Thank you for the further history on this wee lass.
Now as I hinted before and I am sure you can appreciate, chickens are a prey species they often will hide that they are unwell for as long as possible (since telling everyone that you are ill will make you a target for predation). That means the signs we are seeing now are likely to be associated with advanced disease here. Therefore, you have taken that correct first step of isolating this bird from the others (because while I am glad there is no other signs of other birds being affected, airborne pathogens can spread within the airspace quite easily)
Now when a bird demonstrates difficulties in breathing, we do have a few considerations. If she is the only bird affected in a group, it is possible to have a partial airway obstruction (if something is lodged). More commonly we see this with respiratory disease agents like Infectious Coryza, Acute Fowl Cholera (Pasteurella multocida), Influenza, ILT IRT, infectious bronchitis, Chlamydiosis, and mycoplasma (Mycoplasma gallisepticum). Furthermore, we can see localized throat based disease due to Trichomoniasis (canker), Fowl pox (wet form causes canker lesions in the throat) and Syngamus trachea (gape worm) infestations.
If you are comfortable handling your bird, then you can potentially narrow down a few of these differentials at home. Now I am not sure how far down the throat you have looked but we do want to be checking for any signs of gapeworms (even if she is wormed we can see resistance), the plaques of Fowl Pox or discharge of canker. If you can't see anything, since you can only look down a chicken throat so far, then you can try the "Q-tip test". To do this, you need to place the bird in your lap, gently open her beak, and swab a Q-tip down her throat (twirl it as you do this). Twirl as you bring it back up, and if she's got gapeworm, you'll see thin, red strings on the q-tip. This way you will know if this is the cause. But if you end up with a cheesy discharge then canker or pox would be higher on the differential list. And if there is any other discharge, the bacterial causes would be suspect. And once you have identified what is present, you will be in a better position to know if you are treating them appropriately.
Otherwise, further measures to pinpoint the causative agent and increase your treatment success, you do want to consider involving your vet. This should especially be considered since she is already in respiratory distress with this (and her pale comb tells us she likely has too low blood-oxygen levels as well as her severe signs will make her a fragile patient to try and treat symptomatically alone). Furthermore, the vet would be able to listen to her lungs, examine her airway, and help you determine the disease local and therefore rule out some of these agents. Furthermore together you can collect some respiratory secretions from this bird to be cultured. This will tell you what agents are present and what treatment will actually clear them . As well, you might consider having a fecal exam performed as well to tell you if parasites are playing a role (directly or via compromising the immune system). And of course, if this bird is struggling, they can put her on oxygen therapy to help her to breathe easier while being treated.
Furthermore, once you have samples for culture, you can consider a broad spectrum treatment to try and tackle as many of the bacterial causes as possible. There are a range of options that would include, erythromycin, oxytetracycline, or macrolides. Other options may be tilmicosin, tylosin, spiramycin, or fluoroquinolones (especially useful if Mycoplasma is suspected).
Otherwise, supportive care is that key facet that we need to make sure you are addressing. If you think she is sounding congested in the upper airway, you can consider a bit of steam treatment here. You can achieve this by putting her in a carrier in the bathroom while you run a hot shower. Or if you have a nebulizer/humidifier you can set up a wee steam tent for her (by putting her in a carrier and covering it and the humidifier with a bed sheet). This can just help reduce some of those airway clogging secretions.
If her fluid and nutrition intake is on the decline, then we do want to initiate tempting, hand feeding or even syringe feeding at this stage As I am sure you can appreciate, dehydration can weaken a bird and contribute to worsening illness and cause additional issues. In regards XXXXX XXXXX intake, we want her water intake to be ~ ½ a cup daily. While you are keeping an eye on this, you also want to monitor her for signs of dehydration (skin tenting or sunken eyes). To maintain hydration, in a drinking bird, you can offer water with electrolytes instead of plain water. There are readily available electrolyte solutions available on the market (ie. Vi-tal) or you can use Pedialyte or diluted Gatorade (diluted 50/50 with water). You can offer these in a bowl or if she aren't drinking then you can administer fluids (and hand feeding) via towel restraining and a syringe or dropper. Wrap bird “burrito style” and hold securely upright in lap. You can drip water on top of the beak, as reflex will cause them to catch the droplets with their tongue. (some will even drink from the syringe directly). In doing this, do make sure not to get water into the nares.
Feeding wise, offer favorite foods. You can also get Nutrical paste to supplement her diet (either mixed in food, water, or via syringe) which will provide extra calories or nutrition. Offer fresh foods, high in nutrition and water content like cucumbers, Romaine, grapes, melon, oranges, etc. Hard boiled eggs mashed shell and all are extremely nutritious and delicious to birds and cooked brown rice is good for them too.
If you are comfortable hand feeding birds you can make a handfeeding paste with handfeeding powder (ie Nupreen Hand Feeding Formula) and your electrolyte solution. Ideally, if you haven’t hand fed a chicken before, you should have your vet show you how to do this safely (as aspiration is a serious risk that is best avoided). Do monitor the crop by gently palpating to make sure its emptying into the gut, (normally 2-3 hours post eating). If it feels more like a hard tennis ball, that is an indication of dehydration and crop impaction or crop stasis. Give fluids and massage crop. But if it doesn’t improve, then veterinary intervention may be required.
Overall, there are a range of agents that can be to blame for the signs you are seeing. In her case specifically, the signs of respiratory distress and poor oxygenation tell us that this is already severe and advanced (even if she is only just showing signs now) and having a vet's aid (+/- oxygen supplementation) at this point needs to be considered to give her the best chance of survival. Depending on their findings, they will be able to give you an idea of her survival chances. If there is hope for her then they can guide you on the above diagnostic steps to determine which is to blame to make sure you treat as effectively and economically as possible. Otherwise, broad spectrum antibiotics can be tried and supportive care coupled with isolation/strict hygiene are important to get her through this illness and prevent spread to any other birds.
I hope this information is helpful.
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