Eradication of lice in cattle15 August 2002
Pediculosis in cattle occurs throughout the world, and is more common in cattle than in any other domestic animal (Urquhart et al. 1987). Two species of lice are endemic in Norwegian cattle, biting lice (Damalinia (Bovicola) bovis (Linnaeus 1758)) and sucking lice (Linognathus vituli (Linnaeus 1758)). Cattle lice cause irritation and restlessness, but there are conflicting opinions concerning their economic effects on livestock production. Fadok (1984) and Loomis (1986) maintained that lice can have a significant effect on milk production and weight gain. Gibney et al (1985), Nickel (1971) and Scharff (1962) found a significant effect on weight gain. However, Chalmers & Charleston (1980 a) found no significant difference in weight gain or haematocrit levels between louse-infected and louse-free cattle. Other authors have also not found significant effects on growth rate (Kettle 1974, Cummins & Tweedle 1977, Cummins & Graham 1982). In recent years, hide damage caused by lice has been increasingly recognised as a significant effect of lice infestations (Bugby et al 1990, Webster & Bugby 1990). The damage is described as areas of grain loss up to 3mm diameter that are seen on dyed crust leather (Bugby et al 1990). Historically, lice control has been obtained through the use of various insecticides (Drummond et al 1986, Hiepe 1988, Losson 1990, Wall & Shearer 1997), but toxicological problems, the environmental impact and residual effects have limited the practical use of topical insecticides. Different types of pyrethroids and new avermectin derivates without residual problems are at present recommended for dairy cattle in lactation. Louse infestations usually pass unnoticed until high numbers of lice occur on certain body regions or on the tail. Consequently, animals may not be treated at all or only treated in the late winter when the louse population has increased and possible economic consequences are already present. If the farmers are aware of the louse problem, they usually treat with insecticides to control lice during the late autumn, which is the time when the animals are housed after a period on pasture and the lice populations are still low (Scott 1988). In recent years, new strategies for the control of ectoparasites have been developed (Hiepe 1988). Eradication is a well-known method to control mange mite (Sarcoptes scabiei var suis Mègnin 1880) in pig production (Ebbesen & Henriksen 1986). Eradication of cattle lice in a single herd based on organophosphorus insecticides has also been previously described (Anthony et al. 1963). Eradication is defined as the elimination of an infectious agent from the animals and their environment and the establishment of precautions to prevent reinfection (Alexander 1986). The present study was part of a large field investigation that was conducted to evaluate the extent and impact of lice infestations in cattle and to determine the effectiveness and economic consequences of eradication as a control strategy. This paper presents the results of the clinical evaluation of the lice eradication programme and assess the effectiveness of eradication as a control strategy for lice in cattle. Materials and methods Thirty-three dairy herds were observed over a period of two and a half years. The cohort was open and included all animals leaving or entering the herds at any time. All animals, except for bulls weighing more than 400kg live weight kept in boxes, were examined three times, in March 1994, in March 1995 and in March 1996. Between the first and the second examination, in the second third of 1994, the herds in the main group (28 herds) were treated against lice. The herds were divided into two groups. One group was treated with deltamethrin and the other group with flumethrin as the main medicament in the eradication programme. The survey in March 1994 recorded the situation in the herds before eradication. The surveys in March 1995 and in March 1996 represented the clinical evaluation of the eradication programme. Five of the herds took part in a pilot study. These herds were examined for the first time in November 1993 and the eradication started immediately after the examination. The herds in the pilot group were followed for two and a half years after treatment, and examined using the same procedure as for the herds in the main group. The selection criteria were: * No registered lice problems and no systematic control of lice in the year preceding the commencement of the study * No signs of ringworm * The herd had to be a member of the Dairy Cow Recording System (Husdyrkontrollen) The number of lactating cows in the herds varied from 8 to 50. The total number of animals in all age classes varied from 22 to 128 recorded at the examination time in March 1994. Procedures The eradication programme was conducted according to the following treatment scheme: 1) All cattle were dosed twice with deltamethrin (Coopersect vet) or flumethrin (Bayticol vet) on day 1 and day 21 (2 mg/kg pour-on flumethrin or 100 mg/animal pour-on deltamethrin). These doses corresponded to the recommended doses for these preparations in the Norwegian market. The herds were randomised into two groups for comparison of flumethrin and deltamethrin. It was recommend that all animals should be clipped before treatment. 