Viruses can cause cancer by hijacking our cells. Some major culprits include HPV, EBV, HBV, and HTLV-1. These sneaky invaders mess with our DNA and cell signaling, leading to uncontrolled growth.

Each virus has its favorite targets. HPV goes for cervical cells, EBV attacks lymphocytes, HBV hits the liver, and HTLV-1 infects T-cells. Understanding how they spread and who's at risk helps us fight back against these cancer-causing troublemakers.

Oncogenic Viruses

Major Human Oncogenic Viruses

• Human papillomavirus (HPV) consists of double-stranded DNA belonging to Papillomaviridae family
  • Over 200 types identified
  • Approximately 40 types can infect the genital tract
• Epstein-Barr virus (EBV) comprises double-stranded DNA virus of Herpesviridae family
  • Also known as human herpesvirus 4
  • Infects B lymphocytes and epithelial cells
• Hepatitis B virus (HBV) contains partially double-stranded DNA of Hepadnaviridae family
  • Primarily infects hepatocytes in the liver
• Human T-cell leukemia virus type 1 (HTLV-1) encompasses single-stranded RNA retrovirus
  • Mainly infects CD4+ T lymphocytes

Additional Oncogenic Viruses and Mechanisms

• Other notable oncogenic viruses include hepatitis C virus (HCV), Kaposi's sarcoma-associated herpesvirus (KSHV), and Merkel cell polyomavirus (MCPyV)
• Viruses induce cellular transformation through various mechanisms
  • Expression of viral oncoproteins (E6 and E7 in HPV)
  • Integration into the host genome (HBV)
  • Manipulation of cellular signaling pathways (NF-κB activation by HTLV-1)

Cancer Types Associated with Viruses

HPV-Associated Cancers

• Cervical cancer serves as primary cancer associated with HPV
  • High-risk HPV types 16 and 18 cause approximately 70% of cervical cancer cases worldwide
• Other HPV-related cancers include:
  • Anogenital cancers (anal, vulvar, vaginal, penile)
  • Oropharyngeal cancers

EBV-Associated Cancers

• Various lymphomas linked to EBV:
  • Burkitt's lymphoma
  • Hodgkin's lymphoma
  • Diffuse large B-cell lymphoma
• Nasopharyngeal carcinoma strongly associated with EBV
• Subset of gastric carcinomas, particularly in Asia, linked to EBV infection

HBV and HCV-Associated Cancers

• Hepatocellular carcinoma (HCC) represents major cancer caused by HBV
  • Chronic HBV infection significantly increases HCC risk
  • Cirrhosis often precedes HCC development
• HCV also causes hepatocellular carcinoma
  • Associated with some B-cell lymphomas

HTLV-1-Associated Cancers

• Adult T-cell leukemia/lymphoma (ATLL) caused by HTLV-1
  • Rare and aggressive form of blood cancer
• HTLV-1 also associated with neurological condition called HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP)

Epidemiology of Virus-Associated Cancers

Global Burden and Distribution

• Virus-associated cancers account for 15-20% of all human cancers worldwide
  • Significant variations in prevalence across geographic regions
• HPV-associated cancers have high global burden
  • Cervical cancer ranks as fourth most common cancer in women worldwide
  • Particularly affects low- and middle-income countries
• EBV infection ubiquitous with over 90% of adults worldwide seropositive
  • EBV-associated cancers show distinct geographic patterns
  • Burkitt's lymphoma endemic in equatorial Africa
  • Nasopharyngeal carcinoma highly incident in Southern China and Southeast Asia

Prevalence of Specific Virus-Associated Cancers

• Chronic HBV infection affects approximately 257 million people globally
  • Highest prevalence in Western Pacific and African regions
  • HBV-related hepatocellular carcinoma accounts for significant proportion of liver cancer cases
• HTLV-1 infection endemic in certain geographic areas
  • Southwestern Japan, Caribbean, parts of South America and Africa
  • Lifetime risk of developing ATLL in HTLV-1 carriers estimated at 2-7%

Factors Influencing Global Burden

• Vaccination coverage impacts virus-associated cancer prevalence (HPV vaccine)
• Screening programs affect early detection and prevention (cervical cancer screening)
• Access to healthcare leads to disparities between developed and developing countries
• Environmental and lifestyle factors contribute to regional variations in cancer incidence

Risk Factors for Oncogenic Viruses

Transmission Routes

• Sexual transmission serves as primary route for HPV
  • Risk factors include multiple sexual partners, early sexual debut, unprotected sexual activity
  • Vertical transmission from mother to child during childbirth possible but less common
• EBV primarily transmitted through saliva ("kissing disease")
  • Can also spread through blood and organ transplantation
• HBV transmitted through blood, bodily fluids, and vertical transmission
• HTLV-1 spread through breast milk, sexual contact, blood transfusions, and sharing contaminated needles
  • Prolonged breastfeeding (>6 months) significantly increases risk of mother-to-child transmission

Co-factors and Environmental Risks

• Co-infection with HIV increases risk of developing virus-associated cancers due to immunosuppression
• Behavioral and environmental factors act as co-factors in virus-induced carcinogenesis
  • Alcohol consumption
  • Smoking
  • Exposure to certain chemicals (aflatoxin for HBV-related HCC)
• Genetic susceptibility influences development of virus-associated cancers (EBV-related nasopharyngeal carcinoma)
• Immunosuppression increases risk of virus-associated cancers (post-transplant lymphoproliferative disorders)