Editor’s note: This is the first of a two-part article. Replacement heifers, whether they are raised on the farm, purchased from other dairies or contract-raised by growers, are critical to herd productivity because they represent the future milking and breeding stock in all dairy operations. The goal should be to provide an environment for heifers to develop full lactation potential at the desired age with minimal expense. Animal health and well-being play vital roles in achieving this potential, and one disease that can influence future productivity is mastitis.
Unfortunately, most producers regard young heifers as uninfected, and the presence of mastitis is not observed until freshening or until the first clinical flare-up in early lactation. Thus, animals may carry intramammary infections for a year or more before they are diagnosed with mastitis.
The greatest development of milk-producing tissue in the udder occurs during the first pregnancy, so it is important to protect the mammary gland from pathogenic microorganisms to ensure maximum milk production during the first lactation.
Louisiana researchers found that if bred heifers infected with Staphylococcus aureus were left untreated, they produced 10 percent less milk in early lactation than those receiving therapy.
Research in New Zealand has shown that Staph. aureus mastitis in heifers results in significant production losses during the first lactation, which carries over into the subsequent lactation, even if infected quarters are successfully treated.
Prevalence of heifer mastitis and somatic cell counts
Researchers became interested in heifer mastitis in the mid-1980s after several dairy producers complained that a large percentage of their heifers were freshening with clinical mastitis.
Subsequent study of breeding-age animals revealed that intramammary infections may be diagnosed as early as 6 months old, and infections persist throughout pregnancy and into lactation. Other studies demonstrated that greater than 90 percent of breeding-age and bred heifers (12 to 24 months old) may be infected.
Most of the infections were shown to be caused by the coagulase-negative staphylococci (Staphylococcus chromogenes and Staphylococcus hyicus), followed by Staph. aureus (20 percent). Mixed isolates of coagulase-negative staphylococci (CNS) and Streptococcus species were also found.
Somatic cell counts (SCC) are used to assess udder health status of mature, lactating cows, and this parameter has been examined in heifer mammary secretions. In secretions from uninfected quarters, counts are approximately five million cells per milliliter (mL).
The volume of mammary secretion is very low in breeding-age animals; thus, somatic cells become concentrated, resulting in high SCC. However, counts may be 20 million cells per mL in quarters infected with Staph. aureus and over 10 million cells per mL in those infected with the CNS and Streptococcus species.
Such elevated SCC in infected quarters over a long period of time suggests that these mammary glands would be in a state of chronic inflammation, which would adversely affect development of milk-producing tissues.
In fact, histological analysis of mammary tissues obtained from bred heifers chronically infected with Staph. aureus demonstrated that the potential for milk production was reduced significantly compared with tissues from uninfected quarters.
Efficacy of nonlactating dry cow antibiotic treatment
Because of the high level of infection commonly found in heifers at some dairies, especially mastitis caused by Staphylococcus aureus, infected quarters should be treated. 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.
Greater than 90 percent of mastitis-causing staphylococci are generally killed by the drug preparations used. From a practical standpoint, the administration of antibiotics by a parenteral route would be preferred; however, neither subcutaneous nor intramuscular injections of drugs have been found to cure intramammary infections.
Parenteral treatment is ineffective because sufficient antibiotic does not pass into the mammary gland to be bactericidal. Thus, intramammary infusion is the route of choice.
Prior to treatment, heifers should be restrained in a squeeze chute equipped with a head gate. Teat ends should be scrubbed with cotton balls soaked in 70 percent alcohol or with the pledgets accompanying mastitis tubes in order to sanitize the teat orifice prior to infusion.
While administering the antibiotic, the partial insertion technique must be used to avoid stretching the teat canal and the sphincter muscle as well as to avoid the introduction of bacterial contaminants. All quarters of each animal should be treated to cure existing Staph. aureus infections and to prevent new ones.
After infusion, teats should be immersed in a barrier teat dip to seal the teat orifice and prevent entrance of contaminating bacteria.
The cure rate for Staph. aureus mastitis after use of nonlactating cow therapy in heifers is more than 90 percent. Therapies evaluated have included: a product containing 1 million international units (IU) of penicillin G and 1 gram of streptomycin, a product containing 300 milligrams (mg) cephapirin benzathine and a product containing 400 mg novobiocin with 200,000 IU of penicillin G.
This is far greater than the 25 percent cure rate observed after mature cows are treated during lactation for this disease using conventional lactating cow therapy. 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, a condition which would allow for better drug distribution and better contact with colonized bacteria.
