Myxoviruses: Structure, Function, and Impact on Health

Myxoviruses are RNA viruses that can cause common cold, mumps, and measles in humans. These are RNA viruses and all of the types are unsegmented except influenza. These are helical in symmetry and RNA is negative sense RNA. The influenza virus is pleomorphic and the viral envelope can occur in spherical and filamentous forms. These are of two types, orthomyxovirus and paramyxovirus.

Orthomyxovirus

• It is an influenza virus. It is an RNA virus and the strand is negative sense.
• The incubation period is 18 to 72 hours. Humans, animals, and birds act as the reservoir.
• The mode of infection is inhalation of droplets or coming in contact with formites.
• The specialty of RNA is that it is segmented. Influenza A and B have 8 segments each whereas influenza C has 7 segments.
• The structure of the Orthomyxovirus virus consists of:
  • Hemagglutinin A- It is triangular and its function is to attach to the body via the sialic acid receptor. It attaches to the respiratory epithelium and RBC membrane.
  • Neuraminidase- It is lesser in number than hemagglutinin. It is mushroom-shaped and its function is to help in the release of the virus. 
  • M1 protein-It is present below the envelope and its function is to maintain the structural integrity of the virus.
  • M2 protein- It is an ion channel and it assists in the uncoating of virus.
    • These viruses show a unique phenomenon called antigenic shift and antigenic drift.
      • Antigenic shift is a sudden change. It is also called as genetic reassortment because there is an exchange of RNA segments between influenza A. Antigenic shift can be a major cause of major epidemics and pandemics. It is seen in influenza A virus.
      • Antigenic drift is a gradual change. It occurs due to mutation in H&N. It is seen in influenza A and influenza B.

Pathogenesis of Orthomyxovirus

• Attachment - This is the first step and in this, the influenza virus enters the body. The hemagglutinin attaches to the sialic acid receptor.
• Endocytosis- This process refers to the internalization of the virus. It occurs by the formation of an endosome.
• Acidification of Endosomes- It is done by uncoating and the M2 ion channel allows acid influx.
• Conversion of negative-sense RNA into positive-sense -RNA occurs by RNA-dependent RNA polymerase.
• Release- The Neuraminidase enzyme present in the virus helps to release the viral content. The neuraminidase is sialidase in nature.

Clinical features of Orthomyxovirus

• The infection will start with prodromal symptoms like fever, sore throat, headache, cough, fatigue, and myalgia.
• Running nose is usually absent in these patients.
• GI symptoms are very rare.
• The most common extrapulmonary symptom is myalgia.
• Reyes syndrome or Jamshedpur fever is present in kids.
• The most common complication is pneumonia.

Lab Diagnosis

• Nasopharyngeal swab is used to take specimens.
• Antibody detection tests like ELISA and HAI are used.
• Antigen detection tests like DIF.
• The most important test is the Hemagglutination inhibition test.

Treatment of Orthomyxovirus

• NA inhibitors-
  • Tamiflu and Oseltamivir are given via oral route. 
  • Zanamivir via nasal route
• M2 inhibitors-
  • Amantadine and Rimantadine are used only for Influenza A.
• Vaccination is available in the market. Live nasal spray is given to patients who have better immunity and no contraindications.
• Inactivated vaccine can be given to healthcare workers and two doses are given annually.

Also Read: Understanding Platelet Disorders

Paramyxovirus

• It consists of measles, mumps, RSV virus, parainfluenza, and Nipah virus.
• The paramyxovirus has hemagglutinin and neuraminidase on the same spike. There is a protein called F protein or fusion protein which will be present in the virus.

1. Measles virus
   • It is also called rubeola and the incubation period is 10 days in children.
   • The mode of transmission is inhalation of respiratory droplets.
   • The main period of infection is 4 days before rash and four days after rash. A single serotype can give lifelong community immunity.
   • The prodromal symptoms are fever, cough, coryza, and conjunctivitis.
   • The pathognomonic sign is Koplik spots which start occurring at 12 days. The Koplik spots are present near the molars and these are white-colored dots on the erythematous base. The rashes will develop two to three days after Koplik spots.
   • The rashes are maculopapular and these start behind the ear to the face and follow the rest of the body.
   • Fever and Koplik spots disappear with the appearance of the rash.

The Complications are:

• Secondary bacterial infections like otitis media and bronchopulmonary pneumonia.
• Due to measles GI difficulties like diarrhea can occur.
• The CNS manifestations are post-measles encephalitis and subacute sclerosing panencephalitis.
• In pregnancy intrauterine death can occur. 

Lab Diagnosis

• The nasopharyngeal swab will be used as a sample.
• Reverse transcriptase PCR
• Intracytoplasmic or intranuclear inclusions or giant cells will be found.

2. Mumps

• The mode of transmission is droplets or saliva and it is mainly seen in winter and spring seasons. 
• The infective period is one week before two and one week after the symptoms.

The Clinical Features Are:

• Fever, fatigue, and bilateral parotitis.
• The most common manifestation is Bilateral parotitis.
• The second most common manifestation is unilateral orchitis which is known as sterility.
• The other manifestations are pancreatitis and aseptic meningitis.
• The pain increases on consuming the citrus juices.

Diagnosis of Paramyxovirus

• Antigen detection tests and antibody detection tests like ELISA.
• RT PCR

3. Respiratory Syncytial Virus (RSV)

• It is mainly prevalent in the rainy season and the main age group that gets infected is 6 weeks to six months.
• The mode of transmission is inhalation and fomites.

Clinical Features of Paramyxovirus

• In infants it is the most common cause of lower respiratory tract infection (LRTI).
• The patient will present with fever, rhinorrhea, cough, dyspnea, and wheezing. 
• In adults it can cause upper respiratory tract infection.

Diagnosis of Paramyxovirus

• The antigen detection test and antibody detection test are used.
• RT PCR
• It has a cytopathic effect.
• The multinucleated giant cell is a unique feature.

Treatment of Paramyxovirus

• The newer drug named Palivizumab is given.
• For serious infections, Ribavirin is available.

4. Parainfluenza

• It is prevalent in the winter season.
• It causes common cold.
• Its most important clinical manifestation is Croup or laryngotracheal bronchitis.
• In severe cases it can lead to pneumonia.
• The most common complication is Otitis media.

5. NIPAH Virus

• This virus is famous for its epidemics in Kerala, in 2018.
• It affects the nervous system mainly and has a 60 to 70% mortality rate.
• The pigs are amplifiers and the disease gets transmitted to humans from pigs.

Clinical features of Paramyxovirus

• Encephalitis
• Fever

Diagnosis is done by PCR.

Also Read: Primary Biliary Cholangitis : Pathophysiology, Clinical Features

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