Discipline: mastitis; Key words: treatment, culling, cow-specific, simulation model.
Costs associated with clinical udder infection can be divided into treatment costs, costs of mortality through culling (replacement of culled animals) and indirect costs from production losses. Intervention for clinical cases usually consists of (mainly within udder) antibiotic treatment. However, in light of its connection with antimicrobial resistance, it may be prudent to search for alternative interventions. For instance, culling cows with clinical mastitis could be a viable alternative. However, in contrast to antibiotic treatment, culling of infected animals is rarely considered an intervention, but rather a consequence. In studies, culling was considered in the context of optimal replacement decisions, where it led to only a slightly earlier optimal replacement time for cows with clinical udder infection; therefore, antibiotic treatment was recommended over culling in most cases. Other studies also evaluated the optimal replacement time in economic terms, comparing results for cows with and without udder infection. In addition to antibiotic treatment or culling of cows with clinical mastitis, it may also be economically beneficial to reduce the transmission of mastitis-causing pathogens; for instance, by improving hygiene or bio-security. However, if pathogen transmission is contagious, the number of new cases depends on the number of infected animals. Reducing the transmission of udder infection-causing pathogens may therefore lead to a long-term indirect cost reduction due to a decreased number of new cases. Consequently, the long-term effects of reducing udder infection must include transmission as a variable as well. These implications were further studied by Dr M. Gussmann and colleagues. The overall aim of their study was to compare different intervention strategies for clinical udder infections. Their results were published in the Journal of Dairy Science, Volume102 of 2019, pages 1483 to1493, the title being: Economic and epidemiological impact of different intervention strategies for clinical contagious mastitis
The researchers conducted a simulation study which represented a Danish dairy cattle herd with udder infection caused mostly by Staphylococcus aureus and nine different methods of intervention. A standard intervention of three days of treatment consisting of within udder injections for all clinical cases was used. Two of the methods reflected the use of more antibiotics and six reflected cow-specific treatment or culling decisions. For these methods, the cost and effectiveness of culling as an intervention was assessed.
The results showed that nearly all methods could reduce the number of udder infection cases (e.g., a median of 37 clinical cases with the extended within udder treatment over five day method and 30 clinical cases with the cow culled with recovery probability below 50%) compared with the standard method (median of 42 clinical cases). This happened alongside either increased antibiotic usage (e.g., from a median of 123 treatment days up to 179 treatment days with the five day method) or an increased number of cows culled in relation to udder infection (e.g., from a median of 16 up to 24 cows with the cow culled with recovery probability below 50% method). Methods with more antibiotics or reactive culling had a slightly higher net income (e.g., €190 median net income with the five day method or €197 with the cow culled with recovery probability below 50% method, compared with €188 for the standard method). This shows that a cow-specific clinical intervention approach can be cost-effective in reducing within udder infection incidence.
In conclusion, the results suggested that cow-specific intervention methods (including reactive culling) can be economically beneficial in the long term, even more so than an intervention with increased antibiotic treatment. The increased income from milk, together with the reduced number of udder infection cases, compensated for the extra intervention costs, though economic benefits also depended on the transmission level in the herd. In addition, only one method could not reduce the number of clinical cases compared with the standard three day within udder treatment. The incidence was decreased either at the cost of an increased use of antibiotics or at the cost of the number of cows culled in relation to udder infection. Therefore, the farmer has to choose the right balance between treatment and culling for a specific herd, taking into account their antimicrobial usage and longevity goals, as well as the main causative pathogen in the herd. The results have shown that cow-specific treatment or culling decisions will, in most cases, reduce the incidence of clinical infection while increasing income in the long term. The best option to achieve a stable and low number of infection cases would probably be to reduce within udder transmission.