Mapping the spread of methamphetamine abuse in California from 1995 to 2008

Abstract

Objectives: From 1983 to 2008, the incidence of methamphetamine abuse and dependence (MA) presenting at hospitals in California increased 13-fold. We assessed whether this growth could be characterized as a drug epidemic.

Methods: We geocoded MA discharges to residential zip codes from 1995 through 2008. We related discharges to population and environmental characteristics using Bayesian Poisson conditional autoregressive models, correcting for small area effects and spatial misalignment and enabling an assessment of contagion between areas.

Results: MA incidence increased exponentially in 3 phases interrupted by implementation of laws limiting access to methamphetamine precursors. MA growth from 1999 through 2008 was 17% per year. MA was greatest in areas with larger White or Hispanic low-income populations, small household sizes, and good connections to highway systems. Spatial misalignment was a source of bias in estimated effects. Spatial autocorrelation was substantial, accounting for approximately 80% of error variance in the model.

Conclusions: From 1995 through 2008, MA exhibited signs of growth and spatial spread characteristic of drug epidemics, spreading most rapidly through low-income White and Hispanic populations living outside dense urban areas.

FIGURE 1—

Methamphetamine abuse and dependence (MA) discharges in California, per 10 000 persons in 1983 through 2008. Note. Fitted exponentials for 3 successive growth phases are as follows: MA = 0.749e0.230t, MA = 2.014e0.252t, MA = 3.824e0.165t; entire period: MA = 1.889e0.069t.

FIGURE 2—

Methamphetamine abuse and dependence (MA) discharges in California by geographic distribution for selected years. Note. Geographic distribution years chosen as first year of available zip code data (1995), recent peak of epidemic (2005), and most recent year (2008). Edge corrected inverse distance weights using zip code centroids and a variable search radius of 25 points.

FIGURE 3—

Posterior estimated growth of relative incidence rates of methamphetamine abuse and dependence. Note. Relative rates across zip codes are scaled to state average and bracketed by 2.5%–97.5% credible intervals.

FIGURE 4—

Posterior estimated growth of relative incidence rates of methamphetamine abuse and dependence, (a) geographic distribution for selected years and (b) geographic distribution of overall change: San Francisco Bay Area, 1995–2008. Note. Bay area maps show pattern of positive spatial autocorrelation.

FIGURE 5—

Posterior relative incidence rates of methamphetamine abuse and dependence for 4 model covariates, household income, population density, household size, and race/ethnicity: San Francisco Bay Area, 2008. Note. HH = household; Pop = population; RR = relative rate. HH income range = $6640 (RR = 2.03) to $181 180 (RR = 0.16); population density quintiles = 0–7.72 (RR = 1.04) to 2356–20 120 (RR = 0.95) persons/km²; HH size = 1.00 (RR = 0.24) to 6.74 (RR = 1.93) persons/HH; racial/ethnic group composition = no Whites or Hispanics (RR = 0.99) to 41.8% White and 97.6% Hispanic (RR = 2.65).