Avian Viruses...
What They Are and How They Cause Disease

Understanding what viruses are and how they function will help individuals develop a perspective of how viruses can affect birds.Viruses are the most basic of all organisms. They contain only nucleic acid (DNA or RNA) and a protein coat cover. Some more complex viruses are enclosed in a protective envelope derived from the infected cells own cell membrane. Viruses require the help of other cells to reproduce. Bacteria, fungi, chlamydia are more complex and have a nuclear body (DNA or RNA) and cytoplasm that contain components that convert nutrients into energy to drive the functions of the cell. These organisms can reproduce on their own. The nucleus of a cell can be viewed as the control center and the cytoplasm the factory. Viruses have a control center but no cytoplasm therefore a virus is dependent on the cell it infects to provide the factory it needs to produce energy or reproduce itself. Viruses are very small. For example, it would take 21,517 Psittacine Beak and Feather Disease (PBFD) viral particles to cover the tip of a needle. These tiny particles can survive in the environment for months to years and can be spread to other birds in a home or aviary via contaminated fingernails, skin, hair, jewelry, cloths, food bowls, perches etc.

• Attachment of virus to a receptor on the bird's cell
• Penetration into the interior of the bird's cell
• Uncoating or removal of the protein coat releasing the viral nucleic acid into the infected cell
• Transcription/translation of the virus' nucleic acid, resulting in the infected cell producing new copies of the viral protein coat and nucleic acid
• Assembly of protein coat and nucleic acid into a new viral particle
• Release of the new viral particle from the cell.

The protein coat has binding sites that recognize receptors on the cells of susceptible bird species. A bird is considered susceptible to a viral infection if a receptor on its cell allows the virus to attach. If this receptor is not present then the bird will not be infected even if it has been exposed to the virus. For example, a duck may not be susceptible to a parrot virus or vice versa but an Amazon parrot would be susceptible to a Macaw virus. Disinfectants work by destroying these sites on the virus thus preventing the virus from initially binding to the cell.

• The virus can directly induce lesions in an organ by causing the rupture of infected cells
• The virus may stimulate the bird's immune response, which then destroys the virus-infected cells
• The virus may damage the bird's cells causing them to become neoplastic (cancer).
• The virus may establish itself in the bird such that the infected bird becomes a persistently infected carrier of the virus.

• The virus uses the cell's metabolic functions exclusively for the virus therefore the host cell starves.
• The virus produces products that are toxic to the cell.
• The virus ruptures the cell membrane thus destroying the cell.
• The virus alters the cell wall causing the cell to loose vital fluid and die.
• The virus may change the DNA of the cell causing it to become cancerous.

The result of exposure to a virus depends on the age, species and condition of the bird and the characteristics of the particular virus. A healthy bird that is exposed to a virus to which it is susceptible can become infected. The infected bird may remain clinically normal or it can become obviously diseased. If the bird remains clinically normal and develops a subclinical infection, it may destroy the virus and be protected from future infections, or it may remain persistently infected (carrier state). Likewise, a clinically infected bird (showing obvious signs of disease) may mount an effective immunologic response to destroy the virus and develop protection from future infections, or it may recover from the disease and remain persistently infected (a carrier) or it may die. Young birds are more susceptible to viral infections because their immune system is not fully developed likewise very old birds are more susceptible due to an age related decrease in immune system function. Birds are most resistant to disease during their reproductively active years. If all other factors are equal, a healthy bird exposed to a small number of viral particles is less likely to become ill than the same bird exposed to a large number of particles. Maintaining numerous birds in close proximity will increase the likelihood that a virus can be transmitted from bird to bird. Depending on the type of infection, birds may or may not produce antibodies. Antibodies produced in response to viral infection help clear the infection and protect the body from future infections.

• Preening
• Rubbing
• Inhalation of aerosols
• coitus (mating)
• insect or animal bites
• contamination of egg
• ingestion of contaminated feces
• contact with contaminated fomites
• ingestion of contaminated food or water
• ingestion by neonates of regurgitated food from infected parent
• fighting related injuries
• insects-mechanical vector/biologic vector
• from infected hen to egg in utero (while egg is in the reproductive tract)
• contamination of egg by infected hen immediately after laying

Diseases are frequently caused by the interaction of more than one type of infectious agent, including bacteria, fungi, viruses and parasites. A virus may cause damage to a bird allowing other pathogens (particularly bacteria and fungi) to colonize damaged tissues. The bird may clear the viral infection that initiated the disease process, and it is a bacteria or fungi that is detected on diagnostic testing of the ill bird. In these cases, the virus is referred to as the PRIMARY infection and the bacteria is referred to as the SECONDARY infection. Therefore, many birds that are diagnosed with bacterial/fungal infections may have initially had a viral infection.

