Proceedings of the FLICS Conference, Launceston, June 2001

Transcription

1 206

2 New technology 207

3 208

4 Sheep susceptibility and immune response to lice: potential for manipulation P.J. James and G.S. Nattrass South Australian Research and Development Institute, 33 Flemington St., Glenside, South Australia Summary There is currently no alternative to chemical methods for controlling infestations of sheep lice (Bovicola ovis). Utilisation of host response to parasites through selection of resistant types, vaccination or nutritional supplementation is an appealing option. Evidence is presented of substantial and repeatable variation amongst sheep in susceptibility to lice, together with preliminary information indicating a possible association with susceptibility to gastrointestinal parasites. It is now clear that despite their surface feeding habit, B. ovis stimulate an immune response in sheep. This response may play a part in regulating the size of louse populations. Studies are needed to clarify the nature of the sheep response to lice and to determine the genetic relationship of lice resistance to production parameters and other disease resistance traits. The recent production of a high quality cdna expression library will aid in these studies. Keywords Phthiraptera, Bovicola ovis, sheep, parasite resistance, immunity, cdna library. Introduction Alternatives to chemicals for controlling parasites are required to minimise problems from resistance, residues in animal products and occupational exposure to pesticides. However, currently control of louse infestations relies almost totally on chemical methods. A possible alternative or complementary method is to utilise variability in susceptibility between sheep or to harness the natural regulatory responses of animals. This could be through breeding or the use of genotypes with increased parasite resistance, vaccination or the utilisation of nutritional supplementation to enhance immune response. There are well developed IPM programs which incorporate these approaches for other sheep parasites (Eady et al. 1997, Raadsma 1999). However, there has been limited investigation of the potential for their use against sheep lice. In this paper we present evidence of substantial variability amongst sheep in susceptibility to lice, show that lice stimulate an immune response in sheep and present evidence that suggests this response may play a part in regulating numbers of lice on sheep. Differences in susceptibility between breeds and strains Although it is commonly observed that some sheep in a flock have more lice than others, this does not necessarily indicate differences in susceptibility. Lice spread slowly amongst sheep and the animals with more lice in naturally infested mobs may simply be the ones that became infested first, those that were missed at dipping or had lousicide poorly applied or those that were not closely shorn and had more lice which survived after shearing. We have conducted two studies in which all sheep have been subjected to standardised louse challenge. These studies suggest that there is substantial and repeatable variation present, both between and within breeds of sheep. Studies with American breeds of sheep Polypay and Columbia ewes, six to eight months of age at the beginning of the study were infested with 35 lice (20 female, 10 male and 5 nymphs) on two occasions one month apart and a further 20 lice four months later. They were shorn after 11 months and each ewe again infested with 35 lice on two occasions, 3 months and 5 months after shearing. Lice were counted at 69 body sites at approximately monthly intervals over two years (James et al. 1998). (The Polypay breed was developed from crosses of Finn, Rambouillet, Dorset and Targhee sheep. The ewes in this study had a mature bodyweight of 61 kg and cut 3.8 kg of 24 µ wool at their hogget shearing. The Columbia breed was developed in 1912 from 209

