Latent Disease
By Dr. Jeanne Smith DVM Avian Health Services

There are many infectious diseases of pet birds that can cause a latent infection. Latent infections are ones that do not manifest with clinical signs so an infected bird can appear perfectly normal. Usually the term “latently infected” refers to a bird that may eventually start showing disease, but it can also refer to a bird that may never actually get sick itself with the disease. Usually the term “asymptomatic carrier” is used to describe birds that never get sick themselves. These latent infections are big problems with multiple bird owners or breeders because you can think you are bringing a healthy bird into your flock when you are actually bringing in a contagious disease that can spread to your other birds. When people start asking about an incubation period or how long after exposure a bird will get sick, latent diseases really complicate things. As we talk about some of these diseases you’ll see that a bird can be exposed, infected, and may show clinical signs as soon as a few days to a week or as long as years later. This makes it very difficult to track where the infection came from.

There are now some fairly reliable tests for some of these diseases. The big species of psittacines, which are more valuable, are often tested. The real problem with amplification of these diseases comes from the small species, such as budgies, cockatiels, and lovebirds, which are rarely tested because it is not economically feasible. In addition, the small species are often colony bred so they are exposed to many other birds, I recommend keeping small species as far from your large species as possible and hand feeding small species in another room from the large species (washing your hands in between groups and not sharing feeding utensils). If you have a few psittacine pets in your home and you bring a small species bird in as a pet, test it before exposing your other pets to it.

Each of these diseases could warrant a talk just about it. So I’m going to focus on the latent aspects and diagnosis of a latent infection.

Chlamydiosis Almost all of you will be familiar with clamydiosis or psittacosis. Small species birds are commonly latently infected with this disease, and they can remain latently infected their entire life. It is especially a problem in pet shops where multiple birds from multiple premises are mingled together at a stressful time for them. Usually these birds are recently weaned, shipped off and grouped with unfamiliar birds. That is stressful. Stress plays a role in latent diseases. Birds that are latently infected can be safe for other birds for large periods of time because their immune system may be keeping the disease in check. I f the disease is dormant, they may not be shedding the organism into the environment, so they aren’t infective to other birds. Under stress, the immune system is less effective and latently infected birds are much more likely to shed organisms and be infective. I usually recommend that people do a DNA probe on a small species bird purchased at a pet shop if they have other bird pets or any immunosuppressed people in the household. This is not an indictment of the pet shop. The best run, most careful pet shop in the world can’t predict whether their source flocks are all free of chlamydiosis. The DNA probe can be run on blood in an asymptomatic bird, it can be run on a mouth/vent swab in a bird showing signs of illness. Research has shown that when a bird is ill the most reliable sample is from the mouth/vent. When a bird recovers, it’s blood will remain positive after the swabs go negative. The blood may remain positive for months. The DNA probe is a very sensitive and specific test, but it isn’t 100%. Theoretically , an asymptomatic bird could have chlamydiosis somewhere in it’s body that you’d expect it to be activated and circulating.

Big species psittacines are generally less likely to be latently infected with chlamydia. They are more likely to become ill once infected, usually 7 to 14 days of exposure. Flock treatment with tetracycline's eliminates most of the infection, but not 100%. In two flocks of cockatiels where I have had the opportunity to do DNA tests on blood from every bird in the flock, I have found about 4% to remain positive after a 45 day treatment.

Bordatella avium This is a disease that not many aviculturists are aware of yet, but I am seeing it with increasing frequency. This bacteria causes sinus infections, lockjaw, and can go into pneumonia. It has been well known in the poultry industry for many years, but it is only in recent years that reports have appeared in the pet bird literature. It is very difficult to diagnose because it does not grow well in culture in competition with other bacteria and yet it often is complicated by secondary infections with other opportunistic bacteria. So you’ll have a flock (usually babies in the nursery) with sinus infections spreading through the group and culture a garden variety of opportunistic bacteria. Several cultures later you might isolate the bordatella. And this is from birds showing clinical signs. Trying to culture suspected latently infected parents is even worse. I suspect B.avium whenever I see sinusitis that seems to be spreading bird to bird or if I see lockjaw and I only get a variety of opportunistic bacteria. I increase my suspicions if we treat with the indicated antibiotics and the birds don’t respond the way they should. If it was only an opportunistic bacteria they would be noticeably better by the third day of medication, completely well by 7-14 days. If that doesn’t happen, worry about B.avium. B.avium is notorious for not responding well to treatment. If you do isolate it and run a sensitivity it will look sensitive to almost all antibiotics. But when you use those antibiotics you get very poor response. I usually find I have to use two of the antibiotics it is most sensitive to and treat for as much as 4-6 weeks. Some birds never do improve, Once a bird recovers you can’t tell if it remains latently infected, because the diagnostics are so unreliable. Not every bird will remain infected, the problem is you can’t tell who is and who isn’t. I believe this disease is becoming more common because it goes undiagnosed and then recovered or exposed birds go off to other breeding operations. Cockatiels are by far the most likely birds to bring it into a nursery, from there it spreads to any species of chicks in the nursery. It requires direct physical contact bird to bird or handling of one bird after another without washing your hands. I’ve been involved in outbreaks where simply washing your hands between birds stopped the spread. Research in turkeys has shown that housing turkeys individually in cages adjacent to each other was enough to prevent infection. One psittacine flock I’ve worked with has birds individually housed in wire cages separated only by 1 inch of space between them and spread was stopped when handlers washed their hands between each bird.

