Dairy cows are at the highest risk of developing clinical and subclinical diseases and disorders in the first few weeks postpartum [1-4]. It is believed that more than 50% cows suffer from at least one or more disorders during this period. Mastitis, lameness, retained placenta, metritis, ketosis, and hypocalcemia are among the most common diseases in dairy cattle. The median prevalence of clinical mastitis is around 20-25 cases per 100 cows per year . In some countries and regions, the overall mastitis at cow level can be as high as 60% or more . Milk production loss in combination with treatment costs and culling related to mastitis constitute approximately $116-$325 /case . Lameness is a condition associated with lesions of hind limb, which is caused by pathogens invading the hooves. Research shows that lameness in dairy cows adversely affect milk production, reproductive performance, longevity, and the general well-being of the animal. The prevalence of lameness among most dairy herds varies from 20-40% [8, 9]. Lameness is the third largest cause of economic loss in the dairy cattle industry, after mastitis, and reproductive disorders.
Around the time of calving, the body of a dairy cow makes many biological and physiological changes and adjustments to prepare for calving, colostrum production and lactation [2, 10, 11]. The immune system of the dairy cow, in particular, undergoes numerous changes in order to help the cow successfully deliver and protect the cow from infectious challenges postpartum. One such change is the transfer of immunoglobulin G proteins, especially subclass IgG1, to colostrum, leading to a reduction of IgG1 in the maternal blood [12-14]. The transfer of IgG1 to colostrum and the feeding of newborn calves with colostrum provides the calves passive immune protection for their first few weeks of life following their birth. As the level of IgG1 and IgG2 should maintain an optimal balance to protect animals from intracellular and extracellular pathogens, a disruption of this balance compromises the immune protection of the periparturient dairy cows.
Another significant change related to the immune system is the elevated inflammatory responses during the periparturient period . Inflammation is the first immune response of an organism when facing a microbial infection or a tissue injury. Studies have shown that a certain degree of inflammation is necessary to initiate calving, assist placental expulsion and protect the dams from postpartum microbial invasion . However, excessive and persistent postpartum pro-inflammatory response has been linked to increased disease risk and decreased milk production [17, 18]. Metabolic changes accompanied with a pro-inflammatory state alter the homeostasis of the immune system, including IgG1/IgG2 balance, exposing dairy cows to increased disease risk in early lactation .
Considering the critical role of immune functions in the overall health, production, and reproduction performance of dairy cows, simple and convenient tests that can allow rapid evaluation of the immune status and conditions of dairy cows will provide a valuable tool for dairy farm management. Although laboratory tools and tests are available to measure and evaluate different cellular and molecular components of the immune system, these are typically not suitable for on-site farm testing. Here we report the use of a newly developed, D2Dx™ immunity test, to monitor the immune status change in dairy cows during the transition period [20, 21]. This is a single-step test requiring only the mixing of a testing reagent with a small volume of un-diluted and un-treated blood serum or plasma providing a test result within 30 seconds. This quick test can be conveniently performed on-site or in house at small local veterinary clinics thereby avoiding having to send samples out for offsite processing and waiting for test results to be returned.
The principle of the test is illustrated in Figure 1. This test uses a gold nanoparticle probe to evaluate the relative quantity of IgG1 and IgG2 in a blood sample. The nanoparticle is designed to interact with bovine IgG1 and IgG2 through two different modes: upon interacting with IgG2, the nanoparticle forms large aggregates; with IgG1, the protein will bind to the nanoparticle surface to form a protein layer, however, will not cause nanoparticle aggregate formation. The nanoparticle aggregates are detected by monitoring the color change of the assay solution using a handheld reader device, CT-100 from Nano Discovery Inc. The color change is due to the surface plasmon resonance shift in aggregated gold nanoparticles, a phenomenon that has been well studied and established for biosensing and diagnostic test applications [22-25]. In the presence of both IgG1 and IgG2, as in the case of bovine blood samples, the two proteins will compete to interact with the nanoparticles through the two different binding modes. The degree of color change is reflective of the relative quantities of IgG1 and IgG2 in a sample.
In this study, we tested blood serum samples from 230 dairy cows collected during their periparturient period, 14-7 days before and after calving. The 115 of the cows had mastitis and/or lameness, whereas the other 115 of the cows had no observed clinical problems.