2) The barn was mechanically cleaned and subsequently sprayed with a 0.4 % solution of heptenophos (Ragadan vet). 3) Precautions were established to eliminate the risk of reinfections. The basic principles were the pre-treatment of all introduced animals and the use of special clothes and boots for visitors. 4) All animals introduced to the herds and animals in direct contact with those, were treated on day 1 and day 21. In the main group of 28 herds, the eradication was performed between September and December 1994. The examinations of the herds were performed according to standard clinical examination developed for this study. All animals in the herds were inspected except for bulls over 400kg live weight kept in boxes. The examination was undertaken using a fine-toothed comb and a halogen lamp. The predilection sites of lice, namely the neck, shoulders, dewlap, rump and tail, were examined closely. The differentiation between D bovis and L vituli was made on the colour and body shape of the lice (Wall & Shearer 1997). The diagnosis of pediculosis was based on the identification of one or more lice on an animal. Statistical methods All analyses were performed at herd level. The comparisons of different preparations, the effect of eradication and risk of reinfections with the two lice species and the effect of different risk factors were undertaken using EpiInfo (Dean et al. 1996). Results The situation before eradication is described in Table 1. D bovis was present in 27 of the herds examined in March 1994 and in four herds in the pilot group examined in December 1993. L vituli was present in 11 of the herds examined in March 1994 and in three herds examined in December 1993. Both D bovis and L vituli were present in ten herds in the main group in March and in two herds in the pilot group in December. On the individual level, D bovis was present in 27% of the animals and L vituli in 5% of the animals. Short nosed sucking lice (Haematopinus eurysternus (Denny 1842)) and little blue cattle lice (Solenopotes capilatus (Enderlein 1904)) were not present, and these two species are not endemic in Norway (Gjerde 1994). Five herds took part in a pilot study. These herds were examined and lice eradication carried out in December 1993, and these herds were followed to March 1996. Herd characteristics, louse status before treatment and results of the eradication are described in Table 2. In March 1995, three to six months after eradication, lice were present in five herds. D bovis was present in all five herds, and L vituli was present in three of the five herds (Table 3). In all infected herds, only one or two animals had pediculosis. Shortcomings in the eradication programme were identified in three of the herds that had lice after treatment. The most common error was failing to treat one or a few animals at one or both of the treatments. Only ten farmers had clipped all animals in the herd before treatment. The 23 herds in the main group that succeeded in eradicating lice were re-examined after twelve months to evaluate the risk of reinfection. In March 1996, 15-18 months after the eradication programme, lice were present in a total of eight of these herds. Six herds were reinfected with D bovis, while three herds were reinfected with L vituli (Table 3). Fifteen herds in the main group were free of pediculosis at the final examination. The difference between deltamethrin and flumethrin was assessed on the basis of the surveys in March 1995 and 1996. The results are presented in Table 5. There was no significant difference between the two groups at any examination time. Discussion The results of the survey in March 1994 indicated the same level of lice infestation in cattle in Norway as in other countries in Northern Europe. Christensson et al (1994) found lice in 25 out of 27 farms examined in survey conducted in Sweden and in 29% of the examined animals. Surveys from Scotland and the Netherlands indicated a similar level (Titchener 1983, Hendrikx 1992). All these results indicate that lice are a significant and underestimated problem in untreated herds. In the present study, the eradication programme succeeded in 28 of 33 herds, assessed three to six months after treatment. D bovis was present in all the five affected herds and L vituli in three of the affected herds. These results did not imply any significant difference between the two species. The eradication programme was not carried out correctly in three of the five herds that did not succeed in eradicating lice. The most critical mistake was to omit animals at one or both treatment times. This mistake was not detected in any of the herds that were free of lice three to six months after eradication. The result indicated that