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Drug Resistance Chapter 55 Drug Resistance Ramanan Laxminarayan, Zulfiqar A. Bhutta, Adriano Duse, Philip Jenkins, Thomas O’Brien, Iruka N. Okeke, Ariel Pablo-Mendez, and Keith P. Klugman The control of infectious diseases is seriously threatened by the insufficient controls on drug prescribing; inadequate steady increase in the number of micro-organisms that are resist- compliance with treatment regimens; poor dosing; lack of ant to antimicrobial agents—often to a wide range of these infection control; increasing frequency and speed of travel, agents. Resistant infections lead to increased morbidity and pro- which lead to the rapid spread of resistant organisms; and longed hospital stays, as well as to prolonged periods during insufficient incentives for patients, physicians, or even govern- which individuals are infectious and can spread their infections ments to care about increasing resistance. It is important to dis- to other individuals (Holmberg, Solomon, and Blake 1987; tinguish between risk factors for the emergence of resistance Rubin and others 1999). The problem is particularly severe in (de novo resistance) and those for the spread of resistance (pri- developing countries, where the burden of infectious diseases is mary resistance). relatively greater and where patients with a resistant infection are The molecular basis of resistance may give a clue to the less likely to have access to or be able to afford expensive second- likelihood of resistance emerging. If a single DNA base pair line treatments, which typically have more complex regimens mutation leads to the development of resistance, then its selec- than first-line drugs. Furthermore, the presence of exacerbating tion is likely to be widespread, especially if the biological fitness factors,such as poor hygiene,unreliable water supplies,civil con- cost of the mutation is low. De novo or acquired resistance flicts, and increased numbers of immunocompromised patients results in the appearance of a resistant strain in a single patient. attributable to the ongoing HIV epidemic, can further increase Subsequent transmission of such resistant strains from an infec- the burden of antimicrobial resistance by facilitating the spread tious case to other persons leads to disease that is drug resistant of resistant pathogens. In this chapter, we discuss the causes and from the outset, a phenomenon known as primary resistance burdenof drugresistanceandevaluateinterventionsthataddress (IUATLD 1998).Independent, cumulative events result in mul- theresistanceproblemindevelopingcountries.Althoughanum- tidrug-resistant bacteria or tuberculosis (MDR-TB). Both the ber of the interventions we discuss are relevant to drug resistance creation and the transmission of drug resistance contribute to in HIV/AIDS and other forms of antiviral resistance, chapter 18 its prevalence in a given population. This mechanism also includes a more in-depth discussion of this subject. holds true in the case of antimalarials; that is, resistance devel- ops when malaria parasites encounter drug concentrations that are strong enough to eradicate the susceptible parasite popula- RISK FACTORS tion, but they fail to inhibit the multiplication of naturally occurring resistant strains. Commonly used antimalarial drugs Drug Use in Humans are not mutagenic. The evolution of drug resistance is facilitated by a number of fac- In the case of tuberculosis, spontaneous mutations leading tors, including increasing use of antibiotics and antimalarials; to drug resistance occur rarely in Mycobacterium tuberculosis, 1031 and multidrug regimens can prevent the emergence of clinical countries, without controls on over-the-counter use, has led to drug resistance (Cohn, Middlebrook, and Russell 1959). some of the highest rates of resistance in the world, as was seen Resistance is thus an avertable phenomenon resulting from with penicillin resistance in Vietnam. Relatively wealthy coun- inadequate treatment, which, in turn, is often the result of an tries such as the Republic of Korea and Japan also have lax con- irregular drug supply, prescription of inappropriate regimens, trol and even greater access to funds to purchase antibiotics or poor adherence resulting from a lack of supervision. In the (Song and others 1999). Patterns of resistance differ by antimi- case of malaria, the widespread misuse of chloroquine as pro- crobial class, and resistance to several classes has been linked to phylaxis is believed to be an important factor in the emergence particular patterns of use in developing countries. Macrolide and spread of resistance to this drug. use in children in China may be preferred to the use of beta- Despite conventional wisdom, the highest rates of antibiotic lactams, which are known to be associated in