THE FACTS 1

2006

RESPIRATORY DISEASE IN EXOTICS AND SMALL MAMMALS

JOHN CHITTY BVetMed, CertZooMed, MRCVS

Reproduced with permission: Veterinary Times, Vol 35 No.38, 10 October 2005. 

Introduction

RESPIRATORY disease is a common problem in exotic species in general practice, whether avian, reptilian or small mammal. Different species have different disease susceptibilities.

These vary according to:

Anatomy

The complicated sinus system of birds coupled with a tendency to form solid pus means that upper respiratory disease (URD) and sinusitis are common problems in many avian species. Reptiles lack a muco-ciliary escalator meaning they find it hard to remove respiratory discharges. This is one factor contributing to lower respiratory disease (LRD) in snakes. Rabbits suffer from compression of lungs and upper respiratory passages. Breeding to produce extremes of shape (especially “squashed nose” breeds) is a major factor in both upper and lower respiratory disease.

Immunity and “Pathogens”

There are few primary pathogens found in respiratory syndromes. Snake paramyxovirus may be an exception as may Bordetella bronchiseptica in guinea-pigs. Other infectious agents may appear to act as pathogens in stressed or immunologically naïve animals, eg Pasteurella in rabbits or chelonid herpesvirus in tortoises. Mixing of species in households may, therefore, hold dangers, eg mixing tortoise species will allow exchange of herpesviruses and Mycoplasma spp between these species. While one tortoise may cope with its own adapted organisms, another may not. Mixing of rabbits and guinea-pigs allows rabbits (who will often carry Bordetella as part of the normal respiratory flora) to pass these organisms to guinea-pigs that don’t. The susceptibility of different species to different organisms is amply illustrated by the incidence of aspergillosis in different bird species. Aspergillus spp spores are found in the environment and disease represents either lowered immunity to the organism or overwhelming infection. Grey parrots, Gyr falcons and Goshawks are some of the species that appear very susceptible and disease is commonly seen in them. Peregrines and cockatoos are much more resistant so disease is rarely seen in these species.

Husbandry

When we think of husbandry-related disease we often think of reptile problems and this is often the case in respiratory disease. These problems (especially URD in tortoises and LRD in snakes) are often the results of low temperatures. Inappropriate humidity levels will contribute to respiratory disease in a range of species, both reptilian and avian. Exposure to irritants, especially cigarette smoke, air fresheners, cooking fumes, etc will often cause direct irritant effects or predispose to infection especially aspergillosis in birds. Some may be linked to the development of allergy and/or asthma although this diagnosis remains controversial in exotic animal medicine. Other irritants may include dusty bedding in rabbit or rodent hutches/cages. Rotting organic matter in bird aviaries provides a direct source of Aspergillus spores. Sudden changes in husbandry, eg rapid temperature changes or mixing of individuals, will cause stress and therefore, affect immunity. Underlying systemic infection may also play a part – the young Grey parrot with an airsac aspergilloma may seem a clear diagnosis. However, if on haematology there are few if any white blood cells then be aware that it may be suffering from immunosuppression caused by circovirus infection.

Diagnosis

A range of causes cause a range of diseases, often with similar symptoms. Accurate diagnosis is, therefore, essential and often complicated – after all, the best therapeutic agents in the world cannot work if used for the wrong condition! It is also vital to explore the many underlying causes. As stated earlier, there are few primary pathogens and, while your diagnosis of a bacterial pneumonia in a boa constrictor may be correct and you may be using an appropriate antibacterial, there is no chance of success unless the snake is provided accommodation with appropriate temperature ranges, appropriate humidity ranges, and the ability to climb (arboreal snakes often require this so gravity can assist in the clearance of respiratory discharges). The most common question asked in exotics practice is not so much “what has it got?”, but “why has it got it?”

The following are often required as part of a comprehensive respiratory investigation:

History/signalment

This often provides many of the clues as to “why”. Full details of husbandry and diet are required.

