Of all the possible causes for extensive hair loss in cats, one is perfectly normal. This is what is commonly referred to as "blowing coat". It can be difficult to discern what the cause is when massive shedding occurs. Not all hair loss is due to normal blowing coat. History and close examination can help determine likely causes, and diagnostics available to your veterinarian can further identify a definitive diagnosis. It is important to pursue a cause if you are confident that the hair loss is not a normal "blowing coat" event.
Cats have four types of hair. Three are specific to the coat, while the fourth, whiskers, are of course the tactile hairs on the face. The coat is made up of guard hairs, awn hairs, and undercoat. Guard hairs, also known as primary hairs, are the longer, courser hairs that act not only to help insulate, but to provide another sense of touch for spatial information. Awn hairs are thin, medium length hairs that primarily serve to protect the body and for insulation effects. The undercoat is mostly composed of secondary hairs, which are the thinnest and shortest, and the most critical to maintaining body temperature.
Blowing coat is not the same as routine shedding when applied to strictly indoor cats. An indoor environment typically provides a more uniform temperature and photoperiod, especially in distinct seasonal regions. Because of that, indoor cats tend to shed more consistently throughout the year, rather than the more extensive spring and fall shedding experienced by outdoor cats. A longer coated cat that lives outdoors may show more extensive coat loss in the spring, more closely appearing to blow coat.
The normal growth and loss of hair is usually what dictates the occurrence and rate of shedding. An exception to this is when hormone fluctuations impact that rhythm stimulating a more sudden and heavier loss of coat that can be blowing coat. This is particularly evident for an intact queen who has just been pregnant. Blowing coat can be a normal, healthy side effect of those reproductive hormones. What is not normal are hormone fluctuations such as diabetes mellitus, hyperthyroidism, or excessive levels of adrenal hormones, also known as Cushing's disease.
Normal hair growth and loss follows a progressive shift from anagen (stage where the hair follicle is actively producing hair) to catogen (new hair production ceases and follicle becomes dormant) and finally to telogen (a transitional stage where the follicle becomes active again in preparation for the anagen phase). Eighty to ninety percent of the lifecycle of a hair is spent in the anagen phase. All the hair on the body will be in one of these three phases concurrently. The percentage of hairs in anagen, or catagen or telogen on any given day determines when shedding occurs, the length of the hair coat, and the extent of the undercoat.
Breed differences account for the variations in hair length, amount of undercoat, and degrees of shedding. Persians have more awn hairs and secondary hairs relative to guard hairs, than sphinx cats that have virtually no guard or awn hairs, with slow growing secondary hairs, and the vast majority of hairs are in the telogen stage.
Age also influences the type and quality of coat in cats. Kittens have undercoat with virtually no guard hair. This is why they are so soft and fluffy.
Spaying or neutering cats will impact the coat as well as it directly impacts hormones. Some adult cats that have been fixed will have more undercoat than other intact cats within their breed.
Other events that can trigger extensive, sudden hair loss include fever, stress, anesthesia, surgery, and shock. The hair loss does not happen at the time of the event however, and may be delayed 1 to 3 months after the fact. That is because the hormonal signals that dictate the normal hair growth rhythms get interrupted which then changes large numbers of hair follicles to the telogen stage, resulting in handfuls of hair falling out. This is referred to as telogen defluxion. It is necessary that the recent history of the cat be evaluated to determine if some stress or unusual event triggered the hair loss, particularly is there is not skin disease associated.
Cats' hair coat requires protein to maintain normal structure and function. Up to 35% of the daily protein intake is used for growth and maintenance of the coat. This is extremely important for long-haired cats. Other nutrients that directly impact skin and coat health include omega-3 fatty acids EPA and DHA, omega-6 fatty acids, vitamins, certain amino acids, chelated zinc and copper; and a synergistic complex of specific antioxidants.
Extensive hair loss in cats can represent a disease condition, ranging from external parasites, to ringworm, allergy, illness or hormonal imbalance and a few things in-between. External parasites such as fleas, demodex mites, cheyletiella mites, and otodectes (ear mites) are also possible causes. Two species of Demodex that can be tricky to diagnose are Demodex gatoi and Demodex cati. Occasionally Demodex cati is found with generalized demodectic mange, which can also flag another, underlying, disease such as diabetes or viruses such as feline leukemia or feline immunodeficiency virus.
In cases where hair loss is due to blowing coat, the skin beneath is healthy. If the skin exposed by hair loss is scabby, especially oily, infected, etc, it is not a case of blowing coat.
In the event of blown coat due to pregnancy, or lactation, the coat will re-grow fully in a couple months. Extensive hair loss from other causes may take up to two years to fully re-grow.