rare instances resistance in the pneumococcus bacterium globally are not for with serious anaphylactic reactions, and in Beijing and penicillins or macrolides, which usually require multiple DNA Shanghai, the highest global rates of macrolide resistance are mutations or the import of foreign genes, respectively, but for encountered in nasopharyngeal isolates from children (Wang sulfamethoxazole-trimethoprim, which can be selected from and others 1998; Yang, Zhang, and McGee 2001). Tetracycline among a population of susceptible pneumococci by a single use remains widespread in developing countries, and poor base change in the dihydrofolate reductase gene (Adrian and African countries, such as the Central African Republic, may Klugman 1997). The direct selection of resistance following have higher rates of resistance to tetracycline than to beta- exposure of children carrying pneumococci has been shown in lactams or macrolides (Rowe and others 2000). a prospective study in Malawi to occur in 42 percent of children The relationship between compliance and resistance emer- exposed to sulfadoxine-pyremethamine for a week and in 38 gence in the treatment of acute and largely self-limiting infec- percent of children a month after exposure to drug treatment tions is less robust than in the case of chronic infections such as for malaria (Feikin and others 2000). tuberculosis (TB). It is likely that resistance selection occurs Evolutionary biology suggests that drug selection pressure is more readily in the commensal flora (for example, the pneu- an important factor in the emergence and spread of drug resist- mococcal flora of the nasopharynx) than among the organisms ance. Although the relationship between antimicrobial use and causing the acute infection. Thus, shorter courses (and reduced drug resistance (in the pneumococcus, for example) is well compliance) may reduce the selection of resistance in com- established in developed countries (Bronzwaer and others mensal flora. In contrast, in TB, selection takes place in the 2002), direct evidence to support this hypothesis is less forth- infecting pathogen, and poor compliance is associated with the coming in developing countries because of a lack of data on selection of resistant strains. antibiotic use. Resistance to antimicrobials is less likely to arise in the poorest developing countries simply because of the lower levels of antibiotic use associated with poorer socioeconomic Antibiotic Use in Animals status. For instance, India—a large country with scant control Many developed countries use antibiotics for veterinary uses, over antibiotic prescribing—has very low rates of resistance both for improving feed efficiency and rate of weight gain among systemic isolates of pneumococci, at least in rural areas (subtherapeutic use) and for disease prevention and treatment (INCLEN 1999). These low rates exist despite wide antibiotic (therapeutic use) (Levy 1992). Although the extent of anti- availability, probably because extreme poverty limits the dura- biotic use in animals in developing countries is unknown, one tion of antibiotic exposure for the treatment of acute pneumo- study from Kenya reported that tetracyclines, sulfonamides, coccal infections. Rising incomes and increased affordability of and aminoglycosides were the most commonly used antimi- antibiotics will likely change this low incidence of resistance; crobials for veterinary purposes (Mitema and others 2001). the same may be true of quinolones, which are widely available Over 90 percent of the antibiotics used were for therapeutic at relatively affordable prices, even in semirural and rural pop- purposes, and there was no evidence of use for growth ulations. This trend may be responsible for the emergence of promotion. nalidixic acid resistance to Shigella in Bangladesh and fluoro- There is strong evidence that the use of antibiotics in farm quinolone resistance to Salmonella typhi in India. animals promotes the development of drug-resistant bacteria Recent evidence suggests that shorter courses of antibiotics in animals (Aarestrup and others 2001). Because routes for the may select for less resistance in the pneumococcus compared movement of these resistant bacteria to humans are available, with longer courses (when patients comply with those courses) there is sufficient circumstantial evidence that drug resistance (Schrag and others 2001). Very low levels of resistance have also in bacteria associated with food animals can influence the level been found in isolated rural African communities (Mthwalo of resistance in bacteria that cause human diseases (Wegener and others 1998). This observation, however, should not lead and others 1999). Furthermore, mathematical models indicate to complacency. Increased access to
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