They can be susceptible to lice, mites, fleas and worms. One week a month add Apple Cider Vinegar to their drinking water. About 20ml to every litre. This helps with all internal parasites and overall general health.
Normally, the dust baths that chickens take in a dry patch of dirt or sand, preferably enhanced with some wood ash and food-grade diatomaceous earth, will keep them mite-free, but often in extremely hot and humid areas where mite populations flourish or during long periods of wet weather when the chickens might not have the opportunity to bathe, the mites will get the upper hand.
Sprinkle food-grade Diatomaceous Earth and fireplace ash in the dust bath area, on the floor of the coop and rub some into the roosts.
Feed hens raw minced garlic.
Spray hens with a 10% garlic juice/water solution.
Mix 1 Cup of cooking oil, 2 Cups of water and 1 T dishwashing liquid and spray coop interior.
Mix cooking oil and water and leave a bowl in your coop to 'trap' the mites.
Mix garlic juice, thyme and lavender essential oils into water (1/10 ratio oils/water)
(Poultry scientists found this mixture achieved a 100% kill rate over 24 hours)
Bay, cinnamon, clove, coriander or spearmint essential oils in water sprayed on the hens.
This also deters other pests