Lecture 1 -- Retrovirus (Feline leukopenia + Feline immunodeficiency virus)

These flashcards cover key concepts regarding feline and canine infectious diseases, specifically focusing on Feline leukaemia virus (FeLV), Feline immunodeficiency virus (FIV), and their implications in veterinary medicine.

What are the key structural features of a retrovirus?

• Enveloped viruses with an RNA genome

What are the characteristics of an enveloped viruses?

Envelope is made of lipids, which are dissolved by soap, detergents, alcohol and disinfectants, disrupting the viral envelope and inactivating the virus.

What are the characteristics of a RNA viruses?

RNA viruses require an RNA polymerase to replicate their genome BUT enzyme lacks proofreading activity = Tend to mutate

List out the key structure of a retrovirus

• The RNA genome of a virus and reverse transcriptase are enclosed within a protein coat (capsid)

• In enveloped viruses, this nucleocapsid is further surrounded by a lipid envelope

• Embedded within this lipid envelope are viral glycoproteins, which are essential for attachment to host cell receptors and entry into the host cell

Retrovirus Life Cycle

How does a retrovirus enter a host cell?

It binds to specific receptors on the target cell → The viral envelope fuses with the host cell membrane, releasing the viral core into the cytoplasm.

What is reverse transcription in retroviruses?

Viral reverse transcriptase converts viral RNA into DNA

What happens during integration?

Once viral DNA enters the host nucleus, integrase inserts it into the host genome →The integrated DNA is called a provirus

What happens once the provirus is integrated?

The provirus acts like a host gene → Host machinery transcribes DNA into viral mRNA (for protein) and genomic RNA (for new virus particles).

What are the main subtypes of FeLV?

Subtypes A, B, and C

Which subtype is transmissible?

Only Type A is transmissible between cats

How do Types B and C arise?

• Type B: Recombination of Type A with endogenous retroviruses

• Type C: Mutation of Type A genome

What are the main routes of FeLV transmission?

Require close contact

• Saliva, urine, feces, milk, and vertical (transplacental) transmission

Explain the feline leukopenia pathogenesis

Describe the antigen and antibody levels in FeLV during primary viraemia

• Antigen: High

• Antibody: Negative

• Explanation: Virus is replicating rapidly; antibodies have not developed yet.

Describe the antigen and antibody levels in FeLV during persistent viraemia

• Antigen: Positive

  • Explanation: Ongoing infection

• Antibody: Positive

  • Explanation: Immune system produces antibodies as virus persists

• Viral load: High

Describe the antigen and antibody levels in FeLV during persistent latent infection

• Antigen: Negative

  • Explanation: Strong immunity so the virus is cleared → No circulating antigen

• Antibody: Positive

  • Explanation: Antibodies remain from previous exposure but do not indicate current infection

What factors influence susceptibility to persistent FeLV infection?

• Virus dose/route

  • Contact with persistently infected cats

  • In utero

• Age

  • Young cats more susceptible

• Immune status

  • Virus-Neutralizing antibodies

  • Maternal derived antibodies

What are the main clinical syndromes caused by FeLV?

“RAIN”

• Neoplasia

• Immunodeficiency

• Anemia

• Reproductive failure

How can FeLV cause neoplasia?

Retroviral DNA inserts near proto-oncogenes or disrupts tumor suppressor genes, increasing cell proliferation

What are the four main types of FeLV-associated lymphoma and their associated symptoms?

1. Mediastinal

   • Respiratory signs e.g. dyspnoea or tachypnoea

   • Regurgitation and weight loss if the lymphoma interferes with the oesophagus

2. Multicentric

   • Any organ my be involved

     • Epidural → Posterior paralysis

     • Kidneys → renal failure

3. Alimentary 消化道

   • Anorexia, weight loss, vomiting/diarrhea, often anemic

4. Leukemic (bone marrow)

   • Pyrexia, weakness, anorexia

Describe primary anemia caused by FeLV.

