Symptoms and Prevention and Treatment Methods of Deep Mycoses in Dogs

Deep mycoses refer to infections caused by fungi that invade the skin, mucous membranes and internal organs. There are many species of pathogenic fungi. These pathogens are widely present in soil, animal feces and milk. Dogs are mainly infected through contact with contaminated soil or via the respiratory and digestive tracts; only a few fungi are contagious through contact. Deep mycoses can occur in various organs throughout the body, and their pathological features are: granulomatous inflammation, necrosis, abscesses, ulcers, fistulas, connective tissue proliferation and nodule formation. The widespread addition of antibacterial drugs to feed may be an important cause of deep mycoses in dogs.

The several main deep mycoses that dogs are prone to are:

1. Histoplasmosis

Its pathogen is Histoplasma capsulatum. It grows in soil and infection mainly occurs via the respiratory tract. Affected dogs mainly show persistent, difficult-to-cure intractable coughing and diarrhea, with no sputum discharge. Affected dogs have anorexia, emaciation, irregular fever, vomiting and dermatitis, and palpation of the abdominal wall often reveals enlarged mesenteric lymph nodes. In chronic cases, ulcers of the buccal mucosa and enlargement of the tonsils are sometimes seen.

When dogs are found clinically to show intractable coughing or diarrhea, this disease should be considered. If conditions permit, an intradermal test with histoplasmin should be performed, and chest X-ray fluoroscopy, serum complement fixation test, agar diffusion test, etc. can also be done for diagnosis.

Amphotericin B has a very good therapeutic effect on this disease. The total dosage is 4 mg/kg body weight, divided into 10 doses, injected once every two days. Before use, first dissolve it with water for injection, then dilute it with glucose saline to 0.1% (mg) and inject intravenously. The total amount must not exceed 5 mg/kg body weight, otherwise it may damage the kidneys. Clotrimazole (antifungal No. 1) also has a good therapeutic effect on this disease, with a dosage of 0.75-1.5 g, taken orally in two divided doses.

2. Coccidioidomycosis

Its pathogen is Coccidioides. Infection mainly occurs via the respiratory tract. Granulomas develop in the lungs, bronchi or mediastinal lymph nodes of affected dogs. Affected dogs show elevated body temperature, coughing, difficulty breathing, poor appetite, emaciation and diarrhea. When the joints are affected, lameness and muscle atrophy appear.

This disease spreads rapidly among dog populations. When suspicious symptoms appear in dogs, further chest X-ray photography should be carried out, and if conditions permit, coccidioidin intradermal testing and serum complement fixation testing can be done.

The treatment method is the same as for histoplasmosis.

3. Blastomycosis

Its pathogen is Blastomyces dermatitidis.

This disease can be divided into two types: the systemic type and the cutaneous type. The systemic type mainly manifests as lung disease. Affected dogs are depressed, febrile, anorexic, emaciated and coughing. On necropsy of dead dogs, all lobes of the lungs have nodules and abscesses; the lungs appear grayish-white or pale red with mottled markings and show focal or diffuse consolidation. The center of the granulomatous nodules undergoes necrosis but does not calcify. When the lesions spread to the surrounding area, they can cause enlargement and suppuration of the bronchi and mediastinal lymph nodes, and may even cause pleurisy. Cutaneous blastomycosis manifests as single or multiple cutaneous granulomas, which eventually undergo central liquefactive necrosis and ulceration.

Non-calcified nodules or consolidation in the lungs seen on chest X-ray photographs are helpful for the diagnosis of this disease. It is best to perform a biopsy of the skin nodules. When microscopic examination of pus or sputum reveals single or budding spherical cells with thick, refractive cell walls (double-layered outline), a definitive diagnosis can be made. Once this disease spreads, it is difficult to cure. Amphotericin B can be used for treatment in the early stage. Skin nodules can be surgically removed.

4. Cryptococcosis

Its pathogen is Cryptococcus neoformans. It exists in soil and can be isolated from soil, bird droppings, fruit and milk.

Cryptococcosis in dogs mainly affects the brain, meninges, paranasal sinuses, as well as the lungs, spleen, muscles, joints and skin, causing ataxia, circling movements, abnormal behavior, lameness, hyperesthesia and nasal discharge in affected dogs. On necropsy, small suppurative foci are found in the paranasal sinuses, nasal turbinates, nasal cavity and brain, and the meninges show mucopurulent inflammation. Subcutaneous granulomas can be seen in areas such as the ears, eyelids and feet.

When affected dogs are clinically found to have unexplained respiratory and central nervous system symptoms, the possibility of cryptococcosis should be considered. Further pathogen examination, serological tests and histopathological examination, if they can be performed, are all helpful for the definitive diagnosis of this disease.

The treatment method is the same as for histoplasmosis.

5. Sporotrichosis

The pathogen is Sporothrix schenckii, and infection occurs through wounds.

The lesions mainly involve the skin, often occurring on the extremities and spreading along the lymphatic vessels, forming a typical band-like swelling. The primary site is a firm, inelastic, movable, non-tender nodule or granuloma. After the hair over the nodule falls out, exudate can be seen attached to it; after drying, a scab forms. Some nodules may form abscesses, which then rupture to form ulcers.

This disease can be suspected based on the above symptoms. For a definitive diagnosis, material can be collected from unruptured abscesses for isolation and culture. If the pathogen is isolated, a definitive diagnosis can be made.

Iodides have a specific therapeutic effect on this disease. Potassium iodide can be given orally at a daily dose of 4.4 mg/kg body weight. Sodium iodide can also be injected intravenously. Oral griseofulvin at a dosage of 20 mg/kg body weight per day can be used for 2-3 weeks. For skin ulcers, a 0.2% iodine solution (with 2% potassium iodide added) can be used for warm compresses. Local application of 5-10% potassium iodide ointment often achieves good therapeutic results.

6. Candidiasis (thrush)

Its pathogen is Candida albicans. It often exists in the digestive tract of healthy animals. Young animals and those with weak constitutions, especially animals that have long been fed antibacterial additives or treated with antibiotics for a long time, are all susceptible to infection and disease.

The clinical feature of affected dogs is the formation of one large or several small raised soft plaques on the oral and esophageal mucosa. The surface of the soft plaques is covered with a yellowish-white pseudomembrane, and after peeling it off, a congested surface that bleeds easily is exposed.

For dogs with the above symptoms, scrapings from mucosal and skin lesions are examined microscopically. If yeast cells and tangled mycelium are found, a definitive diagnosis can be made.

Early detection and early treatment are of great significance for improving the therapeutic effect, and the treatment method can refer to that for histoplasmosis. To prevent this disease, the most important thing is to remove various predisposing factors. Long-term use of antibacterial additives should be stopped. The use of antifungal agents for too long should also be stopped to avoid the growth of Candida. Nystatin can also be given orally at irregular intervals at a dosage of 600,000-1,000,000 units/day for 5-7 consecutive days.