Progress in the British hypodermosis eradication programme: the role of serological surveillance.

The British warble fly eradication programme has resulted in the numbers of infested cattle falling from an estimated four million in 1978 to zero in 1991. In 1982, when about 705 herds were infested the disease was made notifiable, and serological surveillance was begun in 1988. In spring 1991, 227,000 blood samples were tested by using the ELISA technique and no warbled animals were found. In addition no clinical cases were reported during 1991.

Immunological and feeding studies on antigens derived from the biting fly, Stomoxys calcitrans.

Pairs of rabbits were immunised with three antigenic preparations derived from Stomoxys calcitrans gut, abdominal section and whole flies. Immunoblotting studies demonstrated that a humoral response was mounted against eight antigens from the gut preparation and 12 each from the abdominal and whole fly preparations. In vitro feeding experiments showed higher mortality between Days 4 and 7 in the group of flies which had fed upon blood from rabbits inoculated with the gut derived antigen. This group also produced the lowest percentage of viable eggs (15.5%).

Transmission of bovine virus diarrhoea virus by blood feeding flies.

Three species of blood-feeding flies (Stomoxys calcitrans, Haematopota pluvialis and Hydrotaea irritans) were fed for five minutes on a bullock persistently infected with bovine virus diarrhoea virus (BVDV) containing 10(4.5)TCID50 non-cytopathic BVDV/ml serum, then subsequently fed on BVDV-free seronegative animals maintained in isolation. Virus was isolated from recipient animals between days 5 and 10 using H pluvialis, and up to 72 hours after transmission with S calcitrans; virus isolation was negative using H irritans. Positive seroconversion results obtained with H pluvialis gave a steadily increasing antibody titre. BVDV was recovered from flies 96 hours (four days) after an infective feed for H pluvialis and S calcitrans, but for two hours only for H irritans.

The serological incidence of Hypoderma bovis in cattle in England and Wales in spring 1989.

Sera from 152,434 cattle held on 6175 farms in England and Wales were tested for the presence of antibodies to Hypoderma bovis during the 10 weeks starting on February 6, 1989. Thirty-nine positive animals (25.6 animals/100,000 tested) were identified on 21 of the farms (0.34 per cent). In comparison, the survey in 1988 examined 74,502 cattle on 3087 farms and found 29 positive animals (38.9/100,000 tested) on 18 farms (0.58 per cent).

Cyromazine pour-on for the prevention of cutaneous myiasis of sheep.

The results of trials on eight farms to assess the efficacy of two pour-on formulations containing cyromazine for the prevention of cutaneous myiasis of sheep are presented; data from trials on sheep with larval implants and on sheep kept in cages with adult flies are also reported. The incidence of cutaneous myiasis was reduced by between 87 per cent and 100 per cent for eight weeks when a formulation containing 6 per cent w/v cyromazine was used at an application rate of 60 to 85 mg of active ingredient/kg bodyweight. When a formulation containing 10 per cent w/v was used at an application rate of 50 to 100 mg/kg, the incidence of the condition was reduced by between 90 per cent and 100 per cent for eight weeks. Studies of sheep with larval implants, using the formulation containing 10 per cent w/v cyromazine at 50, 100 and 150 mg/kg bodyweight gave variable results with some animals at each dose rate having lost protection by the seventh week. When sheep were treated with the formulation containing 10 per cent w/v cyromazine at 50 or 100 mg/kg and exposed to adult flies in fly-proof cages they were completely protected for nine and eight weeks, respectively.

Serological survey of the incidence of Hypoderma bovis in cattle in 1988.

Sera from 74,502 cattle from 3087 farms in England and Wales were tested for the presence of antibodies against Hypoderma bovis in the spring of 1988. Twenty-nine positive sera were identified on 18 premises and these animals were treated; an examination of 6030 sera taken from 108 neighbouring herds identified another 17 seropositive animals on 10 farms in Devon, Cornwall, Lancashire, Shropshire and Powys, indicating that these counties still harbour populations of warble fly.

Progress in warble fly eradication.

Warble flies (Fig. 1) are bee-like insects, belonging to the parasitic fly family Oestridae, which spend their entire 10-month larval period inside the body of the host animal, only emerging in the summer to complete their development to the free-flying adult stage (Fig. 2). In 1978, when almost 40% of cattle in Britain were infested, the Ministry of Agriculture, Fisheries and Food (MAFF) initiated a warble fly eradication scheme. We are now in the final stages of this plan, with infestation rates down to 0.01%. This represents a massive saving to the farming industry in terms of less damaged hides and improved production of milk and beef. It also represents an enormous improvement in animal welfare; cattle in the past often carried 20-30 of these large (2 cm long) grubs in pus-filled 'warble' lumps on their backs. Now they are never seen in most parts of the country. In this review, David Tany discusses the progress of warble fly control, warning that the problem could reappear unless stringent measures are taken o eradicate remaining foci and o control imported cattle.

Cattle fly control using controlled-release insecticides.

Field trials were carried out from 1980 to 1984 on the use of a controlled-release pesticidal fly control technique on farms in Sussex, England, with a recurrent infectious keratoconjunctivitis problem related to fly attack. Pesticide impregnated p.v.c. ear-tags provided control of biting and disease-carrying fly species throughout the season. Over 90% control of flies, compared with untreated herds, was attained using 2 tags impregnated with cypermethrin, fenvalerate or flucythrinate, although control using a single tag was unsatisfactory. In all cases infectious keratoconjunctivitis was almost completely eliminated.

Persistence of antibody in calves after an infection with Hypoderma bovis.

The antibody titre of seven calves, naturally infected with Hypoderma bovis, was estimated by an enzyme-linked immunosorbent assay (ELISA) on sera collected from June to early December 1983 following infection in 1982. It was found that the antibody was lost from the circulation of these animals about 14 weeks after the disappearance of the infection. It is suggested that the presence of antibody in the sera of calves sampled in the late autumn and winter indicated that a new infection had been acquired.

Distribution of cattle warble flies in Britain: a larval survey.

A nationwide survey was conducted of the relative distribution and life cycles of the two cattle warble fly species Hypoderma bovis and H lineatum to complement information derived from examining livestock at markets and hide damage at slaughter. Warble fly larvae were collected from cattle at abattoirs in 59 counties in England, Wales and Scotland from February 1 to June 30, 1979. H bovis was commonest in all areas except the Western isles of Scotland. There was no evidence of cross-infestation between cow and deer warble species.

Studies on the feeding patterns of the sheep headfly Hydrotaea irritans (Diptera, Muscidae) in Great Britain.

Serum-precipitin tests carried out on the stomach contents of laboratory maintained and field-caught Hydrotaea irritans showed that a positive result could be obtained for at least seven to 10 days after feeding. Eye, nasal and salivary secretions, even if undiluted, did not give a precipitin response in these tests. Field results from over 3000 flies thus indicate that 20 to 55% of the active fly population had taken a blood meal within the previous 10 days: 75 to 100% of these had fed on cattle blood and 15% or less on sheep blood. Significantly lower feeding levels were obtained from flies collected within plantations, indicating a protracted resting phase in woodland. From "fly feeding areas" high blood-feeding levels were shown from early July, as ovarian development begins. Early feeding rates were similar in both sexes.