5 crosses of Rambouillet and Lincoln sheep. In this study ewes had mature body weight of 67 kg and cut 4.9 kg of 26 µ wool.). There were clear differences between breeds in the densities of lice that developed with densities approximately ten times higher on the Polypays than the Columbias at most inspections (Figure 1). The other notable point was that some of the sheep appeared relatively resistant to infestation. Despite having lice applied on five occasions and penning with other infested sheep, seven of the Columbias and one of the Polypays failed to become infested. Figure 1. Mean (± se) number of lice per part on Columbia ( ) and Polypay ( ) ewes following equivalent louse challenge. (L = dates when lice were applied). Mean number of lice per part L L winter L L Shorn L summer winter Year 1 Year 2 summer Shorn Studies with Merinos Forty 12 month-old Merino wethers made up of equal numbers of two bloodlines from the Bungaree and Collinsville family groups (Roberts et al. 1975) were infested with 35 lice on two occasions a month apart as described for the American sheep. The initial infestation was in June at which time the sheep were carrying 7 months wool. They were shorn in December and again 12 months later. Lice were counted at 24 sites along the sides of the sheep at intervals over two years (Figure 2). All sheep developed infestations and louse numbers built to a mean count of 2.34±0.36 and 3.03±0.49 per part for the Bungaree and Collinsville sheep respectively before shearing at the end of year 1. There was some evidence that the density of lice in the Collinsville sheep was increasing faster than in the Bungaree sheep at the counts prior to the first shearing. Louse numbers appeared to plateau in July of the second year and did not increase further at the counts in September and November. The density of lice in the Collinsville sheep was significantly higher than the Bungaree sheep at the last count before shearing (p<0.05). It should be noted that the bloodlines included in the study were not sampled to provide a family group comparison, but simply to increase the variability of the genetic background of Merinos under study. Thus the bloodlines should not be taken as necessarily representative of the two family groups. McGuirk et al. (1978) demonstrated large differences between bloodlines within Merino strains with respect to susceptibility to fleece rot and flystrike and it is likely that similar variability exists with relation to susceptibility to lice. The magnitude of the difference in louse densities between the two studies may indicate that Merinos are relatively susceptible to lice. All sheep in the study with Merinos became infested whereas in the study with the American breeds some sheep did not become infested. In addition, the densities of lice that developed in the Merinos were approximately 20 times higher than the maximum densities observed in the Polypays and Columbias, despite that less lice were applied to the Merinos. However, this conclusion should be drawn with caution as the Merinos were paddocked outdoors whereas the Polypays and Columbias were penned indoors and the strains of lice may have differed. Differences amongst sheep within strains There were also substantial differences among sheep within breeds and a high degree of repeatability in densities at different times, both between and within years, in both studies. For the American sheep the 210

6 repeatablility of louse counts between years was 0.82 and the Spearman's correlation coefficient for sheep rank for louse counts was 0.91 (p<0.001). With the Merinos, the repeatability between years was 0.70 and the rank correlation was As is the case with many parasites there was a clumped distribution with the majority of lice found on a small number of heavily infested animals. In the American study, 62.5% of lice were found on the 12.5% most heavily infested sheep. In the Merinos the aggregation was not as marked. However, 51.2% of the lice were found on the 29.7% % most heavily infested animals. These observations suggest that a significant reduction in the within generation susceptibility of the flock could be achieved if the most susceptible animals could be identified and culled. Figure 2. Mean counts of lice per part (±se) on Merino sheep of the Collinsville ( ( ) family groups following equivalent louse challenge. ) and Bungaree Mean count/part Shorn Day from infestation Shorn Association with resistance to internal parasites Many of the immune responses invoked by internal and external parasites are similar, and both B.ovis and trichostrongylid helminths feed at epithelial surfaces (Sinclair et al. 1989, Balic et al. 2000). To investigate the possibility of an association between resistance to lice and resistance to trichostrongylid parasites, faecal samples were collected from the sheep used in the Merino louse susceptibility study on three occasions at the end of the second year and assessed for faecal egg count (FEC) (James et al. submitted). After collection of the third faecal sample to assess natural worm burdens, the sheep were drenched with 15 ml cydectin. A bulk faecal egg count was carried out to ensure efficacy of the drench and the sheep dosed 25 days later with 40,000 larvae administered per os. The dosed larvae were cultured from faeces collected from a sheep with nematode burdens consisting of 84% Trichostrongylus vitrinus, 1% T.axei and 15% Ostertagia sp. Faecal samples were collected and faecal egg counts carried out four and six weeks after challenge. Pearson correlations for the association of natural (challenge) faecal egg counts with louse counts in years 1, 2 and at the start of the third year of the study were 0.25 (0.04), 0.45 (0.25) and 0.38 (0.36), respectively. This suggests that resistance to lice and resistance to gastrointestinal parasites may be related. However, whether this is due to interaction of the effects of the parasites or to correlation in the underlying mechanisms of resistance requires clarification. We have no genetic information on resistance to lice and even if it is shown to be highly heritable it is unlikely that sheep breeders would be willing subject their animals to deliberate louse challenge in order to select for improved lice resistance. An immunologically based test or the use of genetic typing is a possibility for the future, but will require a significant research effort. However, many breeders already select for resistance to internal parasites. If the genetic association between resistance to internal and external parasites reflects the phenotypic associations indicated to date, such breeders may also be increasing resistance to lice. 211

