PROGRESS ON ERADICATION OF WARBLE FLY IN THE UK.

There are two species of warble fly within the UK, Hypoderma bovis and H. lineatum. Of these the former is the predominant species in Britain. Both cause serious myiasis to cattle and therefore are of extreme economic importance. To implement any kind of control measure against a pest it is essential that the biology and lifecycle of the species in question is comprehensively studied and understood. Not until this has been established can an effective control scheme be planned. Once the flies background is known the decisions on which specific aspects of the flies lifecycle is to be targeted needs to be elucidated. Various factors need to be integrated for an effective control method, these include economic viability, environmental safety, effects on non-target species, practicality of implementation, and the use of recent technology to continually increase the effectiveness of the control scheme. The ensuing review will endeavour to discuss all these factors with regards to the successful control of warble fly in the UK and the need for constant surveillance against this important economic pest. The first part of this discussion will reveal the flies lifecycle and how this causes pathogenesis to cattle, and then how a knowledge of the lifecycle has allowed it to be exploited by a control campaign.

The lifecycle of both species will be described here as the differences between the species are not considerable. H. bovis is approximately 15mm in length and H. lineatum 13mm in length (Monnig 1950). Both species are hairy, those on the head and thorax being yellowish-white in H. lineatum and greenish to reddish yellow in H. bovis. The adult flies have no functional mouth parts and as expected are short lived, living only three to five days (Kettle 1990). Both species are univoltine and H lineatum adults can be found from March until the end of May, whilst H. bovis emerge from around June to mid-September (Tarry 1980). Mating occurs very soon after emergence often within 24 hours and females are ready to oviposit twenty minutes after copulation (Broce 1985).

There is a clear distinction between the two species in both ovipositing behaviour and the number of eggs per hair laid on cattle. H. lineatum approaches cattle by a series of hops along the ground and then crawls up the leg of the cow. This behaviour is probably unnoticed by the cattle and in America this behaviour has given the fly a colloquial name of " heel fly ". In contrast to this H. bovis flies to and lands directly onto the cattle and the noise of its approach seems to be recognised by cattle causing them to attempt evasive action. These evasive attempts are known as " gadding " and result in decreased yield from cattle in either milk or biomass due to feeding disturbances and in severe cases bad cuts and broken bones due to collision with barbed wire and fence posts (Tarry 1986).

The eggs, measuring 1mm in length, of H. lineatum are deposited in a row of several eggs attached by a petiole to the hairs below the hocks. The eggs of H. bovis are laid singly on the body and above the hocks (Urqhart 1992). Egg production has been estimated to range from 500-800 and the first instar stages are less than 1mm long. Entry into the tissue of the cow is via the hair follicle and is aided by the use of proteolytic enzymes and the use of paired mouth hooks. From this point of invasion the larvae either travel to the submucosa of the oesophagus, in the case of H. lineatum, or to the epidural fat of the spinal canal. In February to March the larvae migrate again to the skin of the back in areas roughly twelve inches either side of the mid-line. The larvae, which are now 10mm in length moult to the 2nd instar stage. It is this stage that causes the characteristic raised skin nodules called warbles. The larvae cut a hole in the cows skin through which it is able to respire. The larvae are equipped with backwardly pointing spines which makes manual removal at this stage difficult (Zumpt 1965). The larvae moults to the 3rd instar stage and then to the prepupa in the warble. The larvae are approximately 30mm long when mature and the prepupa exits the warble by the breathing aperture and falls to the ground. Here it seeks cover, H.bovis has been reported burrowing beneath loose soil (Gregson 1958). The metamorphosis from pupa to adult fly takes from two to eight weeks, emergence occurs from March to June according to species and climatic conditions, these adults then continue the cycle.

It is evident from the flies lifecycle that both adult and larval stages cause significant economic loss from both dairy and beef cattle. The gadding behaviour of cows has been discussed but severe pathogenesis is also caused by the migrating larvae. The paths through the hosts body leave tracts which become infected with a substance called butchers jelly. These tracts have to be removed by cutting and it is this trimming that causes further loss of meat, a loss that is compounded by increased man hours in meat preparation. The warbles themselves cause damage to the hide either by direct puncture through the hide or by weak scar tissue which greatly reduces the versatility of the hide for manufacture of quality leather goods. The combined economic cost of these Hypoderma derived symptoms was estimated at 13,000,000 in 1977 (Tarry 1986).

Attempts to control warble fly had been attempted in the years prior to World War II. However The Warble Fly Order which instructed farmers to treat all warbled cattle was revoked in 1964 (Wright 1979). This was mainly because the time and man power needed to implement the control by derris applications was unacceptable to farmers. These applications needed to be repeated monthly to visibly infected animals due to derris having a poor residual action. When the order was revoked in 1964 a publicity campaign by the Ministry of Fisheries and Food was engaged, introducing farmers to the new systemically acting organophosphorus insecticides. This campaign was successful and caused a dropped of approximately 34% in the years 1960-1966 with regards to damage hides. There was some re-emergence of warble infestation in following years which have been shown to be closely related to climatic conditions (Tarry 1980). Due to this upward trend and the knowledge that a practical solution in the form of organophosphorus existed the Warble Fly Order 1978 was introduced. This order required farmers to treat infected cattle with organophosphorus insecticides and these herds also had movement restrictions enforced on them.