Clinical abnormalities, necropsy findings and microscopic abnormalities may suggest that a bird was infected by a virus; however, clinical signs or lesions that absolutely confirm that a particular type of viral infection occurred are rare. A thorough necropsy with collection of necessary diagnostic samples should be performed on any bird that dies. Diagnostic samples may include portions of tissues placed in formalin for microscopic evaluation, samples of blood for bacterial or viral isolation and swabs or samples of abnormal tissues for viral, bacterial or fungal isolation.

The goal of maintaining any bird in captivity is to insure that it is in the best possible condition. Few effective vaccines exist against the viruses that infect companion and aviary birds. Careless exposure of a single bird within a flock can result in viral outbreak that may affect the entire flock.

• Use clean, disinfected feeding utensil for each bird
• Quarantine all new birds minimum 30 days if captive bred; 90 days for all others
• Isolated any sick birds ASAP
• Do not allow fecal contamination of food/water bowels
• Disinfect food/water bowls daily
• Disinfect cage area as often as possible
• Limit outside visitation of aviary
• Use disinfectant foot baths on entry/exit from aviary
• Veterinary evaluation of sick birds
• Necropsy examination of dead birds - Place bird in refrigerator as soon after death as possible - DO NOT FREEZE REMAINS - this damages tissues making microscopic evaluation difficult.
• Limit exposure to noxious fumes esp. cigarette smoke.
• Provide nutritionally, complete diet.
• Relatively stress free environment
• >Do not overcrowd
• Do not mix babies from different nests/parents
• Wash hands between handling birds
• Eliminate vermin
• Maintain good air circulation
• Reduce fecal and food accumulations

Viruses are susceptible to inactivation when they are outside the bird's body. The destruction or removal of the virus from the bird's environment is one of the best methods for control of viral transmission. This can be achieved by a combination of:

• Washing - to remove viral particles from the environment
• Painting - permanently fixes virus to the surface
• Disinfection - destroys the receptors that allow a virus to bind to a cell or destroys the viral nucleic acid

• Chlorinated compounds - widely available, inexpensive, low residual toxicity. In general a 1:32 dilution (1/2 cup bleach per gallon water) is sufficient. These compounds are rapidly inactivated by organic material and sunlight, require frequent mixing-every few hours-to maintain an active solution. The solutions and fumes they produce can be irritating to skin, eyes and respiratory tract therefore should only be used in well ventilated areas.
• Chlorine dioxide - similar to bleach and in some studies may be superior to bleach but is rapidly inactivated by organic material and sunlight.
• Chlorhexidine gluconate - relitively non-toxic to skin, often used to cleanse wounds, non-corrosive and good activity against many bacteria, yeast (Candida) and some enveloped viruses, however, should not be considered a reliable viricide. Inactive in presence of organic material and limited stability.
• Glutaraldehyde - rapidly inactivate many bacteria (including mycobacteria-avian tuberculosis), viruses and chlamydia, active in presence of organic debris, and stable as working solution from 2-4 weeks. These compounds are infrequently used because of widespread side effects of irritation to skin, respiratory tract and eyes especially following long term exposure.
• Iodines - used for cleaning wounds and skin and is effective against many bacteria (not Pseudomonas), some viruses and fungi. Rapidly inactivated by organic material.
• Phenols - inactivate many bacteria (including mycobacteria and Pseudomonas), fungi and some viruses, inexpensive but are irritating to skin, eyes and respiratory tract, toxic to cats and reptiles.
• Quaternary Ammonium Compounds - inexpensive, relatively safe, and inactivate many bacteria, some viruses and chlamydia. May be inactivated by organic debris, and are ineffective against spores, mycobacteria, fungi, nonenveloped viruses and Pseudomonas. Hard to rinse from some surfaces and leave a slimy residue. Not recommended for use on objects that would be in direct contact with birds i.e. feeding utensils, food or water bowls.
• Wood tar distillates - low toxicity but poor disinfectant.
• Alcohols - 70% ethyl alcohol inactivates many bacteria and viruses but require minimum of 20 minutes contact time. Dissolve plastics, glues, and rubber.

