**6. The optimum treatment schedule for maximizing efficacy of nonlactating cow therapy**

The question arises as to when is the best time to treat bred heifers for optimizing cures against *Staph. aureus* mastitis. A 2-year study involving 233 Jersey heifers was designed to answer this question (Owens et al., 2001). In this trial, heifers were quarter sampled shortly after they were confirmed pregnant and at 4-week intervals thereafter. At the initial sampling, 56.5% of quarters were infected with some type of organism, and 15.4% of quarters were infected with *Staph*. *aureus*. After the initial sampling, animals were treated with a one-time infusion of 1 of 5 nonlactating cow infusion products during the first (0 - 90 days), second (91 - 180 days), or third (181 - 270 days) trimester of pregnancy. Products evaluated were: 1) a combination of 1 million units of penicillin and 1 gm dihydrostreptomycin; 2) 300 mg cephapirin benzathine; 3) a combination of 400 mg novobiocin and 200,000 units of penicillin G; 4) 300 mg tilmicosin; and 5) 250 mg cephalonium (this last product was only infused during the first trimester of gestation). Results showed that cure rates for the 5 products were high, ranging from 67 to 100%, and significantly higher than the spontaneous cure rate (25%) observed in untreated control quarters. No differences were observed among the three treatment time periods during gestation. However, fewer new *Staph. aureus* infections occurred after treatment in the group infused during the 3rd trimester, indicating that treatment during this time will reduce incidence of new IMI after infusion and continuing to calving. Because therapy during the first, second, or third trimester of gestation had no effect on treatment efficacy, the timing of treatment is best determined by what is most convenient for the management practices of a particular dairy. For example, heifers could be treated: 1) at time of artificial insemination; 2) during routine rectal palpation to determine pregnancy status; or 3) when moved to a new location in preparation for calving. Treatment should be administered no less than 45 days prior to expected calving date to prevent antibiotic residues at calving.

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

distribution and greater contact with colonized bacteria.

**nonlactating cow therapy**

**6. The optimum treatment schedule for maximizing efficacy of** 

The question arises as to when is the best time to treat bred heifers for optimizing cures against *Staph. aureus* mastitis. A 2-year study involving 233 Jersey heifers was designed to answer this question (Owens et al., 2001). In this trial, heifers were quarter sampled shortly after they were confirmed pregnant and at 4-week intervals thereafter. At the initial sampling, 56.5% of quarters were infected with some type of organism, and 15.4% of quarters were infected with *Staph*. *aureus*. After the initial sampling, animals were treated with a one-time infusion of 1 of 5 nonlactating cow infusion products during the first (0 - 90 days), second (91 - 180 days), or third (181 - 270 days) trimester of pregnancy. Products evaluated were: 1) a combination of 1 million units of penicillin and 1 gm dihydrostreptomycin; 2) 300 mg cephapirin benzathine; 3) a combination of 400 mg novobiocin and 200,000 units of penicillin G; 4) 300 mg tilmicosin; and 5) 250 mg cephalonium (this last product was only infused during the first trimester of gestation). Results showed that cure rates for the 5 products were high, ranging from 67 to 100%, and significantly higher than the spontaneous cure rate (25%) observed in untreated control quarters. No differences were observed among the three treatment time periods during gestation. However, fewer new *Staph. aureus* infections occurred after treatment in the group infused during the 3rd trimester, indicating that treatment during this time will reduce incidence of new IMI after infusion and continuing to calving. Because therapy during the first, second, or third trimester of gestation had no effect on treatment efficacy, the timing of treatment is best determined by what is most convenient for the management practices of a

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

Sampimon & Sol (2005) studied the efficacy of nonlactating cow therapy (600 mg cloxacillin) administered intramammarily 8-10 weeks prepartum in 1) 5 low prevalence farms in which less than 15% of heifers had SCC of >150,000/ml at the beginning of the trial, and 2) 8 high prevalence farms in which greater than 15% of heifers had SCC of >150,000/ml at the beginning of the trial. Results showed that in high prevalence farms, treated heifers produced significantly more milk during the first 100 days of lactation compared with untreated controls; however, in low prevalence farms, treatment had no effect on production. In both groups of farms, SCC and incidence of clinical mastitis were significantly lower in heifers receiving antibiotic therapy. Authors concluded that nonlactating cow therapy was beneficial in high prevalence farms, but not in low prevalence farms.

The treatment of heifers during pregnancy with a nonlactating cow product is advantageous because: 1) the cure rate is higher than during lactation, especially against *Staph. aureus*; 2) there are no milk losses during therapy; 3) the risk of antibiotic residues is minimal; 4) SCC at calving is reduced; 5) new IMI with environmental streptococci is prevented; and 6) milk production is increased by approximately 10% in successfully treated animals in some herds.
