by Darrel K. Styles, DVM
(Note: This article was written in 1996. Therefore, newer drugs may be available. Ed.)
This is the last in a series of articles on antimicrobial agents. This installment deals with drugs that combat fungal, parasitic, and protozoal infections. These categories are not necessarily related, but I am grouping them together for convenience's sake.
Antifungal agents have been steadily improving over the past decade. In the past, these agents have been quite toxic and not necessarily all that effective. The problem with combating fungal diseases is that the fungi are similar in cell structure and metabolism to us and our avian patients. Therefore, any agent that harms the fungi often would harm the patient.
Most of the antifungal agents work in much the same way. They target the fact that the fungi have the sterol ergosterol in their cell membranes in much the same way we and birds have cholesterol in our cell membranes. This cell wall difference gives us the differentiating tool to attack the fungi without causing undue harm to the host. Most of the drugs interfere with the function of ergosterol in the cell membrane and make the fungal cell membrane susceptible to disruption and therefore cell death.
Letís now discuss the different fungal types. In general, the fungi may be divided into two broad categories; yeast-like fungi and hyphal-forming fungi.
Yeast-like fungi are those like Candida. The fungi typically do not form long, stringy projections but instead maintain a spherical or ovoid shape. However, they may form small projections call pseudo-hyphae. The yeast-like fungi tend to reproduce by simple binary fission but can also propagate by spores like other fungi.
The hyphal fungi are those like Aspergillus, which tend to form long, stringy projections almost like roots. The hyphal fungi reproduce by forming fruiting bodies that give rise to spores. These spores then seed the next generation.
The reason for making the distinction between the two categories of fungi is that they behave differently. Therefore, the medications employed against them must be specific for the fungal type.
One of the oldest antifungal agents is Amphotericin B, which is known by its trade name Fungizone. This drug, although highly effective, tends to be extremely toxic and must be used either topically or given intravenously (a difficult feat in avian patients). Amphotericin B is very effective against hyphal forming fungi like Aspergillus. But due to its toxic nature, it has not been all that useful in avian patients except for the topical cream form for oral fungal lesions. It also has been employed as an aerosol for tracheal and lung/air sac fungal infections.
Another old drug is flucytosine known by the trade name Ancobon. This drug can be given orally but seems to be efficacious only if the fungal infection is easily accessible by the circulatory system. If, however, the fungi is in the airways, which it typically is, these areas are not well vascularized. The body walls off the fungus in the form of granulomas, and the drug has difficulty penetrating the walled off region.
Another old standby that we still use today is Nystatin suspension. The drug is very effective against gastrointestinal Candida infections. However, the drug must contact the fungi. It is not absorbed across the GI lining and is therefore limited in its use. It is, however, the most popular neonatal antifungal agent due to its efficacy and mildness.
The new class of antifungal agents that is very effective is called the conazoles. These include many of the antifungal drugs we are familiar with today, such as ketoconazole (Nizoral) and difluconazole (Diflucan). These drugs are used to treat both systemic or airway and gastrointestinal fungal infections.
Ketoconazole comes as tablets in the U.S., which can be easily converted into a stable suspension. Ketoconazole is highly effective against the yeast-like fungi, such as Candida.
Difluconazole is an injectable medication which can also be aerosolized. It is highly effective against hyphal-forming fungi, such as Aspergillus. (More recently, some vets are seeing good results with Lamisil. Ed.)
The safety margin on the conazole drugs is wide. And they are very effective if the fungal infection is caught early.
Before we leave the antifungals, let me say a word about chlorhexidine (Nolvasan). Categorically, you can not treat a Candida infection with water treatment by Nolvasan. Nolvasan can be used as a preventative measure when severe Candida infections occur in the flock. Nolvasan will limit and control the spread of Candida but will not treat active infections.
One last point, Aspergillus is not a contagious disease. Aspergillus is everywhere in the environment and only when we have a stressed and immunocompromised host will it infect the bird. A bird with Aspergillus cannot give it to another bird, but it does indicate that there may be a problem in the environment.
In our psittacine birds, we fortunately are not plagued with many parasites. However, this is not the case in fowl and softbills.
We can divide parasitic infections into two broad categories: endoparasites, such as worms, and ectoparasites, such as lice. Endoparasites may be further divided into nematodes, such as roundworms, trematodes, such as flukes, and cestodes, such as tapeworms.
