FUNGAL DISEASES IN DOG - PART 2: CANDIDA

24/07/2024

CANDIDA FUNGAL DISEASE

Introduction

Candidiasis is a disease caused by species of fungi belonging to the genus Candida. Among them, Candida albicans (C. albicans) is the most commonly isolated species in dogs. This commensal organism typically resides in the gastrointestinal tract, upper respiratory tract, and urogenital tract. Candida infections occur when there is an overgrowth of Candida fungi in the dog's body, which can lead to localized skin and mucosal infections. In many cases, C. albicans can also cause systemic infections that threaten life, primarily in cases with underlying immunosuppressive disorders.

Characteristics of the Pathogen

Candida fungi are very small and translucent. In artificial culture environments (in vitro), they exhibit many different phenotypes depending on their life cycle and environmental factors. The yeast form can convert into a filamentous form (hyphae) that is opaque white. If faced with adverse conditions, the fungi can transform into a spore form called chlamydospores, which can persist for a very long time. Conversely, under favorable conditions, the fungi can grow quite large.

C. albicans develops in both yeast and filamentous forms on YPD agar (Garnhami, 2015)

Epidemiology

Candida albicans is a type of yeast that consumes glucose and is present as part of the normal microbiota in the nose, gastrointestinal tract, and external genitalia of many animal species. C. albicans only causes disease when given the opportunity. Factors that may predispose the host to infection include immunosuppression due to corticosteroid use, the concurrent presence of autoimmune diseases, and diseases caused by Ehrlichia canis.

Candida albicans commonly causes disease in birds and can lead to mucosal infections in the mouth, esophagus, and crop. Limited infections in the intestinal mucosa have been described in pigs and foals. Systemic Candida infections have also been reported in cattle, calves, and foals after prolonged treatment with antibiotics or corticosteroids. In cats and dogs, Candida infections are rare but are associated with upper respiratory and oral diseases, eye lesions, gastrointestinal issues, and urinary bladder inflammation.

Pathogenesis

Candida albicans is a fungus that exists in the body as small colonies. Under normal conditions, this yeast is kept in balance by beneficial bacteria and other microorganisms in the dog's intestines. When the dog's immune system is not functioning properly, the balance between beneficial bacteria and C. albicans in the small intestine is disrupted, allowing C. albicans to multiply rapidly, grow out of control, and ultimately cause damage to the intestinal mucosa. C. albicans can alter its pathology from a unicellular organism to a branched filamentous form, which can reproduce quickly. It uses root-like structures resembling tentacles to penetrate the intestinal wall. When the intestinal mucosa is damaged, Candida can enter the bloodstream and spread to other parts of the body, causing chronic infections that are very difficult to eliminate because Candida has a tough connective tissue coating that protects it from the effects of the immune system and some antifungal medications.

Candida albicans in its environmental form (Josep Allué, 2023)

Electron microscopy showing C. albicans penetrating the intestinal wall (Fiosson, 2023)

Pathology

Symptoms

Candida infections in dogs can manifest in various ways. Symptoms may include: accumulation of yellow, brown, or black debris in one or both ears; excessive head shaking; scratching excessively at the ears or other parts of the body; skin rashes, heat, and infections; excessive licking of the paws, legs, tail, or other areas; hair loss, dull or greasy fur, and a foul odor; urinary Candida infections can occur in cats and sometimes in dogs, especially those with urinary catheters. Disseminated Candida infections often present clinical signs related to the affected body systems (gastrointestinal, reproductive).

Lesions

Gross lesions on the skin and mucosa are one or more raised white nodules covered by scales. The development of histopathology includes signs of orthokeratotic hyperkeratosis (thickening of the stratum corneum). The reactive process includes infiltrative surface skin inflammation and inflammation of the hair follicles, with a prominent involvement of macrophages and lymphocytes.

Dog infected with Candida. (A) Hair loss and scaling on the face, ears, and legs. (B) Pustules and pigmented spots on the abdomen. (C) Gray-white discharge in the oral cavity (Declara et al., 2022).

Diagnosis

Clinical Diagnosis

Candida infections can range from mild to severe. Cutaneous infections may affect the ears, skin, paws, and other areas, with dogs exhibiting abnormal behaviors such as excessive licking and biting of a specific area on their bodies. Severe cases are rare in dogs and are associated with upper respiratory and oral diseases, purulent pleuritis, eye lesions, gastrointestinal issues, and urinary bladder inflammation.

Laboratory Diagnosis

Isolate and culture fungi from scrapings or biopsy samples from mucosal lesions or urine sediment. Candida fungi grow on standard agar media. Observe the morphology of the fungi from samples taken from the ears, skin, gastrointestinal tract, or reproductive system: C. albicans appears as oval yeast cells (3–7 μm in diameter) with thin walls, or they may appear in chains, forming pseudohyphae when daughter cells remain attached after division. True hyphae, which are regular and filamentous, can also be observed.

Skin biopsy sample from a dog infected with Candida. A: H&E staining shows layers of necrotic epithelial cells with a neutrophilic infiltrate associated with hyperkeratosis, right-sided epidermal hyperplasia, dermal edema, and perivascular lymphocytic dermatitis. B: PAS staining shows the presence of hyphae (arrow) in the stratum corneum (Declara et al., 2022).

Dogs with urinary Candida infections may show fungal shadows in the bladder on ultrasound. If there is a suspicion of an infection inside the ear, an otoscope should be used to examine the ear canal. Samples can also be taken from inside and around the ear for testing.

Prevention and Treatment

Treatment

Topical application of Nystatin ointment, Amphotericin B, or 1% iodine solution is useful in treating oral or skin fungal infections. Systemic or urinary infections are treated with Fluconazole for 4–6 weeks; 1% Clotrimazole can be administered intravenously slowly over 1 hour for urinary tract infections. Terbinafine hydrochloride at 30 mg/kg, once daily, can be combined with Econazole nitrate shampoo at 10 mg/ml, 2-3 times a week.

Prevention

In dogs with a poorly functioning immune system, C. albicans can overgrow and cause disease. Conversely, an overactive immune system can lead to allergic reactions, which can also result in illness. Therefore, when dogs have severe allergies, medication to reduce the allergic response is necessary. Other contributing factors to the disease that should be noted include mold in food, environmental factors, transmission from the mother dog, and diabetes.

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Source: Nguyen Duc Hien, Nguyen Ngoc Phu Vinh, Huynh Minh Tri, Pham Minh Thu, Tran Khanh Long (2023). The disease is common in dogs. Can Tho University Press.