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Table 5 Examples of study types evaluating the human health risk posed by infections with antimicrobial resistant foodborne or zoonotic pathogens

From: Antimicrobial drug use in food-producing animals and associated human health risks: what, and how strong, is the evidence?

Reference

Study

Key findings relevant to antimicrobial resistance

Typea

subjects

country

time period

bacterial target

relevant study outcomes

sample size

controls

comments

GRADE level: Controlled trial

Not applicable because of ethical concerns

GRADE level: observational study

[87]

CC

human patients

U.S.

1999–2001

Salmonella Newport; MDRAmpC resistant

risk factors for infection

34 cases; 37 infected controls and 94 community controls

Salmonella patients with susceptible Newport infection; healthy community controls

followed up by investigation of cattle isolates and risk factors for shedding on dairy farms; regional in scope (New England)

Association of infection with Salmonella Newport MDRAmpC with dairy farm exposure; bacterial isolates from humans and cattle were indistinguishable or closely related (based on antibiotic resistance profile and PFGE pattern)

[88]

CC

human patients

U.S.

2002–2003

Salmonella Newport; MDRAmpC resistant

risk factors for infection

215 case patients; 54 with MDRAmpC resistant strain and 146 pansusceptible; 1154 community controls

healthy community controls; volunteers without diarrhea in the previous 28 days

FoodNet study; national in scope (8 of 9 FoodNet sites)

Association of infection with Salmonella Newport MDRAmpC with consumption of uncooked ground beef or home prepared runny scrambled eggs or omelets during the 5 days before onset of illness

[89]

CC

human patients in Wisconsin

U.S.

2003–2005

Salmonella Newport MDR AmpC

associations between antimicrobial resistance and reported exposures

268 isolates from Wisconsin and 402 from the remaining U.S.

Cases in Wisconsin vs. rest of the U.S.

Controls were collected 2003–2004

Infections with multidrug-resistant Salmonella Newport strains were associated with exposures to cattle, farms and unpasteurized milk

[90]

CC

human patients in California

U.S.

1985

Salmonella Newport (chloramphenicol resistant)

risk factors for infection

45 patients and 89 matched controls

healthy volunteers

Epidemic Salmonella strain was isolated from hamburger products eaten by cases, as well as the abattoirs where the animals were slaughtered, dairies that sent the animals to slaughter, and ill dairy cows.

Association of infection with resistant Salmonella Newport isolates with eating ground beef during the week before onset and penicillin and tetracycline use during the month before onset of symptoms; chloramphenicol resistant Salmonella from dairy farms were associated with chloramphenicol use on those dairies

[91]

CC

human patients from national database

Netherlands

2003–2005

MRSA (non-typable)

risk factors for infection

35 cases, 76 controls

MRSA infections with other strains

Analysis aimed to exclude secondary cases

Living in rural areas and contact with pigs and cattle were associated with case patients

[92]

CS

veal calf farmers

Netherlands

2007–2008

MRSA ST 398

risk factors for carriage

102 farms, 390 questionnaires, 2151 nasal swabs from calves

farmers negative for MRSA ST 398

nasal swabs taken from farmers and farm calves; farms randomly selected

Human MRSA ST 398 carriage was associated with intensity of animal contact and number of positive animals on farm; calves were more likely to be carriers when treated with antibiotics, and good farm hygiene had a protective effect.

[93]

CS

swine farmers

Belgium

2007

MRSA ST 398

risk factors for carriage

49 swine farms, 127 persons on the farms participated and 1500 pigs were sampled

farmers negative for MRSA ST 398

nasal swabs and wound samples; farms randomly collected

MRSA ST 398 carriage by farmers was associated with prevalence among pigs on the farm, having regular contact with pigs, dogs, and horses, and use of protective equipment.

GRADE level: other

[22]

phylogenetic study

bacterial isolates

various

1997–2011

MRSA CC398

evolution and transmission dynamics including transmission between species

n/a

none

Phylogenetic and population genetic analysis

human and animal populations of CC398 are separate, but with multiple transfers back and forth; same data suggesting onward transmission of animal-associated strains in community settings (e.g., hospitals); different tetracycline resistance genes showed different evolutionary patterns

[94]

outbreak

human patients with rare strain

Denmark

1998

Salmonella Typhimurium DT104

source of multidrug resistant infection outbreak

27 cases

none

rare strain isolated from patients and pork samples; pork samples were traced back to herd; resistant infections in humans were more difficult to treat

A swine herd was the likely source of a human outbreak with MDR Salmonella Typhimurium DT104

[95]

outbreak

human patients infected with rare strains

U.S.

1983

Salmonella Newport

source of multidrug resistant infection outbreak

18 cases

none

rare isolate strain found in human patients and cattle

hamburger from beef cattle fed sub therapeutic chlortetracycline for growth promotion likely outbreak source

[96]

human case report

human patient infected with rare strain

U.S.

1998

Salmonella Typhimurium

source of multidrug resistant infection

1 case

none

Infection traced back to farm;

The farm was identified as the source of a multidrug resistant Salmonella isolate from a human case patient

[97]

human case report

MRSA cluster around pig farmer

Netherlands

2004

MRSA

source of multidrug resistant infection

pig farmer, family and co-workers, pigs

none

animals on farm sampled

Infection with MRSA traced back to farm, onward transmission in farm family

[98]

correlation study

Isolates from swine and swine workers

U.S.

Unspecified

MRSA

risk factors for MRSA colonization

20 workers and 299 swine

none

animals and humans sampled

Colonization with MRSA ST 398 was very common in swine herds, similar isolates in humans and animals

[99]

correlation study

isolates from farm personnel and milk samples

Hungary

2002–2004

MRSA

risk factors for MRSA colonization

1 farm, 12 workers sampled, 595 milk samples

none

MRSA strains were subtyped

MRSA isolates from workers in close contact with cows were indistinguishable from cow isolates

  1. a CC case-control study (or case-case study where controls are cases of infection with other strains), CS cross-sectional study