Further Investigation

Almost always necessary and many tests may be required. The following should be considered:

Cytology and bacteriology/sensitivity

The upper respiratory tract can be sampled directly via nares or choana (in birds/reptiles) while tracheal or lung washes are very valuable tools in all species. Endoscopy will often allow direct sampling of lesions giving more precise results especially for abscessated lesions.

Haematology/biochemistry

These provide little direct help in diagnosis of respiratory disease. However, they will give an idea of systemic response to infection, underlying immunosuppression or underlying disease. The latter is important in the tortoise with URTD where respiratory symptoms are often the result of recrudescence of infection in a run-down tortoise, Haematology will often show lowered white blood cell counts and biochemistry may reveal signs of dehydration, follicular stasis or renal/liver disease. Serology may be useful in some species, especially paramyxovirus in snakes or Encephalitozoon cuniculi in rabbits.

Newer tools

MRI and CT are proving to be of great value where accessible. Hopefully increased availability will allow more patients to benefit.Obviously many of these investigations require general anaesthesia and this may constitute a risk in the animal with respiratory compromise. Agents should be chosen that are easily reversed and cause minimal respiratory or cardiovascular compromise. Oxygen must always be provided and mechanical ventilation is frequently necessary. Accurate and constant monitoring is vital and we have found capnography to be an excellent tool.

Therapy

Naturally, therapy should be based on diagnosis. However, therapy may have to be initiated before some test results are available or to stabilise a patient too critical to allow full investigation at that stage. An ideal preliminary therapy should have the following characteristics:
• broad-spectrum activity against a range of infectious agents;
• correction of underlying husbandry defects;
• provide physical relief of symptoms and support of respiratory membranes;
• removal of obstructions and relief of dyspnoea; and
• be non-irritant and non-stressful to the patient.

Underlying factors

As described earlier there are many potential underlying factors. These will, hopefully, be identified at examination and during history-taking. Stressors should be removed, as should obvious causes of respiratory irritation. Hospitalisation should be tailored to provide correct temperature and humidity for the species being treated. Snakes should be provided with space to stretch out and, if necessary, to climb. Coupage of snakes allows for physical removal of discharges in the absence of a mucociliary escalator.

Systemic

Broad-spectrum antibiotics are frequently used, however they are rarely effective on their own, especially when abscessation is present or where the main infectious agent is non-bacterial. However, they may be useful where there is a risk of systemic invasion of bacteria or where bacterial infection may be a complicating factor of a fungal or viral infection.

It may also be deemed unnecessary to use systemic antibiosis in URTD where there are no systemic signs, especially as penetration of drugs into the upper respiratory tract may be poor. Antibiotic choice is based ideally on culture and sensitivity but, initially, is based on likely pathogens in that species and tolerance of drug in that species – rabbits and rodents being cases where antibiotic choice may be restricted. It is, of course, very tempting to reach straight for the fluoroquinolone – they are comparatively safe, broad-spectrum and penetrate well into the respiratory tract. However, other anti-bacterials are also useful eg oxytetracycline or potentiated sulphonamides and may be more appropriate where there is abscessation.

Where fungal disease is suspected, itraconazole or terbinafine are extremely useful. It is worth remembering that some Grey parrots appear very sensitive to itraconazole so it is worth avoiding this drug in this species.

Other systemic agents with potential effects on the respiratory system, eg bronchodilators or diuretics have not been properly evaluated in exotic species although in individual cases may be of some benefit to the individual patient.

Corticosteroids may be extremely useful in cases of asthma or respiratory irritation. However, they may also cause profound immunosuppression and a diabetes mellitus-like syndrome in birds. Their use, therefore, must be restricted to cases where it is felt there is no evidence of infectious disease (especially not zoonotic disease a corticosteroid dose is very likely to induce shedding of Chlamydophila psittaci in a carrier even if it is not the immediate cause of disease at that time) and ultra-short acting agents only should be used.

Non-steroidal anti-inflammatory agents may be of use in reducing pulmonary inflammation over the longer term in a range of species.

