The small bird that is going light: avian gastric yeast and the other causes of wasting

Quick answer

Most of the causes of wasting in a small bird are treatable, and most of them need an oral medication given daily for weeks. Getting the diagnosis is the hard part.

"Going light" is the aviary term for a bird that eats well and keeps losing weight. It is not a diagnosis, it is a pattern, and in budgerigars, canaries, finches and cockatiels the most commonly missed cause of it is a yeast called Macrorhabdus ornithogaster.

The disease was called megabacteria for years because the organism is a large rod. It is a fungus, it lives at the junction between the two stomachs, it interferes with digestion, and it produces exactly the picture owners describe as "he eats all day and he is skin and bone".

• Feathers hide the body completely. A bird that looks thin has already lost a great deal of muscle.
• Weigh daily. It is the only early detection method that works in a small bird.
• Whole undigested seed in the droppings is a significant finding and narrows the list considerably.
• A single negative faecal test does not clear the bird, because shedding is intermittent.
• Black or tarry droppings mean gastric bleeding and are an emergency.

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Black, tarry or plum-coloured droppings in a small bird mean bleeding into the stomach.

In a bird this size the total blood volume is tiny, and a bleeding proventricular ulcer can kill within hours. Do not wait for an appointment slot: telephone an avian vet, keep the bird warm and quiet, and go.
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Decide in 60 seconds

Why weight is the only early sign you can see

A small bird's body is a small object inside a large arrangement of feathers. A budgie can lose a substantial proportion of its pectoral muscle without changing shape to the eye, because the contour feathers keep the outline the same.

Metabolically these birds live close to the edge. Mass-specific metabolic rate rises as body size falls, so a finch burns energy far faster per gram than a macaw and carries proportionally less in reserve. A small bird that eats nothing for a day is in serious trouble, and one that absorbs its food poorly declines steadily rather than dramatically.

Put those two facts together and the conclusion is unavoidable. By the time you can see that a small bird is thin, the disease has been running for weeks.

The keel is where you check.

Feel gently along the midline of the chest. The keel is the ridge of the breastbone, and the pectoral muscle sits either side of it. In a bird in good condition, that muscle is level with or slightly rounded above the ridge. In a wasting bird the ridge becomes sharp and prominent with a hollow on each side. Learn what your own bird feels like while it is well.

The scale beats the fingers.

A flat scale with a perch on it, the same time every day, and the number written down. This single habit finds more small-bird disease than any other thing an owner does.

Weigh at the same point in the day, ideally in the morning before the first substantial feed, on the same scale. Record the number. A single reading tells you almost nothing because a full crop and a recent dropping both move it. A downward line across a week tells you a great deal.

Never assume a stable weight means a well bird if the droppings have changed. Fluid retention and an enlarged proventriculus can hold the number up while muscle disappears.

Macrorhabdosis, in detail

Where it lives and what it does.

Birds have two stomachs. The proventriculus secretes acid and enzymes, and the ventriculus, or gizzard, grinds. Macrorhabdus colonises the narrow junction between them. The organism disrupts acid secretion, so gastric pH rises, protein digestion falls off, and the lining becomes inflamed and eventually ulcerated.

That mechanism explains every sign. Food is poorly broken down, so seed appears whole in the droppings and the bird gets less from every meal. The bird responds by eating more, which is why owners describe an excellent appetite. Muscle is catabolised to make up the shortfall. Inflammation and ulceration eventually produce vomiting and, in the late stages, bleeding.

Who gets it.

Budgerigars most of all. Canaries, finches including Gouldians, cockatiels, lovebirds and parrotlets are all affected. It occurs in larger parrots and in poultry, but the small seed-eating species dominate.

How it spreads.

By ingestion of the organism in droppings or regurgitated food. That includes parent birds feeding chicks, shared food and water dishes, and contaminated aviary surfaces. This is why one bird going light in a collection is a collection problem.

Why it appears suddenly in a bird that has been fine for years.

Carriage without disease is common. Stress, a concurrent illness, a change of home, breeding, or a poor diet can tip a carrier into clinical disease. So "he was fine until we moved house" is a consistent history, not an argument against the diagnosis.

Diagnosis.

A wet preparation of a fresh dropping examined under the microscope can show the characteristic large rod-shaped organisms, and a Gram stain will also show them. The catch is intermittent shedding, so several samples collected over several days are far better than one. PCR testing is more sensitive where a laboratory offers it. A crop or proventricular wash may be taken. Radiographs may show a dilated proventriculus. Definitive confirmation is histological.

Treatment.

This needs an avian vet and a specific antifungal. Amphotericin B given orally is the mainstay in most places, and the course is measured in weeks. Antifungals aimed at Candida, including nystatin, are generally reported not to work against this organism, so a course of the wrong antifungal is a common reason an owner is told the bird "did not respond".

Relapse is common and repeat courses are often needed. In a collection the whole group is usually treated and the environment cleaned, because reinfection from the aviary undoes an individual cure.

Availability differs by country. In many places amphotericin B for oral avian use has to be compounded by a pharmacy, which takes time to arrange. Sodium benzoate in drinking water circulates as a home remedy, its evidence is weak, and the margin between a dose someone recommends online and a toxic one is not wide. Do not use it unsupervised.

