while some are absorbed in very small quantities, if at all. Bacitracin, streptomycin, 
neomycin, and polymyxin are members of the latter group, while penicillin, chloram- 
phenicol, carbomycin, tetracycline, and related cyclinesareall rapidly absorbed from the 
gastrointestinal (G.I.) tract. Absorption apparently occurs from all sections of the 
gastrointestinal tract. The last-named group of antibiotics are also absorbed when applied 
topically and by lung tissue from aerosol or fine dusts. 
In general and within broad limits the larger the continual oral dose the higher the 
concentration in the blood serum and other tissues. However, such a relationship is 
commonly not found after the administration of a single dose. Maximum concentration 
in blood serum occurs about 1 to3hours after ingestion in animals with simple stomachs, 
but requires 8 hours in calves. The maximum blood concentration usually occurs within 
1 hour after intravenous or intramuscular administration (I.M.), although there is a large 
difference among species in this respect. The absorbable antibiotics can persist in the 
bloodstream and other tissues for at least 24 hours after ingestion in simple stomached 
animals and up to 7 days in the serum, liver, and kidney of calves. The length of per- 
sistence depends on the size of the oral dose (1,2,3,4,5,6,7). 
The extent and rate of absorption are influenced by many factors. Most of these have 
been studied using humans, a few using laboratory animals (rat, rabbit, chicken, dog, and 
cat), but none of these factors have been studied using large farm animals. In most studies 
the presence of food has increased the extentof absorption although with some antibiotics 
the opposite has been found (5). Concurrent administration of aluminum hydroxide jels 
and other antiacids has been shown to reduce the absorption of the cycline antibiotics. 
Erythromycin is destroyed by gastric secretions and must be given in protected tablet 
form. 
The amount of calcium in the diet affected the extent of absorption of oxy- and 
chlortetracycline. When chickens were given this antibiotic at 200 gm./ton of feed 
(+20 mg./kg. body weight), the concentration in the blood plasma on the low calcium diet 
was double that obtained on a regular diet (0.10 vs. 0.20uwg. chlortetracycline per ml. 
plasma). Similar results were obtained at dietary antibiotic levels two and four times 
that amount. The concentration of chlortetracycline inthe feces was also less when on the 
low calcium diet (6,7,8). Similar results have been obtained in swine where diets low in 
calcium (0.06 percent) doubled the chlortetracycline levels in blood plasma (0.54 to 1.06 
ug. chlortetracycline/ml.) compared to normal diets containing 0.70 percent calcium (9). 
The addition of phosphorus to diets also increased the absorption of chlortetracycline 
(6, 10). 
Simultaneous administration of citric acid has increased blood levels of the cyclines 
in rats, guinea pigs, dogs, chickens, andpigs but not in man (2,5,6,7,10). In view of all the 
animal data showing this effect the early work with humans needs to be repeated. Other 
effective adjuvants were trisodium citrate, malic, tartaric, malonic, pyruvic, and lactic 
acids, monosodium phosphate, tricarballyic acid, terephthalic acid, camphoric acid, and 
ethylenediamine tetraacetic acid (EDTA). These compounds increased blood-serum levels 
of chlortetracycline by 47 to 114 percent in chickens (6). These and other less effective 
adjuvants all are capable of complexing calcium ions. 
The simultaneous administration of various multivalent metallic ions with the 
cyclines has lowered their blood levels (6, 11). The substitution of calcium sulfate for 
calcium carbonate in poultry and swine diets resulted in increased levels of chlortetra- 
cycline when the antibiotic was fed (6,7,9). Diets low in calcium are now in use to help 
increase the effectiveness of the cyclines in treatment of poultry diseases. 
Additional but more precise information on factors affecting absorption have been 
obtained by the use of ligated intestinal loops where the antibiotic, adjuvant, and metallic 
ions were injected into the loop. Subsequent levels of the antibiotic in serum, liver, kid- 
ney, and loop contents were used to estimate extent of absorption (10). Total absorption 
of oxytetracycline from the loop varied from 20 to 45 percent with an average of 30.5 
percent. 
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