Why Is My Runt Cat So Small?

What is a runt kitten?

A runt kitten refers to a kitten that is significantly smaller or weaker than the rest of the litter at birth. The runt is usually easy to identify as the smallest kitten of the group. There are a couple potential reasons a kitten may be born small:

Genetics – Some cat breeds are predisposed to having smaller kittens, such as Siamese cats. If the parents are smaller in size, the kittens may inherit those genes and be born smaller as well. Certain genetic conditions may also cause a kitten to be undersized.

Pregnancy complications – Issues during pregnancy like poor nutrition, stress, crowding in the womb, or birth order can result in a runt kitten. Later born kittens tend to be smaller. The runt may have difficulty competing for food and be underdeveloped.

So in summary, runt kittens are simply smaller or weaker at birth, which may be attributed to genetic factors from the parents or complications during the pregnancy and birth. Their small size does not necessarily indicate they will have long term health problems, though they do require extra care initially.

Genetic causes

There are several genetic reasons a kitten may be born a runt. In litters with multiple kittens, fetal crowding in the uterus can prevent proper nutrients from reaching all fetuses, resulting in smaller or weaker kittens (Source). Being born last in a large litter also puts a kitten at risk of being underdeveloped and smaller. Additionally, some purebred cats are prone to having smaller kittens due to genetic factors being concentrated from inbreeding. For example, Siamese and Burmese cats often have at least one small kitten per litter. The genetics of the parents, especially the mother, play a large role in determining kitten size and health.

Pregnancy complications

Pregnancy complications like illness and malnutrition can stunt a kitten’s growth in the womb, leading to a much smaller size at birth. Intestinal parasites like toxoplasmosis are one of the most common culprits. When a pregnant cat ingests infected feces or transmits congenitally, the parasite enters the fetal bloodstream and causes damage. Kittens born with toxoplasmosis may have health issues like respiratory problems, vision loss, and nervous system damage.

Malnutrition during pregnancy prevents proper development and growth of kittens in the womb. A mother cat who is underweight, deficient in nutrients, or unable to keep food down due to illness will deliver small or undersized kittens. Providing a pregnant cat with a high-quality diet, avoiding food aversions, and controlling nausea can help avoid malnutrition.

Other issues like birth defects, poor placental function, and overcrowding in the womb can also result in one or more kittens being born significantly smaller. Monitoring a cat’s pregnancy, health, and diet can help minimize complications and developmental issues leading to runts.

Long term prognosis

The long term prognosis for a runt kitten’s growth and development can vary quite a bit depending on the underlying cause. Many runts do experience catch up growth and can develop into normal sized, healthy adult cats. According to one veterinarian, if a runt survives to 6-8 weeks of age, they often continue to thrive and have a normal life expectancy https://cat-world.com/runt-of-a-cat-litter/.

However, some runts may struggle to reach normal adult size due to congenital defects, illness, or poor nutrition in early life. Their growth plates may close early, stunting their growth. These cats tend to remain undersized as adults. With extra care and vet monitoring, many still live normal lifespans, but their small size can predispose them to issues like dental overcrowding, bladder stones, and difficulty regulating body temperature.

Runts at higher risk for long term issues include those with genetic disorders, congenital defects, or a very low birth weight. These cats may be more prone to chronic health problems. However, any runt that survives past the critical first few months has a good chance of catching up developmentally and living a full life.

Special care

Runt kittens often require extra care and attention in their first few weeks to help them thrive. One of the most important things is supplemental feeding with a kitten milk replacer. Since runts can have trouble competing with their larger littermates, they need more frequent feedings with a high-calorie formula to help them gain weight. Feed the runt kitten every 2-3 hours if possible.1

It’s also crucial to keep the runt kitten warm, as their small size makes them prone to getting chilled. Make sure they have a heated bed or blanket and keep the ambient temperature around 85°F. Weigh them daily to ensure they are gaining weight appropriately. Runts may need veterinary care if they fail to thrive, so monitor their growth and development closely.

