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Mastitis (inflammation of the mammary gland) occurs when the lactating queen's mammary gland(s) becomes inflamed, blocked or infected. Mastitis 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 an 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: If the milk duct is not properly cleared blockages may occur. These 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 is often seen at weaning.
- Pain, heat, and swelling of the affected gland(s)
- The milk may be bloody, yellow or thick
- The queen may refuse to let her kittens nurse from the affected gland
- The queen may become depressed and lose her appetite and become dehydrated
- The queen may be lethargic
- 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.
Mastitis can come on quickly and without warning which is why it is important to keep a watchful eye over the queen's mammary glands during lactation so quick action can be taken if any changes are observed.
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 permit 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 it may be recommended that the kittens continue to nurse from all but that gland, the physical condition of the cat, and if the mastitis is septic or non-septic. What does appear to be commonly recommended is to avoid kittens nursing from a gland which has become infected, as infection can be passed onto the kittens, which can result in sickness or death. 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.
How is mastitis diagnosed?
- A tentative diagnosis may be made on the clinical signs your cat is displaying.
- Bacterial culture and sensitivity of the affected milk.
- Microscopic examination of the discharge for the presence of white blood cells.
Treatment depends on the severity of the condition.
- Broad spectrum antibiotics may be given to the queen until bacterial culture results are back in, and then a more appropriate antibiotic will be given.
- Pain medication may be given to the queen.
- Treatment of dehydration if necessary.
- Kittens may also be prescribed antibiotics.
- Abscesses will need to be lanced and drained.
- The gangrenous tissue will need to be debrided (removal of necrotic material).
- Application of a warm compress several times a day to assist with milk drainage.
- Manually expressing of the affected mammary glands to remove infected milk.
- If nonseptic mastitis occurs at weaning, reducing 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