Few vaccines have been developed and tested for use in companion and aviary birds. When a bird recovers from a viral infection, it does so because its immune system produces specific antibodies and specialized immune system cells that react to the surface proteins that compose a virus' coat or envelope. Vaccination is intended to induce a similar immune response. Depending on the particular virus, a vaccine may initiate an immune response that will prevent the virus from infecting the bird or it may allow a bird to be infected but produce a minor, rather than a severe, disease. In either case, the bird should develop an active immune response following vaccination. Over time, the response will decrease and administration of a booster vaccination will be necessary. Vaccines may fail to produce effective immune response if the vaccinate bird is immunocompromised, already infected, or undergoing immunosuppressive therapy (steroids, some antibiotics). Any vaccine may be expected to cause an inappropriate reaction in a certain percentage of birds. These reactions can vary in severity from a sore/scab at injection site to lethargy or rarely death.

• Infection - process of a virus entering a bird's cell and reproducing
• Disease - abnormal changes that occur in the bird's cells as a result of infection. The severity of the disease that occurs depends on the bird's age, nutritional status, stress level and environmental conditions.
• Carrier state - a subcliniclly infected bird that shows no outward signs of virus infection but can spread the virus to other birds.
• Types of Infection:
  • Subclinical - No signs of disease. Immune system controls infection and birds develop antibodies.
  • Peracute - Very rapid progression of disease. Infected birds recover or die within hours to days of infection. Recovered birds usually develop antibodies.
  • Acute - Rapid progression of disease. Infected birds recover or die within days to weeks after infection. Recovered birds usually develop antibodies.
  • Persistent - Chronic, Latent, or Slow
    • Chronic - Long-term infection with persistent shedding of virus. May persist for months to years. Birds may or may not develop antibodies.
    • Latent - Long-term infection with intermittent shedding of virus. May persist for months to years. Birds may or may not develop antibodies. No disease or shedding of virus during dormant state. Birds shed virus and may develop disease when virus is activated.
    • Slow - Progressive disease over months to years eventually causing the death of the bird. Virus reproduces over a long time period. No signs of disease early on.
• Fomite - Inanimate object responsible for spread of disease i.e. cage toy, cleaning utensils, feeding syringes, clothing, human skin.
• Biologic vector - living organism that becomes infected with virus from bird then carries that virus to another bird i.e. Western Equine Encephalitis infects mosquito when mosquito bites horse then infects bird when same infected mosquito bites a bird.
• Mechanical vector - living organism that spreads virus by carrying it on its body from one bird to another i.e. poxvirus infections
• Disinfectant - will destroy many disease causing organisms on the surface of inanimate objects. Some organisms and heat-resistant spores may be resistant.
• Sterilant - will destroy all microbial organisms including heat resistant spores. Achieved by autoclaving, boiling or exposure to toxic chemicals.
• Germicide - Kills a specific group of organisms when used as directed.
• Sanitizer - reduces surface microorganisms to an acceptable level on inanimate objects.
• Antiseptic - reduces surface microorganisms to an acceptable level on skin.

Active compound and trade names of common disinfectants.

From: Ritchie, WR. Avian Viruses: Function and Control. Wingers Publishing, INC. Lake Worth, Florida, 1995.
The majority of information in this article was obtained from Ritchie, WR. Avian Viruses: Function and Control. Wingers Publishing, INC. Lake Worth, Florida, 1995.

Psittacine Beak and Feather Disease

Psittacine Beak and Feather Disease (PBFD) is a chronic disease characterized by feather dystrophy and loss, beak deformity and ultimately, death. The cause of PBFD is a newly discovered virus in the family Circoviridae. The disease probably originated in Australia and spread to other continents as a result of the shipment of birds for the pet bird trade. The disease has been reported in Australia, North America, Europe and Asia. Most species of parrots are susceptible to the virus however the species most commonly affected are cockatoos, African Grey Parrots, Eclectus Parrots and Lovebirds. The first clinically detectable sign of PBFD is the appearance of necrotic, abnormally formed feathers. Generally, PBFD is a disease of young birds (up to 3 years of age) however, it has been reported in birds over 20 years old that had been clinically normal most of their life. Peracute, acute and chronic disease patterns occur.

