Fever coat is often associated with pregnant or nursing gerbils, or their runty pups. Occassionaly gerbils on very poor diets or parasites can develop similiar coats. It is usually caused by a nutritional deficiency. It is most obvious on black gerbils, but other self colors are noticably affected.
In pregnant or nursing females, the growing pups demand too much from the mothers reserves, depleting her of vitamins, minerals, and fat. The fur will grow in normally after her last litter is born. Some lighter colors like Lilac are more prone to fading while the mother is pregnant, or nursing.
Runty pups who are signifigantly smaller than their siblings often struggle to obtain enough milk in the early weeks as their coat starts to grow in. Although they get much stronger once they start eating solid food and are able to consume enough food to grow a proper coat, often the coat is fully grown in by the time they recover. Often this causes black gerbils to look slate or blue colored. Upon molting their first coat they'll grown in their proper coats.
Gerbils with a parasites can often have poor coats as well.
Photo by Carraig Stanwyck