Coccidiosis Coccidiosis is caused by protozoa, unicellular parasites. Endoparasites Worms living in the intestines of chickens fall mainly into four categories. Blackhead A protozoan parasite, Histomonas meleagridis. Riboflavin Young chicks, as early as 1-week-old, exhibit curling of the toes, inability to walk and sometimes diarrhoea. Vitamin E Deficiency Vitamin E deficiency in chickens affects the brain. Vitamin D3 Deficiency (Rickets, "Rubber Legs") Rubber legs Gapeworms hit free range flocks While gamekeepers are constantly on the lookout for an attack of gapeworms in pheasants and partridges being reared for commercial shoots. Chronic respiratory Disease (CRD) (Airsacculitis) The underlying cause of CRD is Mycoplasma gallisepticum (Mg). The condition is frequently triggered by respiratory viruses such as ND and IB and subsequently complicated by bacterial invasion. Chickens and Turkeys Infectious Coryza The bacterium causing this disease is Hemophilus paragallinarum. Species affected Chickens appear to be the only natural hosts of H. paragallinarum. Aspergillosis (Fungal Pneumonia) The disease is caused by a fungus, Aspergillus fumigatus. Species affected Young chicks are very susceptible. Turkey poults, pheasant chicks, quail chicks, ducklings, and goslings may also become infected.