• Peracute - occurs in neonatal parrots. Clinical signs include diarrhea, lethargy, rapid weight loss and sudden death. Appears to be particularly common in young cockatoos and African Grey Parrots. Birds may die before any signs of abnormal feathers are seen. A correct diagnosis may be missed in birds that die from a peracute infection if a complete necropsy and thorough microscopic analysis of the tissues are not performed.
• Acute - occurs in young birds during the time of their first feather formation after the replacement of neonatal down. These infections are characterized by several days of depression followed by sudden changes in developing feathers including: necrosis, fracture, bending, hemorrhage, or premature shedding of diseased feathers. The severity of feather changes depends on the number of feathers in a developmental stage at the time of infection.
• Chronic - occurs in birds that survive the acute phase of the disease. There is a progressive appearance of abnormal feathers during each molt. Signs include: retention of feather sheaths, hemorrhage within the pulp cavity and fractures of the feather shaft. Short clubbed feathers, curled feathers, and circumferential constrictions of the feather shaft may also be present. If birds live long enough, they develop baldness as feather follicles become inactive. Certain species of birds may develop abnormally colored feathers. PBFD positive African Grey Parrots may show red body feathers in place of the normal gray feathers. Typically, beak deformities occur in birds following a protracted course of PBFD where substantial feather changes have occured. These changes include progressive elongation of the beak with necrosis of the palate area and transverse or longitudinal fractures of the beak. Usually, the upper beak is more affected than the lower beak.

The virus is spread from hen to the egg, hen to chicks being parent raised, and inhalation or ingestion of infected feather dust or feces. Viral particles in feather dust or dried feces can easily spread on clothing, nest materials, feeding formula or feeding utensils, nets, bird carriers, food dishes and air currents. The virus particles can remain viable in the environment for months, long after the infected bird is gone. Many birds infected with PBFD die with in 6-12 months of onset of clinical signs however, some birds have been known to survive 10-15 years. Death usually occurs from secondary bacterial, fungal, parasitic, chlamydial, or viral infections.

PBFD should be suspected in any bird that shows progressive feather loss and abnormal feather development. However one can not determine that a bird is infected with the virus just by examination of the feathers. Other disease processes can result in feathers with a similar appearance. Any factor that disrupts the blood supply to the developing feather, including trauma; bacterial or fungal infection of the feather follicles; other viral infections; malnutrition; hormone reactions; and some drug reactions, can cause feather changes similar to those seen with PBFD. Identical feather lesions to those caused by PBFD can be produced by pinching developing feathers at or near the level of the pulp cap. On the other hand, birds can carry the virus in their blood but have perfectly normal feathers. Until recently, the primary method of diagnosing PBFD was the demonstration of viral particles in the cells of the feather follicle and /or shaft. This required a surgical biopsy of the affected feather and its associated follicle. Since PBFD virus does not affect all feathers at the same time, this test could give a false negative result if the tissue sampled did not contain the virus. A more recent test, DNA probe analysis, tests for the presence of PBFD virus in the bird's blood. This is the only effective method available for detecting the PBFD virus in a bird before feather lesions are present. The test requires a small amount of blood. Some birds may become infected with the virus, test positive, remain infected but never show clinical signs. Other birds, become infected, test positive, fight off the infection and subsequently test negative. Therefore, it is recommended to retest any PBFD positive bird 90 days after the initial test. If this bird tests positive a second time then it is infected however if subsequent test is negative, then the bird was able to eliminate the virus and is not infected. Since the virus survives in the environment, the DNA probe can also be used to test samples of feces and/or feather dust taken from the surfaces in the environment.

If a bird from a breeding aviary tests positive for PBFD, remove the bird from the environment as quickly as possible. The bird can potentially shed large numbers of viral particles that can be spread to other birds by wind or on cloths, skin, hair or inanimate objects. All areas, supplies, and equipment should be cleaned, disinfected then tested with the DNA probe as mentioned above to determine if any infective material remains. In addition, I recommend testing of all birds exposed to the positive bird as well as any offspring of the infected bird. Retest the positive birds in 90 days to determine if the birds were permanently infected.

If a companion bird tests positive, then the bird should never be exposed to other birds outside of the household. Many infected birds can live a long life when provided a healthy, relatively stress free environment, good diet and regular veterinary care. Remember, you can carry the virus on your cloths.

To prevent spread of this disease, DNA probe testing is recommended on all cockatoos, Lovebirds, Eclectus Parrots, African Grey Parrots and any birds showing feather lesions consistent with PBFD as part of a routine pre-purchase examination, prior to sale from an aviary or pet shop and prior to entry in to a new home/aviary. Research is ongoing for development of a vaccine against PBFD.

The primary information source for this article was the text book written by Dr. Branson Ritchie, Avian Viruses: Function and Control published in 1995 from Wingers Publishing Inc., Lake Worth, Florida. Branson Ritchie, DVM, PHD is an Associate Professor of Avian and Zoologic Medicine at The University of Georgia in Athens. He and his associates have been primarily responsible for the research into the cause of PBFD and for development of testing procedures for detection of this virus and are presently working on a vaccine for prevention of this disease.