In the last decades, pigeon AIDS - a disease complex caused by circovirus attacking the immune system - is getting more and more widespread. Unfortunately there is no vaccine against circoviruses yet so the only way of defence is to follow the rules of disease control and hygiene.
The virus spreads mainly by droplet transmission but nestlings can be infected via crop milk as well. Following the peroral or aerogen infection the virus breaks into the bloodstream and causes viraemia. Then it starts replicating in almost every organ with different intensity. It mostly infects the leukocytes of young pigeons, primarily in the thymus and bursa, thus damaging the animals' immune system and making way for secondary infections (bacterial, fungal, parasitical and viral). The virus is also able to replicate in epithelial cells, so in the bowel too. As a result, it is excreted into the environment by dropping, further infecting the whole loft this way.
The virus causes the most serious problem when it infects young (a few weeks or months old) pigeons. The clinical course is determined by the secondary infections (bacterial, fungal, parasitical or viral) caused by the absence of leukocytes and as a result, a weak resistance. A good example of this is the defect seen in pictures where symptoms of infection caused by Trichomonas parasites can be seen in case of circoviral infection. Accordingly if, for example the animal is infected by Salmonella then the leading symptoms will be pathological thinness, lethargy, watery stool and nodule formation which can be seen during dissection, whereas in case of infection by Paramyxovirus we will see the neurological symptoms that are the characteristics of this virus. As the virus paralyzes the pigeon's immune system, the animals often die of banal diseases. Tumour formation throughout the body is also considered as a common symptom.
Figure 1 a) and b): Oral ulcers covered by yellowish accretions due to the infection caused by circovirus are frequent. This bird is also suffering from a disease caused by Trichomonas.
Figure 2; Dissection finding: Circovirus infection with secondary Trichomonas infection.
MWe are frequently faced with diseases against which the loft has been vaccinated (e.g pox, paramyxovirus). The reason behind this is that the inadequately working immune system is incapable of giving a proper immune response to vaccination. Therefore vaccines that would otherwise be effective and are of exceptional quality and vaccination protocols advised by the manufacturers do not provide the necessary result. In this case it is advised to introduce double vaccination (done in a 3-week period) in lofts infected by circovirus.
The immune system of older pigeons is different from that of the young ones. Due to this, the main place for virus replication is the bone marrow and the epithelial tissues so the forming disease is different as well. As a result of replication in the bone marrow the number of leukocytes greatly decreases. The greatest scale of loss among leukocytes is that of natural killer cells, thus the most frequent form of disease is generalised tumour formation throughout the body. Tumours can be formed even at the strangest places which may lead to diseases that are quite astounding. One of the most memorable cases of our laboratory was when we were examining an animal that was thin to the bones. The owner claimed that the pigeon had good appetite but it vomited the food previously eaten. During the pathological dissection we found out that there had been a tumour induced by circovirus under the crop which had been blocking the way of food so it couldn't reach the intestinal tract.
Due to replication in epithelial tissues, abnormal feather formation may occur. This is visible mainly at the time of moulting when a large number of feathers are stuck in the calamus, or feathers with missing vanes or unopened feathers appear. In the calamus and the surrounding epithelial tissues inflammation - caused by secondary bacteria - occurs frequently due to the replication of the virus. Because of secondary infections and epithelial degradation, bleeding occurs frequent thus bloody feathers may appear on the animal. This symptom is important for differential-diagnosis as infection caused by quill mites may also lead to similar problems.
Figure 3; Abnormal feather formation caused by feather follicles infected by circovirus. The feathers produced by these inflamed follicles are characterised by retained feather sheath and blood and tissue debris spread into the calamus. These feathers can be pulled easily from the follicles.
Figure 4: Abnormal feather formation
Only the suspicion of circovirus infection can be verified with traditional pathological examination. For the exact diagnostic of the virus, special laboratory examinations are needed. Due to the aggressive replication of the virus it is detectable in every infected tissue of the pigeon (liver, spleen, kidney, thymus, lung, etc.). Special techniques are needed such as PCR or electron microscope for this purpose. The virus can be made visible by using the latter method. However, it is a time and resource consuming method. In contrast, the PCR is more affordable and provides results in a few hours and is even suitable for the examination of live animals because it is able to detect the circovirus from blood and dropping. The most beneficial and simple way is to detect the circovirus from dropping as sampling is simple and there are no specific transport conditions as the circovirus is very resistant and is excreted with dropping in a great amount.
APrevention by vaccines against porcine circovirus has been successful for years now and scientists are working on vaccines against pigeon circovirus. Unfortunately, currently there is no vaccine to prevent circovirus in Hungary or anywhere else in the world. However, we are glad to inform you that our laboratory has been taking part in the development of a vaccine for 5 years now. In 3-6 years the first experimental vaccines are expected to reach such a stage that after registration they will be available for commercial use.
Due to the above mentioned reasons there is a particular emphasis on classical disease control which can be summarised as follows. In case of lofts free of circovirus the aim is to maintain that state. Therefore it is necessary to quarantine new animals (for a minimum of 21 days). They need to be put in a separate room, far from the rest of the loft. During the time of the quarantine neither the new animal, nor its dropping, feather or any other object contacted by the animal (feeder, drinker) can be reached by the rest of the loft. Wild or extraneous pigeons should be excluded from the loft as well. the screening of new animals should be performed during the quarantine period because even one infected pigeon can easily harm the whole loft or make it difficult to maintain the good status of a pigeon loft.
In case of lofts infected by circovirus the aim is to moderate the loss. As circovirus significantly decreases the resistance of pigeons, the emphasis should be on avoiding secondary infections (bacterial, fungal, parasitical and viral) and other immune suppressive effects (parasitical, stress). As an immune system weakened by circovirus is unable to give a proper immune response to vaccination, the animals should be vaccinated twice a year - differring from the vaccination protocol advised by the manufacturer - in order to avoid other diseases. Furthermore, we can do a lot for the infected loft by providing the best possible housing and nutrition conditions for the animals. To sum it up, it can be said that a prudently designed loft-health programme should be applied in the case of lofts infected by circovirus. This should include the constant control of every pathogen and stress effect endangering the loft's resistance based on laboratory examination, the properly timed treatments and vaccinations, and the complete observation of disease control rules, including quarantine. By taking all of this into account, a good performance can be reached even in the case of lofts infected by circovirus.