Pathogenic RNA Viruses: Enveloped, Unsegmented Negative ssRNA Viruses

Pathogenic RNA Viruses

Overview of Enveloped, Unsegmented Negative ssRNA Viruses

Enveloped, unsegmented negative-sense single-stranded RNA (ssRNA) viruses are a group of medically important viruses that cause significant human diseases. This group includes members of the families Paramyxoviridae, Rhabdoviridae, and Filoviridae. These viruses are characterized by their enveloped structure and a genome consisting of a single, unsegmented strand of negative-sense RNA.

• Paramyxoviridae: Includes genera such as Morbillivirus (measles), Respirovirus (parainfluenza), Rubulavirus (mumps), and Pneumovirus (respiratory syncytial virus).

• Rhabdoviridae: Most notably includes the rabies virus.

• Filoviridae: Includes Ebola and Marburg viruses, which cause hemorrhagic fevers.

These viruses are responsible for a range of diseases, from mild respiratory infections to severe, often fatal, systemic illnesses.

Paramyxoviridae

Morbillivirus: Measles

The measles virus (Morbillivirus) is a highly contagious, enveloped, helical, negative-sense ssRNA virus. Measles is primarily a childhood infection but can affect individuals of any age.

• Transmission: Spread via respiratory droplets; requires large, dense populations for sustained transmission.

• Pathogenesis: Infects the respiratory tract, then disseminates throughout the body.

• Clinical Features: Characterized by a skin rash, fever, and the appearance of Koplik’s spots on the oral mucosa, which are diagnostic in early stages.

• Complications: Pneumonia, encephalitis, and subacute sclerosing panencephalitis (SSPE).

• Prevention: Effective vaccination (MMR vaccine) has eliminated endemic measles in some regions, but outbreaks still occur.

Table: Comparison of Measles and Rubella

Disease	Causative Agent	Primary Patient(s)	Complications	Skin Rash	Koplik’s Spots
Measles (rubeola)	Paramyxoviridae: Morbillivirus	Child	Pneumonia, encephalitis, SSPE	Extensive	Present
Rubella (German measles)	Togaviridae: Rubivirus	Child, fetus	Birth defects	Mild	Absent

Diagnosis, Treatment, and Prevention of Measles

• Diagnosis: Based on clinical signs, especially Koplik’s spots.

• Treatment: Anti-measles immunoglobulin if given soon after exposure; vitamin A supplementation reduces severity in children.

• Prevention: MMR vaccine administered in childhood.

Parainfluenza Viruses

Parainfluenza viruses (HPIVs) are divided into four types and cause respiratory tract infections, especially in children.

• HPIV-1 and HPIV-2: Cause croup (laryngotracheobronchitis), a severe inflammation of the upper airway.

• HPIV-3: Associated with lower respiratory tract infections.

• HPIV-4: Causes mild upper respiratory infections.

• Treatment: Supportive; no specific antivirals. Severe cases may require airway management.

Rubulavirus: Mumps

The mumps virus causes mumps, a disease characterized by painful swelling of the parotid salivary glands.

• Transmission: Respiratory secretions.

• Clinical Features: Parotitis, orchitis, meningitis, pancreatitis, and possible deafness.

• Prevention: MMR vaccine has nearly eradicated mumps in industrialized countries.

Pneumovirus: Respiratory Syncytial Virus (RSV)

RSV is a leading cause of lower respiratory tract disease in infants and young children. It is highly contagious and can cause severe bronchiolitis and pneumonia.

• Pathogenesis: Causes formation of syncytia (multinucleated giant cells) in the lungs, leading to airway obstruction and dyspnea.

• Clinical Features: Severe in infants; mild or asymptomatic in older children and adults.

• Treatment: Supportive care; ribavirin may be used in severe cases.

• Prevention: Hand hygiene is critical in healthcare settings.

Rhabdoviridae

Rabies Virus

Rabies is a fatal zoonotic disease caused by the rabies virus, which infects the central nervous system.

• Transmission: Usually via the bite of an infected animal (dogs, bats, etc.).

• Pathogenesis: Virus travels from the site of entry along peripheral nerves to the CNS, causing encephalitis.

• Clinical Features: Hydrophobia (fear of water), seizures, disorientation, hallucinations, paralysis, and death from respiratory failure.

Diagnosis, Treatment, and Prevention of Rabies

• Diagnosis: Neurological symptoms; postmortem identification of Negri bodies in brain tissue.

• Treatment: Immediate wound cleaning, administration of human rabies immune globulin (HRIG), and vaccination with human diploid cell vaccine (HDCV).

• Prevention: Vaccination of domestic animals and at-risk humans.

Filoviridae

Filoviruses: Ebola and Marburg Viruses

Filoviruses are filamentous, enveloped, negative-sense ssRNA viruses that cause severe hemorrhagic fevers in humans and nonhuman primates.

• Natural Reservoir: Thought to be fruit bats.

• Transmission: Initial mode to humans is unclear; spreads person-to-person via contaminated body fluids.

• Pathogenesis: Virions attack macrophages and liver cells, leading to uncontrolled bleeding and multi-organ failure.

• Clinical Features: Up to 90% mortality in some outbreaks; symptoms include fever, hemorrhage, and shock.

• Diagnosis: Clinical symptoms, confirmed by ELISA or PCR.

• Treatment: Supportive care with fluid replacement; vaccine (rVSV-ZEBOV) available for Ebola.

Summary Table: Key Features of Major Enveloped, Unsegmented Negative ssRNA Viruses

Virus Family	Representative Diseases	Transmission	Key Features	Prevention
Paramyxoviridae	Measles, Mumps, Parainfluenza, RSV	Respiratory droplets	Syncytia formation, childhood infections	MMR vaccine, hygiene
Rhabdoviridae	Rabies	Animal bites	Neurological symptoms, Negri bodies	Animal vaccination, post-exposure prophylaxis
Filoviridae	Ebola, Marburg	Body fluids	Hemorrhagic fever, high mortality	Vaccine (Ebola), infection control