Simian Immunodeficiency Virus (SIV) and Simian AIDS – NAVLE Study Guide

Overview and Clinical Importance

Simian immunodeficiency virus (SIV) is a lentivirus belonging to the family Retroviridae that naturally infects over 45 species of nonhuman primates. While SIV is typically non-pathogenic in its natural African primate hosts due to coevolution, cross-species transmission to non-natural hosts, particularly Asian macaques, results in a severe immunodeficiency syndrome known as simian AIDS (SAIDS). This disease closely parallels human AIDS and serves as the primary animal model for HIV research. For NAVLE preparation, understanding SIV is essential for questions on primate medicine, comparative immunology, and zoonotic disease potential.

Etiology and Viral Classification

Viral Structure and Genome

SIV is a single-stranded, positive-sense RNA virus measuring approximately 80-100 nanometers in diameter. Like all lentiviruses, SIV possesses:

• Lipid envelope derived from host cell membrane containing viral glycoproteins gp120 and gp41
• Matrix protein (MA/p17) beneath the envelope
• Conical capsid (CA/p24) containing two copies of viral RNA and reverse transcriptase
• Nucleocapsid proteins (NC) complexed with viral genomic RNA

The SIV genome contains standard retroviral genes (gag, pol, env) plus accessory genes (tat, rev, nef, vif, vpr). The reverse transcriptase enzyme enables conversion of viral RNA to DNA, which integrates into the host genome–a hallmark of lentivirus pathogenesis.

SIV Strains and Host Species

Over 40 distinct SIV strains have been identified, typically named after their natural host species. Key strains include:

Pathophysiology and Immune Response

Mechanism of Infection

SIV primarily targets CD4+ T lymphocytes and macrophages. The infection process involves:

• Viral envelope glycoprotein gp120 binding to CD4 receptor and CCR5/CXCR4 co-receptors
• Membrane fusion mediated by gp41, allowing viral core entry into cytoplasm
• Reverse transcription of viral RNA to DNA by reverse transcriptase
• Integration of proviral DNA into host genome via viral integrase
• Transcription, translation, and assembly of new viral particles
• Budding of immature virions from cell membrane; proteolytic maturation into infectious particles

Infected CD4+ T cells typically undergo apoptosis within 24 hours of infection. The rate of cell death exceeds immune system replacement capacity, leading to progressive CD4+ T cell depletion and immunodeficiency.

Natural Host Adaptation vs. Pathogenic Infection

Natural Hosts (African Primates):

• High viral loads but minimal immune activation
• Reduced CCR5 expression on CD4+ T cells limits infection
• Preserved gut-associated lymphoid tissue (GALT) prevents microbial translocation
• Low immune activation prevents chronic inflammation
• Rapid turnover of infected cells without massive CD4+ depletion

Susceptible Hosts (Asian Macaques):

• Massive acute viremia with peak viral loads 10-100 times higher than natural hosts
• Severe depletion of gut CD4+ T cells within days of infection
• Chronic immune activation and inflammation
• Progressive CD4+ T cell decline leading to immunodeficiency
• Development of opportunistic infections and neoplasia

Clinical Manifestations of Simian AIDS

Acute Phase (First 2-4 Weeks Post-Infection)

In susceptible macaque species, acute SIV infection may present with:

• Pyrexia (elevated body temperature)
• Lymphadenopathy (enlarged lymph nodes)
• Transient rash
• Mild anorexia and lethargy
• Peak viremia at 7-14 days post-infection

Important: Many infected animals remain clinically asymptomatic during acute phase, making early detection challenging without laboratory testing.

Chronic/Latent Phase (Variable Duration)

This phase can last weeks to years depending on viral strain and host genetics. Clinical signs include:

• Persistent generalized lymphadenopathy
• Gradual weight loss despite normal appetite
• Intermittent diarrhea
• Decreased activity levels
• Progressive CD4+ T cell decline (monitored via blood work)

Simian AIDS (End-Stage Disease)

When CD4+ T cell counts drop below critical levels (typically less than 200 cells/µL), animals develop full-blown SAIDS characterized by:

Constitutional Signs:

• Severe wasting (18% or greater body weight loss over 2 weeks)
• Profound anorexia
• Chronic debilitating diarrhea
• Dehydration and poor body condition
• Fever of unknown origin

Respiratory Manifestations:

• Dyspnea (labored breathing)
• Tachypnea
• Nasal discharge
• Wheezing or rales on auscultation

Neurological Signs:

• Ataxia and motor deficits
• Behavioral changes
• Seizures (in severe cases)
• Cognitive impairment

Opportunistic Infections and Complications

Pulmonary Opportunistic Infections

Pneumocystis jirovecii (formerly P. carinii) Pneumonia:

• MOST COMMON opportunistic infection in SIV-infected macaques
• Presents with progressive dyspnea, tachypnea, exercise intolerance
• Radiographic findings: diffuse interstitial infiltrates
• Gross pathology: multifocal tan-white nodules 1-10 mm diameter throughout lungs
• Histopathology: foamy pink alveolar exudate, type II pneumocyte hyperplasia, extensive lymphocytic infiltration

Other Pulmonary Pathogens:

• Mycobacterium avium-intracellulare complex (MAC): granulomatous pneumonia
• Cytomegalovirus (CMV): interstitial pneumonitis with characteristic owl's eye inclusions
• Cryptosporidium parvum: respiratory cryptosporidiosis
• Bacterial pneumonia: Streptococcus, Klebsiella species

