20 Dec 2010
Marie Kubiak
Job TitleMarie Kubiak discusses a preventable disease caused when captive birds, originally from warmer climes, are subjected to the UK’s winter temperatures
DURING winter, as temperatures drop and overnight ground frosts become the norm, birds of prey with wing tip oedema are likely to start being presented to veterinary surgeons around the country.
This condition, although recognised for many decades, was only reported in veterinary literature in the late 1980s (Forbes and Harcourt-Brown, 1991).
Diagnosis has continued to increase as the presence of disease has become more widely accepted and both falconers and vets become better at recognising the clinical manifestations.
Wing tip oedema typically affects birds from the Falconiformes order, which includes hawks, falcons, eagles and buzzards, but sporadic cases have also been reported in owls. All birds in the order should be considered susceptible, but as cold temperatures appear to be the inciting factor, the disease tends to affect birds of prey from warmer climes that are not adapted to UK winter temperatures.
Harris hawks (Parabuteo unicinctus) are the most commonly presented species with this condition, reflecting not only their overall popularity as captive raptors but also an inherent predisposition (Figure 1).
This species’ natural range covers the southern United States and parts of South America. Temperatures in the Harris hawk’s natural habitat rarely drop below 5°C, yet in UK captivity this species is frequently exposed to temperatures below freezing in the winter months.
Lanner falcons (Falco biarmicus) are another vulnerable species, primarily originating from southern Europe and Africa. Lanner falcons are far less commonly kept as UK captive birds, so presentation is significantly less frequent.
Juvenile birds are most commonly affected as they tend to have a lower bodyweight and less dense plumage, therefore are less tolerant of cold. It has also been suggested that younger birds, with less flight experience, have a lower muscle mass and less well-developed circulation associated with musculoskeletal tissues. One survey reported that 86 per cent of all cases involved first-year birds, with only four per cent of cases seen in birds older than two years (Forbes, 1992).
Birds with any pre-existing wing pathology should also be considered higher risk, as existing compromise to blood flow or any alteration in wing position can predispose to wing tip oedema. In particular, abnormal abduction of the distal wing is a significant risk factor, because when the metacarpus is held away from the body it is more susceptible to temperature variation in line with the environment, without the benefit of the buffering effects of close association with tissue at core temperature.
Cases appear to be precipitated by freezing conditions and incidence is further exacerbated when a cold snap is preceded by a period of damp weather. Low ground temperatures lead to cooling of the extremities with the metacarpal region seeming to suffer the most. The exact pathogenesis is poorly understood, but appears to be related to compromised blood supply to the wing tip. This is believed to be as a result of vessel constriction in response to exposure to cold temperatures. Viral, bacterial, toxic and traumatic causes have been investigated, but with no supportive findings for any of these factors as yet.
Histopathology of affected tissue shows a non-specific inflammatory reaction that appears to be consistent with a causative factor of extreme temperatures with no other influences identified (Forbes, 1992). The vascular disruption that results from the vascular changes leads to accumulation of a sterile transudate subcutaneously, with distinctive pitting oedema identified between feathers (Figure 2).
Conscientious falconers may detect these cold, swollen metacarpi before other obvious signs develop. However, most birds are presented once disease is further advanced, with non-specific signs of wing discomfort – usually as an abducted or dropped wing and poor flight performance (Figure 3).
If the condition remains undetected at this stage it can develop to dry gangrene as a result of avascular necrosis of the distal wing. The distal wing progressively becomes dark and dry, and eventually the entire wing tip breaks off, taking several of the primary flight feathers with it. This stage usually takes three to eight weeks from the initial insult to the tissue. The first indication an owner may have is to find the wing tissue and feathers on the floor of the housing.
Differential diagnoses include carpal bursitis (blaine), septic arthritis, neoplasia and trauma.
Carpal bursitis is most commonly seen in tethered birds that have been attempting to fly away from their perch, either due to intolerance of recent tethering or in response to an environmental stimulus or stressor triggering flight. Shortly after taking off they are suddenly pulled back by their leash and may crash land on the ground, disorientated and frantically beating their wings.
Repeated blunt trauma over the carpus leads to an inflammatory response and an initially sterile transudate within the superficial tendon sheath. The swelling in these cases is localised rather than diffuse, and pitting oedema is not a characteristic.
Septic arthritis lesions are uncommon, but invariably involve joints, rather than a diffuse area of distal wing, and are noticeably warm on palpation.
