New World Screwworm Back From the Dead After 50 Years


On September 30th, the New World screwworm, Cochliomyia hominivorax, was confirmed to be infecting Key deer living in the National Key Deer Refuge in the Florida Keys [1].  This is the first report of a screwworm myiasis cluster affecting multiple animals in 50 years, as self-sustaining screwworm populations were eradicated in the United States by 1966 [2].  This horrible disease has the potential to cause large economic losses for farmers due to potential livestock loss and trade restrictions imposed if the screwworm were to reestablish itself in livestock populations [1].  It can also have devastating effects on wildlife, where surveillance and access to veterinary care is more limited than in production animals.  As of October 19th, nearly 10% of the entire population of the endangered Key deer have been humanely euthanized due to infection with screwworm maggots [3].  The Florida Commissioner of Agriculture has declared an agricultural state of emergency in Monroe County and eradication efforts have been implemented to prevent screwworms from spreading to other areas of the United States [3].

What is New World Screwworm?

C. hominivorax (translates to “man eater”) are flies similar in size to the common housefly but have orange eyes [1].  Gravid females search for hosts on which to lay their eggs – in open wounds or orifices of warm-blooded animals [4].  One female fly can lay up to 400 eggs at a time and eggs hatch within 24 hours into immature C. hominivorax maggots [4]. Unlike other maggots which feed on necrotic tissue, C. hominivorax maggots feed on living tissue [4]. Infestations with these larvae, which resemble screws (hence the name), can cause major damage to soft tissue and even to bone as the larvae use their hook-like mandibles to eat for five to seven days before they detach to pupate [5]. Untreated animals can die within a week or two from secondary infections and/or toxicity as they are literally eaten alive by the screwworm maggots [2, 4].

What are Key deer?

Key deer (Odocoilues virginianus clavium) are the smallest subspecies of the North American white-tailed deer and are the only large herbivore found in the Florida Keys [6].  These endangered miniature deer had a population as low as a few dozen animals in the 1950s but have slightly bounced back since the establishment of the National Key Deer Refuge and the listing of the deer as an endangered species in 1967 [6].  The population had grown to around 1,000 before the screwworm was reported in the refuge on Big Pine Key, approximately 30 miles east of Key West [7].  Adult screwworm flies have now been reported in many of the major Keys that make up the range of the Key Deer [3]. Many of the deer found were so infected beyond treatment that euthanasia was the only option.

Can New World Screwworm infect humans?

Although rare, humans can be susceptible to myiasis from screwworms [7].  An example of such a case in the medical literature, details an event in 2014 of an adult woman in her 20s who was vacationing in the Dominican Republic.  After falling asleep on the beach after a night of drinking, she awoke the next day to an uncomfortable sensation of movement in her ear.  She removed a fly from her ear and flew home the next day.  The discomfort gave way to acute pain and bloody discharge which she initially chalked up to air pressure changes associated with the flight home.  A day after returning home, she went to a doctor where many moving screwworms were found in her external ear canal.  An otolaryngologist had to manually remove all the larvae, debride the ear canal, and reconstruct the eardrum, which was perforated by the screwworms.  During the operation, it was noticed that the larvae permeated the soft tissue in the ear all the way to the temporal bone, luckily without penetrating the middle ear [5].  This is one of many stories that illustrate the severity of screwworms, and the reasons why its reemergence is of major concern.  With concerns of local Zika virus transmission, Florida residents and vacationers do not need another disease to be concerned with in the state.

What can be done to control New World Screwworm?

