RETROVIRIDAE. Part II- LENTIVIRUSES PETER H. RUSSELL, BVSc, PhD, FRCPath, MRCVS Department of Pathology and Infectious Diseases, The Royal Veterinary College, Royal College Street, London NW1 OTU. Web siteWeb site

Objective Students should be able to: describe the differing immune-complex or dysfunction diseases associated with these viruses: Visna Maedi and caprine arthritis/encephalitis virus, equine infectious anaemia, feline immunodeficiency virus (FIV), bovine immunodeficiency virus. compare and contrast the diagnosis and pathogenesis of FIV and FeLV.

FELINE IMMUNODEFICIENCY VIRUS Once considered to be a major cause of lymphadenopathy and immunosuppression in cats, but now field work shows this is untrue. The virus is neither a death sentence to cats. Nor zoonotic.

Pathogenesis and clinical signs:

1)Regressive transient infection with no CD4 decline - subclinical, most UK cases.

2) Typical, progressive CD4 decline, 3 idealised stages Stage 1. High circulating virus for 1-10 weeks. Lymphoid depletion as a result of virus infection and destruction of double positive CD4CD8 Tcells. Thymic aplasia (most important in kittens). Stage 2. Asymptomatic carrier stage with a decrease in circulating virus effected by CD8 cells and polyclonal antibody. Paracortical T cells and lymphoid follicles expand in lymph nodes, spleen and thymus, this may be visible as white nodules.

3)Rapidly progressing infection with high virus and rapid depletion of CD4 cells in 40% of cats after vaginal exposure

Clinical signs of virulent viruses: Weight loss, pyrexia, anaemia, stomatitis, abortion, chronic rhinitis, recurrent cystitis, vomiting, recurrent skin infections, diarrhoea, abnormally high level of B cell lymphomas ‑ in combinations but not all together.

Control: No vaccine. Early experimental vaccines exacerbated disease. If a cat is healthy and positive get a confirmatory test done at Glasgow. If the cat is positive then keep it isolated. Addie now considers the cats protection league's policy of killing any healthy ELISA positive cat to be unnecessary.

BOVINE IMMUNODEFICIENCY VIRUS MAFF consider it to be absent from UK and of little clinical relevance.

VISNA ‑ MAEDI VIRUS (V-M V) (or Maedi-Visna) The virus: Isolates are neurotropic or pneumotropic and cause chronic wasting disease in sheep. Variants which escape neutralisation arise during the infection (as with HIV and Equine Anaemia Virus). Visna = Icelandic for wasting; Maedi = Icelandic for dyspnoea.

Immunity and epidemiology: Infected animals remain carriers. Transmission is via aerosol, milk, or colostrum. The crowded 6 month winter housing as practised in Iceland allowed spread not only of V-MV but also of Johne's disease and SPA. Infection was introduced into the UK by French exotic breeds and carriers occur in one third of flocks containing imported sheep (particularly Texel).

CAPRINE ARTHRITIS/ENCEPHALITIS VIRUS This is caused by a distinct virus which is serologically related to V ‑ MV but has antigenic and genetic differences in its envelope. It causes an encephalitis of kids or an insidious polyarthritis of adults.

EQUINE INFECTIOUS ANAEMIA (EIA) (Swamp fever) EIA is an exotic type III immune- complex haemolytic disease of horses of great importance in the USA. The disease is notifiable in the UK.

Immunity and epidemiology: Animals can be virus-carriers for several years despite having antibody. Transmission is by the mechanical transfer of leucocytes via mosquitoes, flies, syringe-needles, semen and milk.

Control: UK. Horses can only be imported from virus-free premises. However EIA is widespread in damp areas of Germany and Italy and so could enter the UK.

Summary of both lectures The retroviruses all contain RT and all integrate and mutate, but do not cause acute disease. This replication results in latency and persistence and chronic diseases eg tumours, immunosuppression and immune complex disease. FeLV, but not FIV, is the most important immunosuppressive viral diseases of cats in the UK and suspect cats are blood tested for antigen.

Summary of both lectures (cont.) The viruses are labile and cell associated and so close contact (eg in cat colonies or winter –housed sheep), or sources of leucocytes, eg blood, milk and semen, transfer them Vaccines are not usual except to FeLV.

Summary of both lectures (cont.) Visna maedi virus of sheep is endemic and causes a T-cell based disease whereas EIA is exotic and causes complement-mediated haemolysis. Both involve bouts of variant viruses and immune complex disease. The bovine viruses are of little importance in the UK to date.