Chrysomya bezziana infestation

All warm-blooded terrestrial animals are susceptible to Chrysomya bezziana, including humans, birds, wildlife and livestock. In Africa, C. bezziana infestations have been recorded on buck, impala, lions, rhinos and elephants (Zumpt, 1965). Pre-disposing factors include wounds such as cuts and scratches, while the umbilical of the new-born, the vulva of post-parturition females, scrotum of post-castration or de-horned and branded cattle are especially vulnerable. Where exotic animals are maintained in a C. bezziana endemic area such as a zoo, many other animal species not normally exposed as hosts can be infested by C. bezziana. For example, at the Zoo Negara in Malaysia, kangaroos, wallabies and polar bears have been recorded as hosts (Spradbery and Vanniasingham, 1980).

Clinical Diagnosis

Signs of infestation include a ragged lesion containing larvae of Chrysomya bezziana, persistent licking of the lesion by the host, an initial hypersensitivity after which sensitivity decreases. Restlessness, lethargy, inappetence with a decrease in growth rate, anaemia and hypoproteinaemia, and intermittent irritation and pyrexia characterize the condition. The lesion can extend into the body cavities causing peritonitis, sinusitis and pleuritis depending on the site of infestation. There is a foul-smelling and characteristically pungent odour associated with C. bezziana-infested lesions.

Lesions

Early larval invasion of the damaged epidermis by newly-hatched larvae create small cavities 5-10mm in diameter with the larvae bathed in a serous fluid and visibly active. Within 24 hours the cavities are enlarged and extend into the subcutaneous tissue and muscles. Progressive liquifactive necrosis of the tissues continues as the larvae grow and the lesion becomes cavernous with ragged edges. In the depths of the lesion are a seething, pulsating mass of larvae covered in liquefied tissue and blood.

Differential Diagnosis

Myiasis due to Chrysomya bezziana must be distinguished from myiases due to other species of carrion blowflies (Calliphorinae). In the sheep blowflies, Lucilia cuprina and L. sericata, the larvae feed superficially on the surface of the wound or fleece and sustained by the serous exudate from the host animal. If disturbed, C. bezziana larvae retract deeper into the wound while Lucilia tend to evacuate the site and burrow into surrounding wool. Any lesions, especially following invasive husbandry such as castration or de-horning, should be explored with a view to the possibility of C. bezziana infestation. Diagnosis of a screw-worm fly infestation is normally made after detecting larvae and collecting specimens for storage in 80 per cent ethanol for microscopic examination in the laboratory (Spradbery, 2002a).

Laboratory diagnosis

Histologically, two phases are evident: firstly of necrosis, neutrophil infiltration and haemorrhage associated with tissue invasion and growth of larvae; and then a fibroplastic healing phase in which mast cells and eosinophils are prominent. Biochemical changes include initial neutrophilia, anaemia and decreased serum protein with progressive increase in serum globulins (Humphrey et al., 1980). Specimens of first to third instar larvae (and including pupae and adults) from infested wounds can be readily diagnosed by reference to Spradbery (2002a) and first-instar larvae by Szpila et al. (2014). A technique for identifying geographical races of C. bezziana adults based on wing morphometrics has been described (Hall et al., 2014).

Local Control (vaccination, restriction of movement, regulation)

At present, the only means for control of Chrysomya bezziana infestations of livestock are regular inspections and treatment of infested stock using a variety of insecticides. Wardhaugh and Mahon (2002) reviewed the use of insecticides as an integral part of the deployment of the sterile insect release method (SIRM) for control and eradication of C. bezziana. The range of potential treatment chemicals has been reviewed by James et al. (2005) and James et al. (2014) tested the efficacy of Australian-registered chemical formulations against C. bezziana in animal and laboratory evaluations. The sterile insect release method (SIRM) / sterile insect technique (SIT) has been experimentally evaluated for deployment against C. bezziana in field trials in Papua New Guinea (Spradbery et al., 1989) and Malaysia (Mahon, 2002) and is included in the Australian response plan for a C. bezziana incursion (Animal Health Australia AUSVETPLAN, 2007). A potential vaccine for C. bezziana has been explored by an Indonesian/Australian team (Sukarsih Partoutomo et al., 2000; Willadsen, 2000). Livestock movement controls should reduce the rate of spread of a C. bezziana outbreak and is an integral component of the Australian response plan for a screw-worm fly invasion (Ausvetplan, 2007). An effective bait-trapping procedure for surveillance has been developed to capture adults of C. bezziana (Urech et al., 2012, 2014) and, with a real-time PCR assay, can be used to detect very low numbers of C. bezziana in bulk fly catches from insect traps (Jarrett et al., 2010).

National and International Control Policy (vaccination programmes, quarantine regulation)

Old World Screw-worm fly, C. bezziana, is subject to strict global quarantine restrictions as set down in the OIE Terrestrial Animal Health Code to prevent the spread of the pest to new geographical regions. Australia is free of screw-worm fly but with a climate suitable for C. bezziana (Sutherst et al., 1989) and there are national quarantine restrictions and an extensive surveillance program as part of the Australian emergency preparedness plan for screw-worm fly (Animal Health Australia AUSVETPLAN, 2007).

The risks of a C. bezziana incursion into Australia and recommended surveillance requirements have been recently reviewed (Beckett et al., 2014).

21/07/15 Original text by:

Dr J. Philip Spradbery, XCS Consulting Pty Ltd, GPO Box 2566, Canberra, ACT 2600, Australia.