Infection transmission, pig-man resistance

David Burch Pig health
Infection transmission, pig-man resistance

I have been working on this subject recently to see if it can be quantified and these are my results in relation to human infections and antimicrobial use.

Human infections

Let’s look at the number of human infections and the incidence of disease and likely antibiotic treatment (from Prescott et al, 20051; Mossad, 20102; Foxman, 20023). Most of the data is based on United States figures but they are also likely to apply or be similar in Europe also.

Upper respiratory tract infections are the most common and approximately 75% of cases are treated with antibiotics. Pneumonias are also treated with antibiotics, often the more advanced ones. Sexually transmitted and urinary tract infections are also common. A lower number of cases of food and water borne infections are reported and possibly cause 5-7 times the number of infections but are not reported or treated with antibiotics. Overall, approximately 16.34% of the human population receive antibiotics each year. This relates to 16,340 people for every 100,000 population. I am sure more complete EU data will be developed and available over the forthcoming years.

Regarding the transmission of pig infections, the major ones considered were, Streptococcus suis, Staphylococcus aureus (MRSA), Salmonella Typhimurium, Campylobacter coli, and commensal bacteria Escherichia coli and Enterococcus spp, which have both been associated mainly with nosocomial (hospital) infections (Barlow et al, 20034; DANMAP 2010,20115; Burch, 20026; AHVLA, 20127; Nijsten et al, 19968; VLA, 20109; Danmap 2006, (2007)10.

The transmission of infections from pigs directly to humans, who are working with pigs, such as farmers, veterinarians and even slaughter men can be quite high. The incidence appears to fall away rapidly from farmers (83%) to families (4.3%), in the case of MRSA, so that transmission to the general public is actually very low causing 0.002% incidence of infections in the population in Denmark. Salmonella Typhimurium is carried by many animal species, so it is hard to attribute infections from pigs to man but based on UK phage types a 28.4% attribution can be made or only 7.4% of all human salmonella infections. On a population basis, this amounts to 0.0011% of reported cases. Campylobacter coli from pigs has been shown to have a minimal or almost zero transmission rate to man. Escherichia coli and Enterococcus spp are considered primarily commensal organisms and therefore not directly associated with infections. It has been shown, by looking at the antimicrobial resistance profiles of E. coli directly to pig farmers, that there is very low colonisation rate of 4%. When the antimicrobial resistance profiles of healthy human and pig isolates of these commensal organisms are compared, the similarity is comparatively low, especially in comparison with hospital isolates, where many of these infections arise. The bacteria have to get to man via food/meat, which is usually cooked and the organisms destroyed so transmission is relatively low in comparison with direct contact.

On this basis, it can be estimated that infections caused by pigs occur in 0.0031% of the human population or 3.1 people / 100,000. They would be causing 0.019% of the 16.34% human infections and that is the figure for antimicrobial resistance that pigs potentially cause, if all these infections were causing antimicrobial resistance. As resistance to ‘critical antimicrobials’ (fluoroquinolones and 3rd and 4th generation cephalosporins) is of major concern, this averages out at about 2% in pigs, then this is 0.00034% contribution of  resistance to man.

Before the European Commission comes out with their future guidelines, the relevant bodies, such as EFSA, EMA and ECDC should determine the rate of transmission of infections and antimicrobial resistance of other animal species to man in comparison with antibacterial use in man and carry out a true risk assessment and not use assumptions of 33-50%, which have been used by some ill-informed agencies, recently.

What are your thoughts?

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