Introduction

Black flies, also called buffalo gnats or turkey gnats, are blood-feeding insects in the insect family Simuliidae. These pest gnats (2-5mm) are not to be confused with “biting gnats” commonly encountered along the coast of Mississippi, which are very tiny (1mm or less) (Figure 1). Unlike mosquitoes which breed in stagnant water, immature black flies breed in fast-flowing water, from which the adults emerge in great swarms (usually) in spring or early summer. Black flies are notorious nuisance pests in certain parts of the world and are vectors of the agent of onchocerciasis in tropical areas, leading to a condition called “river blindness.” Nuisance biting from black flies by itself can be serious, even fatal, with many reports of livestock and poultry deaths due to black fly biting.1–4 Outbreaks were a major problem in the southern U.S. during the early 20th Century,1 but had disappeared until recently. This re-emergence may be due to elimination of harsh, environmentally persistent pesticides and improving water quality of creeks and rivers.

Figure 1
Figure 1.Black fly (left) versus biting gnat (right)

Photo courtesy Jerome Goddard, Ph.D., used with permission.

Black fly bite reactions include papules, erythematous wheals, indurations, and extensive swelling of the affected area (Figure 2).5,6 In addition, adenopathy (in this case, “black fly stiff neck”) may occur from bites occurring on the face, neck, or scalp.2 There have been many reports of human cutaneous reactions to black fly biting in the northern U.S, and other parts of the world,2,7–9 but only a few from the southern U.S. where black flies have typically not been a problem.

Figure 2
Figure 2.A typical black fly bite reaction

Photo courtesy Jerome Goddard, Ph.D., used with permission.

That all changed in 2009, when the Mississippi Health Department and the Mississippi State University Extension Service began getting reports of serious buffalo gnat and turkey gnat biting incidents, particularly during spring and early summer. A literature and popular press search in 2016 revealed three online articles and papers referencing black fly nuisance biting in Mississippi―one was a newspaper article in The Natchez Democrat describing the attack of black flies in the local community10; another was a Gunowners Blog reporting severe black fly problems and asking for control measures for this pest11; and lastly, a physician at the University of Mississippi Medical Center published an article in 2013 about a clinical reaction in an older patient bitten by a black fly. That paper described sequelae from the black fly bite as a systemic reaction due to sensitivity or toxicity from the attack.12 The MSU Extension service continues to get reports of intense and severe black fly attacks in (primarily) southern Mississippi, including deaths of backyard poultry. In one recent case, a chicken killed by black flies upon necropsy revealed dozens of black flies packed in the oral cavity and esophagus of the dead bird.13

Black fly problems in Mississippi are likely to get worse. In 2009-10, black fly outbreaks were mostly seasonal, lasting from March through May, but recent research and surveillance in Mississippi has shown nearly year-round emergence of the pests in Amite and Covington Counties (Figure 3). In fact, there is an annual emergence of black flies from Okatoma Creek near Seminary, Mississippi every year in late December,14 which this author calls “Christmas black flies.” Physicians should be aware of these pests and the lesions they produce because they are almost certainly to encounter patients suffering from black fly bites.

Figure 3
Figure 3.Black flies can be found year-round.

Photo courtesy Jerome Goddard, Ph.D., used with permission.

Treatment and Control

Black fly bites can result in papular urticarial lesions, which cause intense itching and localized swelling. These lesions may be treated topically with a low-to-medium dose corticosteroid creams. Pruritus can be managed with oral nonsedating antihistamines.15 Should cellulitis or a more severe, febrile illness occur such as black fly fever, additional supportive care and systemic antibiotic may become necessary.16 There have been reports of unusual systemic reactions to black fly bites, including one report of acute inflammatory demyelinating polyneuropathy (Guillian-Barre syndrome), for which supportive care was indicated.9 Though rare, black fly bites may cause a systemic reaction consistent with anaphylaxis requiring treatment with intramuscular epinephrine at a dose of 0.01 mg/kg per single dose, at a concentration of 1:1000.

As for control, there are technologies available to treat streams, creeks, and rivers with a bacterial insecticide, Bacillus thuringiensis israeliensis,17 but these can be expensive and difficult to implement. The best treatment is prevention, for which a variety of insect repellents may help. When outdoors during times of black fly emergences, patients should apply insect repellents that contain N,N-diethyl-3-methylbenzamide (DEET), picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol (PMD), or 2-undecanone.


Acknowledgement

Dr. Patricia Stewart, Allergist-Immunologist, Mississippi Center of Advanced Medicine, Madison, MS, kindly provided the treatment recommendations.

Funding

This work is partially supported by Crop Protection and Pest Management, Extension Implementation Program, award no. 2021-70006-35580 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.