The gray creature slowly lumbers through the yard, its black eyes easily piercing night's cloak of darkness. This American marsupial, carrying babies in its pouch, seems harmless enough to the casual observer. Who would think that the lowly opossum is responsible for many hundreds of avian deaths every year? Of course, it does not kill birds through any violent act. No, it causes deaths in a much more insidious fashion.
The Virginia opossum, Didelphis virginiana, is the definitive host of the organism responsible for causing such devastating disease in parrots, primarily Old World species (those of Australian, Asian and African origin), such as cockatoos, eclectus, African grey parrots, parakeets, and Poicephalus sp. The critter that causes sarcocystosis, or sarc, is a one-celled coccidian organism called Sarcocystis falcatula. This parasite requires both a definitive host (the opossum), where it undergoes sexual reproduction, and an intermediate host (usually a herbivore or insectivore), where it undergoes asexual reproduction. The opossum becomes infected when it consumes an animal that has mature cysts in the muscle tissues.
The normal intermediate hosts is the cowbird, Molothrus ater, and the grackle, Quiscalus quiscula. The opossum consumes these birds as normal prey items. It is when the sarc organism is ingested by certain species of exotic birds that big problems may occur.
This disease is primarily seen in warmer areas of the United States, where opossums normally reside, especially Florida, Texas and California. Although it is most commonly diagnosed in outdoor aviary birds, the occasional pet parrot may also succumb to this parasitic disease. It also appears to be seasonal, with most cases occurring in the fall and winter.
Many people believe that if they can prevent opossums from invading their yard, by using hot wires, having aggressive dogs, trapping, by fencing or other methods that keep opossums away, they cannot have problems with this organism. How wrong they are! The opossums can be quite far away, and all it takes is a fly or cockroach that has visited an infected 'possom poop to mechanically transmit the organism to their birds. In the last few years, Florida has been invaded by a new, accidentally imported flying cockroach. As if we didn't have enough gross bugs, now we can be strafed by flying roaches! Bleah!
Many aviculturists are quite diligent in protecting their property against the lowly 'possum, however, if their neighbors do nothing to deter these marsupials, it may be quite difficult to control this disease. In addition to keeping opossums away, it is vital to provide strict insect control to prevent the organism from coming into contact with susceptible parrots.
In outdoor aviaries, it is safest to use fine screening material around the flight cages to keep out roaches and flies. This isn't foolproof however, because often roaches can sneak in between the door and jam, or along the floor or ceiling. In addition to screening the flights, it is advisable to hire a professional exterminator who is familiar with applying pesticides around birds. In Florida, where I practice, bugs are a way of life, and it is necessary to keep up with pest control continually to prevent infections. This is even more important when dealing with aviaries, because leftover food and droppings are great enticements for bugs. Products containing pyrethrins as the active ingredient are usually safe to use around birds, as is 5% carbaryl dust (Sevin dust). Ivermectin can also be used in an aviary to help control insects. Please contact your avian veterinarian to discuss how to use these products safely around your birds, and do not try to apply them without professional advice.
One question that I am often asked about concerning sarcocystosis is this: Why is there a predilection for this disease in Old World birds? Actually, the answer makes complete sense when you understand this parasite. Opossums are found throughout the Americas. Parrots from Central and South America seem to have evolved with exposure to these parasites, and can better tolerate infection. However, this is not to say that New World birds never die from sarc, as cases have been documented in adult and nestlings. Old World birds, coming from parts of the world that do not have native opossums or the sarc organism, have evolved without being exposed to the parasite and are naive hosts.
We have also seen this occur in other species with other diseases, as well. For example, a type of filarid worm is native to the United States, and its host is the grackle. It rarely causes problems with native birds, however, this worm has caused great losses among juvenile emus. Emus, being large, flightless birds from Australia, have no inherent immunity to the filarid worms. The emu chick, if bitten by a gnat that has bitten an infected grackle, will develop neurological signs and will likely die.
The penguin also suffers by being transplanted from its normal habitat. Malaria causes tremendous problems in penguins at many zoological parks and gardens. Because these flightless birds have never been exposed to malaria in their native cold climates, they have no immunity to it.
Instead of succumbing to sarc, many macaws, conures and amazons may have cysts in muscle tissue that are found incidentally at necropsy and histopathology when a bird has died of other causes.
Let's now go through exactly how this parasite gets into a susceptible bird and what it does once it gets in there. When the opossum sheds the parasite in the feces, it may be mechanically transported by cockroaches or flies that have visited opossum droppings. If these insects then find their way into an aviary, there are several ways that they can spread disease to psittacines. The bugs may crawl through a dish of bird food and spread the microscopic organisms. If a susceptible parrot then consumes the food, it may develop the disease. Another way that a bird may become infected is if it ingests a roach or fly that has been in contact with infected opossum feces. Cockatoos, in particular, seem to enjoy catching and eating bugs. Rarely, a bird may have direct contact with opossum feces, if a possum defecates on top of an aviary or bird cage.
Within hours of the organism entering the host, the sarc sporocyst releases sporozoites, and it begins multiplying in the intestinal tract walls. Within two days, the organism may be found in the intestines, liver and lungs. Then the organism is spread through the bloodstream to other internal organs and muscles. Most of the parasites may be found in the lungs, and fewer numbers may be found in other tissues. After the organism undergoes further replications, the parasite may then form cysts in muscle tissue.