Nasal/Sinus Flushing

Nasal or sinus flushing is extremely useful in many exotic species. It enables instillation of an antimicrobial directly into the site of infection and will also enable physical flushing of pus and debris and the clearing of nares that may provide a “feel good” factor. This may be vital in rabbits that have great difficulty in mouth-breathing so may become very distressed if the nasal passages are blocked. Part of the flushing process involves the physical unplugging of the nares and this in itself is often useful.

Almost any non-irritant anti-microbial solution may be used. A mixture of 1ml 2.5 percent injectable enrofloxacin diluted in 20ml saline is appropriate for a 1kg bird. In tortoise URTD 0.1ml oxytetracycline injectable solution may be flushed into each nostril.

We have now started using a new disinfectant agent from South Africa called F10SC Veterinary Disinfectant (Health & Hygiene Pty). This is a mixture of disinfectant agents providing a very broad-spectrum of activity against a range of potential agents. When used at a 1:250 dilution it appears to cause little or no irritation to the mucous membranes (it was initially used for the sterilisation of drip lines and catheters). In our clinic we have used it in the following manner for URTD:

Nebulisation

This technique is useful for both upper and lower respiratory disease. It enables:

Penetration of anti-microbials to the site of infection

While this is probably true for URTD there is some doubt about this in LRTD. Certainly where there is lung consolidation or abscessation, nebulisation may have little ability to penetrate the lesions. However, other effects of nebulisation may still have benefits in these cases.

Expectorant

Nebulisation acts as an excellent expectorant and aids greatly in the clearing of discharges. In snakes, nebulisation should be combined with coupage as, in the author’s experience, it makes loosening of discharges and tracheal blockage more likely unless aid is given in expelling these materials.

Hydration of mucous membranes

As anyone who has suffered from a cold while flying or sitting in air conditioned rooms will tell you, drying of mucous membranes is uncomfortable and has the effect of allowing infection to penetrate deeper. Keeping membranes hydrated provides a “feel good” as well as assisting in membrane integrity.

The simplest means of providing some of these aims is “steaming” and for many rabbits or parrots with problems, being placed in a steamy bathroom provides some short-term relief. This is, however, not a long-term answer nor should drugs be “steamed” into patients as the heat and uncertain delivery system will result in underdosing.

Aromatics such as Olbas Oil should also be avoided as it is very easy to overdose these biologically active compounds in small animals.

There are many drugs proposed as suitable for nebulisation. Often, in avian medicine, these have been used as a route for systemically toxic drugs (eg, gentamycin or amphotericin) as absorption from the air sacs is poor. While this is great for the patient it does raise huge health and safety issues – someone has to get the patient out of the nebulisation chamber! Also, it is very unwise to send home a bird on nebulisation therapy if the nebulised drugs could be toxic to the owner. As aspergillosis therapy typically lasts at least six months this can mean a very long hospitalisation period.

We have, therefore, used F10SC Veterinary Disinfectant in this situation as well. In South Africa it has good human safety trial results and appears relatively safe to users. It is also broad-spectrum in activity and surfactant inclusion benefits its properties as an expectorant.

A 1:250 dilution is again used and nebulisation periods of 20-45 minutes 2-3 times daily are used depending on species and condition.

Typical human asthmatics’ nebulisers are often effective in this therapy. While many texts advise use of ultrasonic nebulisers producing very small droplets however, for most situations this is unnecessary. Aspergillosis in birds is one of the most common reasons for this type of therapy. The spores of this fungus measure 2-5 microns in diameter.

Therefore it is unnecessary for the nebulisers to produce droplets smaller than 2 microns in order for the agents to penetrate as far as the spores. Where fungal spread has entered air passages narrower than this, there will generally be such consolidation that nebulisation alone would not be effective.

 

Respiratory problems are often frustrating in diagnosis and therapy. Attention to underlying causes and multimodal therapies have greatly helped in improving treatment success rates.

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