claim that the presumptive human health benefits of employing such an approach
is unsubstantiated by meaningful clinical data (Pfaller 2006 ; Phillips 2007 ).
Wassenaar et al. ( 2007 ) addressed the concern that fluoroquinolone-resistant
Campylobacter infections could result in more severe disease than susceptible
strains. In a detailed analysis of the apparent link between fluoroquinolone resis-
tance inC. jejuniand its presumed increased virulence, Wassenaar et al. ( 2007 )
argued that there were no significant differences in the duration of disease between
susceptible and resistant infections. However, for both resistant and susceptible
infections, disease symptoms were prolonged by 1–2 days on an average in British
patients that had recently travelled abroad compared to those who had not travelled.
This finding illustrates that even this seemingly straightforward issue may not be as
clear-cut as it originally appeared.
Threlfall et al. ( 2006 ) examined changes in the occurrence of antimicrobial
resistance in other zoonotic pathogens,Salmonella entericaserotypes Enteritidis
and Typhimurium obtained from human infections in England and Wales (2000,
2002 and 2004). These investigators showed that the incidence of strains of
S. enteritidiswith resistance to nalidixic acid, coupled with decreased susceptibility
to ciprofloxacin, had more than doubled between 2000 and 2004. In contrast, the
overall levels of resistance inS. typhimuriumhad fallen by approximately 25%.
These data demonstrated that changes in the incidence of resistance did not
correlate with veterinary usage (as measured by veterinary sales of antimicrobial
drugs in the UK). Furthermore, the study showed that:
l ForS. enteritidisthe important factors associated with an increased incidence of
resistance were foreign travel and the consumption of imported foods contami-
nated with drug-resistant strains.
l ForS. typhimuriumthe most important factor was an overall decline in the
occurrence of multiple drug-resistantS. typhimuriumdefinitive phage type 104,
reflecting a change in serotype prevalence.
In their final analysis, Threlfall et al. ( 2006 ) concluded that changes in the
incidence of resistance in predominantSalmonella spp. in human patients in
England and Wales are multifactorial. The important point is that resistance rates
were linked to serotype prevalence rather than to antibiotic usage.
Wassenaar and Silley ( 2008 ) also reviewed susceptibility profiles for a range of
host-specific pathogens and antimicrobial compounds, in an attempt to identify
lessons learned when considering bacterial pathogens that differed in host specific-
ity. Their survey included bacterial pathogens that were limited to a human host,
those specific to particular food producing animals and those that occur in both host
types. From their examination ofStaphylococcus aureus, E. coliandSalmonella
enterica, they noted that Salmonella resistance profiles seemed to be strongly
correlated to specific serovars (where serovar, or serotype, is defined as the identi-
fication of surface antigens that allow the epidemiologic classification of organisms
at the sub-species level). Fluctuations in the dominant serotype were the most
important factor determining theSalmonellaresistance level. Furthermore, they
found that multidrug resistance was a more severe problem in human pathogens
240 M. Martinez and P. Silley