Effect of a second infusion of an internal teat sealant four weeks after drying off

Mastitis continues to be a major financial and animal welfare concern for dairy producers globally. Interventions at the end of lactation (drying off) including internal teat sealants (ITS) and antibiotics which reduce the risk of new intra-mammary infection over the dry period, resulting in reduced clinical and subclinical mastitis incidence in the subsequent lactation. However, some ITS treated cows are diagnosed with clinical mastitis over the dry period or early in the subsequent lactation and loss of ITS may be a contributing factor. This study tested the effect of infusion of ITS (Interseal, Elanco New Zealand) at approximately 4 weeks following drying off in reducing the risk of subsequent subclinical and clinical mastitis in cows with a SCC of < 250,000 cells/mL at the last DHI test of lactation and cows > 4 years old. Cows from 23 spring calving herds in the Waikato region of New Zealand were randomly assigned to ITS or antibiotic dry cow therapy (DCT) at dry off (Dry Off treatment) and ITS or no treatment (negative control) at 4 weeks after drying off (Week 4 treatment) in a 2 by 2 factorial design. Cows were monitored until 30 d after calving for clinical mastitis, and the SCC from the first DHI test of the season for each cow were retrieved for analyses. Treatment groups were balanced for age, breed, previous SCC, milk yield, and lactation length. Cows treated with ITS at dry off were 2.1 (1.1-4.1) times more likely to get clinical mastitis first 4 weeks of the dry period than DCT treated cows. ITS administration at Week 4 reduced the cow-level clinical mastitis incidence in the first 30 d of lactation compared with negative control (OR = 0.59 (95% CI = 0.38-0.91)). There was no effect of dry off treatment on clinical mastitis incidence in the first 30 d of lactation. The geometric mean DHI SCC for DCT treated cows was 34 (95%CI = 31-53) vs 48 (95%CI = 43-53) x 1,000 cells/mL for ITS treated cows, with subclinical prevalence (i.e., SCC >200,000 cells/mL) being 10.3 (95% CI = 8.3-12.7) % vs 14.9 (95% CI = 12.3-17.9) % for DCT and ITS treated cows, respectively. There was no effect of Week 4 ITS administration on first DHI test SCC or subclinical mastitis prevalence. There was no interaction between Dry Off treatment and Week 4 treatment for any of the outcomes. We conclude that administration of ITS 4 weeks after drying off in low SCC, older cows following drying off with ITS or DCT alone resulted in a ∼40% decrease in cow-level early lactation clinical mastitis. Antibiotic DCT administration at dry off resulted in fewer cases of clinical mastitis in the first 4 weeks of the dry period, and a 31% relative reduction in subclinical mastitis prevalence at the first DHI test. Week 4 ITS administration is an intervention to reduce clinical mastitis in herds or sub-populations deemed at higher risk of clinical mastitis.