Avian gastric yeast in parrots: weight loss with a full food bowl

Quick answer

A small parrot in normal feather. Plumage hides body condition completely, which is why the weight loss caused by this disease is usually found late.

A budgerigar or cockatiel that is eating well, sitting in front of a full bowl, and steadily losing weight is one of the most recognisable pictures in avian medicine, and avian gastric yeast is near the top of the list of causes.

The organism is called Macrorhabdus ornithogaster. It was named megabacteria for decades because it is a large rod shape and everyone assumed it was a bacterium. It is a yeast, and that single misclassification is still causing birds to be treated with antibiotics that cannot possibly work.

• The classic picture is chronic weight loss with a normal or increased appetite, known in budgerigar keeping as going light.
• Whole undigested seed in the droppings, and vomiting with mucus, are the other two core signs.
• Dark, tarry or coffee-coloured droppings mean the stomach is bleeding and the bird needs help now.
• Antibiotics do not treat a yeast. Neither does nystatin, which is prescribed for a different organism.
• Many birds carry it without signs and shed it only intermittently, so one clear faecal sample does not clear the bird.

Decide in 60 seconds

Why this organism causes wasting

Where it sits.

A bird's stomach has two parts. The proventriculus is glandular and produces acid and enzymes. The ventriculus, the gizzard, is muscular and grinds. Between them is a narrow waist called the isthmus.

Macrorhabdus colonises that isthmus and the surrounding proventricular lining. It is a physically large organism and it builds up in a layer over the tissue that is supposed to be secreting acid.

What that does to digestion.

Acid output falls. The pH in the stomach rises. Protein digestion, which depends on an acid environment to activate the enzymes, becomes inefficient, and the grinding action of the gizzard is working on material that has not been chemically prepared.

The bird therefore extracts less from the same food. It responds the way any animal does to a calorie shortfall, by eating more. That is the origin of the paradox that fools people: the bowl empties, the bird is at the bowl constantly, and the bird is starving.

Why the droppings change.

Seeds that were not properly softened and broken down pass out recognisably whole. This is not the same as a bird husking seed neatly and dropping the shells, which is normal. These are intact kernels in the faecal portion of the dropping.

Why it bleeds.

Over time the colonised lining ulcerates. Ulceration in the proventriculus bleeds into the gut, and blood digested during its passage comes out black and tarry rather than red. In a small bird this is a large proportional blood loss and it is an emergency.

Why some birds are fine and others die.

Carriage without disease is common. Clinical illness tends to appear alongside something else: a stressful move, a poor all-seed diet, overcrowding, breeding, cold, or another infection. This is why a bird can live with the organism for a long time and then decline apparently out of nowhere after a house move or a new bird arriving.

Which birds get it

Budgerigars are the classic host, and the old aviculture term going light describes precisely this disease in that species.

Cockatiels, lovebirds, parrotlets and small conures are all regularly affected. Canaries and finches get it as well, so a mixed household matters.

Larger parrots are affected less often but not never. A grey, an Amazon or a macaw with unexplained wasting deserves the test even though the odds are lower.

The organism also occurs in poultry and other birds, which is worth knowing on a smallholding where a pet bird shares airspace or handlers with chickens.

What healthy looks like, and how to measure it

A bird in good condition sits upright, holds its feathers close, and is interested in the room. Compare against your own bird's photographs, not against a picture of somebody else's.

Weigh, and write it down.

Use a digital kitchen scale that reads in single grams, with a perch or a small tub taped on top. Weigh at the same time of day, ideally in the morning before the first big feed, because a full crop distorts the reading.

Weekly is enough for a well bird. Daily is right for a bird under investigation or in treatment. What matters is the trend line, not any one number, and a steady fall over three or four weeks is meaningful even if each week's change looks small.

Feel the keel.

