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Picornaviridae

  • Icosahedral, 28-30 nm (small size), single stranded RNA, linear positive sense, nonenveloped
  • Three genera containing viruses infecting humans
Genus Species
Enterovirus Polio virus 1-3
Coxsackieviruses A1-24 except 23
Coxsackieviruses B1-6
Echoviruses 1-34 except 10 & 28
Enteroviruses 68-71
Rhinovirus Rhinoviruses 1-100
Hepatovirus Hepatitis A virus ( earlier called as enterovirus 72)
 
Differences between coxcasckie a & b viruses
  A B
Pathological changes induced by inoculation of suckling mice Generalized myositis
Flaccid paralysis
Death within one week
Patchy focal myositis
Spastic paralysis
Localized lesions in the liver, pancreas, myocardium
Number of types 23 (1-24 except 23) 6 (1-6)
 
Human enteroviruses and associated clinical syndromes
Syndrome Poliovirus
1-3
Coxsackievirus Echovirus
1-34
Enterovirus
68-71
A1-24 B1-6
Neurologic
Aseptic meningitis
Paralysis
Encephalitis
 
1-3
1-3
-
 
Many
7,9
2,5-7,9
 
1-6
2-5
1-5
 
Many
2,4,6,9,11,30
2,6,9,19
 
71
70,71
70,71
Skin & mucosa
Herpangina
Hand, foot & mouth disease
Exanthems
 
-
-
-
 
2-6,8,10
5,10,16
Many
 
-
-
5
 
-
-
2,4,6,9,11,16,18
 
-
71
-
Cardiac & muscular
Pleurodynia/ Bornholm disease/ Epidemic myalgia
 
-
-
 
-
-
 
1-5
1-5
 
1,6,9
1,6,9,19
 
-
-
Ocular
Acute haemorrhagic conjunctivitis
 
-
 
 
24
 
-
 
-
 
 
70
Respiratory
Colds
Pneumonia
Pneumonia of infants
 
-
-
-
 
21,24
-
9,16
 
1,3,4,5
4,5
-
 
4,9,11,20,25
-
-
 
-
68
-
Gastrointestinal
Diarrhea
 
 
Hepatitis
 
-
 
 
-
 
18,20-22,24
 
4,9
 
-
 
 
5
 
Many
 
 
4,9
 
-
 
 
-
Undifferentiated febrile illness 1-3 - 1-6 - -
Generalized disease of infants - - 1-5 - -
Diabetes mellitus - - 3,4 - -
 
a. Polioviruses
 
i. Consists of three types (1-3) based on neutralization tests
  • Type1: epidemic
  • Type2: endemic infections
  • Type3: occasionally associated with epidemics
ii. Natural infections occurs only in man
 
iii. Spread by faeco-oral route and droplet infection
 
iv. Pathogenesis
 
  • Virus multiplies in the tonsils & payer’s patches→ spread to regional lymph nodes (cervical, mesenteric)→ viraemia→ disseminated throughout the body including spinal cord & brain
v. Clinical features
  • Inapparent infection 90-95% of individuals
  • Minor illness/ abortive poliomyelitis Influenza like illness, 4-8%
  • Non-paralytic poliomyelitis
  1. Headache, neck stiffness, back pain, complete recovery; 1-2%
  2. Paralytic poliomyelitis

1. Flaccid paralysis, may be spinal, bulbar or bulbospinal; 0.1-2%
vi. Laboratory diagnosis
  • Isolation: tissue culture, CPE: cell retraction, cytoplasmic granularity, nuclear pyknosis
vii. Prophylaxis
  • Inactivated polio (Salk) vaccine
  • Live attenuated polio (Sabin) vaccine
b. Rhinoviruses-Picornaviruses
  1. Differentiated from the enteroviruses by their acid lability (therefore not able to infect intestinal tract) and their optimal temperature for replication (33°C)
  2. Consist of more than 100 serotypes
  3. Most important cause of common cold (>50% of common colds)
  4. Infection transmitted by droplet infection
  5. Incubation period: 2-4 days. Rhinorrhea, nasal obstruction, sneezing, sore throat, cough, headache, malaise, mild fever. Symptoms subside in about a week
c. Rhabdoviridae
Bullet shaped, 75x180nm, single stranded RNA, linear, non-segmented, negative-sense, enveloped
Two genera-Vesiculovirus: infections in animals and Lyssavirus: rabies virus
 
d. Rabies virus
Rabies is a natural infection of dogs, foxes, wolves etc.
 
Animal susceptibility to rabies
Very high High Moderate Low
Foxes
Coyotes
Jakals
Wolves
Cotton rats
 
Hamsters
Shunks
Raccoons
Cats
Bats
Rabbits
Cattle
Dogs
Sheeps
Goats
Horses
Nonhuman primates
Opossums
 
 
Man acquires infection by the bite of the rabid dog or other animals
Rarely infection can occur following licks on abraded skin and intact mucosa
Bite of the animal results in deposition of rabies infected saliva in the muscle→ virus replicates locally→ infects peripheral nerves→ within the nerve fibres it travels along the axon towards the central nervous system (speed of 3mm hour)→ in CNS it multiplies and produces encephalitis→ virus then spreads outwards along the nerve trunks to various parts of the body including salivary glands→ shed in saliva
Man is not highly susceptible, the incidence of human rabies after bites by known rabid dogs is about 15%

a. Clinical features
  1. Incubation period: 1-2 months, shorter in children and than in adults, shorter in persons bitten on the face or head than those bitten on the legs (related to the distance virus has to travel to reach brain)
  2. Malaise, headache, fever, paraesthesia at the site of bite followed by anxiety, hyperactivity, aggression, convulsions, hydrophobia. Finally patient develops coma and death.
  3. Disease once developed is always fatal in about 4-14 days
  4. Virus is secreted in saliva, urine and other secretions
b. Immune response
  1. Rabies virus ascends to the brain along the nerves and does not come in contact with immune system. It is only after virus spreads form the CNS to different parts of the body that antibodies are formed.
  2. By this time it is too late as irreversible damage of the neurons have already occurred
  3. Antibody is protective if it is present before exposure (pre-exposure vaccination) or after exposure (post-exposure vaccination or passive immunisation) thereby preventing the binding of the virus to the nerve fibers at the site of inoculation  
c. Laboratory diagnosis
  1. Demonstration of Negri bodies (intracytoplasmic, round, oval, eosinophilic with basophilic inner granules) by Seller’s stain.  Max. in pyramidal cells of Ammon’s horn, Purkinje cells of hippocampus, brain stem, cerebellum
  2. Demonstration of antigen by direct immunofluorescence
  3. Antemortem: salivary, corneal smears, skin biopsy from nape of the neck
  4. Postmortem: impression smears of cut surface of salivary glands, hippocampus, brain stem, cerebellum
  5. Detection of genomic RNA or viral mRNA: PCR, DNA probes
d. Rabies vaccines
i. Neural vaccines
  1. Pasteur vaccine, Fermi vaccine, Semple vaccine, Beta-propiolactone vaccine
  2. Suckling mouse brain vaccine
ii. Non-neural vaccine
  1. Duck egg vaccine-Flurry strain and Kelev strain
  2. Cell culture vaccine
  3. First generation: Human diploid cell vaccine (HDCV)
  4. Second generation vaccine: Purified chick embryo cell vaccine (PCEC), Purified vero cell rabies vaccine (PVRV) 




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