Trematode or fluke infections are uncommon in psittacine birds with the notable exception of some imported cockatoos, which succumb to the disease because of its cryptic diagnosis. Cestode or tapeworm infections are not uncommon in imported Psittacus species, such as Timneh and Congo greys. Nematode infections are sometimes found in birds housed outdoors and are invariably roundworm (ascarid) or Capillaria infections. For soft-billed birds and fowl, there are a multitude of endoparasitic pests which are beyond the scope of this article.
Almost all of the anthelmintics are toxic to some degree and therefore must be used carefully. Letís begin with an overview of deworming medications for nematodes.
Levamisole (Ripercol) is a commonly used medication for nematodes. It can be given orally by gavage or in the water with great care. Levamisole seems to have a wide margin of safety where different species of birds are concerned. This cannot be said of other anthelmintics.
Another common medication is Ivermectin. This medication is highly effective against most endo- and some ectoparasites. It can be given safely orally to a wide number of species, topically with very small birds, and by injection with great care in some species where it has been used previously.
I had one prominent avian vet tell me that he killed a shipment of Vinaceus Amazons with injected Ivermectin when he had no problems in other Amazon species. Due to anecdotes like this, I almost always give this drug orally to psittacines. I have given it by injection to softbills where I know how the drug will behave.
Ivermectin must be diluted to the proper concentration because it is quite toxic. I prefer using the preparation Eqvalan for horses, which is more water soluble. Just prior to administration, I dilute the drug in water instead of propylene glycol, which some vets use. Ivermectin is not only effective against worms, it can be used to treat air-sac mites and scaly mites quite effectively.
The bendazoles, such as fenbendazole and thiabendazole, also have been used for nematode infections but not as commonly as Ivermectin. There can be some toxicity with these drugs in certain species.
The last worm drug commonly used is Pyrantel Pamoate. This drug is very safe and is only for intestinal nematodes.
For cestode infections (tapeworms), there is really only one reliable, safe, and highly effective drug and that is praziquantel (Droncit). This drug can be given orally or by injection. Within an hour to several hours post administration, long tape ribbons may be observed issuing from the cloaca.
Ectoparasites are not that common in household pets but are seen occasionally with outdoor aviary birds. Lice and mites may be effectively controlled with applications of 5% Sevin (Carbaryl) dust on the bird and in the environment. You must ensure that the insecticide is 5% and not more concentrated. As mentioned previously, air-sac mites and scaly mites are usually controlled with Ivermectin.
Protozoa are simple single-celled organisms that are more complex than bacteria and typically have complex life cycles.
The class of antiprotozoal drugs is pitifully small, even in human medicine. There has been increasing interest in this field due to the increase in exotic protozoal parasites and AIDS patients. Just several years ago, a major U.S. metropolitan city with state-of-the-art water purification had a severe epidemic of the water-borne protozoan Cryptosporidia causing some deaths and multiple illnesses. However, we have very few drugs in our armamentarium to combat these agents.
In our avian patients, we worry about several protozoal infections: Sarcocystis, Giardia, Coccidia, Plasmodium, and Trichomonas.
We can control Giardia infections somewhat with the drug metronidazole, also known as Flagyl. There were more effective treatments than this drug, but the FDA pulled them off the market. Flagyl is difficult to administer on a flock basis but can be given orally or by injection to an individual bird.
Coccidia can be controlled with either sulfa drugs, such as Bactrim, or the poultry agent Amprolium. Coccidial infections are not common in psittacine birds but are very common in softbills where they can result in severe losses.
Trichomonas or frounce is a common infection of columbiforms (pigeons and doves) and raptors (birds of prey). A new drug called carnidazole (Spartrix) is very effective in controlling this problem.
For Sarcocystis, there is little we can do except watch the birds die. Some sulfa drugs may be preventative but none have been established to be curative.
Fungal infections can be avoided by maintaining good hygienic procedures and cleaning the environment. Parasitic infections can be controlled by placing most nonground-dwelling species in suspended wire caging and carefully monitoring ground-dwelling or free-ranging species. Protozoal infections can be avoided by controlling the vectors, such as wild birds, insects, and mammalian carriers.
In concluding this set of articles concerning pharmacotherapy, I hope I have given you some insight into how and why vets choose a particular medication and how some of the drugs work. One point I wish to reiterate is to please avoid medicating unless directly supervised by a veterinarian. More than once, I have seen worse problems induced by the cure than the cause. Medicines are not a panacea. They are simply tools to help the animalís own system rid itself of the pathogen.
The drugs discussed in this article can be especially toxic and should be used with great care.