The keel is the ridge of bone down the centre of the chest. On a bird in good condition the breast muscle on either side is full and rounded, so the keel feels like a shallow ridge between two soft mounds. As muscle is lost the keel becomes a sharp prominent edge with hollows beside it.

Do this with the bird stepped onto your hand, using one fingertip, without gripping the chest. Birds have no diaphragm and breathe by moving the sternum, so a hand closed around the chest stops them breathing.

Look at the droppings properly.

A normal parrot dropping has three parts: a coiled dark faecal portion, a white or cream urate portion, and a small amount of clear liquid urine. Learn what your bird's normal looks like on white paper, because that is the only way to notice a change in the faecal colour or the appearance of undigested seed.

Know what your bird eats and how much.

Weigh out the daily ration rather than topping the bowl up. A bird that is quietly eating twice as much as it used to is giving you information, and a bowl that is refilled whenever it looks low hides that completely.

Changes that mean act today

Black or tarry droppings.

Digested blood. Same-day veterinary attention. In a budgerigar-sized bird the volume of blood involved is small in absolute terms and large in relative terms.

Vomiting that is not courtship.

Courtship regurgitation is a deliberate, rhythmic head-bobbing behaviour directed at a mirror, a toy, a favourite person or a cage mate, and the bird looks pleased with itself. Illness vomiting is involuntary, often accompanied by mucus, and the material ends up flicked over the head feathers, the cage bars and the surrounding wall.

A bird with wet, matted feathers around the face and neck and sticky material on the cage bars has been vomiting, whatever the owner has seen.

A bird sitting on the cage floor, fluffed, or with eyes half closed.

This is late. Warmth, quiet, no chasing, and immediate veterinary care.

A sudden fall in weight after weeks of slow decline.

Wasting diseases tend to accelerate. A bird that had been losing a little each week and then drops sharply has crossed a line, and appetite often fails at the same time.

The look-alikes, and what separates them

The practical point is that several of these need the same first step: a fresh dropping sample, a weight, and an avian vet who will look down a microscope.

How it is diagnosed

Faecal cytology.

A fresh dropping is examined under a microscope, usually after staining. The organism is large and rod-shaped and, once seen, is unmistakable. A sample that has dried out or sat for hours is much less useful, so a dropping produced in the last hour is worth the effort of collecting properly.

Why one negative is not a clear result.

Shedding is intermittent. A bird can be colonised and produce clean samples for days. Collecting droppings over three consecutive days, or pooling several from one day, raises the chance of finding it considerably. If clinical suspicion is high and cytology is negative, ask about repeating rather than concluding.

PCR.

A DNA test on faeces is more sensitive than looking, and is the better option where the picture fits but samples keep coming back clear. Availability and cost vary a great deal by country.

Crop or proventricular wash.

A sample taken directly under sedation, used where faecal testing has not resolved the question.

Imaging and blood work.

Radiographs help separate this from proventricular dilatation disease and from a foreign body, and identify metal in the gut. Blood work assesses how much reserve the bird has and whether the liver or kidneys are involved.

Post-mortem examination.

If a bird dies undiagnosed and others share the household, a post-mortem is genuinely worth doing. It gives the surviving birds a diagnosis, and in an aviary situation it changes what happens to everyone else.

Treatment, and the two mistakes that waste months

It is a yeast, so it needs an antifungal.

The medication most used in birds for this organism is amphotericin B given by mouth. It has to be dosed by body weight, it needs an accurate scale and an accurate syringe, and it needs a course long enough to clear the organism rather than just quieten it. Availability differs by country and in some places it comes only from a compounding pharmacy on a vet's prescription.

Do not attempt to source or dose this yourself. The margin for error in a bird weighing tens of grams is unforgiving.

Mistake one: antibiotics.

The name megabacteria has outlived its accuracy by decades. Birds are still put on courses of antibiotics for this, which cannot work against a yeast, and which disturb the normal gut flora at the same time. Weeks are lost and the bird arrives at the correct treatment thinner.

