25.3 Viral Infections of the Circulatory and Lymphatic Systems

Viral Infections of the Circulatory and Lymphatic Systems

Viruses that infect the circulatory and lymphatic systems range from relatively common infections like mononucleosis to life-threatening diseases like HIV/AIDS. Because these systems circulate throughout the entire body, viral infections here can spread widely and affect multiple organs. This section covers herpesviruses (EBV, CMV), arthropod-borne viruses (yellow fever, dengue, chikungunya), and HIV/AIDS.

Common Circulatory and Lymphatic Viruses

Epstein-Barr Virus (EBV) is a herpesvirus transmitted primarily through saliva, which is why the disease it causes, infectious mononucleosis, is nicknamed the "kissing disease." It can also spread through blood transfusions and organ transplants.

Symptoms of infectious mononucleosis include fever, sore throat, swollen lymph nodes (lymphadenopathy), and prolonged fatigue. In some cases, EBV can lead to more serious complications like hepatitis or splenic rupture, which is why patients are told to avoid contact sports during recovery.

Treatment is mainly supportive: rest, hydration, and over-the-counter pain relievers. Antiviral medications may be used in severe cases, but they aren't standard for typical mono.

Cytomegalovirus (CMV) is another herpesvirus, transmitted through close contact with body fluids (saliva, blood, urine, semen, breast milk). It can also pass vertically from mother to fetus and through organ transplantation or blood transfusion.

In healthy individuals, CMV is often asymptomatic. The real danger is for immunocompromised patients, where CMV can cause fever, fatigue, muscle aches, enlarged liver and spleen, pneumonia, and retinitis (inflammation of the retina that can lead to blindness). Congenital CMV (passed to a fetus during pregnancy) can cause birth defects and developmental issues, making it one of the leading infectious causes of disability in newborns.

Treatment for immunocompromised patients or severe cases involves antiviral medications like ganciclovir or valganciclovir. Mild cases in healthy individuals typically need only supportive care.

Characteristics of Arthropod-Borne Viral Diseases

These three diseases are all caused by arboviruses (arthropod-borne viruses) transmitted by mosquito bites, primarily from Aedes species. They share overlapping symptoms but differ in key ways.

Yellow Fever

• Transmitted by Aedes and Haemagogus mosquitoes
• Symptoms: fever, chills, headache, muscle aches, nausea, vomiting, and jaundice (yellowing of the skin, which gives the disease its name) in severe cases
• A safe, effective vaccine exists (the 17D vaccine), making yellow fever the only one of these three with a widely available vaccine
• Treatment is supportive care; there are no specific antivirals

Dengue Fever

• Transmitted by Aedes mosquitoes (especially Aedes aegypti)
• Symptoms: high fever, severe headache, retro-orbital pain (pain behind the eyes), muscle and joint pain, nausea, vomiting, and skin rash
• Can progress to dengue hemorrhagic fever, a severe and potentially fatal form involving plasma leakage, bleeding, and organ damage
• Treatment involves supportive care with careful fluid management and pain relief (avoid aspirin, which can worsen bleeding)

Chikungunya Fever

• Transmitted by Aedes mosquitoes
• Symptoms: sudden onset of high fever, severe joint pain, headache, muscle pain, and rash
• The hallmark feature is severe joint pain (polyarthralgia) that can persist for weeks to months after the acute infection resolves
• Treatment is supportive: pain relief and rest

Comparing the three:

Stages and Management of HIV/AIDS

Human immunodeficiency virus (HIV) attacks CD4 T cells (helper T cells), which are critical for coordinating the immune response. Over time, the destruction of these cells leaves the body unable to fight off infections. HIV infection progresses through three distinct stages:

1. Acute HIV infection occurs 2–4 weeks after exposure. The patient experiences flu-like symptoms (fever, rash, swollen lymph nodes, sore throat). Viral load in the blood is extremely high during this stage (viremia), making the person highly contagious.
2. Chronic (clinical latency) HIV infection can last years. Symptoms are absent or mild, but the virus continues replicating at low levels. CD4 T-cell counts gradually decline over time.
3. AIDS (Acquired Immunodeficiency Syndrome) is diagnosed when the CD4 T-cell count drops below 200 cells/mm³ (normal is roughly 500–1,500 cells/mm³). At this point, the immune system is severely compromised, and the patient becomes vulnerable to opportunistic infections (e.g., Pneumocystis pneumonia, toxoplasmosis, Kaposi sarcoma).

Diagnostic Methods

• Antibody tests (ELISA for screening, Western blot for confirmation) detect HIV antibodies in the blood. These are the most common initial screening tools but may miss very early infections before antibodies develop (the "window period").
• Antigen/antibody combination tests (4th generation) detect both HIV antibodies and the p24 antigen, which appears earlier than antibodies. This shortens the window period.
• Nucleic acid tests (NAT) detect HIV RNA directly in the blood. These are used for early detection (within 10–33 days of exposure) and for monitoring viral load during treatment.

Current Management Approaches

• Antiretroviral therapy (ART) is the cornerstone of HIV treatment. It uses a combination of drugs (typically three or more from different classes) to suppress viral replication. ART does not cure HIV, but it can reduce viral load to undetectable levels, maintain CD4 counts, and prevent progression to AIDS.
• Regular monitoring tracks CD4 T-cell counts and viral load to assess treatment effectiveness. Patients are also screened for opportunistic infections and other complications.
• Prophylaxis for opportunistic infections is prescribed when CD4 counts drop below certain thresholds (e.g., prophylaxis for Pneumocystis pneumonia when CD4 drops below 200 cells/mm³).
• Supportive care includes nutritional support, mental health services, and substance abuse treatment as needed.

Viral Pathogenesis and Host Interactions

Understanding how viruses interact with the host helps explain why different viruses cause different diseases.

Viral tropism refers to which specific cell types or tissues a virus can infect. This is determined by the receptors on host cells that the virus can bind to. For example, HIV has tropism for CD4 T cells because it binds the CD4 receptor (along with co-receptors CCR5 or CXCR4). Tropism directly shapes where in the body a virus causes damage.

Virulence factors are features that help a virus cause disease and evade host defenses. These can include proteins that block interferon signaling, mechanisms to hide from antibody detection, or the ability to mutate rapidly (as HIV does, which is why vaccine development has been so difficult).

Immunosuppression caused by certain viral infections (most notably HIV) creates a dangerous cycle: the virus weakens the immune system, which then can't control the virus or fight off other pathogens. This is why opportunistic infections are the hallmark of AIDS rather than direct damage from HIV itself.