Vet monitoring

It is crucial to take runt kittens to the vet regularly for monitoring, as they can be prone to health issues due to their small size. The vet will check the kitten’s weight, growth rate, organ development, and look for any potential problems during exams. Monitoring development closely allows early intervention if the kitten falls behind. One source recommends weighing daily to ensure proper weight gain (https://cat-world.com/runt-of-a-cat-litter/).

During the first few weeks, the vet may recommend supplemental feedings with kitten formula and provide guidance on nutrition. Later exams will include vaccinations and routine wellness checks. More frequent vet visits may be needed compared to the rest of the litter. Close monitoring by a vet in the early months can help set the kitten up for future health and development.


Socializing a runt kitten with its litter mates and humans is crucial for its development. Kittens have a prime socialization window before 12 weeks of age, when they are most receptive to forming social bonds and learning appropriate behaviors [1]. Runts in particular may need extra help socializing since they can be pushed aside by larger litter mates. Make sure the runt gets equal time playing and interacting with humans. Hand-feeding, petting, and playing with toys are great ways to socialize a runt kitten. If you only have a single runt kitten, spend extra time cuddling, grooming, and allowing supervised exploration to avoid behavior problems from lack of socialization [2]. With patience and effort during this critical window, runt kittens can become well-socialized and affectionate pets.

Common health issues

Runt kittens are prone to certain health conditions due to their small size and underdevelopment. One of the most common issues is respiratory infections. Runts often have underdeveloped lungs and weaker immune systems, making them more susceptible to viral and bacterial respiratory infections like feline calicivirus and feline herpesvirus 1. These infections can cause symptoms like sneezing, coughing, runny nose and eyes, lethargy, and loss of appetite. If not treated promptly with antibiotics, antivirals, and supportive care, respiratory infections can be life-threatening in runts. Keeping the environment clean, avoiding stressors, and vaccinating the kitten can help prevent respiratory illness.

Other common conditions include hypoglycemia, or low blood sugar, which can cause seizures and coma in severe cases. Runts may also be prone to digestive issues like diarrhea if they have trouble competing for food against larger litter mates. Providing supplemental feeding and monitoring their food intake is important. Congenital defects, like heart murmurs, cleft palate, and blindness, are also more common due to stunted fetal development. Regular vet checkups are crucial to monitor a runt kitten’s health and development.

Life expectancy

The life expectancy of a runt kitten can vary depending on several factors. According to the National Post, if a runt survives to 6-8 weeks of age, it will likely grow close to full size and have a normal lifespan.[1] Cat World notes that the average indoor cat lifespan is 10-15 years.[2] So while runt kittens may face additional health challenges early on, with proper care and vet monitoring, their life expectancy can be normal.

Some key factors that influence a runt kitten’s lifespan include the underlying cause of their small size, their ability to thrive and gain weight, and any ongoing health issues. Providing excellent nutrition, socialization, vet care, and a safe environment are important to support their development and longevity.

In summary, the average lifespan of a runt kitten is similar to other cats at 10-15 years. But their long-term prognosis depends on overcoming any initial hurdles and receiving attentive care as they grow.

When to be concerned

Runt kittens require extra monitoring and care. There are certain signs that indicate an emergency requiring immediate veterinary care:

  • Not eating or refusing to nurse for 12 hours or more (See source)
  • Dehydration – Dry gums, weakness, sunken eyes (See source)
  • Hypothermia – Body temperature below 95°F (See source)
  • Difficulty breathing or respiratory distress
  • Unresponsive to stimulation
  • Bleeding from nose or mouth
  • Seizures
  • Weakness or inability to stand

If you observe any of these emergency signs in a runt kitten, seek veterinary care immediately. With prompt treatment, many issues can be managed, but delays can be life threatening for vulnerable runts.

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