Diagnosis of Viral Diseases

DIAGNOSIS OF VIRAL DISEASES

There are well over 1000 known viruses of vertebrate and it is impossible for a single

laboratory to have all the resources required for the diagnosis of all these viruses.

Laboratories tend to specialize on particular viruses and serve as reference laboratories.

Stages of viral disease diagnosis:

1. Clinic: presumptive diagnosis based on clinical signs and history

2. Pathology: Observed lesions and pathological changes at gross and histopathological

levels. History may provide clues.

3. Microbiological diagnosis: confirmatory diagnosis

Principles of microbiological diagnosis:

i. Isolation of viruses

ii. Detection of viral nucleic acid/specific genes

iii. Detection of viral antigen

iv. Detection of specific virus-induced antibody

Sample collection:

a. Samples must be collected at from the right site and at the right time

b. Samples to be collected must relate to the clinical signs and pathological changes

observed

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c. Samples must be collected as soon as clinical signs are observed

d. Knowledge of the pathogenesis of the disease may dictate the type of sample to be

collected

e. Proper labeling of samples for identification and to avoid confusion

f. Samples must be sent to the laboratory with history and tentative diagnosis

g. Transport samples with ice packs (4 oC) if transit time is less than 24 hours

h. For transit above 24 hours, use dry ice at -70 OC.

i. Long term storage is achieved with liquid nitrogen at -196 OC.

j. Use transport medium containing buffer (isotonic saline) with bovine albumin/foetal calf

serum (protein to prolong virus survival), antibiotic and antifungal agent (to prevent

contaminants)

Samples for diagnosis of viral diseases:

 Respiratory tract infection: nasal swab, tracheal swab, nasopharyngeal aspirate, lung

tissue.

 Enteric infection: Faeces, rectal swab.

 Genital tract infection: prepucial washing, semen, genital swab.

 Eye infection: Conjunctival swab.

 Skin infection: Vesicular fluid, epithelial scrapings, biopsy of solid lesions.

 Central nervous system: cerebrospinal fluid, faeces, nasal swab, brain tissue.

 Generalized infection: Nasal swab, faeces, blood leukocytes.

 Post mortem examination: relevant organ.

 Every case: Blood for serum to be used in serology.