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Neglected bacterial disease has big impact on welfare of working animals

A horse with Epizootic. Lymphangitis. The thoracic (brisket area) skin and subcutaneous tissue are thickened with purulent foci (abscesses).

A horse with Epizootic. Lymphangitis. The thoracic (brisket area) skin and subcutaneous tissue are thickened with abscesses. © AFIP

The challenges in trying to treat and control an equine disease affecting working animals in poorer parts of the world are described in the latest issue of Equine Disease Quarterly.

Caused by a soil fungus, Epizootic Lymphangitis has been eradicated from many countries, but remains a problem for equids, particularly in northern Africa, Asia, and the Middle East, says Dr Karen Reed, the head of animal welfare and research for British-based charity The Brooke, which works to improve the working lives of equines around the globe.

The disease is a systemic infection of equids, caused by the soil fungus Histoplasma capsulatum var. farciminosum.

Donkeys are less commonly affected than horses and mules. It has been reported in camels, cattle and anecdotally in humans.

It is contagious, spreading between animals through inhalation, skin contact with infected discharges, contaminated objects such as harness, and insect vectors. Skin wounds are common entry sites for the organism.

Reed says three forms of the disease exist: skin, eye and respiratory. The skin-related form is most common, causing a chronic, suppurative, ulcerating pyogranulomatous dermatitis and lymphangitis.

Initial nodules appear anywhere on the body but commonly on lower limbs, the chest, and neck. Nodules rupture, discharging thick pus, the ulcerated lesions subsequently scarring and healing. Lesions progress locally along lymphatics, which become beaded and rope-like with enlarged regional lymph nodes. Repeated cycles of ulcerating and healing nodules occur.

Keratoconjunctivitis with mucopurulent discharge characterizes the eye form and is more common in donkeys. The respiratory form results in a mucopurulent nasal discharge and subsequent coughing and difficult breathing.

In all three forms the disease’s chronic nature leads to debility and anorexia. Working animals can no longer perform, leading to abandonment by their owners, especially where veterinary care is unavailable or unaffordable or euthanasia not easily accepted.

Some animals appear to recover, but the mechanism of immunity and the possibility of carrier states remains unclear, Reed says.

Field diagnosis of epizootic lymphangitis is by organism identification in a smear of material aspirated from an unruptured nodule.

Organisms are usually surrounded by a “halo” when Gram-stained, useful in rapid differentiation from glanders (which is difficult to differentiate clinically). Culture of the organism is possible but challenging. A Histofarcin skin test (similar to tuberculin and mallein tests) has been developed but requires further validation. Serological techniques have been described but are not commercially available.

A polymerase chain reaction (PCR) has been developed for H. capsulatum var. capsulatum, which may prove useful in the future.

Treatment is challenging, she says. Amphotericin B is the drug of choice but, since most cases occur in working animals owned by poor individuals in developing countries, modern antifungals are rarely available or affordable.

More commonly, systemic iodides (oral potassium iodide or intravenous sodium iodide) and local excision of nodules and topical iodine tincture are used.

Treatment is lengthy (3-4 weeks) and owner compliance is challenging. Treatment early in the disease increases success rates.

Historically, disease control focused on culling infected animals, strict hygiene, and movement controls to limit disease spread. These methods are usually impractical in endemic areas, particularly in developing countries where control commonly concentrates on educating owners about wound prevention; vector control; management of harness, tack, and equipment; early presentation for treatment; and encouraging euthanasia for advanced cases.

“This neglected disease has a significant welfare impact on working equids and has economic importance to many resource-poor owners with inadequate access to animal health services who rely on animals for their livelihoods,” Reed says.

“More research is required to understand fully transmission routes, risk factors and immunity, and develop animal-side diagnostics and simple, affordable, transportable, and stable therapeutics.”

Equine Disease Quarterly is funded by Lloyd’s, London, brokers and their Kentucky agents.

Horsetalk.co.nz

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