Gastrointestinal Opportunistic Infections

• Cryptosporidium: chronic watery diarrhea, intestinal villous atrophy
• Microsporidia: malabsorption, chronic diarrhea
• CMV colitis: bloody diarrhea, ulcerative colitis
• Mycobacterium avium complex: granulomatous enteritis
• Salmonella species: bacteremia and enterocolitis

Central Nervous System Opportunistic Infections

• Toxoplasma gondii: focal necrotizing encephalitis, ring-enhancing lesions
• SIV encephalitis: direct viral invasion causing meningoencephalomyelitis, multinucleated giant cells
• CMV encephalitis: periventricular involvement

Neoplasia

• B-cell lymphomas: non-Hodgkin lymphoma, primary CNS lymphoma
• Simian virus 40 (SV40)-associated tumors in immunocompromised animals

Diagnosis

Clinical Pathology Findings

Specific SIV Diagnostic Tests

1. Serologic Testing:

• ELISA (Enzyme-Linked Immunosorbent Assay): screening test for anti-SIV antibodies
• Western blot: confirmatory test detecting antibodies to specific SIV proteins (gp120, gp41, p24)
• Seroconversion typically occurs 2-6 weeks post-infection
• Note: Antibody tests cannot detect infection during 'window period' before seroconversion

2. Molecular Diagnostics:

• PCR (Polymerase Chain Reaction): detects proviral DNA in peripheral blood mononuclear cells (PBMCs)
• Quantitative RT-PCR (viral load testing): measures plasma viral RNA copies per mL; critical for monitoring disease progression
• Set-point viral load (typically measured 6 months post-infection) predicts disease progression rate

3. Virus Isolation:

• Co-culture of patient PBMCs with susceptible cell lines
• Labor-intensive; primarily used for research rather than routine diagnosis

Diagnostic Imaging

• Thoracic radiographs: interstitial to alveolar pulmonary infiltrates with Pneumocystis or other respiratory infections
• Computed tomography (CT): for detection of CNS lesions, lymphadenopathy, or neoplasia

Gross and Histopathology

Lymphoid Tissues:

• Follicular hyperplasia and paracortical expansion in early disease
• Lymphoid depletion and involution in late-stage disease
• Multinucleated giant cells containing SIV antigens

Respiratory System:

• Primary retroviral pneumonia: syncytial giant cells, alveolar damage, viral antigens in macrophages
• Opportunistic pneumonias with organism-specific histologic findings

Central Nervous System:

• SIV encephalitis: perivascular lymphocytic cuffing, microglial nodules, multinucleated giant cells
• White matter vacuolation and gliosis

Treatment and Management

Antiretroviral Therapy

Important NAVLE Concept: While antiretroviral drugs used for human HIV are being researched in SIV models, there is NO approved clinical antiretroviral therapy for routine treatment of SIV in captive primates outside of research settings.

Experimental antiretroviral agents studied in research include:

• Nucleoside reverse transcriptase inhibitors (NRTIs): Tenofovir, Emtricitabine
• Non-nucleoside reverse transcriptase inhibitors (NNRTIs): Efavirenz
• Protease inhibitors: Ritonavir, Atazanavir
• Integrase inhibitors: Raltegravir

Supportive Care

• Nutritional support: High-calorie diets, appetite stimulants, assisted feeding if necessary
• Fluid therapy: IV or subcutaneous fluids for dehydration
• Prophylaxis against opportunistic infections in research settings (not standard practice)

Treatment of Opportunistic Infections

Prognosis

Prognosis for SIV-infected Asian macaques is grave. Without intervention:

• Rapid progressors: develop AIDS within 3-6 months
• Typical progressors: develop AIDS within 1-2 years
• Long-term non-progressors (rare): maintain stable CD4+ counts for years
• Median survival after development of SAIDS: 3-6 months

Factors affecting prognosis:

• Viral strain virulence
• Route of infection (intravenous worse than mucosal)
• Inoculum dose
• Host genetics (MHC haplotype influences disease progression)
• Age at infection (younger animals may progress faster)

Prevention and Biosafety

Colony Management

• Test-and-remove strategy: regular serological screening of breeding colonies
• Strict segregation of SIV-positive and SIV-negative animals
• Prevent cross-species housing between African and Asian primates
• Quarantine and testing of all newly acquired animals
• Minimize bite wounds and aggressive interactions

Transmission Prevention

Natural transmission routes:

• Sexual contact (most common in wild populations)
• Bite wounds and aggressive encounters
• Vertical transmission (mother to infant in utero or through breast milk)
• Iatrogenic transmission through contaminated needles or surgical instruments

Note: SIV is NOT transmitted through casual contact, food sharing, or aerosol routes under normal circumstances.

Human Safety and Zoonotic Potential

CRITICAL NAVLE CONCEPT: While SIV is the progenitor of HIV-1 and HIV-2, there have been EXTREMELY FEW documented cases of SIV transmission to humans (laboratory exposures only). SIV does NOT readily establish productive infection in humans due to species-specific barriers.

Biosafety Level 2+ precautions required:

• Personal protective equipment (PPE): gloves, gowns, face shields
• Minimize needle stick injuries
• Bite prevention protocols
• Proper sharps disposal
• Post-exposure prophylaxis protocols should be in place