Chickens’ guts almost always have some worms. This isn’t a problem until the worms become too many and cause health issues, decreased laying, and even death.
How to know a chicken has worms
A chicken with too many worms will pass yellow foamy diarrhoea.
The Chicken worms often are ingested when the bird swallows chicken dropping that has worm eggs or larvae in it. The more chickens stay enclosed in the same area, the higher number of worms in the area and hence in the chickens.
How often should chickens be dewormed?
Chicken should be dewormed the first time at 8 weeks and monthly thereafter. It is almost impossible to have a worm specific Anthelmintics (dewormer). Instead what you get are broadspectrum dewormers. Since each dewormer targets a different type of worms, it is important to keep changing antihelminthics. For example, if in the first month you use a piperazine based dewormer such as Piperin, Piperamentric, Ascarex, Then change to levamisole such as Ergamisol. The following month use fenbendazole and so on.