Reports of colistin resistance prompting layer producers to rethink E. coli management
Dieter Vancraeynest, DVM, PhD
Pathogenic Escherichia coli leading to colibacillosis is still a leading cause of economic loss in the poultry industry. When a clinical outbreak occurs in broilers and turkeys, veterinarians have several effective antimicrobial agents available. It’s a different story for layers, however. Any antimicrobial used needs to have a 0-day egg withdrawal, which rules out many medications. E. coli resistance is also a problem.
In most EU-member states, colistin is one of the few antimicrobials with a 0-day egg withdrawal. It was also thought to have limited resistance issues. In fact, while colistin is poorly resorbed from the intestine, it has often been the drug of choice for colibacillosis outbreaks in layers. Unfortunately, new information has made layer producers rethink this approach.
Upon conducting a routine surveillance project on antimicrobial resistance in food animals, scientists in China found a major increase in colistin resistance due to a plasmid-mediated resistance gene — MCR-1. The resistance can spread to E. coli strains and even to other types of pathogens such as Klebsiella pneumoniae and Pseudomonas aeruginosa, the investigators reported.
They also found the resistance gene in humans, even though colistin is seldom used in people. In humans, colistin is considered a drug of last resort, used only for multi-drug-resistant infections because it is nephrotoxic and neurotoxic when administered systemically.
Since the report by Chinese scientists, other researchers have revisited collections of bacteria isolated in the past and they’ve found the MCR-1 gene, generally at low frequency, in animal populations around the world. In addition, isolates containing MCR-1 from different human sources are being increasingly reported.
These reports have sparked an intense debate about whether the use of colistin in food animals should be heavily restricted. In fact, the European Medicines Agency has recommended that colistin-containing medicines for animals be reserved as a second-line treatment and their sales minimized throughout the EU. It’s therefore important for the poultry industry worldwide to consider alternative methods of E. coli management with heavy emphasis on E. coli prevention.
A good starting point is to ensure pullets are properly vaccinated against relevant respiratory challenges such as infectious bronchitis and, in areas where it’s a problem, Mycoplasma. These infectious diseases can set the stage for secondary E. coli infection or exacerbate primary E. coli infection.
Vaccination of pullets against E. coli is helpful. Producers have used commercial and autogenous inactivated E. coli vaccines with some success. These vaccines need to be injected, however, which requires more labor and stresses birds. The autogenous vaccines also provide only limited cross-protection, which is sometimes problematic if more than one E. coli type is involved.
A newer option is a live, attenuated vaccine that can be delivered by mass spray or drinking water and helps protect against several strains of E. coli. In a controlled challenge study, commercial layers that received the vaccine (Poulvacâ E. coli) at day of age and again 12 weeks later had a 13% reduction in mortality compared to unvaccinated controls.
It’s important to keep in mind that some of the more pathogenic varieties of E. coli can be present in the environment. For instance, E. coli can be found in litter and water. It can be spread in dust blown around by fans. In addition, the pathogen can be tracked in by people from one poultry house to another.
Biosecurity is therefore imperative to help prevent and control E. coli infections. This includes limiting visitors to layer farms and requiring anyone who must visit to wear clean protective clothing and footwear. Other necessary measures include water sanitation, maintaining proper ventilation to reduce ammonia levels and control of rodents and other pests. Flock stress should be kept to a minimum.
In summary, layer producers may have even fewer options available for treatment of colibacillosis in their flocks and alternative approaches are needed. A combination of vaccination, biosecurity and good overall farm, house and flock management can help prevent the disease or, at the very least, minimize economic losses.
 Liu Y, et al. Emergence of plasmid-mediated colistin resistance mechanism MCR-1 in animals and human beings in China: a microbiological and molecular biological study. Lancet. February 2016;16(2):161-168.
 Quesada A, et al. Detection of plasmid mediated colistin resistance (MCR-1) in Escherichia coli and Salmonella enterica isolated from poultry and swine in Spain. Research in Veterinary Science, Volume 105, April 2016, Pages 134-135.
 Nordmann P, et al. Plasmid-mediated colistin resistance: an additional antibiotic resistance menace. Clin Microbiol Infect. 2016 May;22(5):398-400.
 European Medicines Agency, Science Medicines Health. Countries should reduce use of colistin in animals to decrease the risk of antimicrobial resistance. 27 July 2016.
Accessed July 28, 2016
 Data on file, Study Report No. PPH-0006, Zoetis LLC.
 Data on file, Study Report No. PL-178-2007, Zoetis LLC.
Posted on October 30, 2016