Controlled Human Hookworm Infection: Accelerating Human Hookworm Vaccine Development

Abstract

Background: Controlled human hookworm infection (CHHI) is a central component of a proposed hookworm vaccination-challenge model (HVCM) to test the efficacy of candidate vaccines. Critical to CHHI is the manufacture of Necator americanus infective larvae (NaL3) according to current Good Manufacturing Practice (cGMP) and the determination of an inoculum of NaL3 that is safe and reliably induces patent infection.

Methods: cGMP-grade NaL3 were produced for a phase 1 trial in 20 healthy, hookworm-naïve adults in the United States, who received either 25 or 50 NaL3. Participants were monitored for 12-18 weeks postinfection for safety, tolerability, and patency of N. americanus infection.

Results: Both NaL3 doses were well tolerated. Early manifestations of infection included pruritus, pain, and papulovesicular rash at the application site. Gastrointestinal symptoms and eosinophilia appeared after week 4 postinfection. The 50 NaL3 inoculum induced patent N. americanus infection in 90% of this dose group.

Conclusions: The inoculum of 50 NaL3 was well tolerated and consistently induced patent N. americanus infection suitable for future HVCM trials.

Clinical trials registration: NCT01940757.

Keywords: Necator americanus; challenge model; controlled human hookworm infection; current good manufacturing practice; hookworm vaccine.

Figures

Figure 1.

Representative images of rash induced by application of NaL3 to the forearm. A, Day 7 postapplication of 25 NaL3. B, Day 35 postapplication of 50 NaL3.

Figure 2.

Prevalence of gastrointestinal symptoms by study week. Bar graph represents sum of participants in each dose cohort who experienced abdominal pain, abdominal bloating, nausea, vomiting, or diarrhea that was assessed as being related to controlled human hookworm infection. A, 25 NaL3 cohort. B, 50 NaL3 cohort.

Figure 3.

Time to patency for controlled Necator americanus infection as shown by Kaplan-Meier curves for volunteers exposed to either 25 N. americanus third-stage larvae (NaL3), as shown by the red line, or 50 NaL3, as shown by the blue line. Solid lines represent the proportion of individuals in the dose group with patent N. americanus infection, as detected by saline flotation, while dotted lines represent the proportion with patent infection, as determined by quantitative polymerase chain reaction. Crosses indicate censored data. Abbreviation: qPCR, quantitative polymerase chain reaction.

Figure 4.

Eggs per gram (EPG) of feces, as determined by the McMaster (A) and quantitative polymerase chain reaction (B) methods for controlled human hookworm infection with an inoculum of 25 Necator americanus third-stage larvae (NaL3), as shown by the red line, or 50 NaL3, as shown by the blue line. Each dot represents the median EPG of the inoculum group by study day, and the solid lines show the trend in EPG for each inoculum group using locally weighted regression. The gray shading shows the upper and lower boundaries of the 95% confidence interval around the trend line.

Figure 5.

Receiver operator characteristic curves comparing the accuracy (area under the curve) of quantitative polymerase chain reaction with the conventional McMaster method to indicate patency (eggs in stool) from controlled human hookworm infection with 50 Necator americanus third-stage larvae (NaL3) using the saline flotation technique as the gold standard. Abbreviations: AUC, area under the curve; MM, McMaster; PCR, polymerase chain reaction.

Figure 6.

Peripheral eosinophil counts of volunteers infected with 25 Necator americanus third-stage larvae (NaL3) in red or 50 NaL3 in blue. Dots represent median eosinophil counts by study day, and the lines show the trend of eosinophil counts for each cohort using locally weighted regression. The upper and lower boundaries of the 95% confidence interval around the trend line are depicted by the gray shading.