Mistake two: nystatin.

Nystatin is a reasonable choice for crop candidiasis and is widely stocked, so it gets reached for whenever a yeast is mentioned. It is not the drug for this organism. If your bird is diagnosed with avian gastric yeast and prescribed nystatin, it is worth asking the question.

Sodium benzoate and water treatments.

Adding compounds to drinking water has been tried and is unreliable, because a sick bird's water intake is exactly the thing you cannot control. Medicated water also tastes different, which reduces drinking in a bird that already needs fluid. Treat the bird, not the bowl.

Supportive care matters as much as the drug.

A recovery setup: floor-level food and water, soft footing, low perching, and the carrier already out and familiar so a vet trip costs the bird nothing extra.

A wasted bird needs warmth, because shivering and thermoregulation burn calories it does not have. It needs food within easy reach, which usually means bowls at floor level as well as in their usual place. It needs quiet and it needs the flock, whether that is a cage mate or you.

Where the bird is too weak to maintain intake, an avian vet may teach you to give a hand-rearing formula by crop tube or syringe. This is a skill to be taught in person, because a formula delivered into the airway instead of the crop kills the bird.

Diet correction is part of treatment, not an afterthought.

An all-seed diet leaves a bird deficient in vitamin A and in calcium and short on protein quality, and a deficient bird handles infection badly. Converting to a formulated diet with fresh vegetables is a slow process that should be done once the bird is stable, not during a crisis, because a bird that stops recognising its food as food will stop eating.

Other birds in the house.

Transmission is faecal-oral, and parents pass it to chicks when they regurgitate to feed them. If one bird is diagnosed, the others have been exposed. Ask your vet about testing in-contacts, separating food and water bowls, cleaning droppings promptly, and avoiding shared dishes between cages.

Quarantine every new bird before it joins the household, and include a faecal examination in that period. This is the single measure that keeps the organism out.

What never to do

Do not treat wasting with more seed.

The instinct to give a thin bird more of what it likes is understandable and it makes the underlying deficiency worse. Volume was never the problem.

Do not medicate a bird with anything bought over the counter for another species.

Doses in a bird weighing tens of grams cannot be estimated from a product intended for a dog or a chicken flock, and several common antifungals and antibiotics are toxic to small birds.

Do not assume improvement means cure.

Birds look better before the organism is gone. Stopping a course early is a reliable way to get a relapse that is harder to treat.

Do not put a new bird straight into the cage.

Most household outbreaks trace back to a bird that arrived and went straight in.

Do not delay because the bird is still eating.

Eating is not reassurance in this disease. It is the presenting sign.

What the vet will ask for

• A written weight history, ideally weekly for the past month or more
• Fresh droppings, collected within the hour, on clean paper in a sealed bag
• Photographs of droppings over several days
• The exact diet, including how much is eaten and how often the bowl is refilled
• Where the bird came from, when, and whether it was quarantined
• How many other birds are in the household, and their species and condition
• Any recent change: a move, a new bird, breeding, a cold room, building work
• Whether vomiting is directed at an object or person, or involuntary

Expect a weigh-in, a keel score, examination of the crop and abdomen, and microscopy on the sample. Radiographs and blood work follow if the picture is not clear or the bird is very unwell.

When to get help

Contact an avian or exotics vet the same day for:

• Black, tarry or dark red-brown droppings
• A bird on the cage floor, fluffed, or too weak to perch
• Repeated involuntary vomiting
• A sudden acceleration in weight loss, or an appetite that has now failed

Book an appointment within the week for:

• A steady downward weight trend with a normal appetite
• Whole undigested seed in the droppings
• A newly acquired small parrot that is not gaining as expected
• Any bird in a household where another bird has been diagnosed

Take the weights, the dropping photographs, and a fresh sample. A bird that arrives with three weeks of numbers gets a diagnosis in one visit far more often than a bird that arrives with a description.