In one study, an economic analysis was performed to justify use of the heifer treatment program. Production data collected over the first two months of lactation demonstrated that Staph. aureus- infected heifers receiving nonlactating cow therapy during pregnancy produced an average of 5.5 pounds (2.5 kilograms) more milk per day than herdmates that did not receive treatment.
At the milk price received at that time, the greater milk yield translated to a $42 increase for treated heifers, which was well worth the $5 cost of treatment. Other advantages include a longer productive life and higher income due to quality milk premiums.
Treatment of animals eight to 12 weeks prior to expected calving date with 300 mg cephapirin benzathine resulted in a cure rate of greater than 95 percent. An examination of SCC showed that at the time of treatment, SCC was 15 million per mL but decreased to 4 million per mL one week later and to 700,000 per mL on the day of calving.
If infected quarters were left untreated, heifers freshened with Staph. aureus-infected quarters with an average SCC of 5 million per mL. When these latter animals were treated with lactating cow products immediately after calving, cure rate was only 56 percent.
Thus, cure rates are much greater when nonlactating cow products are administered two to three months prepartum than when a lactating cow product is given shortly after calving.
Efficacy of lactating cow products
Lactating cow products have been used successfully in heifers when treating infections caused by the coagulase-negative staphylococci immediately prior to calving. In one study, quarters of infected heifers were infused one time at approximately one week prepartum with either 200 mg sodium cloxacillin, 200 mg cephapirin sodium or left untreated.
At the time of infusion, approximately 90 percent of heifers were infected in one or more quarters, and if left untreated, 78 percent of animals remained infected at time of calving. However, only 18 percent of the heifers remained infected at calving if they were treated prepartum, regardless of the treatment used.
This study also examined the influence of prepartum antibiotic treatment on subsequent lactational performance and demonstrated that heifers receiving treatment produced approximately 1,000 pounds (455 kg) more milk per lactation than untreated controls.
Prepartum treatment with lactating cow therapy has been shown to be effective for quarters infected with coagulase-negative staphylococci, but waiting until this time to treat chronic Staph. aureus mastitis might be too late.
A mammary gland that has been infected with Staph. aureus for several months to a year will not develop normally, and treatment during the immediate prepartum period would most likely be of little benefit in curing infections or salvaging mammary tissue.
At this point, the tissue damage would have already been done, and affected quarters should have been treated earlier in gestation to cure existing infections, reduce chronic inflammation and allow mammary tissue to develop normally during the later stages of pregnancy.
The optimum treatment schedule
The question arises as to when is the best time to treat bred heifers for optimizing cures against Staph. aureus mastitis. A two-year study involving 175 Jersey heifers was designed to answer this question. In the trial, heifers were sampled shortly after they were confirmed pregnant and at four-week intervals thereafter.
After the initial sampling, animals were treated with a one-time infusion of one of three nonlactating cow infusion products during the first (0 to 90 days), second (91 to 180 days) or third (181 to 270 days) trimester of pregnancy.
Products evaluated were: a combination of 1 million IU of penicillin G and 1 gram streptomycin, 300 mg cephapirin benzathine and a combination of 400 mg novobiocin and 200,000 IU of penicillin G.
Cure rates among treatments indicated that all antibiotics were equally effective in curing infections, and there were no apparent effects of the timing of therapy on cure rate. Treatment efficacy ranged from 83.3 percent to 100 percent.
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 at time of artificial insemination, during routine rectal palpation to determine pregnancy status or when moved to a close-up pen. Treatment should be administered no less than 45 days prior to expected calving date to prevent antibiotic residues at calving.
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 Staphylococcus aureus
2. There are no milk losses during therapy
3. The risk of antibiotic residues is minimal
4. SCC at calving is reduced
5. Milk production is increased by approximately 10 percent in successfully treated cows
Treatment is indicated only in herds experiencing a 5 percent or greater prevalence of heifers calving with clinical mastitis caused by Staphylococcus aureus. The potential for residues at calving should be considered, especially in animals that calve early.
Residue testing should be carried out before mixing milk from treated animals with herd milk. PD
References omitted due to space but are available upon request to editor@progressivedairy.com .
âÂÂExcerpts from University of Idaho Extension website. Article last updated December 21, 2010.
PHOTO:
TOP RIGHT: Replacement heifers, whether they are raised on the farm, purchased from other dairies or contract-raised by growers, are critical to herd productivity because they represent the future milking and breeding stock in all dairy operations. Photo by PD staff.
Stephen Nickerson
Nutritionist
University of Georgia
scn@uga.edu
