Probiotics
With info on Prebiotics at the end of the article

Are probiotics beneficial for the average pet bird?  It looks like most of the time the answer is "no", although there are some circumstances where they appear to be useful. 

What are probiotics? The World Health Organization defines them as "live micro-organisms which, when administered in adequate amounts, confer a health benefit on the host." The benefit they're talking about involves the gut flora, which is the community of microorganisms (bacteria) that normally live in the intestine.  Probiotics are beneficial bacteria that we consume in the hope that they will become part of our gut flora, helping us digest our food and crowding out the bad bacteria that we don't want.

There are certain general types of bacteria that tend to be found in the gut flora of any given animal species, and each individual will have hundreds of different bacteria species in their gut.  But the exact blend of bacteria species is very individual, thanks to the rapid evolution rate of bacteria, and the mix is also influenced by the individual's diet (Wu et al).  Members of the same species are expected to have more similarities in their gut flora than members of a different species, but every individual is unique and even identical twins have dramatically different gut flora (Science News).

Studies indicate that species-specific probiotics are more effective in birds than probiotics that are not species-specific.  For example a probiotic made specifically for cockatiels is expected to be more effective for cockatiels than one made for birds in general or for humans. 

However it appears that probiotics in general do not colonize the gut whether they're species-specific or not, because they are simply not individualized enough to fit in with the existing gut flora (ResearchGate). If they try to take up residence they will be crowded out by the "native" bacteria. But it has been shown that probiotic bacteria can have a beneficial effect on the function of the gut flora while they're passing through the system (McNulty et al; user-friendly explanation on MedicineNet).

These pass-through benefits may be most apparent during times when the normal gut flora has been depleted (like after taking antibiotics). But the probiotics are unlikely to take up residence even at this time. If the normal gut flora has been depleted it will replenish itself over time, from internal reproduction and from picking up the fecal bacteria that's plentiful in the environment.  Stool transplants and other forms of poop-eating are much more effective ways to repopulate the gut than taking oral probiotics (Science-Based Medicine 1, Science-Based Medicine 2). It's not uncommon for pet birds to munch on their own dried poop, and there can actually be some benefits to it since it contains B vitamins as well as beneficial bacteria. If the gut flora have not been depleted, there simply isn't room for the foreign bacteria that we're trying to introduce with oral probiotics.

In humans, oral probiotics appear to be useful for certain specific conditions like irritable bowel syndrome (IBS) (Rowland et al), although Robert Clare reports that IBS has a placebo response rate of about 50% so this is questionable. Skeptics has a discussion that cites many of the available studies. But there is no evidence that probiotics do anything for healthy individuals (PubMedHealth, Genome Medicine, NeuroLogicaBlog).

Because of this lack of evidence, the EU prohibits food and nutritional supplement companies from claiming that their products have health benefits due to their probiotic content, and US companies have run afoul of the FTC for making such claims (Nature).

Birds do not have the same gut flora as humans, so the value of human probiotics is even more questionable for birds than it is for humans. When I looked for information on what type of bacteria occur in birds, different sources sometimes gave radically different answers. Apparently some birds are not even close to each other in the general types of bacteria they have in the gut. This isn't surprising, since birds can have radically different diets and live in very different habitats. Waite & Taylor discuss the microbiota of birds in general, and Hird et al reported on the gut flora of 59 neotropical bird species.  Xenoulis et al compare the gut flora of wild and captive parrots.  There's additional information on parrot gut flora from Darrel Styles and from Susan Orosz. E. coli, a bacteria species that's dangerous to humans, is part of the normal gut flora of some parrot species (including cockatiels) but not others (Styles).

Smith published a review of avian probiotics, which includes the disclosure that "The author is affiliated with Avian Health Products, producing species-specific probiotics." Not surprisingly, the review is in favor of probiotics overall.  But it does point out that even ideal species-specific probiotics may not always do anything:

"In human and animal probiotic research, species specificity of the probiotic strain is considered essential for effectiveness... Since the discovery of epithelial adherence in avian lactobacilli, most researchers have found that strains of lactobacilli with a high level of adherence provide the greatest health benefits. Conversely, nonadherent lactobacilli appear to have little or no benefit in birds... Several studies have been reported in which species-specific, adherent lactobacilli did not show significant beneficial effects.

"Probiotics used in feed must be able to survive the storage conditions used for the feed and exposure to the feed itself. Because the efficacy of probiotics depends on delivery of live organisms at a determined dose level, the viability at the point of ingestion by the bird is very important. Probiotics need to be viability-tested for shelf life, storage conditions, and administration conditions. "

This shelf life testing has probably NOT been done in bird food mixes containing probiotics.

