Dr. Päivi Rajala-Schultz, Department of Veterinary Preventive Medicine, The Ohio State University
Most - if not all - diseases we deal with in dairy production are multi-factorial. In other words, several factors that are often interrelated work together in a complex network to impact the occurrence of a disease. These factors are called determinants of a disease or simply risk factors for a disease. They can be divided into three categories: host, environment, and pathogen related risk factors. Host factors are associated with the cow, such as her age, days in milk, immunological status, disease history, genetics, milk production, etc. Environmental factors can include, for example, geographical location, season of the year, housing (type of stalls, bedding, stocking density, and social groups), and nutrition. Pathogen risk factors are related to infectious diseases and include the ability of the organism to cause severe disease and its ability to be transmitted among animals. Let's take a look at some of these factors.
Transition period, starting two to three weeks prior to calving and continuing two to three weeks after calving, is the most stressful time in the lactation cycle of a dairy cow. She goes through a number of profound physiological changes due to the increasing demands of the fetus and the development of the mammary gland and initiation of milk synthesis. Besides the hormonal and metabolic changes, cows often go through changes in ration as well as in social groups during the transition period. Stress caused by these changes weakens the immune system of the cow and makes her susceptible to diseases. Therefore, it is no surprise that disease occurrence peaks around calving and early lactation.
The most common diseases that occur at and around calving are dystocia (difficult calving), milk fever, retained fetal membranes, metritis (infection of the uterus), mastitis, ketosis, displaced abomasum, and other digestive disorders. It is important to realize that diseases do not occur independently of each other, instead they are often strongly interrelated. A great example of a disease involved in such an intertwined network is milk fever. Hypocalcaemia, i.e., milk fever (either clinical or subclinical), increases the likelihood of cows experiencing difficulties with calving (due to decreased muscular tonicity). This increases the risk of retained fetal membranes and uterine infections, which in turn increases the likelihood of infertility problems later on. A cow with milk fever has difficulties rising up and teat injuries can occur in conjunction with the unsteadiness. Cows with teat injuries have been reported to be 8 times more likely to contract clinical mastitis than cows without them. Cows with milk fever are also at 30% higher risk of experiencing ketosis during the lactation. Cows with ketosis, on the other hand, can be 6 times more likely to develop displaced abomasum than cows without it. This association can go both ways - abomasal disorders can also increase the risk of ketosis. Clinical ketosis increases also the risk of silent heat, cystic ovaries, and other infertility disorders. So, once the disease process starts, there is a cascade of events that can have a profound effect on the entire lactational performance.
Occurrence of most diseases in early lactation is directly or indirectly influenced by nutritional factors. Dry matter intake decreases and cows require more energy than they can consume, resulting in a negative energy balance, mobilization of body tissues, and loss of body condition. This is a normal and necessary process to help the cow meet the energy requirements of the lactation. It is not until some weeks after the peak milk production that dry matter intake reaches its maximum and the cow can regain the lost body weight. In addition to this, cows typically go through several ration changes during the transition period and unless these changes are gradual enough and the rumen microbes are provided enough time to adjust to the new ration and the ration is well balanced to meet the nutritional demands of the cow, digestive disorders and other diseases easily follow.
Mastitis is also an excellent example of a disease with multifactorial background. Risk factors in all three groups (host, agent, and environment) contribute to the occurrence of the disease. The rate of new intramammary infections is typically higher during the dry and transition periods than during the rest of the lactation. This is one of the reasons that has prompted the use of routine antibiotic dry cow therapy in control of the disease. However, focusing the control efforts on the agent is usually not enough. For effective mastitis control, we need to remember also the host (cow) and environmental factors. After the cessation of milking, the teat ends continue to be exposed to environmental pathogens, but the flushing effect of milking is gone. A dry, clean environment is extremely important to a dry cow. Recent studies have shown that even in cows that have been treated with antibiotics at dry-off, the risk of having a new infection during the dry period, or at least one quarter infected with environmental pathogens at calving, increases significantly with increasing milk yield at dry-off. The higher the milk production of the cow at the time she is dried off, the more likely she is to be infected at calving. Increased intramammary pressure from a full gland may cause leaking of milk, and the teat canal is more likely to stay open longer into the dry period. The slower formation of the protective keratin plug in the teat canal allows entry of the environmental pathogens into the mammary gland. It has been shown that cows that leak milk following dry-off are four times more likely to develop clinical mastitis during the dry period. Reducing the level of milk production prior to dry-off may directly help to reduce the rate of new intramammary infections during the transition period and increase the resistance of the mammary gland in the early dry period.
In conclusion, the transition period is probably the most important time in the production cycle of a cow in determining her health, lactation performance, and longevity in the herd. Good balanced nutrition and a dry, clean environment during the transition period can not be emphasized enough in maintaining healthy cows. It is also extremely important to remember that everything works in a complex network and in solving, or preferably in prevention, of health problems the entire, complex picture with all factors need to be considered.