Is There a Pill to Prevent Loiasis (African Eye Worm)?

Yes, there is a pill that can prevent Loiasis, but it is not a universal standard and requires strict medical supervision. An oral medication called diethylcarbamazine (DEC) has been historically used as a weekly preventative pill (chemoprophylaxis) for long-term travelers and expatriates heading to high-risk areas. However, because of severe safety risks for people who might already be infected, the medical community strongly emphasizes that the best and safest prevention is avoiding the bites of the flies that carry the parasite.

Weekly DEC for Prevention

For expatriates, missionaries, and long-term workers living in high-risk zones, taking a weekly 300 mg dose of DEC has been shown to be highly effective at preventing Loa loa infections ^[1][2][3]. DEC works by killing the microscopic forms of the parasite before they can establish a full infection in your body.

Despite its effectiveness, DEC is not an internationally standardized routine recommendation for all travelers ^[4][5]. It is generally considered an “off-label” use—meaning it is primarily approved for treating active infections, but doctors may prescribe it for prevention based on specific, high-risk circumstances rather than broad travel guidelines. Short-term tourists typically do not require DEC and should rely entirely on bite prevention.

The Critical Need for Pre-Screening

If you have previously lived in or traveled to an area in Central or West Africa where Loiasis is common, there is a chance you may have already been exposed to the parasite. This makes taking a preventative pill potentially dangerous without prior testing.

Taking a full preventative dose of DEC when you already have a high number of Loa loa parasites in your blood—a condition called hypermicrofilaremia—can trigger rapid and life-threatening inflammatory reactions, including serious brain swelling (encephalopathy) ^[6][7][8]. Similarly, if you happen to be co-infected with Onchocerca volvulus (the parasite that causes river blindness), taking DEC can cause a severe, systemic reaction known as a Mazzotti reaction ^[9][10].

Because of these risks, mandatory medical screening is required before anyone can safely start DEC therapy for prevention ^[11][12][4]. This typically involves taking a blood smear during the middle of the day. This timing is necessary because the microscopic parasites are “diurnal,” meaning they circulate in the bloodstream in the highest numbers between 10:00 AM and 2:00 PM.

(Note for patients already diagnosed with Loiasis: While unmonitored DEC is dangerous if you have a high parasite load, it is still the primary medication used to treat the disease. Doctors have specific, safe protocols to administer it for treatment, often by using other medications first to slowly lower your parasite count before starting DEC.)

The Safest Defense: Avoiding Bites

Because of the complications and required screening associated with DEC, the undisputed first line of defense against Loiasis relies on personal protective measures to avoid being bitten by Chrysops flies (commonly known as deer flies or mango flies) ^[13]. Even if you are already infected, avoiding bites is crucial to prevent acquiring more parasites.

These flies are active and bite during the daytime, frequently peaking in activity around midday ^[13][14]. To protect yourself effectively, you should:

• Use the right repellents: Repellents containing DEET or citriodiol are proven to significantly reduce landings and bites from Chrysops flies ^[15]. Other common repellents, such as icaridin or IR3535, have shown little to no effectiveness against these specific flies ^[15].
• Wear light-colored clothing: Chrysops flies are highly attracted to dark objects, especially black ^[16]. Wearing long-sleeved, long-legged, light-colored clothing provides a physical barrier and makes you less of a visual target.
• Treat your gear: Treating your clothing, tents, and netting with permethrin adds an extra layer of chemical defense.
• Be aware of your environment: These flies breed in muddy streams and swampy areas near forests, and they are strangely attracted to wood smoke, which is often present near human dwellings ^[13][17].