Black spot - derived from an article by Dr J. Vanderborght (Belgium) - Black spot is caused by the circovirus. Most of the adult birds already have a competent immune system when they are infected. This means that they can develop protective antibodies over a period of some weeks and these will then clear the body from the virus.
Young chicks are most susceptible when there is an outbreak and the mother has not been able to build sufficient protective antibodies to pass on to her eggs. As the hen is not yet cleared from the virus, the virus will replicate in the egg, destroying the immune system, and whatever secondary infection will kill the newly hatched chick.
The enemy to a healthy immune system is stress as the latter gives rise to immune suppression. The onset of the breeding season always brings stress in the breeding room. Immune stimulation measures are OK, but at low dose and then applied intermittently.
Secondary infections will also weaken the immune system.E. Coli is a normal inhabitant of the birds intestinal flora, but it has been shown that domesticated birds do have higher levels in their intestines than their wild relatives. Levels can be reduced with prebiotics, probiotics and good hygiene protocol. Addition of 5-10% of egg yolk powder also has shown to reduce E. Coli numbers, probably because of the protective antibodies they contain.
Black spot is no disaster, except for massive outbreaks, which seem to be the exception rather than the rule. Till now no vaccine is available. Understanding the disease is very important as to prevention and/or interpretation of what we see.