Like all mammals, female dogs and cats produce a special type of milk called colostrum during the first few days following parturition. Colostrum provides both specialized nutrition and passive immunity to newborn puppies and kittens. Passive immunity is provided in the form of immunoglobulins (antibodies) and other bioactive factors that are absorbed across the intestinal mucosa of newborns. Most of these factors are large, intact proteins. Once absorbed into the body, passively acquired antibodies offer protection from a number of infectious diseases. Because the immune system of puppies and kittens is not fully developed until they are about 16 weeks of age, the transfer of this protective immunity from the mother to newborns via colostrum is important for their survival. In addition to antibodies, examples of bioactive factors found in colostrum include lysozyme, a bacteriolytic enzyme that prevents the growth of certain types of bacteria, and bile salt–activated lipase, which aides in the digestion of fat. ^{2. and 3.}

In some species, such as humans, rats, rabbits, and guinea pigs, a significant proportion of passive immunity is acquired prior to birth (in utero). In contrast, puppies and kittens, like pigs, horses, and ruminant species, obtain the greatest proportion of maternally derived antibodies through the colostrum. These differences are due to the types of placentas found in different species, reflecting the number of placental layers that antibodies must transverse to reach developing fetuses. The dog and cat have an endotheliochorial placenta consisting of four layers. This type of placenta allows only about 10% to 20% of passive immunity to be transferred in utero. Therefore, for puppies and kittens, the major proportion of passive immunity is acquired after birth via the colostrum. This emphasizes the importance of immediate nursing and the provision of colostral antibodies and bioactive factors to puppies and kittens immediately after birth.

In older neonates and adult animals, normal digestive processes would result in the complete digestion of the immunological compounds found in colostrum, making them unavailable to the body as immune mediators. However, the intestinal mucosa of newborn dogs and cats is capable of absorbing intact immunoglobulins provided by colostrum. The time during which the newborn’s gastrointestinal tract is permeable to the intact immunoglobulins in colostrum is very short. The term closure refers to the change in the gastrointestinal tract’s absorptive capacity that precludes further absorption of large, intact proteins. The mechanisms behind closure are not fully understood, but they appear to be hormonally mediated, possibly related to increased circulating insulin that appears after the initiation of suckling. ⁴ This limits the ability of the neonatal intestine to absorb intact proteins to about the first 48 hours of life. ⁵ Therefore it is vitally important that newborn puppies and kittens receive adequate colostrum as soon as possible during the first day after birth.

In addition to the immunological benefits of colostrum, the volume of fluid ingested immediately after birth contributes significantly to postnatal circulating volume. ¹ A lack of adequate fluid intake shortly after birth can contribute to circulatory failure in newborns. Water turnover is very high in neonates, necessitating high fluid intake to maintain normal blood volume throughout the neonatal period. ⁵ For this reason, the consistent ingestion of adequate fluids by neonates and the production of sufficient milk volume by the mother are as important as the milk’s nutrient content.

Like the milk of many mammalian species, dog and cat mammary secretions change during lactation to effectively meet the needs of their developing young. Several forms of colostrum are produced during the first 24 to 72 hours after birth, after which the composition slowly transforms to mature milk. The protein content in cat colostrum that is produced on the first day of lactation is very high (greater than 8%); however, protein rapidly declines to about half this value by the third day of lactation. ^{6. and 7.} Protein concentration then slowly increases throughout lactation to again reach a concentration of about 8%. Lipid concentration in cat milk follows a similar pattern. On the first day of lactation, the total lipid concentration is relatively high, but rapidly decreases until the third day of lactation. Values then gradually increase until the forty-second day of lactation, after which they decline slightly. ^{6. and 8.} In addition to the stage of lactation, the queen’s diet can also affect the fat content of queen’s milk. For example, queens fed a food that contained 22% crude fat (dry-matter [DM] basis) produced milk containing up to 17% fat. ⁹ Lastly, lactose concentrations in cat milk stay relatively constant or increase slightly throughout lactation (Table 21-1).

The nutrient pattern of dog’s milk is somewhat different. The most recent study reported that while milk protein is very high on the first day of lactation (>10%), it decreases gradually for the following 3 weeks and then, after day 21, increases slightly until weaning. ¹⁰ This is in contrast to an earlier study that reported a pattern of change that was similar to that of cat’s milk. ¹¹ The lipid content of dog’s milk is higher than that reported for cat’s milk and does not show the dramatic decrease early in lactation that is reported for cat mammary secretions. Because of this higher fat content and possibly due to its slightly higher protein concentration, dog’s milk is higher in energy than cat’s milk. In both species the total energy content of the milk decreases gradually from colostrum to the milk that is produced during midlactation. Energy concentration then increases until weaning in both species. Lactose concentration in dog’s milk is lowest in colostrum and increases gradually until midlactation (see Table 21-1).

