A discussion on ivermectin

The use of ivermectin in treating NTDs is well known, but its potential use in preventing malaria transmission is relatively novel. Ahead of World Malaria Day, BugBitten talks to Carlos Chaccour and Regina Rabinovich, who have published a series Ivermectin to reduce malaria transmission in Malaria Journal exploring the use of ivermectin in malaria prevention.

Q1. Please could you explain how ivermectin works to reduce malaria transmission?

Distributing ivermectin to treat NTDs
Distributing ivermectin to treat NTDs

A few years ago, the New York Times ran a piece about this concept and entitled it: “Human Swallows Pill. Mosquito Bites Human. Mosquito Dies”. Ivermectin is recognized for its great success in the fight against river blindness and elephantiasis but few know that it also kills mosquitoes that feed on those that have taken it.

Female mosquitoes bite because they need the proteins in the blood to produce eggs. If enough people in an area take ivermectin, this could result in massive mortality of the mosquito population. Fewer mosquitos usually equal less malaria.

There is however more complexity as ivermectin can also affect mosquito behaviour and fertility, expanding the impact beyond what could be expected based on mortality alone. All is tightly associated with the time the drug remains in the blood above a certain “mosquito-killing” threshold.

Q2. Don’t we have insecticides that kill mosquitoes already? Why would ivermectin be needed?

Insecticides do not impede evolution. Our current anti-mosquito interventions are mostly used within the home Treated bednets and Indoor Residual Spraying have had enormous success in reducing malaria deaths. This has however put selective pressure on mosquitoes and led to increase biting in areas or times not protected by these measures remarkably outdoors. This behavioural adaptation allows mosquitoes to forage on humans and avoid insecticides.

This drug kills mosquitoes that bite humans. If used appropriately at community level, a large proportion of blood-feeding mosquitoes in an area will die independently of time or place of biting.

Q3. How widespread is the use of ivermectin?

For its current indication in Neglected Tropical Diseases, more than 2.5 billion doses have been distributed in the last 30 years. Some 200 million in 2015.  It is not used for malaria at this time.

Q4. You have reviewed the use of ivermectin extensively, please could you summarise your finding?

Ivermectin should be envisaged as a complementary tool, not as a stand alone measure.

Ivermectin kills all malaria vectors against it has been tested, although there can be differences in susceptibility between species. The lethal effect is quick, most mosquitoes die within a few days of imbibing ivermectin containing-blood. The mosquito mortality is directly related to the amount of drug in the blood at the time of biting. To maximize impact against malaria, two factors will be key (a) the treatment scheme used and (b) the proportion of the population that is treated.

Ivermectin should be envisaged as a complementary tool, not as a stand alone measure.

Q5. What are your recommendations for the future use of ivermectin?

…data from the field is what will drive ultimate decisions.

Our recommendation is that the right data needs to be collected to allow WHO and countries to decide how such a drug would be used by communities, where it would best be deployed, and the overall risk-benefit.  While it is a potential valuable addition to the malaria toolbox, data from the field is what will drive ultimate decisions.

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