• Non-regenerative anaemia

  • because FeLV destroys erythroid precursors in bone marrow

• Can either be red cell aplasia (normal WBC, low RBC) or total marrow aplasia (Low RBC and WBC)

• Often caused by subtype C FeLV

Describe secondary anemia in FeLV infection

• Regenerative anaemia

  • Bone marrow is functional, but production compromised by space-occupying lesions in bone marrow (myeloid tumors)

• Haemolytic anemia can occur, usually mild

What happens if a fetus is infected with FeLV at 3–5 weeks of gestation?

Fetus may be resorbed

What happens if an infected kitten survives to birth?

It is persistently infected - Fading kitten syndrome

What is the risk to kittens of a persistently infected queen?

Most kittens will be FeLV positive → Since they are young, they are more likely to develop persistent infection and clinical signs

What is the “Test and Remove (T&R)” strategy for FeLV?

1. Test all cats in a colony with a lateral flow test.

2. House positive cats separately.

3. Retest all cats 12 weeks later:

   • Cats -ve twice (or +ve only on first test) → can rejoin main colony

   • Cats +ve twice → permanently remove from colony (rehomed or euthanized)

   • Cats negative first, positive second → isolate and retest after 12 weeks

4. Ongoing monitoring: Retest every 6–12 months and all new arrivals

Apart from “Test and remove” strategy, what are the other method to control FeLV?

One food bowl per cat → Reduce friendly contact

Is FeLV vaccination available?

Yes

• Either killed or subunit, or recombination canarypox

• First dose at 8–9 weeks → Second 3 weeks later → Annual boosters

What are the treatment options for FeLV-positive cats?

• Largely supportive care

• No specific antivirals

• Interferon may prolong survival

• May treat some lymphomas with cytotoxic drugs

FIV pathogenesis

What type of virus is FIV?

A lentivirus, related to HIV.

Is FIV zoonotic?

No.

• HIV does not infect cats

• FIV does not infect humans

What is the main route of FIV transmission?

Different from FeLV, FIV is primarily transmitted through bites and aggressive behavior during fights between cats = Unfriendly contact → High viral load in saliva

Which cats are most commonly affected by FIV?

• Male > Female

  • Because transmission is mainly through fighting and biting, which occurs more often in males.

• Free-roaming and feral 野生的 cats

• Cats in unstable colonies

Can FIV be transmitted from queen to kittens like FeLV?

Yes, via placenta or milk — but this is rare in practice

• Most kitten which were born from an FIV infected queen will be negative (completely opposite to FeLV)

Which immune cells does FIV primarily infect? What immune changes develop over time in FIV infection?

• Normally:

  • Antigen-presenting cells (APCs) e.g. dendritic cells, macrophages present antigens via MHC II molecules to CD4+ T cells → CD4⁺ T cell proliferates (clonal expansion) and differentiates into subsets:

    • Th1 → produce IFN-γ → activate macrophages, enhance MHC II

• BUT FIV primarily infects CD4+ T cells → Depletion of CD4 + T cells causes decreased expression of MHC II → Immunodeficient

What occurs during primary viraemia in FIV?

• High viral load in blood BUT low level of antibodies

• Vague flu-like signs:

  • Fever

  • Lymphadenopathy = Enlargement of lymph nodes

  • Malaise = Discomfort

  • Acute enteritis

Why is FIV often missed during primary infection?

• Clinical signs are nonspecific

• Antibodies lag behind viraemia

What happens after the initial viraemia phase?

• The cat controls the virus + Viral levels drop to very low levels

  • Cats remain clinically healthy for 3-5 years, and perhaps for life

How do antibody levels behave during FIV infection?

• Appear after primary viraemia

• Remain high for long periods

• May stay high or fall in terminal disease

What are the clinical signs during the terminal phase of FIV?

• Severe immunodeficiency

  • Opportunistic infections

  • Neoplasia

  • Chronic malaise

  • Chronic infections e.g. not respond to treatment as the animals should

Is there a specific treatment for FIV?

No. Treatment is mainly supportive

How can FIV infection be prevented and controlled?

• Prevent fighting: Keep cats indoors and neutered

• Isolate infected cats 

Is there a vaccine for FIV?

Not licensed in Europe yet