Chrysomya bezziana Villeneuve
(Figures 4-55 and 4-56)
ETYMOLOGY:Chryso = gold + myia = fly; along with bezziana for Dr. Bezzi
HISTORY: In 1910, Dr. Rovere described several cases of traumatic myiasis in cattle from the Congo. He sent some of the adults he reared to a Professor Bezzi in Turin, Italy, who incorrectly identified them as Chrysomya megacephala. Also in 1910, Gedoelst reported similar findings from the Congo, and also referred to the larvae as Chrysomya megacephala. Roubaud and Bouet studied this fly again in West Africa and found its behavior different from other species of Chrysomya in this area. They also believed erroneously that they were dealing with Chrysomya megacephala. Villenueve reexamined the type specimen, which had an incorrect locality designation, and found it conspecific with Chrysomya dux from the Orient. The species which Roubaud and the others were dealing with had to be renamed, so Villeneuve did so in honor of Professor Bezzi, who had already noted that this fly was different from Chrysomya dux (Zumpt, 1965).
GEOGRAPHIC DISTRIBUTION:Chrysomya bezziana, the Old World screw-worm, is a primary myiasis-producing fly that attacks a wide range of warm-blooded animals throughout Africa, the Indian sub-continent, and Southeast Asia from Taiwan in the north to Papua New Guinea in the south. The only continent with a tropical zone still free of primary screw-worm is Australia (Davidson, 1992, Spradbery, 1992). There are secondary myiasis producing flies in Australia that can be of significant importance, such as Chrysomyarufifacies (Figs. 4-55 and 4-56).
LOCATION IN HOST: The larvae of Chrysomya bezziana are obligatory wound parasites and never develop in carcasses or other decomposing matter. The female flies are attracted to the open wounds of man and domesticated animals. Any slight, bleeding wound, even the smallest sore caused by a feeding tick, inflicted accidentally on domestic animals is subject to infestation (Zumpt, 1965).
IDENTIFICATION: Adult Chrysomya bezziana are non-parasitic and as a result will not be observed by the client or the veterinarian. The adults are rarely found in the field (Zumpt, 1965). They are never found feeding on food in open air markets (Patton and Evans, 1929). This fly possesses a dark metallic green or blue body with abdominal segments with narrow dark bands along the posterior margins. The legs are black or partially dark brown; the face is orange-yellow (Zumpt, 1965).
The first larval stage will probably go unnoticed due to its small size, up to 3.0 mm at the time of its molt to the second stage. The second stage is quite similar to the third, but is from 4 to 9 mm in length. The third stage larva is a large larva, up to 18 mm in length. The body is composed of twelve segments which have broad encircling bands of spinules. All three stages are "maggot-like" in their appearance and exhibit cephalopharyngeal sclerites and posterior spiracles that are unique to the species (Zumpt, 1965). The posterior end of the larva has its spiracular plate located in a deep cleft at the end of the eighth abdominal segment. The spiracular plates are large and well separated. The peritreme is wide with a break in its infero-internal border. The button is situated in the break in the peritreme. The three breathing slits are very wide (Patton and Evans, 1929).
The second and third stage larval Chrysomya bezziana are the stages that will be usually observed in the skin of an infested cat by either the owner or the veterinarian. These stages can probably be best identified by an entomologist. Extensive descriptions of these morphologic stages do exist (Patton and Evans, 1929, Zumpt, 1965).
LIFE CYCLE:Chrysomya bezziana has been described as a fly that produces a "particularly vile" myiasis (Davidson, 1985). Female flies lay their eggs in masses of 150 to 500 at the edge or wounds (Harwood and James, 1978) or near body orifices (Davidson, 1985). Larvae develop to the third stage about 2 days following hatching. They burrow deep into the wound to such a depth that only their caudal ends are observable. The entire larval stage lasts 5 to 6 days. Under tropical conditions, the pupal stage lasts 7 to 9 days, however this time will increase in cooler ecosystems (Harwood and James, 1978). The adult flies merge later to mate, locate a new host, and continue the cycle. The female flies mate only once during their lifetime. Under favorable conditions, there may be eight or more generations per year (Zumpt, 1965).
