**5. Efficacy of nonlactating cow antibiotic therapy**

Because of the high level of infection commonly found in breeding age and pregnant heifers, especially mastitis caused by *Staph. aureus*, as well as the associated elevated SCC, antimicrobial therapy should be considered. The testing of various staphylococcal isolates obtained from heifers for susceptibility to antibiotics commonly incorporated into mastitis infusion tubes has shown that antibiotic resistance is usually low (Watts et al., 1995). Greater than 90% of mastitis-causing staphylococci are generally killed by the drug preparations used, based on in vitro sensitivity testing using zone diffusion analysis (Watts et al., 1995). From a practical standpoint, the administration of antibiotics by a parenteral route would be preferred; however, in the author's experience, neither subcutaneous nor intramuscular injections of drugs have been found to cure IMI in heifers because sufficient antibiotic does not pass into the mammary gland to be bactericidal. Thus, intramammary infusion is the route of choice.

In an initial study to evaluate the effectiveness of treatment, several heifers from each of the 4 commercial herds studied by Trinidad et al. (1990c) were randomly selected to receive a single intramammary treatment of a penicillin and dihydrostreptomycin product. Antimicrobial susceptibility testing demonstrated that 97% of the *Staph. aureus* isolates were sensitive to 12 antibiotics, including the product selected for treatment (Trinidad et al., 1990d). Teat ends were sanitized, and a dry cow antibiotic containing 1,000,000 units of penicillin and 1 gram of dihydrostreptomycin was infused into all quarters of 35 heifers using the partial insertion technique; 38 heifers served as untreated controls. Treatments were made at approximately 60 days prior to the calculated calving date.

Results showed that 97.1% of treated heifers (73.2% of quarters) were infected at the time of treatment, but, at calving, infected heifers and quarters in the treatment group were reduced to 40 and 34%, respectively. Antibiotic residues were limited to two heifers that were treated within 3 weeks of calving because estimated dates of parturition were miscalculated, but all quarters were free of antibiotic residue after 5 days; 2.9% of treated quarters had antibiotic residues at time of calving. Of the 38 untreated control heifers, 100% (71.2% of quarters) were infected at initial sampling, and at calving, mastitis in control heifers was reduced only slightly to 97.4%; percentage of infected quarters increased slightly to 77.8%. The mean SCC at calving was also reduced as a result of therapy. For treated heifers, SCC decreased significantly (P < .001) from 11,825 x 103/ml at time of treatment to 3,439 x 103/ml at calving. In the control group, SCC decreased from 11,047 x 103/ml to 5,594 x 103/ml (P > .05, not significant).

342 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

**5. Efficacy of nonlactating cow antibiotic therapy**

lumina were neutrophils.

route of choice.

Leukocyte infiltration into cisternal and parenchymal mammary tissues was also evaluated. Quarters infected with *Staph. aureus* exhibited the greatest tissue leukocytosis, followed by quarters infected with CNS and uninfected quarters. Leukocyte infiltration in gland cistern and secretory tissue for infected quarters was significantly higher (P < .05) than that for uninfected quarters. Leukocytosis into teat cistern tissue was similar for uninfected quarters and those infected with CNS, but significantly lower (P < .05) than quarters with *Staph. aureus* IMI. None of the uninfected quarters or quarters infected with CNS demonstrated marked leukocyte infiltration. However, marked leukocyte infiltration, particularly lymphocytes, into cisternal and parenchymal areas was commonly observed in quarters that were infected with *Staph. aureus*. The majority of leukocytes observed within alveolar

Because of the high level of infection commonly found in breeding age and pregnant heifers, especially mastitis caused by *Staph. aureus*, as well as the associated elevated SCC, antimicrobial therapy should be considered. The testing of various staphylococcal isolates obtained from heifers for susceptibility to antibiotics commonly incorporated into mastitis infusion tubes has shown that antibiotic resistance is usually low (Watts et al., 1995). Greater than 90% of mastitis-causing staphylococci are generally killed by the drug preparations used, based on in vitro sensitivity testing using zone diffusion analysis (Watts et al., 1995). From a practical standpoint, the administration of antibiotics by a parenteral route would be preferred; however, in the author's experience, neither subcutaneous nor intramuscular injections of drugs have been found to cure IMI in heifers because sufficient antibiotic does not pass into the mammary gland to be bactericidal. Thus, intramammary infusion is the

In an initial study to evaluate the effectiveness of treatment, several heifers from each of the 4 commercial herds studied by Trinidad et al. (1990c) were randomly selected to receive a single intramammary treatment of a penicillin and dihydrostreptomycin product. Antimicrobial susceptibility testing demonstrated that 97% of the *Staph. aureus* isolates were sensitive to 12 antibiotics, including the product selected for treatment (Trinidad et al., 1990d). Teat ends were sanitized, and a dry cow antibiotic containing 1,000,000 units of penicillin and 1 gram of dihydrostreptomycin was infused into all quarters of 35 heifers using the partial insertion technique; 38 heifers served as untreated controls. Treatments

Results showed that 97.1% of treated heifers (73.2% of quarters) were infected at the time of treatment, but, at calving, infected heifers and quarters in the treatment group were reduced to 40 and 34%, respectively. Antibiotic residues were limited to two heifers that were treated within 3 weeks of calving because estimated dates of parturition were miscalculated, but all quarters were free of antibiotic residue after 5 days; 2.9% of treated quarters had antibiotic residues at time of calving. Of the 38 untreated control heifers, 100% (71.2% of quarters) were infected at initial sampling, and at calving, mastitis in control heifers was reduced only

were made at approximately 60 days prior to the calculated calving date.