The type of protein found in the milk is also an important consideration. In cats, colostrum protein has a casein-to-whey ratio of about 40:60. ⁶ This ratio shifts to a slight predominance of casein as the colostrum transitions to mature milk, with a final ratio of about 60:40. This shift from a predominance of whey early in lactation to a predominance of casein is also seen in humans and horses. It is significant because the amount of casein in the milk may affect protein digestion, mineral utilization, and the milk’s amino acid composition. ¹² In contrast, the casein-to-whey ratio in dog’s milk is more similar to that of humans and cows in that casein predominates throughout lactation and remains relatively constant at a ratio of 70:30. ¹⁰

Calcium concentrations in dog and cat milk are similar, increasing in both species over the course of lactation. The concentrations of protein and calcium in milk are highly correlated during lactation because casein has a high calcium-binding capacity. ^{10. and 11.} The milk of both dogs and cats also has a relatively high iron concentration. These two species are similar to the rat and several marsupial species in their ability to concentrate iron in their milk at a level that is substantially higher than the concentration found circulating in the mother’s plasma. The high iron content in milk may reflect a high requirement for this mineral early in life. Similar to other nutrients, iron concentration is strongly influenced by the stage of lactation, with values increasing slightly during the first 2 days of lactation and then gradually declining (see Table 21-1). ¹³

The fatty acid profile of mother’s milk has received considerable interest in recent years in response to recognition of the importance of long-chain polyunsaturated fatty acids (PUFAs) to fetal and neonatal development. As discussed in Chapter 20, maternal essential fatty acid (EFA) status is depleted during reproduction, and this effect is exacerbated by an increasing number of parities. ¹⁴ When maternal EFA status is supported by feeding a diet with an improved fatty acid profile, litter size is positively affected. ¹⁵ This effect is most pronounced in females who have had several litters. The same series of studies found that puppies of mothers who were fed a high EFA food that was balanced for both linoleic acid (n-6) and alpha-linolenic acid (n-3) PUFAs had a higher EFA status at birth than did puppies born to mothers fed a diet containing a less favorable EFA profile. This effect was most pronounced for the n-3 PUFA docosahexaenoic acid (DHA), which is necessary for normal retinal and neurological development in neonates.

Enrichment of milk with long-chain PUFAs is dependent upon the types of EFAs that are included in the maternal diet. A study of the effect of the maternal diet on fatty acid profiles in canine mammary secretions found that concentrations of the two 18-carbon parent fatty acids, n-6 and n-3, in milk increased in parallel with an increase in these fatty acids in the maternal diet. ¹⁶ However, an important finding of this study was that the milk did not become enriched with the respective derived fatty acids, arachidonic acid (AA) from linoleic acid, and DHA, eicosapentaenoic acid (EPA), or docosapentaenoic acid (DPA) from alpha-linolenic acid. These findings are in agreement with those reported in humans and suggest that supplementation of maternal diets with linoleic acid and alpha-linolenic acid does not provide an effective approach for increasing the long-chain PUFAs in milk. ¹⁷ Recent studies have shown that while newborn puppies can convert milk alpha-linolenic acid to DHA early in life, they lose this ability after weaning. ¹⁸ Even prior to weaning, the efficiency of conversion is very low, so very large doses of alpha-linolenic acid are necessary to see a significant increase in tissue DHA levels. ¹⁹ Both AA and DHA are essential during perinatal life; DHA is especially crucial for normal neurological and retinal development. ^{20. and 21.} Therefore it is prudent to provide a food to the mother that contains n-3 and n-6 long-chain PUFAs during gestation and lactation to ensure adequate enrichment of her milk with these EFAs. This is the best approach to ensuring a supply of AA and DHA to puppies and kittens during the perinatal period (see Chapter 22, pp. 228-229 for additional information about DHA and development).

The two primary activities of all newborns are eating and sleeping. During the first few weeks of life, puppies and kittens should nurse every few hours, at a minimum of four to six times per day. The frequent intake of small amounts of milk is necessary because of the small size of the neonate’s stomach. Infrequent or weak nursing often signifies chilling, illness, or congenital problems and should be attended to immediately by a knowledgeable breeder or veterinarian. The eyes of puppies and kittens open between 10 and 16 days after birth and their ears begin to function between 15 and 17 days after birth. Normal body temperature for puppies is 94° Fahrenheit (F) to 97° F for the first 2 weeks of life. Normal kitten temperature during this time is about 95° F. By 4 to 5 weeks of age, body temperatures have reached the normal adult temperature in both species (approximately 101.5° F).

