Onchocerciasis and lymphatic filariasis are tropical diseases caused by parasitic worms. Programmes to eliminate both have been complicated by ivermectin-associated serious adverse events in communities having a high intensity of another parasitic nematode which causes loiasis.
Onchocerciasis, also known as ‘river blindness’ is caused by the parasitic worm Onchocerca volvulus. It is transmitted to humans by repeated bites of infected blackflies of the genus Similium producing symptoms which include severe itching, disfiguring skin conditions and visual impairment, including permanent blindness. More than 99% of infected people live in sub-Saharan countries. There is no vaccine to prevent infection but eliminating onchocerciasis involves spraying with insecticides against blackfly larvae supplemented by large-scale distribution of ivermectin.
Lymphatic filariasis or elephantiasis occurs when filarial parasites are transmitted to humans through mosquitoes. Infection is usually acquired in childhood causing hidden damage to the lymphatic system leading to disfiguring manifestations such as lymphoedema, elephantiasis and scrotal swelling in adults. Victims suffer mental, social and financial losses contributing to stigma and poverty. The World Health Organization (WHO) reckons that 1.23 billion people mainly living in Bangladesh, Côte d’Ivoire, Democratic Republic of Congo, India, Indonesia, Myanmar, Nigeria, Nepal, Philippines and the United Republic of Tanzania are at risk of lymphatic filariasis. The WHO’s strategy to beat the disease uses albendazole (400 mg) together with ivermectin (150–200 mcg/kg) or with diethylcarbamazine citrate (DEC) (6 mg/kg). Their target date for achieving elimination is 2020.
Loiasis (loa loa or eye worm infection) is spread by biting flies of the genus Chysops. The parasitic nematodes wander through the subcutaneous tissue but if they stop in one place for a while the host may suffer from local inflammation known as Calabar swellings. However, the worm is most obvious when it crosses the conjunctiva of the eye.
Because onchocerciasis and lymphatic filariasis are co-endemic with loiasis in Central Africa, and because administering ivermectin in communities where more than 20% of the population also has loiasis brings the risk of serious adverse reactions, including encephalitis, a strategy termed “test and (not) treat” has been proposed whereby those with high levels of L. loa microfilariae are identified and excluded from mass drug administration. Until now the assessment of whether or not loiasis is present at high levels in a community involved asking patients about the history of visible worms moving in the lower part of the eye.
A smartphone-based video microscope has been developed that automatically quantifies L. loa microfilariae in whole blood loaded directly into a small glass capillary without the need for conventional sample preparation or staining. The device captures and analyses videos of microfilarial motion using motorised sample scanning and onboard motion detection. The results can be obtained in under two minutes with minimal input from healthcare workers.