In the same study, *Staph. aureus* was isolated from 11 quarters of 6 treated heifers before antibiotic infusion (45.8%), but, at calving, this organism was isolated from only 1 quarter of 1 heifer (4.2%). In the control group, 18 quarters of 10 heifers were infected with *Staph. aureus* at time of treatment (45%). At calving, 6 of the control heifers still had *Staph. aureus* mastitis in 11 quarters (55%). Thus, the overall incidence of IMI was reduced 60% and that caused by *Staph. aureus* was reduced over 90%. From 20 to 26% of quarters with subclinical and clinical *Staph. aureus* mastitis are typically cured after antibiotic therapy (Dodd, 1992). In this study, 90.9% of quarters were cured; thus, therapy in heifers was highly effective in eliminating *S. aureus* mastitis compared with that for lactating cows.

In this study, an economic analysis was performed to justify use of the heifer treatment program (Trinidad et al., 1990c). Production data collected over the first 2 months of lactation demonstrated that *Staph. aureus*-infected heifers receiving nonlactating cow therapy during pregnancy produced an average of 5.5 pounds (2.5 kg) more milk per day than *Staph. aureus*-infected herd mates that did not receive treatment. At the milk price received at that time, the greater milk yield translated to a \$42.00 increase for treated heifers, which was well worth the \$5.00 cost of treatment. Other advantages include a longer productive life and higher income due to quality milk premiums.

Subsequent studies (Owens et al., 1991, 1994) with pregnant and nonpregnant heifers using a cephalosporin-based product formulated for nonlactating cows was also successful. Heifers that were either experimentally or naturally infected with *Staph. aureus* were infused 8 -12 weeks prepartum with one dose of 300 mg of a cephapirin benzathine product and were compared with untreated controls infected with *Staph. aureus*. Results demonstrated that 100% of experimentally induced IMI and 87% of naturally occurring *Staph. aureus* IMI were eliminated in treated heifers at calving, and cured quarters remained negative at biweekly samples collected 2 months into lactation. Quarters remaining infected at calving with *Staph. aureus* were treated with a lactating cow product containing 200 mg of cephapirin benzathine, but the cure rate was lower (50 - 56%). Thus, cure rates were greater when a nonlactating product was administered 8 -12 weeks prepartum than when a lactating cow product was given at time of calving.

After antibiotic infusion, SCC in infected quarters that cured decreased from 15 x 106/ml to 4 x 106/ml 1 week later, and to 700 x 103/ml at calving. In contrast, none of the untreated control quarters infected with *Staph. aureus* cured spontaneously by the time heifers calved, and SCC at calving were 5 x 106/ml. Treated heifers in which *Staph. aureus* IMI were cured yielded a mean of 36.1 lb (16.4 kg) milk/day, and untreated controls that retained *Staph. aureus* IMI yielded 31.9 lb (14.5 kg)/day or 11% less during the first 2 months of lactation.

Generally, spontaneous cure rates for major mastitis pathogens are low. For example, in a subsequent study on heifer mastitis (Owens et al., 1994), spontaneous cure rates for *Staph. aureus* and the environmental streptococci were 9 and 6%, respectively. Thus, treatment is required to cure such infected quarters in these young dairy heifers. New IMI rates in uninfected quarters receiving no therapy over the period from 8 - 10 weeks prepartum were very low for most species of staphylococci. However, new environmental streptococcal IMI were common in previously uninfected, untreated heifers. So, as part of this study (Owens et al., 1994), a trial was initiated to treat quarters found to be negative at 8 – 10 weeks prepartum with cephapirin benzathine to determine if establishment of new environmental streptococcal IMI could be prevented during this 8 – 12 week prepartum period. Results showed that prophylactic treatment of such quarters prepartum reduced new environmental streptococcal IMI at calving by 93%. Thus, in this trial, use of nonlactating cow therapy was effective in preventing new IMI as well as curing existing infections. Reasons for this high cure rate are unclear, but the relatively small secretory tissue area of heifer mammary glands compared with mature cows might allow for greater drug concentrations in the udder of the heifer. Similarly, histological studies have demonstrated less scar tissue and abscess formation in the mammary glands of heifers compared with older cows (Trinidad et al., 1990a), a condition which would allow for better drug distribution and greater contact with colonized bacteria.
