Heifer mastitis has been researched since the 1980s when it was noticed that many heifers were freshening with clinical mastitis. "Studies of breeding age animals revealed that intramammary infections (IMI) may be diagnosed as early as 6 months of age, and infections persist throughout pregnancy and into lactation," says Steve Nickerson, PhD, University of Georgia. Other studies demonstrated that greater than 90% of breeding age and bred heifers (12 to 24 months of age) 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%). Mixed isolates of coagulase-negative staphylococci (CNS) and Streptococcus species were also found.
"We have always focused mastitis control on mature cows," Nickerson states. "However, we ignore the first half of her life when she could have mastitis as a heifer. If the average age at calving is 2.5 years, and a cow lives for 5.5 years and has three lactations, then for 45% of her lifetime, no mastitis control is being applied."
In some herds, S. aureus mastitis prevalence in unbred and bred heifers serves as a source for infecting the milking herd. IMIs in young dairy animals are associated with local inflammation, induration, and extremely high somatic cell counts (SCC). Furthermore, histological analyses have shown that Staph. aureus infections adversely affect the development of milk-producing tissues of heifers.
Nickerson suggests examining heifers for signs of mastitis either at the time of AI or preg check when they are being handled anyway. Visual and manual examination of the developing udders, mammary fluid and teat skin will help identify swollen quarters, abnormal secretions, and presence of teat scabs. Individual swollen quarters with abnormal secretions (clots and flakes) and those with teats exhibiting scabs and abrasions are most likely to be infected and should be treated.
Because of the high level of infection commonly found in heifers at some dairies, especially mastitis caused by Staph. 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.
"Ideally, infected heifers should be separated from uninfected heifers as flies do spread the disease," Nickerson says, "however, from a management standpoint that is really difficult to do."
Treating heifer IMIs
When treating heifers with mastitis tubes, it"s important to properly restrain them in a squeeze chute with a head gate and to thoroughly clean teat ends and the teat orifice. Nickerson says 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.
"Keep in mind that syringe cannulas were designed for adult teats, not little heifer teats," Nickerson explains. "By fully inserting the syringe cannula as opposed to partial insertion, the sphincter muscle is stretched up to 8-fold, keratin is displaced to the side or pushed into the teat cistern, and this leaves a temporary hole or open conduit for bacterial penetration after cannula withdrawal."
If the bacteria are resistant to the drug being infused (such as coliforms), then a new infection is initiated in addition to the one being treated, he adds. "Even with partial insertion, this is possible, so I always infuse teats following treatment with teat seal and follow up with teat dipping using a barrier product to kill any bacterial contaminants at the orifice and to leave a bactericidal film over the orifice." All quarters of each animal should be treated to cure existing Staph. aureus infections and to prevent new ones.
The cure rate for Staph. aureus mastitis after use of nonlactating cow therapy in heifers is more than 90%. This is far greater than the 25% cure rate observed after mature cows are treated during lactation for this disease using conventional lactating cow therapy. "Also, treating heifers and curing them leads to a 10% increase in milk yield during the first lactation," Nickerson notes. It"s believed that 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.
Sources of infections
It"s not entirely clear where IMIs in heifers originate. Nickerson says that sources may include: 1. bacteria that are the normal flora on udder skin, which are in an opportunistic position to colonize the teat end and enter the teat orifice; 2. bacteria harbored in the oral cavities of calves, which suckle other calves; 3. bacteria present in the heifers" environment, such as those found in soil, manure and bedding materials; and 4. bacteria present on biting flies that congregate on teat ends.
Normal flora would be almost impossible to control, as these microorganisms are naturally found on the udders and teat skin. The transfer of mastitis-causing bacteria through cross-suckling
of calves fed mastitis milk can be prevented by housing calves in individual hutches, which is fairly routine. Nickerson says the percentage of IMIs caused by environmental streptococci and coliforms in heifers is low, except just prior to freshening. "Flies have certainly been implicated in the spread of mastitis-causing bacteria among heifers and should be a major focus of control."
The producer has the responsibility of ensuring that an animal remains healthy in his care, and this responsibility should include udder health. "It is his responsibility to culture any new animals that are brought into his herd to avoid the introduction of contagious mastitis-causing microorganisms such as Staph. aureus," Nickerson says. "Some producers and veterinarians worry that sampling heifers for presence of mastitis may destroy the keratin plug, leading to new infections. However, studies designed to test this theory have demonstrated that as long as: 1. teat ends were properly sanitized; 2. samples were taken aseptically; and 3. teats were dipped in a barrier type product after sample collection, there was no effect on the development of new infections.