Dr. Branson Ritchie
Pacheco's disease (PDV) has been reported in psittacine birds from North and South America, Africa, Europe, Australia and Asia. The clinical and microscopic changes that are called Pacheco's disease can be caused by any one of at least three distinct psittacine herpesviruses. It is probable that many other psittacine herpesviruses will be identified. Birds exposed to virulent strains of PDV usually develop clinical signs or die within 3 to 14 days after being exposed to the virus.
Most species of free-ranging and domestic birds are considered susceptible to some strain of herpesvirus, but thus far PDV has only been shown to cause disease in psittacine birds following natural exposure. This is typical for herpesviruses, which tend to be highly host specific. The herpesviruses that cause problems in humans are not known to infect psittacine birds and PDV is not known to infect humans.
Disease progression can vary widely depending on the virulence (aggressiveness) of the infecting strain of virus and the species and condition of the infected bird. In general, Old World psittacine birds (cockatoos, African grey parrots, lovebirds, budgerigars) are considered more resistant to severe forms of the disease than New World psittacines (macaws, Amazon parrots, conures). However, susceptibility does vary widely among individual species within a group.
The fact that many healthy psittacine birds have antibodies against PDV suggests that some infected birds mount an appropriate immune response that prevents the virus from causing progressive disease. It is safest to assume that survivors are infected for life and can intermittently shed the virus. This type of persistent infection is called latency.
Some infected birds die without showing any signs of disease while others die shortly after clinical changes are first noted. Clinical signs may include depression, anorexia, diarrhea (which may or may not contain blood), regurgitation and yellow-green urates suggestive of liver damage. In the final stages of the disease, birds frequently show signs of nervous system damage including difficulty standing or moving, body tremors or seizures. Most psittacine birds die within several hours to two days of showing clinical signs. Birds infected with less aggressive strains of the virus are more likely to survive than those infected with virulent strains, particularly if clinical signs of disease occur. The clinical changes associated with Pacheco's disease may also be caused by bacterial liver disease, chlamydiosis, salmonellosis, liver toxins, avian polyomavirus, reovirus and adenovirus.
Virus spread depends on many factors – the hygiene in the aviary, the species of exposed birds, the distance between enclosures, the strain of the virus and the condition of the flock. Most birds are infected with PDV after they ingest contaminated excrement. Thus, aviary hygiene is critical in preventing PDV outbreaks.
PDV is most frequently transmitted through direct contact with feces, or respiratory secretions from an actively infected bird. Crowding, poor air circulation, accumulation of excrement and stacking of enclosures increase the likelihood of PDV transmission from infected to susceptible birds. This organism is not considered stable when outside the host and is probably inactivated without hours to days in most conditions. Cool, moist conditions and freezing will preserve the infectivity of most viruses.
Pacheco's disease outbreaks usually occur when a new bird has been added to an established group or when stressful events (such as breeding, unseasonable climatic conditions, malnutrition, storms, fires, rodent infestations, etc.) cause a latently infected bird to begin shedding. Exposing unvaccinated birds to situations where they might have encountered birds that were shedding the virus (such as bird fairs or sexing clinics) can also cause an outbreak.