eradication can be an appropriate strategy for the control of lice in cattle, but the programme has to be carried out according to the recommended procedures. The 28 herds (23 from the main group and five from the pilot group) that were free of lice at the examination three to six months after eradication were examined twelve months later. Lice were present in nine of the herds at this second examination. D bovis was present as the only species in six of the herds, L vituli was present as the only species in two herds, and a mixed infection was present in one herd. The results did not indicate any difference between the two lice species in risk of unsuccessful eradication or reinfection. For complete eradication, different strategies can be used according to the nature of the infectious agent. The most important strategies are depopulation, strategic culling, vaccination and systematic medication, or combination of these strategies. An effective medicament and control for reservoirs of the infectious agent are necessary assumptions for the use of systematic medication as an eradication strategy. Eradication as a control strategy has some important advantages compared with other treatment strategies. The animals are constantly free of the infectious agent after treatment, the total consumption of medicaments decreases, and the risk of the development of resistance is reduced. Lice are highly host specific, obligate and permanent ectoparasites (Wall & Shearer 1997). Cattle lice are unable to survive for more than a few days off their host (Matthysse 1946, Wall & Shearer 1997). These features of the biology of the parasites are the basis for developing an eradication method based on systematic medication. The lice population changes during the year. The population increases in the winter and reaches its highest level in the late winter and early spring (Scharff 1962, Chalmers & Charleston 1980 b, Geden et al 1990). Both eradication time and examination times were selected based on this seasonal variation. The eradications were carried out in the autumn, when the louse population was low. This treatment time would be expected to increase the possibility for a successful eradication. The examination time was selected to maximise the possibility to detect unsuccessful eradications or reinfections. Clipping decreases the lice population (Allen & Dicke 1952, Nafstad 1998 b). Based on this observation, clipping was included in the eradication programme. This depopulation of lice before the insecticide treatment was expected to increase the possibility to success. However, only ten farmers clipped all animals in the herd before treatment, and there was no significant difference between clipped and unclipped herds. This result supported the recorded effect of the pyrethroid pour-on medicaments (Titchener 1985, Liebisch 1986), and suggested that clipping was not a necessary part of an eradication programme. The result three to six months after eradication gave an indication of the success of the eradication programme, and the result 15 to 18 months after eradication indicated the risk of reinfection. These findings confirmed that direct animal to animal contact is the most important transfer mechanism (Loomis 1986). Herds of purchasing livestock, or grazing on common pasture with other herds without consistent pre-treatment of introduced animals, had a significantly higher risk of reinfection. Open herds that use pre-treatment did not have a significantly higher risk of reinfection than closed herds, as assessed based on the number of herds included in the present study. These results suggested that eradication can be an appropriate strategy for lice control in open herds, if pre-treatments are performed correctly. Calves and young animals are the age groups most usually purchased and also the age groups with the highest prevalence of lice (Chalmers & Charleston 1980 c, Geden et al 1990, Nafstad 1998 a). Purchase of such animals increased the risk of reinfection in herds not using pre-treatments. Pyrethroids are effective insecticides, and both deltamethrin and flumethrin have been shown to be nearly 100% effective against D bovis and L vituli in controlled studies (Titchener 1985, Liebisch 1986). Many pyrethroids are lipophilic which assists the development of pour-on formulations with good distribution (Losson 1990). Natural pyrethrins are quickly degraded, while synthetic pyrethroides such as flumethrin and deltamethrin have greater stability and a relatively long period of action (Zerba 1988), but they do not affect all developmental stages of the louse life cycle. The eradication programme with two treatments within an interval of 21 days was based on these pharmaceutical properties combined with knowledge of the louse life cycle. Both D bovis and L vituli have an entire egg-to-adult life cycle of about four weeks under normal circumstances (Matthysse 1946, Landcaster 1957, Wall & Shearer 1997). The present study did not detect any difference in effects between deltamethrin and flumethrin.