DISCUSSION 
Antibiotics and sulfonamides can enter the milk supply by the following routes: (1) 
The treatment of cows for mastitis with intramammary preparations; (2) the parenteral 
injection of these drugs for the treatment of mastitis or other diseases; (3) the oral ad- 
ministration of antibiotics in feeds at levels considerably higher than are not recom- 
mended, or the oral: administration of sulfonamides for the treatment of diseases; and 
(4) illegal addition of antibiotics to milk for the preservative effect. 
Many veterinarians and others feel that over-the-counter sale of antibiotic prepara- 
tions has been the greatest contributing factortothe problem of antibiotics in milk (7,21). 
Although this may be true, the direct saleof antibiotics to farmers is not practiced in the 
Scandinavian and European countries; yet antibiotics in milk have been a problem to the 
cheese industry in these countries. As pointed out by Welch, et al. (80), the great bulk of 
the milk on the American market that contains antibiotic residues results from the com- 
plete disregard of the warning on the label of each mastitis preparation to discard milk 
from treated quarters for at least 72 hours after the last treatment. 
The fact that antibiotics can persist inthe udder longer than 72 hours indicates that a 
dairyman could adhere to the recommended use of the antibiotic preparations and yet 
inadvertently contaminate the milk supply. The small amounts present on the fourth day 
after treatment would probably not be detected if only a few quarters were treated at any 
one time. If a large number of quarters were treated, as is often done to eradicate 
Streptococcus agalactiae infection from a herd, there would bea greater possibility of 
having detectable levels in the milk from that herd on the fourth day after treatment. 
The rapid test for penicillin used by dairy plants and F.D.A. for checking interstate 
shipments of milk is sensitive enough to detect 0.05 u./ml. of milk. Penicillin levels 
greater than this amount can cause failures in the manufacture of various dairy products 
(34). If we consider 0.23 u./ml. of milk as the average--median of the range of units in 
figure 3--amount of penicillin that could be present on the fourth day after treatment of 
a quarter, then approximately 22 percent of the quarters in any one herd would have to 
be excreting penicillin on the fourth day before it could be detected in the herd milk by 
this test. If the overnight test for penicillin is used to check the milk, then only about 2 
percent of the quarters would have to be excreting penicillin on the fourth day, since this 
test is about 10 times more sensitive than the rapid test. 
Kelley and Kanegis (40) treated six quarters with 426 mg. of Aureomycin in ointment 
and found the average concentration was 0.32 ug./ml. of milk 96 hours after treatment. 
The test was sensitive to 0.025, uwg./ml. of milk. Consequently, about 8 percent of the 
quarters in any one herd would have to be treated at one time before Aureomycin could 
be detected in the herd milk 4 days after treatment, 
These mathematical manipulations are not presented to indicate that more thana 
3-day persistence of antibiotics in milk is the cause of our present problem. We only 
wish to emphasize a potential problem. As more dairy plants test milk for penicillin and 
other antibiotics, these exceptional cases will be found and should be recognized as such, 
Products that provide therapeutic levels of penicillin in the animal for long periods 
of time have always been desired by veterinarians because of the expense and incon- 
venience of repeated injections at frequent intervals, These products have been supplied 
by the pharmaceutical companies. From the viewpoint of treating mastitis and other 
diseases in dairy cattle, this is desirable; however, from the viewpoint of residues in 
milk, it is undesirable, The long-acting salts of penicillin, such as benzathine, are not 
used in intramammary preparations in this country. Apparently, no difference in per- 
sistence is shown by the presently used sodium, potassium, or procaine salts of peni- 
cillin (26,37). Therefore, a reevaluation of the oil-base vehicles might be indicated since 
most contain aluminum monostearate, which may have a prolonging effect on the per- 
sistence of penicillin in milk (37,63), Unless we are only interested in eliminating peni- 
cillin from milk, the substitution of other antibiotics is not the answer. Broad spectrum 
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