Canine follicular conjunctivitis (CFC) is a relatively common ocular surface condition in practice, but the published data describing its characteristics are still limited and often extrapolated from human ophthalmology. Much of the conceptual framework comes from comparisons with vernal allergic conjunctivitis (VC) and trachoma in people, which share some clinical and immunologic similarities with follicular diseases in dogs. 

Signalment and Clinical Patterns 

CFC can occur in dogs of all ages, but younger dogs tend to be more frequently and more severely affected.¹,² Follicles are particularly common in dogs under 18 months, which mirrors the age distribution of VC in humans, typically affecting patients between 5 and 25 years of age, with onset around 6–7 years.¹ VC often resolves after puberty, when conjunctival sex hormone expression decreases,¹ which may parallel the clinical observation that older dogs are generally less severely affected than juveniles. 

In contrast to VC in humans, where boys are more commonly affected, CFC does not show a clear sex predisposition.¹ Gender susceptibility tends to equalize after puberty in humans and is similarly not a key risk factor in dogs. 

Anatomical factors can influence conjunctival exposure to irritants and allergens.² In dogs, brachycephalic breeds might be suspected as at risk due to ocular prominence, but available clinical data do not support a specific predisposition to CFC. Physiologic exophthalmia may move the eyelids forward and reduce conjunctival sac depth, potentially decreasing retention of foreign material or allergens. 

Clinical Features and Differentials 

CFC is typically bilateral and characterized by the presence of follicles—translucent nodular elevations outlined by fine capillaries.¹ Bilaterality is also a common feature of VC and trachoma,¹ but hallmark lesions differ: VC is associated with giant papillae, and trachoma may exhibit both follicles and papillae.¹ 

VC and trachoma can lead to pseudomembrane formation and conjunctival scarring, while these features have not been reported in CFC.¹ In trachoma, scarring may follow follicular necrosis, while VC may show limbal involvement such as Trantas’ dots.¹ Such changes are not a recognized feature of CFC. 

The distribution of follicles helps orient the clinician. In VC, changes often predominate on the limbal and dorsal bulbar conjunctiva,¹ while in trachoma they are classically palpebral and forniceal. In dogs with CFC, the nictitating membrane (NM) is frequently the most affected site, followed by the palpebral conjunctiva. Supporting a possible allergic component, a recent dog study demonstrated more intense allergic reactions in the ventral conjunctiva and NM, where allergens accumulate.⁴ 

Pathogenesis and Comorbidities 

The role of allergy in follicle formation has been debated. Historically, allergic conjunctivitis has been described as a cause of follicles,¹ but recent data suggest follicles are not common in all forms of canine allergic conjunctivitis.⁴ This raises questions about whether allergy is always the primary driver. Nonetheless, atopy should still be considered a key potential etiologic factor. In clinical cohorts, atopy is a frequent systemic comorbidity, particularly in adults, consistent with the typical age of onset of canine atopic disease.⁴ 

In humans and dogs, allergic conjunctivitis has been associated with atopy with incidences of 25–45% in people and up to 94% in dogs.⁴ VC is classically linked to asthma, allergic rhinitis and atopic dermatitis.¹ Trachoma, in contrast, has a bacterial etiology (Chlamydia trachomatis) and is not associated with atopy.⁵ 

Cytology, Histology and Diagnosis 

Histopathology has been described extensively for VC and trachoma but not fully for CFC. Conjunctival giant papillae in VC reflect mixed inflammatory infiltrates and collagen deposition, whereas trachoma follicles consist mainly of lymphoid tissue.¹ 

In routine practice, diagnosis is clinical and supported by cytology. VC is characterized by mast cells, basophils and eosinophils, while trachoma shows intracytoplasmic inclusions of C. trachomatis.¹ In CFC, cytology typically reveals a lymphoplasmacytic response with minimal eosinophils, consistent across reported cohorts.¹,² Biopsy may be useful in atypical or refractory cases. Ancillary diagnostics—culture, immunofluorescence, enzyme immunoassays, serology, PCR, allergy testing, tear cytology and cytokine analysis—are available but not routinely required.¹ 

Management and Therapeutic Approach 

Topical anti-inflammatory therapy is central to management. Corticosteroids are considered the gold standard in CFC, while in VC they are typically reserved for more severe disease. NSAIDs may suffice for mild cases.³,⁴ Immunosuppressives such as cyclosporine, tacrolimus, montelukast and mitomycin-C are occasionally used in refractory human VC, and may be considered when conventional therapy fails, though veterinary evidence remains limited.³,⁴ 

Mast cell stabilizers and antihistamines are widely used in VC but appear less clearly beneficial in CFC.¹,² Supportive care—including saline irrigation and artificial tears—reduces allergen load, particularly in dogs with deep fornices where debris accumulates.³,⁴ Systemic therapy is generally reserved for patients with concurrent dermatologic disease.¹,² 

Mechanical procedures such as follicular debridement have been proposed in refractory canine cases but may predispose to chronic conjunctivitis and lack strong supporting data.¹ In severe human VC, interventional therapies such as cryotherapy and CO₂ laser have been described, though long-term outcomes are uncertain.³,⁴ Preservation of normal conjunctival immune structures is preferred. 

Prognosis 

With appropriate management, most dogs with CFC show good short-term resolution. Mild cases often respond to topical NSAIDs and supportive care, while more severe cases improve with a course of topical corticosteroids over several weeks. Age, severity and comorbid atopy may influence response, with younger dogs tending to have more significant but still manageable disease. 

References  

1. Cerrada I, Leiva M, Vilao R, Peña T, Ríos J. Follicular conjunctivitis in dogs: A retrospective study (2007–2022). Vet Ophthalmol. 2024 Jul;27(4):310-7. 

2. Hartley C, Hendrix DVH. Diseases and surgery of the canine conjunctiva and nictitating membrane. In: Gelatt KN, Ben-Shlomo G, Gilger BC, Hendrix DVH, Kern TJ, Plummer CE, editors. Veterinary Ophthalmology. Vol 1. 6th ed. Wiley Blackwell; 2021. p. 1045–1081. 

3. Hartley C. The conjunctiva and third eyelid. In: Gould D, McLellan G, editors. BSAVA Manual of Canine and Feline Ophthalmology. 3rd ed. British Small Animal Veterinary Association; 2014. p. 182–199. 

4. Delgado E, Gomes É, Gil S, Lourenço AM. Diagnostic approach and grading scheme for canine allergic conjunctivitis. BMC Vet Res. 2023;19(1):35.