Equine Lymphangitis

Lymphangitis in horses is an inflammation and/or swelling of the distal limb that results from imparement of the lymphatic system, particularly in a limb. It is believed to occur secondary to bacterial infection in many cases, although bacterial culture may be negative.

Also known as Monday Morning Disease (more commonly, MMD refers to Exertional Rhabdomyolysis/Azoturia), Weed and Fat/Big Leg Disease^[1].

This article refers mainly to Sporadic Lymphangitis. Ulceative lymphangitis is referred to in passing, as it is managed in a similar manner. Epizootic Lymphangitis is similar to glanders, but caused by the fungus Histoplasma farciminosum^[2].

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Causes

Corynebacterium pseudotuberculosis has been cultured from some cases (particularly of ulcerative lymphangitis; however, in others, bacterial culture is negative. This may be because:

• a) The micro-organism responsible is difficult to culture (e.g. many Mycoplasma species).
• b) The organism has been effectively eliminated by the immune system and the pathology is due to an excessive immune response after the organism has been cleared.
• c) The organism is not a bacterium but a fungus and therefore very difficult to culture.
• d) There is no causative organism and the disease has another cause.

Of these, a) is thought to be the most likely, and d) the most improbable.

Symptoms

Extreme swelling of a limb, usually a hindlimb, often as far proximally as the hock, or occasionally as far proximally as the stifle. In some cases swelling continued through the udders and milk vein (mares). The swelling is a "pitting oedema", in other words, if pressed, a depression remains in the skin of the limb. The affected leg may reach twice or even three times its normal size.

Lymphangitis is commonly associated with a wound, which may be very minor. It is hypothesised that this has allowed bacterial access to the lymph ducts, see Causes above.

The degree of lameness is variable, but may be sufficient to give the impression of a long bone fracture.

The horse may or may not be pyrexic (fevered).

Bacterial culture is variable. However, if there is a purulent discharge from an obvious wound, positive culture is more likely.

The limb may occasionally ooze serum.

Further diagnostic tests include ultrasonography or radiography to rule out the differential diagnoses of tendinitis or a fracture. Aspiration of a fluid sample for culture may be an option, but is rarely useful.

In Ulcerative Lumphangitis, there may also be "cording" of the lymphatics and the formation of hard nodules and abscesses; occasionally a greenish, malodorous discharge is present. In the USA in particular, the disease may be characterised by multiple small, open sores.^[3]

Treatment

The mainstay of treatment are the administration of broad-spectrum antibiotics (typically potentiated sulphonamides or penicillin and streptomycin, but Doxycycline may be the most effective). If possible, microbial culture and sensitivity testing should be performed, so the most efficacious antibiotic can be chosen^[4]. In addition, anti-inflammatories are important, to reduce the swelling and the inflammatory response. NSAIDs are commonly used (Flunixin is the drug of choice, but phenylbutazone may also be used), as are corticosteroids.

In ulcerative lymphangitis, intravenous iodine salts may also be used; and abscesses should be poulticed and/or lanced.

"Physiotherapy" is also important, particularly maintaining movement by walking out and massage to improve lymphatic drainage and reduce the oedema.

Pressure bandages may also be useful, as may cold hosing in the initial phase.

The above treatment consists of ridding the body of the infection and swelling; however, it has been suggested that the immune system is still very weak during lymphangitis, and if not addressed the disease may recur. Therefore, along with the above treatments, alternative/holistic medicine has been recommended as a more effective treatment. There are a wide range of herbal and homeopathic products commercially available, with varying effectiveness.

Outcome

The initial pain and lameness usually respond rapidly to treatment; however, the swelling may persist for many weeks. In addition, once a horse has had an episode, they appear to be predisposed to recurrence, and may suffer from "filled legs" permanently - i.e. if left in a stable and relatively immobile, poor lymphatic circulation results in a passive oedema of the previously affected limb, that dissipates on exercise.

In some cases, the long-term sequellae may be very frustrating to manage. In such situations, a "holistic" approach may prevent the disease from becoming chronic.

References

RJ Rose & DR Hodgson Manual of Equine Practice Copyright WB Saunders (2000). (p. 180)

1. ^ Abbey Veterinary Group, [1]
2. ^ OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals 2004 Chapter 2.5.13, [2]
3. ^ . RR Pascoe, DC Knottenbelt, Manual of Equine Dermatology Published 1999, Elsevier Health Sciences (p. 109)
4. ^ JA Orsini, Y Elce & B Kraus (2005) "Management of severely infected wounds in the equine patient" Clinical Techniques in Equine Practice, Volume 3, Issue 2, Pages 225-236