5.5 Viral Diseases

This section outlines the causative agents, symptoms, transmission, treatment, and prevention methods for several key viral diseases affecting humans and plants, including Hepatitis, Herpes Simplex, Poliomyelitis, and Cotton Leaf Curl Disease.

Hepatitis

Hepatitis is the inflammation of the liver, generally caused by a viral infection. It can be acute (rapid onset) or chronic (slowly progressing). There are five main types of viral hepatitis.

Comparison of Hepatitis Types

• Hepatitis D: This disease is unique as it is caused by a defective virus (HDV) that requires co-infection with Hepatitis B Virus (HBV) to enter liver cells. HDV uses the HBV surface antigen (HBsAg) as its envelope protein and cannot replicate independently without HBV.

Herpes Simplex

A superficial viral infection causing painful, fluid-filled sores or blisters on the skin or mucous membranes. It is caused by the Herpes Simplex Virus (HSV).

Types of Herpes Simplex

• Oral Herpes (HSV-1)

  • Cause: Herpes simplex virus type-1 (HSV-1).
  • Transmission: Primarily through oral secretions (saliva) or direct contact with sores. Can be spread by sharing items like toothbrushes.
  • Symptoms: Blisters appear on the lips, around the mouth, or sometimes on the face or tongue. Preceded by tingling, itching, or burning.
  • Treatment: Antiviral drugs.
  • Prevention: Avoid contact with the affected area of an infected person.

• Genital Herpes (HSV-2)

  • Cause: Herpes simplex virus type-2 (HSV-2).
  • Transmission: Almost exclusively through sexual contact with an infected person.
  • Symptoms: Sores typically occur on the penis, vagina, buttocks, or anus.
  • Treatment: Antiviral drugs.
  • Prevention: Avoid sexual contact with an infected person.

Poliomyelitis (Polio)

A disease caused by the poliovirus that can lead to permanent paralysis.

• Cause: Poliovirus (an enterovirus).
• Transmission: Faecal-oral route.
• Pathophysiology:
  1. The virus replicates in the oropharynx and intestinal tract.
  2. It spreads to the blood and then the central nervous system (CNS).
  3. In the CNS, it replicates in and destroys motor neurons in the spinal cord.
  4. The death of these neurons results in permanent paralysis of the muscles they control.
• Treatment: No specific antiviral therapy. Physiotherapy for affected muscles is crucial.
• Prevention: Highly preventable through vaccination.
  • Salk vaccine: An injectable polio vaccine (IPV) using a killed virus.
  • Sabin vaccine: An oral polio vaccine (OPV) using a live, attenuated (weakened) virus.

Cotton Leaf Curl Disease (CLCuD)

A serious viral disease affecting cotton plants.

• Cause: Begomoviruses (single-stranded DNA viruses).
• Transmission: Transmitted by an insect vector, the whitefly (Bemisia tabaci).
• Symptoms:
  • Initial: Deep downward cupping of the youngest leaves.
  • Later: Development of cup-shaped, leaf-like structures on the plant.
• Treatment and Prevention: Control is based on using insecticide treatments against the whitefly vector.

Summary Table

Biological Significance: Understanding the transmission routes and causative agents of viral diseases is fundamental to public health. It allows for the development of targeted prevention strategies such as vaccination (e.g., Polio, Hepatitis A/B), hygiene practices, and vector control (e.g., CLCuD), significantly reducing the burden of these diseases on human and agricultural populations.