July 2018

Sudden Death in Backyard Poultry – Consider Black Flies

By Dr. Cindy Bell

Earlier this spring of 2018, the death of a bull and cow in Arkansas was linked to black flies of the insect family Simuliidae.1 In a more recent article dated May 25th 2018, news from Iowa linked high mortality in backyard poultry flocks with black flies.2 Some species may feed preferentially on birds, Simulium flies bite and feed on a wide range of mammalian and avian hosts, including many species of livestock and humans. Simulium meridionale, also known as the “Turkey gnat”, has been linked to an outbreak of acute avian deaths in Louisiana in 2010, which affected pet birds and poultry.3 This particular species is prevalent throughout the Mississippi valley and its distribution extends west into Kansas. In the Louisiana outbreak, sudden deaths occurred even psitticine birds that had been kept in outdoor aviaries.

Death associated with black flies is usually peracute or acute, occurring within 24 hours in cases of severe swarming. Sudden death may be the only problem noted. When swarmed by black flies, birds may exhibit lethargy, anorexia, ruffled feathers, and petechial hemorrhages on skin. Anemia may or may not be a feature.

Simuliotoxicosis is largely a diagnosis of exclusion since gross and histological post-mortem lesions are few and non-specific. Flies may be found within the gastrointestinal tract of dead birds, which can be a helpful clue when the diagnostician is not previously alerted to the presence of black fly swarms. Microscopically, the skin affected by fly bites may have changes consistent with acute allergic dermatitis – edema, congestion, and increased numbers of eosinophils and mast cells. Other organs may have subtle changes consistent with acute shock (e.g. congestion of tissues and pulmonary edema). In previous years, the author of this article has investigated sudden death in backyard chickens from two premises (both in Wisconsin) that experienced high density of flies at the time of the deaths – in both cases, post mortem evaluation of the birds revealed nothing more than small cutaneous hemorrhages.

There are at least two mechanism by which the flies cause death in both birds and mammals – exsanguination and anaphylactic shock. Fatal anemia can occur as a direct result of the flies feeding on the host’s blood. Interestingly, birds that die acutely may show no signs of anemia and blood loss is probably NOT the most common cause of death. Instead, individuals that sustain many bites may die as a result of anaphylaxis secondary to a systemic allergic response to proteins in the fly saliva.

Simulium species of black flies are doubly dangerous to poultry since they are capable of transmitting various blood parasites of birds, including those that cause “avian malaria” (Haemoproteus sp. and Plasmodium sp.), but the hemoparasite most likely to be transmitted by black flies in the US is Leucocytozoon sp. These parasites affect a broad range of wild and domestic birds and many are endemic within the US. Fortunately, most cases of Leucocytozoonosis in poultry are subclinical, although anemia can result from heavy infections.

Based on the life cycle of these flies, which breed only once during the year, the threat in Kansas for 2018 has likely passed as temperatures have soared and large swarms of adult black flies that emerged this spring have already started to die back. In Southern states, black flies are most severe in the spring and die back by early summer. In Northern states, late spring and early summer can be the peak time.

Barriers (e.g. screens) and mechanical control (fans, fly strips) may be sufficient to protect poultry and other livestock from morbidity and mortality associated with black flies. These methods are often safer than chemical insecticides and are uncomplicated by regulations. Historically, carbaryl (also known as Sevin dust) was used widely in poultry for control of arthropod pests and parasites. Permethrin products are recommended since they are generally safer and not as strictly regulated as carbaryl. (Please note, permethrin is highly toxic to cats!)

References and resources:

  1. Black flies – USA (Arkansas): Livestock deaths, ProMED mail, International Society for Infectious Diseases,
  2. Fly bites causing chicken deaths in central Iowa, Ames Tribune,
  3. Avian Simuliotoxicosis: Outbreak in Louisiana Author(s): Rodney W. Schnellbacher, Kali Holder, Tim Morgan, Lane Foil, Hugues Beaufrère, Javier Nevarez, and Thomas N. Tully, Jr. Source: Avian Diseases, 2012; 56(3):616-620.
  4. Hall RD, Gerhardt RR. Flies (Diptera) in Medical and Veterinary Entomology, Mullen GR, Durden LA, Mullen G. eds. 2002, Elsevier Science & Technology, p. 127-146.
  5. Pesticides used for control of poultry insect pests, Mississippi State University Extension,

Dr. Bell is a board-certified veterinary pathologist who has extensive experience in poultry diseases and diagnostics.

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