Viruses are classified based on the Baltimore classification system, which organizes them according to genome type and replication strategy. Among them, negative-stranded RNA viruses form an important group because they are associated with several severe human diseases, including influenza, rabies, Ebola, and measles.
Negative-sense RNA viruses have genomes that are complementary to messenger RNA (mRNA). Unlike positive-sense RNA viruses, their genomes cannot be directly translated into proteins. Instead, they must first be transcribed into positive-sense RNA by a viral RNA-dependent RNA polymerase (RdRp) packaged within the virion.
A simple mnemonic to remember the families of negative-stranded RNA viruses is:
FAB PRO (Fabulous Project)
- F – Filoviridae
- A – Arenaviridae
- B – Bunyaviridae
- P – Paramyxoviridae
- R – Rhabdoviridae
- O – Orthomyxoviridae
General Characteristics of Negative-Stranded RNA Viruses
1. Genome
- Single-stranded RNA (ssRNA), negative polarity.
- Cannot act directly as mRNA.
- Requires RNA-dependent RNA polymerase (RdRp).
2. Structure
- Mostly enveloped viruses.
- Helical nucleocapsid symmetry.
3. Replication
- Takes place in the cytoplasm (except Orthomyxoviridae which replicates in the nucleus).
- RdRp transcribes negative-stranded RNA into positive-stranded RNA, which acts as mRNA.
- Viral proteins and genomic RNA are then assembled into new virions.
4. Transmission
- Many are zoonotic, transmitted via insects, rodents, or direct contact with infected animals.
- Human-to-human transmission occurs in several families (e.g., influenza, measles, Ebola).
- Cause major epidemics and pandemics (e.g., Influenza pandemics, Ebola outbreak, Rabies fatalities).
Families of Negative-Stranded RNA Viruses – Mnemonic FAB PRO
1. Filoviridae
- Structure: Filamentous viruses.
- Important viruses: Ebola virus, Marburg virus.
- Diseases: Severe hemorrhagic fevers with high fatality rates.
- Transmission: Direct contact with body fluids of infected individuals.
- Clinical significance: High biosafety-level pathogens (BSL-4).
2. Arenaviridae
- Structure: Enveloped, “sandy” appearance due to ribosomes inside virions.
- Important viruses: Lassa fever virus, Junín virus.
- Diseases: Hemorrhagic fevers, encephalitis.
- Transmission: Rodent excreta, inhalation of aerosolized particles.
- Clinical significance: Endemic in Africa and South America.
3. Bunyaviridae
- Structure: Enveloped, segmented genome.
- Important viruses: Hantaviruses, Rift Valley fever virus, Crimean-Congo hemorrhagic fever virus.
- Diseases: Hantavirus pulmonary syndrome, hemorrhagic fevers, encephalitis.
- Transmission: Arthropods (mosquitoes, ticks) and rodents.
- Clinical significance: Emerging zoonotic infections with high mortality.
4. Paramyxoviridae
- Structure: Enveloped, non-segmented RNA genome.
- Important viruses: Measles virus, Mumps virus, Respiratory Syncytial Virus (RSV), Parainfluenza viruses.
- Diseases: Measles, mumps, bronchiolitis, pneumonia.
- Transmission: Respiratory droplets.
- Clinical significance: Major childhood infections; measles outbreaks still occur in low-vaccination areas.
5. Rhabdoviridae
- Structure: Bullet-shaped viruses.
- Important viruses: Rabies virus, Vesicular stomatitis virus.
- Diseases: Rabies – a fatal encephalitis.
- Transmission: Animal bites, particularly from dogs, bats, and wild animals.
- Clinical significance: Preventable by rabies vaccination post-exposure.
6. Orthomyxoviridae
- Structure: Enveloped, segmented RNA genome.
- Important viruses: Influenza A, B, and C viruses.
- Diseases: Seasonal influenza, pandemics (e.g., 1918 Spanish flu, 2009 H1N1 pandemic).
- Transmission: Respiratory droplets, aerosols.
- Clinical significance: Antigenic drift and shift lead to frequent epidemics and pandemics.
Clinical Significance
Negative-stranded RNA viruses are medically important because they:
- Cause respiratory infections (Influenza, RSV, Parainfluenza).
- Cause neurological infections (Rabies, Nipah virus – a paramyxovirus).
- Cause hemorrhagic fevers (Ebola, Marburg, Lassa, Crimean-Congo).
- Cause childhood diseases (Measles, Mumps).
Their ability to mutate rapidly makes them difficult to control, requiring continuous vaccine development (e.g., seasonal influenza vaccines).
Diagnostic Approaches
1. Clinical suspicion based on symptoms and exposure history.
2. Laboratory methods:
- RT-PCR (gold standard for viral RNA detection).
- Antigen detection (ELISA).
- Virus isolation in cell culture.
- Serology (antibody detection).
Prevention and Treatment
1. Vaccination: Available for measles, mumps, influenza, rabies, and some paramyxoviruses.
2. Antiviral drugs:
- Oseltamivir, Zanamivir (for influenza).
- Ribavirin (for some arenaviruses and bunyaviruses).
Tabular Overview of Negative-Stranded RNA Viruses
| Family | Key Viruses | Major Diseases | Transmission |
|---|---|---|---|
| Filoviridae | Ebola, Marburg | Hemorrhagic fever | Direct contact with fluids |
| Arenaviridae | Lassa, Junín | Hemorrhagic fever | Rodents |
| Bunyaviridae | Hantavirus, Crimean-Congo | Pulmonary syndrome, hemorrhagic fever | Arthropods, rodents |
| Paramyxoviridae | Measles, Mumps, RSV | Childhood infections, pneumonia | Respiratory droplets |
| Rhabdoviridae | Rabies | Fatal encephalitis | Animal bites |
| Orthomyxoviridae | Influenza A, B, C | Influenza, pandemics | Respiratory droplets |
Frequently Asked Questions (FAQs)
Q1: Why are negative-stranded RNA viruses dangerous?
They include viruses causing hemorrhagic fevers, rabies, and influenza pandemics. Many have high mortality rates and potential for outbreaks.
Q2: Why do negative-stranded RNA viruses need RNA polymerase?
Their genomes cannot act as mRNA. They require RNA-dependent RNA polymerase to transcribe their genome into positive-sense RNA for protein synthesis.
Q3: Which negative-stranded RNA virus causes rabies?
The Rabies virus, belonging to the Rhabdoviridae family.
Q4: Which negative-stranded RNA virus is responsible for pandemics?
The Influenza virus (Orthomyxoviridae) due to antigenic drift and shift.
Q5: Can we prevent infections caused by these viruses?
Yes. Vaccines (for measles, mumps, influenza, rabies) and preventive measures like sanitation, vector control, and post-exposure prophylaxis help reduce risk.
