SCREWWORM MYIASIS

William Edmiston, DVM Eldorado Animal Clinic

Eldorado, Texas

Screwworm Myiasis

Robert B. Moeller Jr. California Animal Health and Food

Safety Laboratory Tulare, CA 93274

rbmoeller@ucdavis.edu 559-688-7543

Nov 2009

Screwworms Economic Impact

• US currently free of screwworms?? (Sept 2007: (NWSW) canine in Mississippi from Trinidad; OCT 2007: (OWSW) canine in

Massachusetts from Singapore ) • $100 million economic losses to livestock producers in US in Gulf

coast region during the first half of the 1900’s • Reintroduction into US --- $ 600-900 million - Losses due to:

* Animal treatments and handling (increased manpower) * Loss of meat products due to tissue damage

• South America estimated losses $3.6 billion annually • Eradication effort in case of extensive reintroduction - $1.2 to $1.5 billion in control program cost

(Cost are SIT program, increase boarder monitoring and education programs)

– US benefit from eradication $900 million annual benefit – Central America livestock $257 million annual benefit from

eradication effort.

Screwworms • New World Screwworms

• Cochliomyia hominivorax • 8-10mm length • Blue to green body • 3 dark longitudinal stripes

• Old World Screwworms • Chrysomya bezziana • 8-12 mm length • Green to blue body • 2 dark longitudinal strips

Screwworm Distribution Historical range

Screwworm Distribution Current range

New World Screwworms History in Western Hemisphere

• 1934 major outbreak in US - 1.3 million animals affected - 200,000 deaths • 1937: Knipling and Bushland

begin working on fly • 1938: Discover that female

breeds only once • 1950 Sterilization of male flies

developed

Knipling

Bushland

History of New World Screwworm Eradication Program

Sanibel Island

1951-1952 1954

History of New World Screwworms

Eradication - Florida 1959 - US 1966 - Mexico 1992 - Central America 2004 - Puerto Rico 1975 - Currently free US Virgin Islands and Jamaica - Cuba and Hispaniola have no eradication programs currently

INFESTED

HIGH RISK

LOW RISK

SCREWWORM OUTBREAK

LIBYA, NORTH AFRICA, 1988-1991

Old World Screwworms Chrysomya bezziana

Screwworms Transmission

• Infect any mammal (birds rarely infected) • Wildlife tend to be reservoir for livestock infections • Female deposits eggs onto wound or mucus membrane

(nose, eye, prepuce, vagina) while feeding • Female lays 100 to 300 eggs/feeding (female can lay 3000

eggs in lifetime)

Screwworm Transmission

• Eggs hatch in 10 to 20 hours • Larva eat flesh and expand wound • Larva mature in 4 to 20 days • Many female flies can lay eggs into wound

(smell attracts more screwworm flies and other flies

• 2o screwworm infection can occur - Cochliomyia macellaria (West) - Chrysomya megacephala (East)

Screwworm Transmission

• After feeding for 4 to 12 days larva drop off

• Burrow into ground • Pupate in soil for 3 to 5

days • Adults emerge

Screwworms • Neonatal animals commonly

affected, umbilical infections common

• Wounds invaded by screwworm flies as small as tick or insect bite

• Gulf coast ear tick (A. maculatum) caused many infected lesions with ear loss in cattle

Screwworms in animals

• Nasal, occular and gingival infestations common

Screwworms Human infections

• Infections of skin primary area of infection

• Nasal passages second most common site

• Poor personal hygiene aids in infestations

• Old people most common individuals infected with larva

Screwworm Identification • Larval identification is the primary method for

mammals • Larvae removed from deepest part of wound to

avoid non-screwworm larvae (Larva tend to be at the margin of viable and nonviable tissues)

• Posterior spiracle open for both screwworms • Look for pigmentation of dorsal tracheal trunks • NWSW: Cochliomyia hominivorax - 2nd instars larva: dorsal tracheal trunks over ½ of

body length, Pigmentation of trunks characteristic - 3rd instars larva: dorsal; trachea; trunks extend

from the 12th to 10th segment. • OWSW: Chrysomya bezziana - Dorsal tracheal trunks pigmented only at 12th

segment • Chrysomya bezziana: anterior spiracle with 4 to 6

lobes separate it from other species

Screwworms: Identification

• Identification of adult and larva screwworms - Usually deep tissue larva not superficial larva • Mab-ELISA testing in development: 97% accurate

identified Cochliomyia hominivorax eggs, larva and adults • PCR technique - Mitochondrial DNA - Random amplified polymorphic DNA-polymerase chain

reaction (RAPD-PCR) • Submit fresh and alcohol fixed larva

Screwworms: Detection

• Physical examination of animals for larva • Insect traps • Detector dogs trained to sniff for odor given off by

infected wounds (one animal)

Screwworm: Treatment

• Manual removal of larva • Flush wound with saline/hydrogen

peroxide/antiseptic • Chloroform, ether, benzol to evacuate

larvae • Boric acid and potassium

permanganate deodorant and cleaning agent

• Antibiotics for secondary bacteria

Screwworms: Treatment

• Old treatments - Diphenylamine 35%; Benzol

35%, Turkey red oil 6.7% and lindane 3%

- 3% lindane and pine oil - 1% dichlorfenthion and

gentian violet • New treatments - 1% fipronil pour-on - Doramectin injectable to

eliminate larvae - Ivermectin injectable to

eliminate larvae

Screwworms: Risk to the United States

• Movement of pets and people from risk countries - Tourist, quest workers, military (pets from screwworm

endemic areas) • Importation of livestock with infections (horses affected) • Air and sea transport carrying flies into US • Intentional release

Sterile Insect Technique • USDA-APHIS participates in International Eradication Programs Against

Screwworms • Screwworms are mass produced in a production facility in southern Mexico, irradiated, and then mass released from airplanes. • New plant open in Panama in 2007 (Pacora, Panama) - $40 million production facility - Able to produce 150 million sterilized flies per week for Panama (Darien Gap region) and eventually Caribbean islands

Tuxtla Gutierrez, Mexico

Pacora, Panama

Colonies of adult screwworms are kept in cages; about 60,000 per cage.

When adults are about 6 days old, eggs are collected and prepared for placement in larval media.

About 1 gram of eggs are placed in the media of dry blood, dry egg, dry

milk, gelling agent and water (kept at about 38 degrees C).

• Trays of larva are kept on large racks, media is added 3-4 times • Mature larvae are removed from the media and pupate (in sawdust) • The larval period last about 6 days

Mature larvae are placed in containers of sawdust where they pupate.

Large numbers of trays of pupae are stored on racks for about 4 days.

1. Pupae are then placed in tubes.

2. The tubes are placed in ‘slots in the wall’ for irradiation. (use Cesium

137 as source) 3. On the other side of the wall are irradiators where the flies are sterilized.

1 2

3

Sterile screwworm pupae are kept chilled in large boxes for up to 2 days before loading to planes for dispersal in the eradication program.

Pupae are released at a rate of about 3,000 or more per nautical mile.

Good Web sites:

• http://smccd.net/accounts/case/insect/ • http://WWW.fao.org • http://www.merck vetmanual.com • http://www.cfsph.iastate.edu • http://www.vet.uga.edu/VPP/gray_book/

http://smccd.net/accounts/case/insect/http://www.fao.org/http://www.cfsph.iastate.edu/

Photos

• Dr Chase • Dr Schlater • Dr Scoda • USDA Grey book

Questions