Trauma to the carpus following ”catching up“ of uncooperative birds within aviaries, or wing beating in a confined space, often leads to wounds over the carpi, with associated swelling and inflammation. These wounds are invariably open initially but it can be more difficult to appraise the initial cause if birds present some time after the injury. Actively inflamed or traumatised tissue should be distinguished relatively easily from the cold, pale tissue of wing tip oedema.
Soft tissue neoplasms are infrequently seen in this location and, although appearance may be outwardly similar, with a pale swollen carpus evident, they do not have the pitting oedema or cold surface associated with wing tip oedema. Spindle cell tumours are the most common neoplasm of this location and typically have a firm, nodular consistency on palpation (Forbes, 1992).
On presentat ion, birds affected by wing tip oedema should be gently warmed and maintained at normal room temperature. Even in very mild cases birds should not be exposed to external temperatures again for a minimum of three weeks following resolution.
Birds still capable of flying should be given regular short flights to help maintain and maximise blood flow to and from the damaged areas. Gentle physiotherapy can be used instead in birds unable to fly, with carpal flexion, extension and massage carried out as frequently as the bird will tolerate. Where distinct oedematous vesicles are present, drainage can be carried out daily in a sterile fashion.
Medical treatment should be carried out concurrently and primarily comprises therapy to improve blood supply to the affected tissue. Isoxsuprine, a beta-adrenoreceptor agonist and alpha-adrenoreceptor antagonist, has been recommended as a vasodilator at 5mg/kg to 10mg/kg orally once daily (Lewis and Greenwood, 1993). In one study (Forbes, 1992), only 21 per cent of birds recovered without loss of primary feathers, even with early aggressive treatment. However, since the introduction of isoxsuprine, recovery rates have increased to 90 per cent.
Propentofylline has also been used at 5mg/kg orally twice daily and is reported to be efficacious in early cases, but less successful than isoxsuprine in more advanced cases. Preparation H (Whitehall Labs), which contains yeast and shark oil, has been used by many avian clinicians in the treatment of avascular lesions. Topical Preparation H has anecdotally been reported to reduce the incidence of metacarpal loss, although a proportion of the noted effects may be related to regular massage of affected areas during application. If used, care should be taken as the oily cream can damage surrounding plumage if applied liberally.
Laser therapy has been advocated as a potential adjunctive treatment, but little data on this therapy in birds is available.
Opportunistic bacterial infections are likely in impaired tissue and prophylactic antibiotic therapy is routinely administered. Common choices include amoxicillin-clavulanate (125mg/ kg orally twice daily) and marbofloxacin (10mg/kg orally twice daily) and are usually provided for a minimum of seven to 10 days. Steroids are no longer advocated due to the lack of evidence for their benefit and the significant immunocompromise associated with corticosteroid administration in avian species, which commonly results in secondary aspergillosis.
Treatment can be successful, with complete recovery of tissue and no feather loss, especially if birds are presented early in disease progress and intensive treatment is rapidly instituted. A 90 per cent recovery rate is reported in these situations. Some cases can be frustrating, with an apparently good initial response to therapy, only for wing tips to be lost weeks later.
Once the wing tips have been lost, treatment options are limited. The lost tip includes not only the distal bones of the wing but also their associated primary flight feathers. Birds that have lost these tips will have permanently compromised flight. Retirement for breeding purposes or euthanasia may then need to be considered.
This is an entirely preventable disease that has only arisen as a result of bringing birds into captive climates they are not capable of adapting to. As treatment cannot guarantee a complete resolution, falconers must be made aware of the potential problem and strategies to prevent it.
No birds should be tethered close to the ground when temperatures below 0°C occur, and susceptible species should be protected when ground temperatures are below 3°C. Birds should not be kept tethered within 18 inches of the ground at all at night or during cold days, from the months of October to April, regardless of expected temperature (unless supplemental heating is provided). It is also important that birds should not be left wet during cold weather following flight in adverse weather or after permitting bathing. It is not difficult to provide birds with access to an elevated sheltered area to get away from ground frosts, or to install supplementary heating that need only keep temperatures at 3°C to 4°C.
• Thanks to Neil Forbes for his original publications and the photographs used in this article.
• Readers should be aware some of the drugs mentioned in this article may not be licensed for use in avian species.
Marie Kubiak
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