7 Other factors influencing susceptibility to lice Cyclic patterns It is often considered that lice undergo seasonal cycles, building up in winter and spring and declining in summer. The summer decline has been attributed to the effects of shearing, high temperatures, solar radiation and rainfall. However, in the studies of Wilkinson et al. (1982) and Niven and Pritchard (1985) louse populations continued to increase through the summer even though mean maximum ambient temperatures were 32 C in the first study and as high as 36.4 C in the second and that sheep were subject to intense solar radiation. Wilkinson et al. (1982) concluded that shearing was probably more important in determining cycles than time of the year per se. There are many instances where cycles in ectoparasite numbers have been shown to be driven by mechanisms other than environment. For example, seasonal patterns in sheep ked populations are at least partially determined by cycles in the development and decline of immunity (Nelson, 1962). Infesting sheep at different times of the year displaced the timing of these cycles. Patterns in the louse densities observed over two years in the studies of James et al. (1998) are shown in Figure 2. Although sheep were housed indoors throughout the study and thus sheltered from rainfall and direct solar radiation, seasonal cycles in louse density, not related to shearing, were strongly evident. Louse populations grew to a peak in spring and then decreased during summer in both years. However, the mean daily, maximum and minimum temperatures at which populations began to decline were 11.5 C, 15.0 C and 8.1 C respectively, well below temperatures likely to cause elevation in skin temperatures and substantially lower that temperatures previously shown to reduce egg laying or cause direct mortality in B. ovis (Murray, 1960; Murray, 1968). Furthermore, the rate of increase in louse numbers began to decline markedly earlier in the season than density in both years of the study. This is consistent with the development of a regulatory response in the host sheep. Experience suggests that similar cycles do not generally occur in Merinos, although we have not carried out experiments to test this. Nutrition and disease It is a common observation amongst sheep owners that sheep in poor condition have heavier louse burdens than those in good condition. This has sometimes been interpreted as suggesting that lice cause low weight gains. However, studies designed to investigate the effects on weight gains caused by lice have shown no effect (Kettle and Lukies, 1982; Wilkinson et al., 1982; Niven and Pritchard, 1985). Counts of lice on lambs from birth until sale were negatively correlated with overall weight gain (rp = , p = 0.018) (James et al. 1998). Between weaning and sale the correlation was (p = 0.013). Scott (1952) showed that populations of B. ovis increased during winter on crossbred sheep on a low nutritional plane, but not on sheep with good nutrition and higher populations of B. bovis have been reported on cattle maintained on low feed rations (Utech et al., 1969). Interestingly, Scott found no effect of nutrition on louse burdens in Merinos, which may indicate that genotype has an effect on this relationship. In the study of James et al. (1998), the lamb which had the heaviest louse counts after weaning was observed to be scouring. In addition, the most prolific sources of lice on the property from which we collected lice for the study were a crippled, bottle fed orphan lamb which later died, and a group of ewes diagnosed with ovine progressive pneumonia. Stress can depress many immune parameters and increase susceptibility to a range of diseases (Dantzer and Kelly, 1989). Sheep in ill health, with low bodyweights or poor nutrition, may be more susceptible to lice because of reduced ability to mount a regulatory response to infestation. Pregnancy In the studies with Polpays and Columbias referred to earlier, half of the ewes of each breed were mated to Hampshire rams in the second year of the study. Densities of lice were higher on mated than unmated ewes during the last third of pregnancy and until the lambs were weaned (Figure 3). However, there was a high degree of variability in louse counts in the pregnant ewes and neither absolute numbers of lice nor changes in louse numbers were significantly different in the two groups at any time (p > 0.05). These ewes were penned and provided a favourable nutritional regime. It would be interesting to repeat this study under paddock conditions. 212