Clinically, there are five different forms of sarcocystosis. The form that causes startling problems in outdoor private aviaries and zoological collections is acute pneumonia. Birds that receive high doses of sporocysts are more likely to develop severe pneumonitis or they may die in the early stages of infection. Most often, there is no warning, and the dead bird may be of normal weight. At necropsy (animal autopsy), the lungs are very wet and foamy, and very red, and often the spleen and liver are enlarged. Birds that are found still alive, are often very ill, weak, depressed and are having difficulty breathing. They may also have neurologic abnormalities. Lab abnormalities may help a vet in making a diagnosis. Radiographs may also be helpful, as they will show changes in the lungs, and an enlarged liver and spleen. It may be difficult to definitively diagnose sarc in a live bird. A new blood test has recently been developed at the University of Miami, using blood plasma. It is a serology test that tests for the sarcocystis antigen (the offending portion of the organism that invokes an immune response). In acute cases, the test may be positive or negative, if the bird has not had time to mount an immune response to the organism. In addition to the serology, it is important to run plasma electrophoresis of the blood, to look for specific patterns that would indicate sarcocystosis.
Recommended treatment is a combination therapy of pyrimethamine and trimethoprim sulfadiazine for at least two weeks. Relapse may occur when medication is discontinued, and the patient should be monitored closely for this possibility.
Another form of the disease involves clinical muscular disease. The sarc organism may cause a mild myositis (muscular inflammation). This form of the disease has been documented in Old World species. It may occur when a bird receives a sublethal dose, and does not die from the acute pneumonitis form, and the bird develops clinical muscular disease instead.
Birds with this form are usually alert but show muscular weakness in the wings, legs or both and may have difficulty walking or flying. Muscle biopsy may confirm disease. Treatment is the same as for the lung form, except the therapy must be continued for at least 6 weeks.
Several cases of sarc invading the heart muscle have been reported in the literature. Myocardial (heart muscle) sarcocystosis may be an incidental finding or it can cause inflammation of the heart muscle. This is an uncommon form of the disease and rarely is diagnosed in a live bird. The cysts are usually found during histopathological examination of the heart.
In New World species that have died of other causes, cysts of the sarc organism may be found incidentally with histopathology of muscle tissue. The cysts may be visible to the naked eye, or in most cases, they are microscopic. The cysts were not responsible for any reaction in any of the reported cases. It is suspected that New World psittacines, that are less susceptible to fatal infections, are more likely to live long enough to develop muscular cysts after exposure to the sarc organism.
Rare cases have been published that describe the sarc organism causing neurological disease, and inflammation of the brain, encephalitis. One case involved an acute progressive disease in a young cockatiel, and other reports involved chronic disease in raptors (Golden Eagle and Northern Goshawk).
An unreported form of sarcocystosis seems to be emerging in Florida. Several Board Certified Avian Specialists have discovered sarcocystis organisms in the feces of clinically ill birds. I am one of those avian vets. Because I didn't believe my own eyes, I have sent the droppings out to an independent parasitology lab for confirmation. This state-of-the-art lab has confirmed that these birds (all cockatoos) had sarc organisms in the feces. They are attempting to discover the species of the coccidia, as they are not sure if it is Sarcocystis falcatula. Other animals, cats and horses, for example, also have their own sarc organism.
In all of the cases, a cockatoo has developed diarrhea, lethargy and decreased appetite. Fecal examination has uncovered the sarc organism. Treatment with standard therapy for coccidia or for sarc has been variable. I have diagnosed this form in both adult and juvenile cockatoos (umbrellas, Bare-eyeds, Moluccans and Goffin's).
I am currently working with several Universities to learn more about this emerging disease. Right now, I have two patients belonging to the same owner, and both cockatoos (a Bare-eyed and a Goffin's) have been clinically ill and are passing sarc organisms in the droppings. They have been treated with several coccidiostats (drugs to control coccidia) and both are improving, gaining weight, and acting better. When first diagnosed, both birds were very ill. One bird has cleared the organism, and the other bird, although improving, still has the organism in the droppings.
Once I have the species of the sarc organism identified, and I have more clinical data, I will be writing this up for a professional publication to let other vets know about this previously unreported form of the disease. The lab I am working with is positive that the birds are passing sarc organisms in the droppings.
Sarcocystosis is a very frustrating disease, both for aviculturists and for avian veterinarians. It is difficult to prevent exposure to the organism, because both opossums and insects must be controlled. And although veterinarians may be able to make a presumptive diagnosis, based on the species of bird and clinical signs, confirming the disease in a live bird is very difficult. After presumptive diagnosis, treatment may be difficult or unrewarding.
The best thing we can do it to educate owners about this disease. Often owners and aviculturists have somewhat nebulous ideas about sarcocystosis and know that there is some vague connection between opossums and disease in birds. By teaching bird owners and veterinarians about this organism, it may be possible to better prevent it from occurring, and it may be possible to diagnose it faster, allowing immediate treatment, based on presumptive diagnosis. Understanding this complex disease is half the battle. So now you know!
Copyright © 2006 Margaret A. Wissman, D.V.M., D.A.B.V.P.
All Rights Reserved
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