Polyomavirus Adult birds very rarely become sick with polyomavirus infections. Once exposed they either clear the infection from their body or a small percentage of them remain latently infected. Research so far indicates that only about 5% of asymptomatic infected birds maintain infection for more than 3-4 months. Usually latently infected parents transmit the virus to their offspring through the egg or by feeding their chicks. Infected chicks then spread the virus chick to chick in a nursery. Breeders can be screened for poylomavirus by a DNA probe done on a blood sample. Another good prevention is to keep a closed nursery—only handfeed your own flock’s chicks. If you don’t test your breeders you have a certain chance of experiencing polyomavirus in your nursery, if you handfeed for one other breeder you double those chances, two other breeders you triple your chances, etc. This is another disease that is common in budgies and lovebirds. It can be one of the causes of French Molt or it can manifest in budgie flocks as Budgie Fledgling Disease. So, again, not handfeeding these species in the same room as large psittacines is advisable.

Proventricular Dilitation (Macaw Wasting Syndrome) This is another latent infection that is on the rise because of the difficulties of diagnosis. Currently it is virtually impossible to identify either a bird that is in early stages of this disease or latently infected. It is a difficult diagnosis in a bird showing clinical signs. Because of this, you can test a new bird you add to your breeding stock for all sorts of things, quarantine, practice good husbandry and sanitation, and still introduce the virus into your flock. There are case reports of birds that appear perfectly normal, have a mate die of PDD, remain in isolation for as much as 10 years, and then when given a new mate the new mate dies of PDD. That’s scary! There is ongoing research at the University of Georgia to try to develop a vaccine and to develop better diagnostic testing. They are working on a DNA probe which promises to be as good a test as the DNA probes for chlamydia or polyomavirus. Right now you can only isolate birds that are tentatively diagnosed or which have had known exposure and wait for a good, accurate test of asymptomatic birds. When an accurate test is available, I’ll recommend new breeding stock be tested before being added to existing flocks.

Although PDD does occur occasionally in chicks less than a year old, it is mostly seen in older birds, suggesting that the disease takes months to manifest. Research at University of Georgia also has shown that birds given an experimental exposure to the virus can remain healthy and normal for over two years but still shed the virus in their feces routinely. It is unknown how many years that will turn out to be, but the case I mentioned above suggests at least 10 years. Research indicates that the virus doesn’t survive outside in the environment and it takes relatively close contact (although not direct contact) to transmit the virus. There is indication that parents pass the virus to chicks by feeding them and Dr. Ritchie at University of Georgia has told me that he suspects the virus can transmit via eggs based on cases he’s been involved with, but it hasn’t been proven yet.

This disease is rare in the small species of psittacines. It has been reported in budgies, but unlike a number of these other latent diseases, PDD is not likely to be brought into your flock with small species.

PBFD (Psittacine Beak and Feather Disease) This virus can be one of the causes of French Molt and is fairly common in the small species. Small species adults may look perfectly normal, but be infected and shedding the virus. In large species, the situation is similar to polyomavirus in that chicks get infected from the parents and spread the infection chick to chick in the nursery They may show early feathering problems, there is also a septicemic form with sudden death, or they may look perfectly normal until their first or second adult molt. Like polyomavirus, asymptomatic birds may clear the infection from their bodies in 3-4 months and a small percentage remain latently infected. With PBFD, however, latently infected adults usually begin showing clinical signs within 2-3 years. There are also indications that large psitticines can harbor and transmit the virus without ever showing clinical signs. There is a DNA probe test that is performed on blood or blood feather pulp that can accurately identify infected birds, whether asymptomatic or not. If an asymptomatic bird tests positive, keep it in isolation and retest at 4 months.

This disease can be prevented by raising small species separate from large species and by testing breeding stock.