Supportive care matters as much as the drug.

Keep the bird warm, keep food within easy reach, feed something it will actually eat rather than something ideal, and weigh daily throughout treatment. A bird that gains weight is responding.

The other causes of going light

A bird in condition: chest full and rounded, feathers smooth, weight carried forward. The comparison you need is with your own bird a month ago, not with a photograph.

Proventricular dilatation disease, associated with avian bornavirus.

The most important look-alike, because whole seed in the droppings and weight loss with a good appetite fit both. PDD more often adds neurological signs: a wobbly gait, head tremors, seizures, or a bird that seems clumsy on the perch. Diagnosis and management are entirely different, and radiographs plus specific testing separate them.

The seed dish full of husks.

Absurdly common and genuinely lethal. Small birds husk seed and drop the shells back into the dish, which then looks full. Blow gently across the surface of the dish every day. If most of it flies away, the bird has been eating from an empty bowl.

An overgrown beak.

A budgie with a beak too long to husk seed will starve in a full cage. Scaly face mite, Knemidokoptes, is a common cause of beak distortion in budgies and produces a characteristic honeycombed crust at the cere and beak margin.

Intestinal parasites.

Trichomonas causes regurgitation, crop signs and weight loss, particularly in budgies. Giardia is a recognised problem in cockatiels and produces poor absorption and loose droppings. Aviary and outdoor birds also get roundworms, coccidia and tapeworms. A faecal examination is quick and cheap.

Chlamydiosis.

Weight loss, lime-green droppings, respiratory signs and a dull bird. Transmissible to people, which makes it a household issue as well.

Heavy metal toxicity.

Zinc and lead from cage components, curtain weights, costume jewellery and old paint. Produces weight loss, regurgitation, unusual dropping colour and sometimes neurological signs.

Tumours, especially in budgerigars.

Renal and gonadal tumours are common in this species. The classic presentation is progressive weight loss with weakness or lameness in one leg, because the mass presses on the nerve supply, sometimes with abdominal swelling and a change in cere colour.

Chronic egg laying.

A hen laying repeatedly depletes calcium and protein reserves and loses condition steadily. Look for a history of eggs, a swollen abdomen, or straining.

Aspergillosis and mycobacteriosis.

Both cause chronic wasting. Aspergillosis usually adds respiratory effort or a change in voice. Mycobacteriosis is a particular concern in finches and produces slow, relentless weight loss.

Air sac mites.

In canaries and Gouldian finches, a clicking or wheezing respiration with tail bobbing and weight loss.

Iodine deficiency in a seed-only budgie.

An enlarged thyroid presses on the oesophagus and airway, producing regurgitation and a squeaky respiration, and the bird loses condition.

Early, established and late

Early.

Weight drifting down. Appetite normal or increased. Droppings may contain whole seed or become more voluminous. The bird still looks and behaves normally. This is where the weight log earns its keep, because nothing else is visible.

Established.

The keel becomes prominent. The bird fluffs at times during the day and sits quietly. Regurgitation may appear, sometimes seen as sticky head feathers or seed stuck around the face. Droppings become less well formed.

Late.

Marked weakness, sitting on the cage floor, dark or tarry droppings from gastric bleeding, and sudden death. Birds at this stage are also at risk from the stress of handling, so the vet may stabilise before examining.

What to do today

The goal of all of this: a bird whose weight sits in a stable range you actually know, on a diet it can digest, in a cage where nobody is eating husks.

What never to do

Do not wait to see whether a small bird picks up. The margin between a bird that is losing weight and a bird that is dying is measured in days.

Do not treat with an over-the-counter antifungal, antibiotic or "tonic" from a pet shop. You will not treat this organism, and you will make the diagnostic samples harder to interpret.

Do not dose sodium benzoate in the water on the basis of a forum post. The evidence is poor and the toxic dose is not far from the suggested one.

Do not assume a normal-looking bird in a group is clear. Carriage is common and shedding is intermittent, so the quiet bird in the corner of the aviary today is often the case in three weeks.

Do not judge food intake by how full the dish looks. Judge it by the weight.

Do not move an affected bird between cages or aviaries. Contaminated dishes and surfaces spread it.

Do not stop treatment early because the bird looks better. Relapse after a shortened course is one of the commonest reasons these cases fail.

When to get help

Contact an avian or exotics veterinarian immediately for:

• Black, tarry or dark plum-coloured droppings
• A bird sitting on the cage floor or unable to perch
• Collapse, or a bird that will not rouse
• Breathing effort or tail bobbing at rest alongside weight loss

Within the week for:

• A downward weight trend over several days
• Whole seed appearing in the droppings
• Regurgitation, or seed stuck to the head feathers
• A prominent keel with hollows either side
• Any bird in a group that is losing condition

Bring these to the appointment: your daily weight log, droppings collected fresh on several different days, a photograph of the cage floor paper, the exact diet including the brand of seed or pellet and how old the bag is, when the bird last moulted or bred, whether any new birds have joined the household and when, what the cage and toys are made of, and whether the bird has ever had a course of any medication.

Ask two things explicitly. Ask whether the sample was examined specifically for Macrorhabdus rather than as a general parasite screen, and ask what the plan is if the first sample is negative. Those two questions are what stop a treatable disease being missed in a bird that is running out of time.