Until recently, New World Screwworms were eradicated from the Southeastern United States since 1957. New World Screwworms were also eradicated throughout Central American countries to the Darien Gap, a nearly untraversable break in the Pan-American Highway, between Panama and Columbia [2].  However, they remain endemic in South America and five Caribbean nations, including the US’s neighbor, Cuba [2, 8].  Eradication efforts success could be primarily attributed to a form of biological control called the sterile insect technique [2]. This technique relies on releasing many male flies that have been sterilized through irradiating screwworm adults and pupae [8]. These sterile male flies mate with multiple female flies. Due to the biology of C. hominivorax, females only mate once in their lifetime.  Females that breed with sterile flies get “tricked” into believing they will reproduce and stop mating [1].  These females die without laying eggs, which breaks the life cycle of the screwworm, leading to eventual population decline and collapse [1, 8]. This form of biological control is appealing because it does not rely on widespread use of potentially toxic chemical insecticides. Additionally, sterile insect technique may also prove effective against controlling other vectors, such as Aedes mosquitoes that carry the Zika virus.  Sterile fly releases have begun on October 11th in efforts to achieve New World Screwworm eradication again [3].

If there is one silver lining to this potential catastrophe, the screwworm appears to be localized to the Florida Keys, where there is only one major roadway in and out of the area. A vehicle checkpoint was setup in Key Largo within 48 hours of confirmation of the presence of screwworm and animals that are being driven north into the Florida mainland are being checked for evidence of myiasis [9]. As of October 22nd, over 1,200 animals have been declared negative by agricultural officials [10].  However, a veterinary hospital on Marathon Key, the island just east of the affected area, has treated nine suspected cases of screwworm in pets.  These cases consist of three dogs, two cats, two pigs, one rabbit, and one tortoise, indicating the wide range of susceptible species to this disease [11].  Hopefully, this hospital and the highway checkpoint will prevent travelers from unknowingly spreading New World screwworms through the movement of infected animals into the continental United States.

Although human roadway transport of New World screwworm is appearing to be contained, adults can potentially fly great distances and spread this disease to other areas.  Adult C. hominivorax flies usually only travel a few miles if there are susceptible hosts present [2], however some sources report that they can travel up to 125 miles before laying their eggs in a host [4].  While it seems unlikely that an adult fly would travel across the open waters of the Gulf of Mexico, Cuba and Miami fall within this maximum range.  There are also many islands within the Florida Keys that an adult may fly to lay its eggs if hosts are present.  This could potentially complicate eradication efforts.

The reemergence of the New World screwworm in Florida is a major concern for the livestock industry and the unfortunate wildlife who may become infested with these parasites, especially the endangered Key deer.  Infection with screwworm is a terrible situation for any unlucky animal and the disease progression is reminiscent of a horror movie.  It is with great hope that eradication of this awful parasite is achieved once again in the United States, and throughout other endemic countries. 


1.              Scutti, S. Florida using 'overwhelming force' to fight flesh-eating screwworms. 2016 October 12, 2016 October 24, 2016]; Available from:

2.              APHIS. New World Screwworm. 2014  October 24, 2016]; Available from:

3.              Herriman, R. Florida screwworm update: Flies detected on multiple Keys, 10% of Key deer euthanized. 2016  October 23, 2016]; Available from:

4.              CDFA. Screwworm. 2016  October 24, 2016]; Available from:

5.              LaCourse, S.M., et al., Pain and bloody ear discharge in a returning traveler. American Journal of Tropical Medicine and Hygiene, 2015. 92(3): p. 599-600.

6.              FWS. Key Deer. 2016 January 20, 2016 October 24, 2016]; Available from:

7.              Kay, J. Florida hopes quarantine and irradiated flies stop screwworm. 2016 October 11, 2016 October 24, 2016]; Available from:

8.              Alphey, L., SIT 2.0: 21st Century genetic technology for the screwworm sterile-insect program. BMC Biology, 2016. 14(1): p. 80.

9.              Atkins, K. Screwworm-infected deer in 'gory condition'. 2016  October 23, 2016]; Available from:

10.           AP. Hundreds of pets cleared in Keys screenings for screwworm. 2016 October 17, 2016 October 24, 2016]; Available from:

11.           Staletovich, J. Screwworm infecting Key deer also found in some sick pets. 2016 October 22, 2016 October 28, 2016]; Available from:


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