8 Age Lice rapidly spread amongst the lambs in study of James et al. (1998), even though not all of the ewes were infested. The maximum mean count per part reached in the lambs was more than three times that for comparable sites on the ewes, and all lambs became infested. This suggests that the lambs were more susceptible. However, the lambs were sired by Hampshire rams and it is possible that there was also an effect of genotype. Figure 3. Mean counts of lice on mated ( ) and unmated ( ) Polypay and Columbia ewes (breeds combined). 0.6 Mean lice per part (± se) Lambing began Lambs weaned Shorn 0 winter summer Immune response Sheep lice feed on the skin surface (Sinclair et al., 1989). Dietary components include lipid secretions, loose scurf, bacteria and squames from the superficial layers of the stratum corneum. Therefore it appears that B. ovis does not penetrate deeply enough to directly contact elements of the host immune system. However, it is now clear that B. ovis stimulates an immune response in sheep and there is evidence that this response may play a role in regulating louse numbers. Hypersensitive response Sheep elicit a Type I hypersensitive or allergic response in their hosts. When the Polypay and Columbia ewes described above were challenged intradermally with soluble B. ovis extract, wheal and flare reactions, characteristic of immediate hypersensitive responses developed within 20 minutes in all instances (James and Moon, 1998). Pfeffer et al., (1997) have demonstrated homocytotophic IgE with specificity for B. ovis in louse infested lambs and hypersensitive response is thought to be the cause of the sheep leather defect known as cockle caused by sheep lice (Heath et al., 1995). Immediate (20 minute) reactions were greater in the more resistant Columbias than in the Polypays (p<0.05). However, the reactions at 24 hours were greater in the Polypays than the Columbias (p<0.001). There was no correlation between louse numbers and the size of reactions at 15 minute, one hour or three hours. However there was a strong correlation between the size of reactions at 24 hours and both louse numbers (r = 0.76 and r = 0.47 for skin thickness and wheal area, respectively) and the frequency of pruritic behaviour (r = 0.57, r = 0.35, respectively). The 24 hr reactions may have been the late phase of a Type I reaction, but the possibility that a Type IV (classical delayed) response was involved could not be completely discounted. More recent studies with Border Leicester x Merino wethers with heavier louse infestations than in the American studies showed evidence of reactions at 48 hours and at 72 hours, although not as strong as at 24 hours. This suggests the development of Type IV or delayed hypersensitivity in some sheep. 213

