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This information is provided by Provet for educational purposes only.

You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

A "runt" is the smallest and weakest young animal in a litter...but why do they occur ?

In a large litter of animals there are usually some individuals that are larger at birth than others, and often there are one or two much smaller individuals. The smallest and weakest youngsters are called "runts", but why do they occur and what are the consequences of being the smallest ?

There are many factors that influence the eventual size of a fetus at birth :

  • Genetic inheritance - predetermines an individuals growth rate and birth size
  • Injury to genetic material in the fetus during pregnancy- eg exposure to XRay radiation
  • Nutrition of the fetus in the uterus
  • Nutrition of the mother during pregnancy (especially late pregnancy)
  • Exposure of the mother to abnormal environmental conditions
  • Developmental abnormalities in the fetus
  • Exposure to toxins
  • Exposure to infections

Low birth rate is an important risk factor in cats for feline wasting syndrome, and in dogs for fading puppy syndrome. Both of these syndromes are associated with increased disease and premature death. In the wild the weakest members of a litter usually die from poor nutrition, disease, by being rejected by the mother or by being caught by a predator. The weakest members of a litter are often more susceptible to infectious diseases due to impaired immune system function. Of course,  in our caring society "runts" are often preferentially selected by potential owners because they look so helpless, and attract sympathy.

Following birth there are several factors that can lead to poor growth rate :

  • Inadequate food intake - insufficient food available, or competition for food from stronger littermates, congenital problems which prevent normal eating or digestion and absorption of food. eg cleft palate, intestinal disease
  • Poor food quality (energy or nutrient deficiency)
  • Infections
  • Congenital abnormalities and diseases
  • Parasitic infections eg toxocara cati, toxocara canis.

All undersize animals should be examined by a veterinarian to eliminate the presence of a serious terminal disease. If there is no evidence of a life-threatening condition owners need to ensure adequate nutrition and treat any underlying diseases, such as parasites. If littermates are pushing a small sibling out at feeding time, separate feeding arrangements may need to be made. 

Although small individuals can often catch up their littermates they rarely achieve the same final size.


Updated October 2013