Blastomycosis in dogs

Perhaps you have heard of ‘blasto’ in dogs, it’s an important disease that all dog owners should be aware of.

Blastomycosis dermatitidis is the most common systemic mycosis of dogs. Systemic meaning that it can affect the whole body, and mycosis, meaning that it is caused by a fungus. It is called ‘dermatitidis’ because it affects primarily the skin, but it can also affect other body parts, such as the vascular system.

Blastomycosis is largely confined to the Ohio, St. Lawrence and Mississippi river-drained areas of North America. It is a soil organism, which can be inhaled by a dog. It is often found growing on dead leaves along a river. The classic case of a dog getting blasto is a young male hunting dog following a trail and sniffing eagerly along a river-bank. It is most common from August to October, since this is when leaves are falling off trees and the mould version of the fungus can grow upon them.

The only way a dog will get blasto is if it inhales a very large number of spores. A healthy dog can fight off a mild infection. However, an overwhelming amount of spores can overwhelm the dog’s immune system, and cause a type of pneumonia known as granulomatous. This means that the reaction is almost entirely made up of a type of immune cell called a macrophage, whose job is to engulf any foreign bodies like bacteria or fungus, and essentially kill them. However, when thousands of macrophages go to the site of the damage and still cannot eliminate the threat, they start to die and cause granulomatous inflammation. The granulomas form in the lung, which take up space, not allowing the lung to work as efficiently. They are essentially like little balls of cement that expand in the lung, making it hard to breathe. The granulomas can sometimes be spectacular, filling up the entire lung, and resembling large tumours.

The granulomas most often spread to the skin, hence the name. The fungus is one of the largest known, and actually blocks tiny blood vessels called capillaries, causing cell death and necrotic ulcers in the skin. However, other common sites affected are the eye and sometimes the bone, as well as the prostate and testes. The affected areas show dramatic lesions. In fact, just by looking at X-rays alone, there is no way to discriminate between blastomycosis which has spread to the bone and a type of bone cancer called osteosarcoma!

What is interesting about the blastomycosis fungus is that it is dimorphic. This means that there are two forms of the fungus. In cold temperatures, such as in its native home of dead leaves, it survives as a mould. In warm temperatures, such as after it has been inhaled by an animal and living in the lungs, it turns into its yeast form. This yeast form is not infectious. For this reason, blastomycosis is not a zoonotic disease, that is, an affected dog cannot give the fungus to its owner.

The symptoms of a blastomycosis infection can range from fever, lethargy and lack of appetite to coughing, eye problems, lameness and skin ulcers. The symptoms may lessen with time, only to get worse again in a few days or weeks.

So, how is blasto treated? Anti-fungals are used to kill the fungus, namely amphotericin, itraconazole, or a combination of both drugs. Dogs that are being treated with corticosteroids for other illnesses are more prone to the disease than untreated dogs, since this treatment actually suppresses the immune system and leaves the dog less able to fight off infections. Thus, the body puts up less of a defence to the fungus, and symptoms are more dramatic in these animals.

To sum up, blastomycosis is a serious disease of dogs, but only to those that are predisposed to it by environment and lifestyle. To get clinical symptoms, either a huge amount of spores needs to be inhaled, or the dog’s immune system must already be suppressed due to sickness or drugs. Antifungals work well as therapy, however if not treated, a serious case can result in death.

Amrita Bannerjee – Pets.ca writer