Few antibiotics available | BRD specimens examined by a U.S. lab showed resistance rose substantially from 2009-11
A recently published survey of U.S. cattle records shows evidence that multidrug resistance within the bacteria that causes bovine respiratory disease is growing.
BRD is a significant disease for the livestock industry and costs of managing it can total $1 billion annually in the U.S.
In Canada, more than 80 percent of drugs are directed at pathogens associated with the disease, also identified as pneumonia.
Treating the disease is already problematic for cattle producers because there are only a few antibiotics available.
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“Ranchers and feedlot operators are already limited in the types of antibiotics that they can use to treat respiratory disease in cattle,” said Brian Lubbers, director of clinical microbiology at Kansas State University’s veterinary diagnostic laboratory.
“Antimicrobial resistance further limits those treatment options.”
Lubbers and a colleague, Gregg Hanzlicek, conducted a survey of BRD records, examining cases in which specimens of bovine lung tissue had been sent to the lab. Most of the samples were from cattle in Kansas and Nebraska.
They documented antimicrobial multidrug resistance in the bacteria Mannheimia haemolytica, which is associated with BRD, isolated from cases over a three-year period, 2009-11.
The results of the survey were published in the Journal of Veterinary Diagnostic Investigation.
“Our survey showed that a high percentage of bacteria (Mannheimia haemolytica) recovered from the lungs of dead cattle was resistant to multiple antibiotics.
“If we define ‘multidrug resistant’ as resistance to three or more antibiotics, the percentages of recovered bacteria that were multidrug resistant were 42 per cent, 46 per cent and 63 per cent in 2009, 2010, and 2011 respectively.”
However, of the specimens recovered from lung tissue of deceased cattle, no single specimen was resistant to all six antimicrobial drugs.
Many factors contribute to the disease:
- poor colostrum transfer to calve
- malnutrition
- dehydration and a suppressed immune system
- environmental stresses such as transportation, poor ventilation, extreme temperatures, crowding and intermingling.
Typical signs of the disease include fever over 40 C, difficulty breathing, nasal discharge, a soft cough, and varying levels of listlessness.
Cattle may be less inclined to clean their coat or may isolate themselves from others. One of the earliest signs of the disease is loss of appetite.
Lubbers said that the bacteria became resistant to antibiotics through one of two ways.
“One path is through mutation of DNA. As bacteria reproduce, mistakes are made in the genes. Sometimes these mistakes kill the bacteria, but sometimes they have no effect.”
He said that if the mistake is in a gene that is the target for an antibiotic, the bacteria may become resistant to that antibiotic.
The other means by which bacteria become resistant is by sharing DNA. He explained that bacteria can carry small “packets” of genes known as plasmids that are shared between bacteria of the same species.
The researchers’ work may have significance to human medicine, even though the bacteria studied, Mannheimia haemolytica, affects cattle and not people.
“The ways in which bacteria develop resistance (mutation, plasmid transfer) are similar whether the host is a human or an animal,” said Lubbers. “As we continue to do further research, we hope to shed light on how antimicrobial resistance can be more effectively limited in both animals and humans.”
The best approach for ranchers protecting their cattle from BRD is preventive maintenance, including vaccinations, reducing stress factors and maintaining optimum physical health.
“I do know that the last new class of antibiotics for use in animals was introduced in the 1970s and new drugs since then have been modifications of existing drugs,” he said.
“I can also tell you that it takes approx-imately 10 years and millions of dollars to develop a new drug. What this means for livestock producers is that we must continue to use and adopt antibiotic best use practices to prolong the effectiveness of the antibiotics we already have.”
Lubbers said that the use of husbandry practices, such as proper housing, adequate nutrition and vaccinations, are always the cornerstones of judicious antibiotic use programs.
“Healthy cattle are one of the best ways that producers can deal with antimicrobial resistance.”
