By Katie Nichols
The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service confirmed three New World screwworm cases in Florida last week.
Three Wild Key Deer on a wildlife refuge in Big Pine Key, Florida are the first locally confirmed cases of New World screwworms in more than three decades. In 2007 and 2010, dogs from Trinidad and Tobago and Venezuela were confirmed carriers of screwworm larvae. Now in Florida, officials have determined the adult flies and larvae are completing their life cycle in the United States.
Adult screwworm fly.
Alabama Cooperative Extension System Veterinarian Dr. Soren Rodning said livestock producers should be attentive to their herds, especially following Hurricane Matthew.
“The current situation reminds us of the need to always remain vigilant for all kinds of foreign animal diseases,” Rodning said. “International travel and natural disasters, including hurricanes, make the spread of foreign animal diseases a real threat we need to be prepared for if and when they do occur.”
Screwworm, or Cochliomyia hominivorax, has not been a problem for producers since the species was eradicated in the 1960s. While screwworms have not been present in the U.S. for many years, the pests still thrive in South America, several Carribean countries, Southeast Asia, India and Africa.
Early Screwworm Eradication
In the sixties, researchers determined the best way to eradicate the species was by releasing sterile males. By using gamma radiation and X-rays, scientists were able to release millions of sterile males at a time, eliminating the species by the end of the decade.
Rodning said with the past effectiveness of this technique, there is a viable chance of controlling the population should another widespread outbreak take place.
“Thankfully, we do have effective means of controlling an outbreak of New World screwworms,” he said. “Hopefully state and federal animal health personnel can limit the impact of this current infestation in Big Pine Key, Florida.”
The USDA worked diligently to eradicate flies in Jamaica, Mexico and parts of Central America. Panama still serves as the buffer zone between infested countries and the screwworm-free U.S.
Screwworm Transmission
New World screwworms can infect all warm-blooded animals, including birds. However, most cases occur in cattle, goats and sheep.
Screwworms are transmitted when female flies lay eggs on the wound of an animal. Wide varieties of wounds can be infected, including tick bites, dehorning or branding wounds, wire cuts and other injuries.
Newborn mammal navels are common infestation sites. Infested wounds often attract other female screwworms, and multiple infestations are common.
Adults have a rapid reproduction rate. Female flies usually only mate once, but can lay more than one batch of eggs at intervals for a few days. New World screwworms usually deposit two masses of eggs, but are capable of laying six to eight batches or more.
Identifying Animal Infestations
Rodning said spotting an infested wound may be difficult in the early stages.
“The most obvious sign is a change in the wound’s appearance—as larvae feed, the wound gradually enlarges and deepens,” he said. “An infested wound also gives off an odor and some bloody discharge.”
In addition to odor and discharge, some movement may be visible inside the wound.
Infested animals generally separate themselves from the herd and lie down in shady areas. Decreased appetite, discomfort and lower milk production are common symptoms.
Untreated animals could succumb to toxicity or secondary infections within seven to 14 days.
Humans can be infested with screwworms, but such infestations are rare. Similar symptoms can be seen in infected humans.
Identification and Treatment
New World screwworm females are larger than the average housefly. The thorax of a screwworm is metallic dark blue to blue-green and the head is reddish-orange. On the back of the thorax, there are three dark stripes.
Screwworm larvae are cylindrical, with one pointed end and one blunt end. There are complete rings of dark brown spines around the body. Younger larvae are creamy white, while fully mature larvae may have a reddish-pink tinge.
Field diagnosis of screwworms is difficult, so Rodning suggests contacting your veterinarian, as well as notifying the State Veterinarian.
“If you see an animal with a larval infection and suspect a screwworm infestation, remove larvae from the wound with tweezers and place them in alcohol,” Rodning said. “Then notify our state veterinarian Dr. Tony Frazier so state or federal personnel may investigate further.”