B060 - SCREW-WORM FLY
Monday, 01 February 2010 09:31
Nature of the disease
Larvae of screw-worm flies (SWF) are obligatory parasites of mammals, including humans. The disease is due to the larvae of the flies which causes lesions known as myasis that can be fatal and causes serious production losses.

There are two screw-worm flies (SWF), the Old World screw-worm (Chrysomia bezziana) and the New World screw-worm (Cochliomyia hominivorax).

Classification
OIE List B disease
Susceptible species
All domestic livestock and companion animal species are susceptible. The greatest economic losses are in cattle, sheep and goats.

SWF strikes also occur in various wildlife species and in humans.

Distribution
The Old World SWF occurs throughout much of Africa, the Middle East, the Indian subcontinent, south east Asia and Papua New Guinea.

The New World SWF is endemic in parts of Central and South America. It has been eliminated from the United States, Mexico and several Central American countries. An outbreak in Libya in 1988 was eradicated after an international effort sponsored by FAO.

Clinical signs (see pictures)
Adult female are attracted by skin wounds or navel of newborns where they lay hundreds of eggs. The myiasis occurs when the eggs hatch and the larvae burrow into body tissue.

Strikes may occur at any site on the body and range from insidious to large gaping wounds. In a heavily infested herd 10-15% of animals may be struck at any one time. Signs include:

  • Ragged, foul-smelling lesion containing SWF larvae (maggots)
  • Constant licking of the lesion by the animal
  • Secondary infections and strikes are common
  • Fever
  • Lethargy and loss of appetite
  • Debilitation
  • Decreased growth rate
  • Mortality rate in newborn calves from navel strike may be as high as 30-50%
Post-mortem findings
Skin lesions will be seen:
  • Ulcerative lesion with liquefactive necrosis of muscle and skin
  • Serous exudate
  • Mass of seething larvae
  • Extension of lesions into body cavities and associated pleuritis, peritonitis or sinusitis is not uncommon
  • Lesions emit a characteristic pungent sickly odour
Differential diagnosis (see pictures)
Any flystrike in cattle should be regarded as being highly suspicious of SWF and should be thoroughly investigated. Other cause of strike in cattle include:
  • The hairy maggot blowfly (Chrysomya rufifacies)
  • The steel-blue blowfly (Chrysomya saffranea)

SWF should also be suspected when there are deep flystrike lesions in sheep or other animals. SWF strikes must be differentiated from the less deep-seated but extensive sheep blowfly (Lucilia cuprina) strikes. Weeping skin sores in imported pet animals should be examined for SWF larvae.

Contamination of the wound by non screw-worm species is possible.

Specimens required for diagnosis (see pictures)
As serological tests are still under development, the diagnosis of screw-worm myasis is based on the identification of the agent which is difficult especially as non screw-worm species may be present. Larvae and adult flies should be sent to specialist entomologists for identification.

Larvae should be collected from the deepest part of the wound. Forceps can be used to remove larvae, but care should be taken not to damage them.

Living specimens can be examined for pigmentation of the dorsal tracheal trunks and then be preserved in 80% ethanol for laboratory examination. A good preservation technique consists in a brief immersion in near-boiling water before placing in ethanol.

Adult flies are rarely seen, but can be caught in wind-oriented traps baited with synthetic attractants (swormlure traps). Sentinel animals can also be used to detect adults. Wounded sentinel animals are highly effective at attracting gravid females who lay their eggs on the edge of the wound.

Transmission
SWF can survive in the right environment both at very low adult density and low host density. The adults flies can survive about 2 weeks in a favorable environment. SWF do not actively migrate but female flies may disperse widely in search of wounded animals (up to 50 kms). The life cycle of  the fly in tropical conditions can be less than 3 weeks, and 15 generations can be produced in one year.

However, rapid long distance spread of the disease is more likely to be due to movement of infested animals than to dispersion of the fly.

Wounding is usually necessary for SWF strikes. These may occur naturally e.g. the navels of newborn animals, be the result of husbandry practices e.g. castration, dehorning, or may be from minor skin penetrations such as tick bites.

Incidence of strikes in livestock is influenced by husbandry practices.

Risk of introduction
SWF may be introduced naturally on air currents or via aircraft. In Australia dead SWF have been collected on returning livestock vessels that have visited endemic areas. It also possible that flies be wind born from Papua New Guinea to Australia. However, experience would suggest that the primary risk of introduction is associated with importation of infested animals.

In the USA, dogs coming from endemic areas have been implicated. SWF infections e.g. in the ear can be difficult to detect. Infested humans are also a potential risk for introducing the fly.

Control / vaccines
Early detection provides best hope for subsequent control and eradication of a SWF incursion.

SWF are highly susceptible to a range of insecticidal chemicals. While the use of insecticides against adult SWF is largely pointless, their use against larvae (e.g. in wound dressings) is an essential element of prevention and treatment. Ivermectin is particularly interesting at dosages of 200 µg/kg in subcutaneous injection to prevent infection of Old World SWF in cattle. Alternatively, spraying or dipping livestock with coumaphos (0.25% aqueous suspension of 50% wettable powder) can be used for prevention.

The Sterile Insect Release Technique (SIT) exploits the fact that females only mate once. SIT involves saturating the environment with artificial reared sterile flies, resulting in the majority of wild females mating with sterile males and producing sterile eggs. This can dramatically reduce SWF populations and if maintained over several generations can achieve eradication. It has been very successful against the New World SWF in the USA.

References
  • BEESLEY WN (1998), The myases In Zoonoses, ed by SR PALMER, Lord SOULSEY and D.I.H. SIMPSON, Oxford University Press, Bath Press, Avon, 1998, p.885-886
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 440p
  • Obligatory Myasis-producing flies In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 652-654
  • Office International des Epizooties, 2002
  • Screw-worm infestation, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 1284-1286