It has been estimated that 90% of adults harbour viruses that have been acquired as a result of infection during earlier life. Many viruses have the property of latency and may reside in the tissues asymptomatically for the remainder of the patient’s life. The oral tissues are a frequent site for symptomatic primary infection and also reactivation of latent viruses. The presence of these viruses often becomes apparent at times when the host immune defence is compromised. This may range from short episodes of localized symptoms in otherwise healthy individuals to widespread prolonged problems in HIV positive patients.

Viruses, which are some of the smallest microorganisms (100–300 nm), consist of a core (genome) containing either DNA or RNA surrounded by a protein shell (capsid) (Fig. 10.1), can cause a range of important human diseases. Certain viruses also possess an outer lipoprotein coat derived from infected host cells. Viruses are obligate intracellular parasites in that they require the protein synthesizing apparatus (ribosomes) of the host cell. Viral replication is a complex process but comprises a number of recognized steps including, adsorption/penetration into the host cell, uncoating, transcription, synthesis of viral components, assembly and finally, release of new virions (Fig. 10.2).

Fig. 10.1 Photomicrograph of HSV-1 showing central capsid and surrounding envelope (original magnification ×100,000).

Fig. 10.2 Stages of viral infection and replication within a host cell.

Laboratory diagnosis

A variety of laboratory methods have been developed for the detection, isolation and identification of viruses within clinical samples (Table 10.1). These techniques involve microscopy, culture, serology and nucleic acid amplification. However, the most appropriate method of sampling depends on the nature of the suspected infection.

Table 10.1 Special investigations used in the diagnosis of orofacial viral infection.

Investigation	For	Against
Electron microscopy of vesicular fluid	Easy sampleRapid result	InsensitiveNot specific
Light microscopy of lesional smear	Easy sample of smear Widely availableRapid result	Not specific
Light microscopy of lesional smear with immunofluorescence	Easy sampleRapid resultSpecific	Not routinely available
Culture of swab of lesion	Easy sampleCan be specificWidely available	Result may not be available for up to 10 days
Antibody titre in venous blood	Requires paired venous samples	Two haematological samples required.Result not available for at least two weeks
PCR amplification of vesicular fluid or swab of lesion	Easy sampleRapid result	Not widely available

Electron microscopy can be used to provide a provisional identification based on the morphological appearance of viral particles but this approach has low specificity and requires additional tests. Routine light microscopy can be used in conjunction with immunofluoresence and monoclonal antibodies to detect the presence of specific viruses (Fig. 10.3). This is a relatively rapid method giving a result within 30 minutes but the technique is not widely available.

Fig. 10.3 Immunofluorescence of a smear showing presence of HSV-1.

Viruses can be grown in tissue culture usually involving baby hamster or monkey kidney fibroblasts. A swab of the lesion should be placed in viral transport medium, which contains at least two antibiotics, usually penicillin and streptomycin, to prevent bacterial growth, combined with an antifungal, such as amphotericin, to eliminate any fungal contamination. The transport medium should also contain serum and a buffer to maintain virus viability. The specimen should be sent to the laboratory promptly although it will not be processed for 24 hours to allow the antimicrobials mentioned above time to have an effect. The first stage of processing is inoculation into a monolayer of tissue culture. The presence of virus is determined in the laboratory by detection of a cytopathic effect, which is seen as the development of multinucleate giant cells (Fig. 10.4). This may take up to 10 days although the cytopathic effect occurs more rapidly in the presence of high numbers of virus particles.

Fig. 10.4 (A) Monolayer of baby hamster kidney fibroblasts inoculated with swab specimen at day 1. (B) Same monolayer at day 7 showing cytopathic effect as destruction of architecture and development of multinucleate cells.

Alternatively, diagnosis may be confirmed retrospectively by the detection of a four-fold rise in antibody titre between acute and convalescent sera taken from the patient. However, this technique has limited clinical benefit in diagnosis due to the prolonged time involved in obtaining a result. Molecular methods are being used increasingly to detect virus DNA or mRNA and in the future it is likely that these will provide a rapid diagnosis.