Disease of cattle
Bovine respiratory disease (BRD) is the most common and economically devastating infectious disease affecting beef cattle in the world. It is a complex, bacterial or viral infection that causes pneumonia in calves which can be fatal. It also affects many other species of feedlot animals like sheep and pigs, but is most prominent in calves. The infection is usually a sum of three codependent factors: stress, an underlying viral infection, and a new bacterial infection. The diagnosis of the disease is complex since there are multiple possible causes and cattle are reluctant to display any indications of pain.
The disease manifests itself most often in calves within four weeks of weaning, when calves are sorted into groups of roughly 10-15 in number, and sold to different feedlots via train, tractor trailer, and truck. Because of this, BRD is most commonly referred to as "shipping fever". It is not known whether the stress itself, co-mingling, or travel conditions are at most to blame, and while studies have identified general stressing factors like transport and cold weather conditions, there is still no conclusive evidence on more specific factors (e.g. distance, transport mode, temperature, or temperature volatility).
Causes.
BRD is a "multi-factorial syndrome" that is dependent on a number of different causes. The pathologic condition commonly arises where the causative organism becomes established by secondary infection, following a primary bacterial or viral infection. 
Stress.
There are many stressors that can help induce BRD, the main one being transportation. During transit, cattle are exposed to many factors such as dehydration and. exhaustion that can lead to anxiety. Poor ventilation and overcrowding also allow bacteria to spread more easily between the calves.
Bacterial Viruses.
Source:
Bacterial Pneumonia.
Source:
Weather.
Fall is the most prominent season for beef cattle, so it is during that time of year when the majority of cattle are grouped together in larger amounts. This increases the risk for BRD along with fluctuations in weather. Studies show that they years with the highest risks for BRD were associated with severe weather environments, rather than average conditions.
Clinical signs and diagnosis.
BRD often develops within 4 weeks of cattle transport. Symptoms include:
Treatment and control.
Because of the number of possible viral/bacterial precursors to BRD, there are a number of treatment options circling around the three main aggravators of the disease: Viruses, Bacteria, and Stress.
Vaccination.
Vaccinations exist for several biological BRD precursors, but the multitude of possible precursors complicates the process of choosing a vaccine regime. Additionally, vaccines are not completely effective in stopping the disease, but are merely helpful in mitigation. Many of the problems with vaccine effectiveness rest with improper use, such as failing to time vaccine doses appropriately, or not administering them before shipping. Vaccines are available for a number of viral/bacterial agents and are often sold in combination with each other.
Antibiotics.
Calves are often bought unvaccinated, so antibiotics can help combat the bacterial factors of the disease. These are given orally, or in the forms of injections, pastes, or electrolytes. The Virginia Cooperative Extension recommends the following antibiotics:
Aside from the strict use of drugs, there are other natural ways to help calves recuperate. Ensuring that they are fed good quality feedstuffs consisting of materials like grass to coax them into consuming the most amount of nutrients possible. Additionally, they should refrain from staying in an area of poor ventilation with fluctuating temperatures to ensure that the infection does not grow worse. 
However, multi-drug resistance has become more prevalent, requiring further studies to target the gene responsible for the resistance.
Stress management.
Stress often serves as the final precursor to BRD. The diseases that make up BRD can persist in a cattle herd for a long period of time before becoming symptomatic, but immune systems weakened by stress can stop controlling the disease. Major sources of stress come from the shipping process and from the co-mingling of cattle.
References.
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