Mastitis (inflammation of the mammary gland) occurs when the lactating queen’s mammary gland(s) becomes inflamed, blocked or infected, it can also occur in cats who have had a pseudopregnancy.
Mastitis can affect a single gland or multiple glands. It is a medical emergency and requires immediate veterinary attention.
Septic mastitis: Damage to the nipples as a result of constant sucking from kittens and abrasions from sharp kitten paws and teeth allow bacteria to enter the mammary gland via the teat. Streptococci, staphylococci, and E. Coli are usually involved in mastitis. 
It is also possible for infection elsewhere to spread to the mammary glands via the bloodstream.
Acute septic mastitis: Infected mammary glands may develop an abscess or become gangrenous.
Nonseptic mastitis: Blockages can occur if the milk duct is not properly cleared which can cause milk to pool in the mammary gland. This forms an ideal environment for bacteria growth and can lead to an infection (septic mastitis). Nonseptic mastitis often occurs when the kittens start to wean.
- Pain, heat, and swelling of the affected gland(s)
- Milk which is bloody, yellow or thick
- Refusal to let the kittens nurse
- Depression or loss of appetite in the queen
- Sick or dying kittens
Symptoms may not be apparent in nonseptic mastitis. The affected gland(s) may be hot, swollen and painful but the cat remains healthy and alert.
The veterinarian will perform a complete physical examination of the cat and obtain a medical history from you. A tentative diagnosis may be made on the clinical signs your cat is displaying as well as a recent history of giving birth. Tests will be necessary to confirm diagnosis which can include:
- Bacterial culture and sensitivity of the affected milk.
- Microscopic examination of the discharge for the presence of white blood cells.
Mastitis is a medical emergency, and you should take your cat to the veterinarian immediately.
There seem to be two different schools of thought in regards to allowing kittens to nurse from a queen with mastitis. Your veterinarian is the best person to advise on how to proceed if you should allow the kittens to continue to nurse from the affected gland(s), or from the queen altogether.
It seems that the decision is based on several factors:
- If the mastitis is confined to one gland, your veterinarian may recommend that the kittens continue to nurse from all but that gland.
- The physical condition of the cat.
- If the mastitis is septic or non-septic.
What does appear to be agreed on is do not allow kittens to nurse from the affected gland, as infection can be passed onto the kittens. Only your own veterinarian can recommend which is the best and safest way to proceed.
If your veterinarian does recommend kittens cease nursing from the mother it may be temporary until she can recover, or permanent. Either way, if the kittens are too young to wean, you will have to bottle feed them with a specially formulated milk designed for kittens. Cow’s milk is not an appropriate substitute for kittens.
- Broad-spectrum antibiotics until bacterial culture results are back in, then a more appropriate antibiotic.
- Pain medication where necessary.
- Fluid therapy to treat dehydration.
- Lance and drain abscesses, debride necrotic material, and oral antibiotics.
- Application of a warm compress several times a day to assist with milk drainage.
- Manually express affected mammary glands to remove infected milk.
- If nonseptic mastitis occurs at weaning, reduce water and food intake may assist in drying up the milk supply. 
 Feline Husbandry; Diseases and Management in the Multiple-Cat Environment: Niels C. Pedersen.
 The Merck Veterinary Manual