High-Altitude Plasmodium falciparum and Plasmodium vivax Reservoirs in Ethiopia Are Not Linked to Recent Travel.

BACKGROUND

The Ethiopian highlands >2000 m above sea level are classified as experiencing very low malaria transmission, but clinical case numbers have increased in recent years. Transmission intensity, and how many infections are imported from the lowlands, are not known.

METHODS

Surveys on subclinical prevalence were conducted in 8 sites across an altitude range of <1900 to 2900 m. Samples were screened by quantitative polymerase chain reaction (qPCR) and microscopy or rapid diagnostic testing (RDT). Data on recent travel were collected from subclinical and clinical individuals.

RESULT: Among 1241 subclinical individuals, P. falciparum prevalence by qPCR decreased from 67% at 1850 m to 16% at 2850 m. P. vivax prevalence ranged from 10% to 34%, with no clear altitude trend; 20.3% of individuals reported recent travel, which did not affect risk of infection. Among 1775 microscopy-positive clinical patients in 8 health centers, recent travel was reported by 2%-16% of patients at altitudes <2500 m, 21% in a health center at 2600 m, and 100% at 2830 m. P. falciparum RDT diagnosis based on LDH detected nearly twice as many subclinical infections than diagnosis based on HRP2.

CONCLUSIONS

High prevalence at altitudes up to 2800 m, and a minority of individuals reporting recent travel, might indicate sustained local transmission in the Ethiopian highlands, though undocumented travel and P. vivax relapses limit the ability to accurately estimate the proportion of infections being imported. Sites up to 2500 m should be considered endemic for malaria, and control interventions need to be intensified.