CLINICAL PRESENTATION AND PATHOGENESIS: Myiasis due to Chrysomyiabezziana is extremely rare in cats. A single case of infestation occurred in a Persian cat from Port Moresby, Papua New Guinea. Secondary infestation with the facultative myiasis-producing flies may complicate treatment and control of the infestation (Davidson, 1985).
The larvae of Chrysomya bezziana are obligatory wound parasites, never developing in carcasses or decomposing organic material. The female flies are attracted to open wounds of man and domesticated and wild animals. Occasionally eggs are deposited on the unbroken, soft skin of various parts of the body, especially if it is contaminated by blood or mucous discharge (Zumpt, 1965). When the larvae hatch, they burrow into the flesh of the host, using their hooked mouthparts to scrape away at the tissues and lacerate the fine blood vessels. Larvae actively feed on the host's blood. Because of the voracious feeding activity and their appetite for host blood, Chrysomya bezziana is often described as being "vampiric" or "blood-hungry." During the blood-sucking phase, only the caudal ends of the maggots with their blackish peritremes remain visible at the surface of the lesion, enabling the larvae to breathe. As many as 3,000 maggots have been observed in some wounds (Davidson, 1985). In untreated wounds, the destructive activity of the larvae may lead to the death of the animals within a very short period of time (Zumpt, 1965).
TREATMENT: Treatment of screwworm infestation involves killing the larvae in the lesions, promoting healing, and preventing secondary reinfestation with larvae of the facultative myiasis-producing flies. The extent of the lesions is determined by clipping the haircoat and removing as many larvae as possible. The larvae that are removed should be killed to prevent them from pupating and developing to the adult fly. The larvae located deep within tissues must be extracted. The dressing should be bland and nontoxic to the cat and should promote healing.
Ivermectin at dosages of 50, 100, and 200 µg per kg administered to infested cattle resulted in 100% larval Chrysomyia. bezziana mortality for at least 6, 12, and 14 days, respectively. Depending upon their age, larvae survived in established strikes following treatment at 200 µg per kg. Larvae up to 2-days-old demonstrated 100% mortality, while older larvae showed greater resistance. At this dosage residual protection lasted 16 to 20 days, two to three times that produced by most insecticide smears. These mortality rates were very conservative; many of the larvae that survived ivermectin therapy failed to develop to the adult stage (Spradbery etal., 1985).The product should not be given to kittens (O'Dair and Shaw, 1991).
EPIZOOTIOLOGY: The effects of infestation with Chrysomya bezziana are particularly devastating. Strike on naturally occurring wounds, e.g., tick bites, and on external body openings can be quite serious. Fly strike also has the ability to follow elective feline surgical procedures (e.g., spays or castrations).
HAZARDS TO OTHER ANIMALS: All warm-blooded animals are subject to infestation by Chrysomya bezziana. The animals most commonly infested are cattle (Zumpt, 1965). Other known hosts include sheep, goats, buffalo, pigs, chickens, dogs and horses. (Patton, 1920, Davidson, 1985). Reports of infestations on wild animals are rare. A survey of the records of the Malaysian National Zoo revealed 91 cases of myiasis in 21 host species during the period 1965 to 1980. Those animals included the honey bear, polar bear, camel, fallow deer, hog deer, red deer, sambar deer, donkey, Asian elephant, gnu, Sumatran horse, striped hyena, red kangaroo, African lion, Asian lion, slow loris, puma, rhinoceros, sheep, Malay tapir, and agile wallaby (Spradbery and Vanniasingham, 1980).
HAZARDS TO HUMANS: Infestations of humans with the larvae of Chrysomya bezziana are very common in India and other parts of Asia (Patton, 1920). This parasite has been reported to occur in humans in Africa (Zumpt, 1965). Children are often subject to infestation on scalp wounds (Davidson, 1985).
CONTROL/PREVENTION: All wounds on domesticated animals should be properly dressed. All elective surgical procedures should be avoided during the fly season.
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Figure 4-55. The larvae of Chrysomyarufifacies from Australia.
Figure 4-56. Two pupal cases of Chrysomyarufifacies from Australia.