BRD diagnosis in feedlots may become easier to make

Agriculture Canada scientists at Lethbridge have developed a rapid and accurate diagnosis of bacteria responsible for bovine respiratory disease in feedlot cattle.

While the concept is not ready for commercial feedlot use, years of work using genomics to identify specific bacteria has shown these microbes can be identified accurately and the correct treatment can be used at the right time.

When cattle arrive at a feedlot, a deep nasal swab collects material to identify the presence of harmful bacteria. Using something as simple as a test strip, a technician could get results quickly and the right treatments could be administered.

“It would be used as a tool to gauge to what extent antimicrobial use would be required,” said senior researcher Tim McAllister, who along with Cheyenne Conrad prepared a report on the innovation.

BRD is the most common and economically important disease affecting feedlot cattle. About 15 percent of cattle in North America are treated for it and it’s responsible for about 70 percent of illnesses and 40 percent of deaths. Costs are estimated at more than $1 billion a year.

The disease is characterized by complex interactions between the cattle’s immune system and bacteria like Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis, as well as viruses like bovine herpes virus-1, parainfluenza-3, bovine viral diarrhea virus and bovine respiratory syncytial virus.

Suppression of the host immune system as a result of stress or viral infection can allow pathogenic bacteria to proliferate within the upper respiratory tract, spread to the lower respiratory tract and cause BRD.

Symptoms are often non-specific and generally occur within the first 40 days of arrival at the feedlot.

Currently, high risk cattle are often treated with antimicrobials to control the disease, whether there are symptoms or not.

Effective antimicrobial stewardship requires using the right antimicrobial target for the right bacteria at the right dosage in the right animal at the right time.

Antimicrobial use can select for resistant bacteria and if they persist within a herd, this reduces the ability of a product to treat or prevent BRD. The Agriculture Canada laboratory has isolated BRD-causing bacteria that are resistant to 10 different antimicrobials.

This is where genomic sequences of bacteria can be used to correctly identify individual strains that may be harmless or cause disease, as well as those showing resistance to commonly used drugs.

They know, for instance, that Mannheimia haemolytica comes in different forms. Serotype 1 and 6 cause disease in cattle in North America.

“Even within serotype 1 and serotype 6 there are different bacteria that are even more potent within those groups,” McAllister said.

The research team at Lethbridge modified a genomic technique used in human medicine known as recombinase polymerase amplification (RPA).

RPA was able to detect the bacteria that cause BRD in less than 30 minutes, representing a significant opportunity to advance and improve diagnostics.

“It is all about matching that specific DNA sequence to the organism of interest,” he said.

Many of the genes that cause resistance in BRD-causing bacteria are identified, so probes were constructed to detect the genes that make bacteria resistant to several of the antimicrobials used to control BRD, including Micotil, Draxxin, Zuprevo, Nulflor, Trimidox, Terramycin and Aureomycin.

Probes were also designed to detect mobile genetic elements that carry genes that necessary for conjugation known as ICE.

These ICE transfer antimicrobial-resistant genes from one bacterium to another, turning bacteria that would normally be killed by antimicrobials into bacteria that are resistant. If these elements are circulating within bacterial populations that cause BRD, it could become much more difficult to treat the disease in cattle.

Further research into these refinements is already underway in a new project funded by Genome Canada.

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