Because puppies and kittens have no shivering reflex for the first 6 days of life, an external heat source is necessary. ²² The dam is the best source of this warmth. After 6 days the puppies and kittens are able to shiver, but they are still very susceptible to chilling. Keeping the environment warm and free from drafts is of utmost importance during the first few weeks of life to prevent hypothermia. It is recommended that the environmental temperature be kept at 70° F during this period, assuming the dam is providing an adequate amount of warmth and protection to the newborns. Newborns should be weighed daily during the first 2 weeks and then every 3 to 4 days until weaning. A helpful guideline is for puppies to gain between 1 and 2 grams (g) per day for every pound (lb) of anticipated adult weight for the first 3 to 4 weeks of life. For example, if the anticipated adult weight of a dog is 25 lb, the puppy should be gaining between 25 and 50 g/day (0.9 to 1.8 ounces [oz]). Kittens usually weigh between 90 and 110 g at birth and should gain between 50 and 100 g (1.8 to 3.5 oz) per week until they are 5 to 6 months of age.

The gastrointestinal tracts of newborn puppies and kittens are uniquely suited to digest and absorb the milk produced by the mother dog and cat, respectively. ²³ Immediately after birth, the ingestion of milk is a potent stimulator for enteric growth and for the development of the intestinal mucosal cells. ^{24. and 25.} Fat and lactose are the primary sources of energy in milk; puppies and kittens have high intestinal lactase activity and are capable of digesting milk fat very early in life. ²³ Similarly, both the type and amount of protein found in the milk are intricately matched to the developmental state of life. Gastric acid production is low in puppies and kittens until they are about 3 weeks of age. However, this does not appear to inhibit their ability to digest milk proteins. The renal capacity of neonates is also not fully developed and is sensitive to excessive or poor quality protein intake. Milk protein is of high quality and at a concentration that is closely matched to the metabolic capabilities of the developing young. Lastly, at birth, the gastrointestinal tract of puppies and kittens is sterile. Microbial colonization begins within the first day of life as the newborns ingest milk. This continues to evolve when solid food is introduced at 3 to 4 weeks of age and as the young attain adulthood. ²⁶

Volume of milk intake is affected by age, rate of growth, and for dogs, breed size. An early study of neonatal Beagle puppies reported that puppies consumed between 160 and 175 g (5 to 6 oz) of milk per day. ²⁷ However, the technique used in the study may underestimate intakes by up to 30%, suggesting that daily intake was substantially higher than this. ²⁸ Naturally, puppies of larger breeds are expected to consume a greater volume of milk, with smaller breeds and kittens consuming less volume. Similarly, the volume of milk that a female dog produces varies with her size. German Shepherds produce about 900 g (32 oz) of milk per day in early lactation, with increases of up to 1700 g (60 oz) per day during peak lactation. ²⁹ In contrast, a much smaller breed, the Dachshund, produces between 100 and 180 g (3 to 6 oz) of milk per day in early lactation. Other influences upon the volume of milk produced are litter size, the age at which supplemental food is introduced, and age of weaning. In healthy puppies and kittens, the dam’s milk supports normal growth until the young are 3 to 4 weeks old. Supplemental feeding with commercial milk replacer is usually not necessary, with the exception of unusually large litters. Even in those cases, dividing the litter into two groups and allowing each group to feed every 3 to 4 hours can often allow adequate intake for all of the puppies or kittens. ²³

After 4 weeks, milk alone no longer provides adequate calories or nutrients for normal development. At approximately the same time, puppies and kittens become increasingly interested in their environment and begin to spend more time awake and playing with each other. The time at which the dam’s milk is no longer solely able to meet the nutrient needs of the offspring corresponds to the time at which the young are becoming interested in trying new foods and when they are developmentally capable of handling the introduction of semisolid food.

Supplemental food should be introduced to puppies and kittens when they are 3 to 4 weeks of age. A commercial food made specifically for weaning puppies or kittens can be used, or a thick gruel can be made by mixing a small amount of warm water with the mother’s food. Cow’s milk should not be used to make the gruel because it is higher in lactose than bitch’s and queen’s milk and may cause diarrhea. Puppies and kittens should also not be fed a homemade “weaning formula.” Although the foods that are used to make these formulas are usually of high nutrient value, many homemade formulas are not nutritionally balanced or complete. The use of this type of formula should be avoided unless its exact nutrient composition is known.

The semisolid food should be provided in a shallow dish, and puppies and kittens can be allowed access to fresh food several times per day. The bowl should be removed after 20 to 30 minutes. At first, little of the semisolid gruel will be consumed, and the litter’s major food source will continue to be the dam’s milk. However, by 5 weeks of age, puppies and kittens are readily consuming semisolid food. The deciduous teeth erupt between 21 and 35 days after birth. By 5 to 6 weeks of age, puppies and kittens are able to chew and consume dry food. Nutritional weaning is usually complete by 6 weeks of age, although some bitches and queens continue to allow their young to nurse for 8 weeks of age or longer (Box 21-1). Puppies will suckle occasionally and will continue to interact with the mother dog at 7 weeks of age even when offered free access to solid food. ³⁰ It is believed that the psychological and emotional benefits of suckling may be as important as the nutritional benefits in puppies that are older than 5 weeks of age. For this reason, complete weaning (behavioral weaning) should not be instituted until puppies and kittens are at least 7 to 8 weeks of age.