p ep lo m ers a re c o m p o sed o f g ly co p ro ­ te in a n d h av e specific epitopes o r re ­ g ions th a t a ttach in a specific w a y to recep to rs o n th e su rface of suscep tib le cells, en ab lin g th e n e w v irions to in ­ fect th ese cells a n d th u s p e rp e tu a te the v iru s in fection cycle. A t p re se n t th e re is n o prac tica l m e th o d o f rem o v in g , o r excising, in te g ra te d p ro v iru ses from infected cells o f liv ing o rgan ism s. A cell in ­ fected w ith a n in te g ra te d re tro v iru s is in fec ted fo r th e le n g th of its lifetim e, as a re all of its p ro g en y cells. T his is of im p o rtan ce in la ten t FeLV infections a n d in th e persis tence of in trace llu la r FeLV d u r in g a n ac tive im m u n e re ­ sponse . B ecause a v ersio n o f th e ir genetic m a te ria l can b ecom e a p a r t o f th e to ta l g enetic in fo rm atio n o f th e cells th e y infect, re tro v iru ses a re a m o n g th e m o st in tim ate p a ras ite s k n o w n in n a tu re .