In 1981 two new implications served to further improve the control campaign. These were the treating of infected cattle again in the autumn to kill any larvae missed in the spring dosing, and the approval of injectable avermectins. Ivermectin is extremely effective in killing Hypoderma larvae but due to residues is not acceptable for use in dairy cattle. This is also the case with other systemic insecticides and at present there is no Hypoderma treatment available for lactating cattle. The table below shows some of the products available at present to treat against Hypoderma.

The only problem at present encountered with the use of ivermectin has been adverse reactions in cattle due to death of migrating larvae or those resident in the oesophagus or spinal region (Sutherland 1990). This point emphasises the importance of detailed knowledge of the flies lifecycle. The adverse reactions can be avoided by careful timing of chemotherapy, in the UK spring treatment will serve to kill the first instar stages before migration can begin (Jackson 1989). The huge advantage of ivermectin is the wide scope of its effects. Many farmers will be able to use ivermectin to administer simultaneous treatment against both hypodermosis and many helminth parasites such as Ostertagia ostertagi.

By 1981 incidence's of hypodermosis had dropped to a level where it could be made a notifiable disease. In 1982 it was illegal to not notify the local Animal Health Office of an infected animal. Within five years the level of infection in UK herds had fallen from approximately 40% to 0.5%. The introduction of an Infected Area legislation which placed a 3km radius of treatment around an infected herd served to further reduce hypodermosis with the national infestation rate being around 0.01% in 1985. The number of farms that were placed under the order in 1982 was 1589. The success of this control programme is due to all those points mentioned in the introduction being integrated successfully, in essence it was the availability of an effective easily applied chemical, farmer co-operation, and an active legislation from MAFF that really turned the tide.

Having eradicated hypodermosis from some regions of the UK, with its presence elsewhere only at low levels the risk of the disease escalating again will become increased. This is because of the difficulties of maintaining vigilance especially as young farmers enter the business who will never have encountered a warble fly problem. The main risk at present is from imported cattle. Due to EC policy, mainly that of non-pedigree cattle being an option and the stoppage of quarantine for foot and mouth disease it is now both easier to import and more economically attractive (Sinclair 1994). An important tool in surveillance of cattle entering from abroad is the Enzyme Linked Immunosorbent Assay (ELISA). This test was first introduced by Engvall and Perlmann in 1971. However to be used for hypodermosis it needed to be adapted and show no cross reaction with other parasites of cattle such as Fasciola hepatica or Ostertgia ostertagi. A system for the test was developed where an extra step to the procedure was found necessary. When testing for either species of Hypoderma a antiglobulin step is placed between the application of the serum and the conjugate. Without this step bovine globulins attached to the Hypoderma antigen on the walls of the microtitre plate and were not presented in a suitable way for antiglobulin conjugate to act upon the substrate (Sinclair & Wassall 1983).

This ELISA test allowed MAFF to randomly screen cattle both within the UK and those being imported. A downward trend can be seen in the number of infected cattle over time in the table below.

The remaining 0.01% infection level for hypodermosis will be extremely difficult to eradicate as a balance between control and the fly population has probably been reached. This low level still poses a real threat as both species are rstrategists with a very high fecundity and reproductive potential. Therefore any relaxing of the current control mechanisms will allow the flies to very quickly increase their population size. To completely eradicate the disease it will probably be necessary to introduce another control mechanism into the campaign. One possibility for this is the sterile male technique, an insect pest management program that showed excellent initial results against Cochliomyia hominivorax (Bush et al.. 1976). Males are exposed to a dose of radiation that is just enough to render them sterile but not to disrupt any of their behavioural habits, when released in large numbers they will mate with the greater number of females who will produce sterile eggs. This technique would be ideal for the UK due to the geographical isolation enjoyed by this country. The draw backs at present are those of cost as production of sterile males is expensive and labour intensive (Kunz et al.. 1990). However were this mechanism employed and sufficient sterile males could be released there is a high likelihood of success in the UK. Once total eradication had occurred resources could be diverted towards surveillance of imported animals.

To summarise, the eradication of warble fly has not yet been achieved and with the present programme it seems unlikely that total eradication will occur. The programme has been hugely successful in controlling hypodermosis and it is important to maintain the necessary awareness of the disease to prevent sudden re-emergence. As many other countries look at the UK's success it is highly likely that eradication schemes will be implemented abroad along similar lines. The widespread use of organophosphorus insecticides against this pest raises the question of resistance at which point the rapid production of alternate methods would be needed. Another recent development concerning organophosphorus insecticides and warble fly treatment is a suggested link with Bovine Spongiform Encephalopathy (BSE). The lack of cases reported abroad of BSE make this link a very tenuous one, although there is evidence of a link with a particular treatment called phosnet. The significance of this treatment and BSE is that Switzerland was the only other European country to insist on its use and it is the only other European country with large-scale BSE. These points all indicate that research into control methods, and there long term consequences of this and other parasites should be continuously studied.

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