HSV-1 most commonly infects the trigeminal ganglia, where it remains latent. If reactivated, it presents as herpes labialis, also known as cold sores. Diagnosis
Histopathology
The histological appearance of a herpetic infection on the mucosa includes degeneration of stratified squamous epithelial cells, the loss of intercellular connections and inflammatory infiltrate around the capillaries of the dermis layer. An intact herpetic vesicle presents as an intraepithelial blister histologically. This vesicle is caused by rupture and distension of the virally epithelial cells by intracellular oedema and coalescence of disrupted cells.Rupturing of the infected cells cause a great number of viral particles to be released, rendering them the ability to affect adjacent epithelial cells and even the sensory axons of the trigeminal nerve. Histologically, these infected cells have an eosinophilic cytoplasm and large, pale vesicular nuclei, appearing swollen under the microscope. The cytoplasms of the infected cells fuse, collectively forming giant cells with many nuclei. The balloon cells and multi-nucleated giant cells can often be identified in smears taken from an intact vesicle or from one which has been recently ruptured.The lamina propria shows a variable inflammatory infiltrate, the density of which depends on the stage and severity of the disease, and inflammatory cells also extend into the epithelium.Cowdry type A bodies are intranuclear inclusion bodies visible under light microscopy. They show electron dense glycoproteins and viral capsids. Both Cowdry type A bodies can both be found in varicella zoster and herpetic gingivostomatitis, making it impossible to distinguish between both eosinophilic bodies.
One way to distinguish between the herpes virus (and hence herpetic gingivostomatitis) and varicella virus is by direct immunohistochemistry using fluorescent antibodies. Differential diagnosis
Diagnosis of HG is very much clinically based. Therefore, it is imperative to rule out other diseases that present similarly, bearing in mind the past medical history of the patient. Some differential diagnoses to bear in mind when considering herpetic gingivostomatitis are:

Teething in infants:  A study mentioned that  "primary  tooth  eruption begins  at  about the  time that  infants  are  losing  maternal antibody protection  against  the  herpes  virus. Also, reports  on  teething  difficulties  have  recorded  symptoms  which  are  remarkably  consistent  with  primary oral  herpetic  infection  such as  fever,  irritability,  sleeplessness,  and difficulty  with eating." Another study highlighted that "younger infants  with  higher residual  levels  of  antibodies  would experience  milder infections  and  these  would be  more likely  to  go unrecognized  or  be  dismissed  as  teething  difficulty." Herpangina: It is a disease that is caused by the Coxackie A virus rather than a herpes virus. In herpangina, ulcers are usually isolated to the soft palate and anterior pillar of the mouth.