The most common causes are:
Aspiration pneumonia due to a kitten inhaling a substance which irritates the lungs. Usually this is milk which has "gone down the wrong way" perhaps due to inexpert hand-rearing or using unsuitable equipment. For this reason, bottle feeding is safest for kittens which can suck. Eye droppers or syringes should only be used if there is no alternative and with the utmost of caution. If a kitten does inhale milk or is rescued from drowning, prompt clearing of the lungs can prevent aspiration pneumonia. Hold the kitten firmly, and shake it sharply downwards supporting its neck (much like clearing a baby’s lungs after birth).
Bacterial pneumonia is caused by a bacterial infection and must be treated with antibiotics as prescribed by the vet. Viral infections can cause pneumonia or can make a kitten susceptible to secondary bacterial infection. Antibiotics cannot kill viruses, but are prescribed to treat those secondary infections.
Symptoms vary depending on the cause and severity of the pneumonia. Aspiration pneumonia occurs very rapidly. Breathing becomes labored and irregular. The kitten may gasp for breath, become limp and have bluish mucous membranes due to lack of oxygen reaching the tissues. The kitten also develops a fever. These kittens must be treated by a vet if they are to survive. They require tube feeding and possibly injected fluids as well as antibiotics. They may also require oxygen to compensate for breathing difficulties.