Antimicrobial drug-resistant Salmonella Typhimurium (Reply to Helms).

In Reply to Helms: In the article by Helms et al., Helms concludes that infections with Salmonella Typhimurium strains resistant to ampicillin, chloramphenicol, streptomycin, sulfonamide, and tetracycline (hereafter referred to as penta-resistant) were associated with higher death rates than infections with non-penta-resistant S. Typhimurium. Helms also concluded that infections with quinolone-resistant (nalidixine-resistant) S. Typhimurium were associated with higher death rates than quinolone-susceptible S. Typhimurium (1).

Table 2 in Helms' article provides information that enables close scrutiny of this conclusion and comparison of the excess mortality associated with penta-resistant, quinolone-susceptible S. Typhimurium with the excess mortality of non-penta-resistant S. Typhimurium (1). In this letter, the Table is based on the original table. However, two additional comparisons have been added: the p values, which are not based on the data but are approximations based on the parameters in the table.

The conclusion is that only quinolone resistance is associated with excess mortality compared with nonresistant isolates. Penta-resistant, quinolone-susceptible S. Typhimurium has a risk ratio of 2.9 (1.1 to 7.9) compared to the ratio of non-penta-resistant isolates 2.1 (1.5 to 2.9). When these figures are compared, the approximate p value is 0.55, which, of course, is far from being significant. Thus, on the basis of the article by Helms, penta resistance may not pose a greater threat to human health than non-penta resistance. However, the measured effect of penta resistance is achieved by the inclusion of quinolone-resistant S. Typhimurium in the group.

Table. Table showing additional comparisons (1) (a)

                                Resistant

                     Deaths/
                     cases      RR (b) (95% CI)

Penta with and       12/283      4.8 (2.2 to 10.5)
without quinolone
Penta with            5/40      13.1 (3.3 to 51.9)
quinolone
Penta without        7/243       2.9 (1.1 to 7.9)
quinolone

                                 Susceptible

                     Deaths/
                     cases       RR (b) (95 % CI)        p value

Penta with and       47/1,764    2.1 (1.5 to 2.9)        0.06
without quinolone
Penta with           47/1,764    2.1 (1.5 to 2.9) (c)    0.01 (d)
quinolone
Penta without        47/1764     2.1 (1.5 to 2.9) (c)    0.55c (d)
quinolone

(a) RR, relative risk: CI, confidence interval.

(b) Adjusted for coexisting conditions.

(c) Compared to the non-penta group.

(d) Approximations based on the parameters from the table.

Jan Dahl, Danish Bacon and Meat Council, Copenhagen, Denmark

Reference

(1.) Helms M, Vastrup P, Gerner-Smidt P, Molbak K. Excess mortality associated with antimicrobial drug-resistant Salmonella Typhimurium. Emerg Infect Dis 2002;8:490-5.

Address for correspondence: Jan Dahl, Danish Bacon and Meat Council, Axeltory 3, 1609 Copenhagen V, Denmark; fax: [phone]; email: [e-mail]