This does not necessarily mean that non-species-specific probiotics are completely useless; it's possible that under the right circumstances, any type of probiotic whatsoever will be better than nothing.  Even the species-specific probiotics are not particularly expected to linger in the digestive tract, and if it's possible for the bacteria to do something useful as they're passing through I don't see why it would have to be a species-specific bacterium. The Primalac company (obviously not an unbiased source) reports beneficial results from using non-species-specific probiotics, in animals ranging from chickens to large mammals. Apparently they have only two probiotic formulas, one for fish and one for everything else.

The Melbourne Bird Vet talks about situations where probiotics are NOT useful. They may be beneficial on overcrowded poultry farms, but not so much for healthy, well-kept pets in a happy home:

"Where animals are not stressed, have an appropriate diet, are not crowded, are not given drugs, do not contract infection or metabolic diseases and live in a clean environment, an ideal level of intestinal bacterial population may be maintained on a rather steady basis. In fact, no differences are generally reported in numerous trials under these ideal conditions."

They also speak out in favor of species-specific probiotics:

"Because of the intimate relationship between the host animal and its bacterial population, it is important that the correct organisms are supplied in probiotic preparation for any given species. Probiotic supplements need to be prepared with particular species in mind and the more types of normal bacteria that can be provided, the better. For use in birds, therefore, multistrain avian-origin probiotic supplements are used."

They go on to talk about a study on handfeeding cockatiel chicks (Performance of neonate cockatiels given cockatiel derived Lactobacillus, not available online and apparently sponsored by Avian Health Products) where no difference in weight gain was found in healthy babies being fed an adequate diet. But when they intentionally exposed the chicks to bad bacteria, the ones getting probiotics did better than the ones that didn't get probiotics. Another study involving quail chicks (apparently under factory farming conditions) found that the ones getting probiotics did better than the others. But these are stressful conditions, so the results aren't surprising.

At present, species-specific probiotics do not exist for most pet bird species. Avian Health Products has probiotics for budgies and cockatiels, and for conures in general. The one for conures is obviously not species-specific, since there are many different conure species and one product can't be specific to all of them. These are apparently the only species-specific probiotics on the market for parrots. 

If you decide to go ahead and use probiotics in spite of their questionable effectiveness, are you actually getting what it says on the label?  In many cases, the answer to that question is "no". A 2011 paper by Weese & Martin found that just two out of 25 veterinary probiotics had labels that accurately described the contents. The SkeptVet has a plain-English summary of the paper.  Many of the products that failed didn't even spell the names of the bacteria right.  Several other studies had similar poor results (Weese, Morovic et al, Labdoor).

This doesn't interfere with the sales hype of course. Even apple cider vinegar is hyped as a source of probiotics because it has Acetobacter bacteria in it, but don't believe this claim unless you're keeping fruit flies as pets.  That's the only life form that's known to have this bacteria in its gut flora. See the ACV article for more information on how overhyped ACV is.  

Prebiotics

The names are similar and their action takes place in the same part of the body, but prebiotics are quite different from probiotics. Probiotics are beneficial microorganisms that we put into the body; prebiotics are substances in foods that promote the growth and activity of the beneficial microorganisms that are already in the gut. Prebiotics are specific types of dietary fiber that can't be digested by the host organism (like a bird or human), but can be fermented and used as a food source by bacteria in the gut.  But not all types of dietary fiber are prebiotic, and different species of gut bacteria may prefer different types of prebiotics.

Oligosaccharides are the best-known type of prebiotics (Slavin). But unfortunately they aren't known to occur in a particularly wide variety of foods. Wikipedia lists the top ten sources as chicory root, Jerusalem artichoke, garlic, leek, onion (all in dry form, which increases the concentration), followed by asparagus, wheat bran, wheat flour, and banana.  Moshfegh et al add barley and rye to the list, with statistics on the amount of oligosaccharide in all these foods.  They say that wheat is the primary source of oligosaccharides in the American diet - it's probably because we eat a lot more wheat than any of those other foods.

Beta glucan (a substance found in grain) is also recognized as a prebiotic. It looks like oats and barley contain more of it than other grains (Science Direct), but it has been observed in numerous other grains and it seems likely that all grains contain at least a little.

It looks like pectin may have prebiotic effects, although Gomez et al report that pectic oligosaccharides are better than plain pectin.  Pectin is part of the cell walls of all plants, so you're getting some of it every time you eat a plant. There's more info on pectin in the Fruit article.

Resistant starch also appears to have prebiotic properties (Zaman & Sarbini). There are several different types, and whole grains are the primary source for one type (Sajilata et al). The other types are found in foods that are much less likely to be fed to birds. However, the information about resistant starch forming when certain foods are heated and then cooled suggests that some resistant starch may form during the production of extruded pellets for pet birds.

More research is needed in this area, and it may turn out that we have more prebiotic fiber in our diet than we realize.

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