9 Hypersensitive responses have been implicated in resistance to other ectoparasites (Kemp and Bourne, 1982; Stromberg and Fisher, 1986) and James and Moon (1998) discuss hypotheses for the possible roles of the immediate and late phase responses in susceptibility to B. ovis. However, at this stage the effect on sheep lice is unclear. Cellular immune response Proliferative responses to B. ovis antigens in lymphocytes collected from peripheral blood and prescapular lymph nodes of infested lambs have been indicated (Bany et al., 1995) providing further evidence of immune recognition of B. ovis by sheep. There was no difference in the degree of proliferation of lymphocytes collected from serum from lymph nodes which do not receive lymph from the skin, suggesting that the cellular response was operating locally at the level of the skin and skin draining lymph nodes. Figure 4. Association between numbers of lice during population decline and titre of anti-b. ovis IgG in washes from the skin surface of infested ewes (from James et al. 1998b) 3 Log (Louse count + 1) Sheep with no lice Log 10 (Skin wash absorbance ratio) Antibody response Antibodies that react with louse extracts were found in the serum of the Polypay and Columbia ewes but there was no difference in antibody titre between infested and non-infested animals (James et al., 1998b). This may have been due to the low louse numbers on the ewes or that the response to B. ovis was obscured by antibodies to other arthropods which cross reacted with B. ovis antigens. However, in the lambs from these ewes, concentrations of anti-b. ovis IgG were significantly higher in infested than naive animals (James et al., 1998a). The lambs were born in late winter when temperatures where they were housed were often below 0 C and it is unlikely that they would have had appreciable exposure to arthropods other than B. ovis in the early part of their lives. In studies with Merinos where infestations were much heavier correlations of 0.36 to 0.60 were found between louse numbers and levels of antilouse IgG, but there was no correlation with serum concentrations of anti-b. ovis IgE (James et al. submitted). Immunity at the skin surface Anti-B. ovis IgG antibodies were found in skin washings from the infested Polypay and Columbia ewes and there was a negative relationship between numbers of lice and the concentration of anti-louse antibody during B. ovis population decline (Figure 4). In addition, Eisemann et al. (1994) showed that B. ovis ingested anti-horse myoglobin antibodies while feeding on the skins of sheep which had been previously vaccinated intramuscularly against horse myoglobin. Furthermore, skin washings collected from sheep during decline in louse densities and added to louse diet fed to lice in laboratory colonies reduced louse reproduction (James et al., 1998b). This was due to a combination of reduced life expectancy of female lice, lower oviposition rate and decreased hatch rate of eggs. 214

10 Although the above findings may suggest regulatory role for IgG antibodies at the skin surface, many biocidal compounds which could affect lice are found on or near the epithelial surface. For example, major basic protein which has toxic effects against helminth parasites, bathes the skin in human atopic dermatitis (Dahl, 1996) and IgA and IgM immunoglobulins have been identified on the surface of sheep skin (Jenkinson et al., 1979). The effects observed on B. ovis could also be due to other isotypes of antibody or other immune effectors correlated with the occurrence of IgG at the skin surface. cdna expression library for B. ovis A cdna expression library has been constructed from the genetic material of adult (male and female) Bovicola ovis. The library has been constructed in the lambda ZAPII vector (Stratagene, USA) as this system can be screened in both procaryote and eucaryote cells. This allows expression in systems which have the ability to perform post translational modifications (eg. glycosylation) which may be important to antigenicity. The primary library had a titre of 5 x 10 5 plaque forming units (pfu) per ml; only 2-3% of these plaques did not contain a cdna insert. The library was subsequently amplified in Escherichia coli and a final titre of 3 x 10 8 pfu obtained. The average insert size in the library was 1.9kb; the largest was 4.5kb and the smallest 0.8kb. Preliminary screening of the cdna library with sera from sheep vaccinated with a louse extract has resulted in the isolation of several immunoreactive clones. These clones have been partially sequenced and compared for DNA sequence homology against the GenBank database. Immunoreactive clones isolated to date have matched known allergens from organisms such as dust mite (Dermatopagoides farinae) and cockroach (Periplaneta americana). Some of these clones were large (90-140kDA), providing further evidence that the library contains a good representation of B. ovis cdnas. However, it is likely that only the most abundant or immunoreactive proteins have been identified to date. More extensive screening, comparing naive, vaccinated and naturally infested sheep and susceptible and resistant animals will be necessary to identify antigens which stimulate a protective response. Conclusions There is now abundant evidence that B. ovis stimulates an immune response in sheep. Increases in louse numbers observed on sheep in ill health or with poor nutrition and the studies with laboratory lice fed skin surface compounds are consistent with immune involvement in the regulation of louse numbers. There are many instances in other host-ectoparasite systems where immune response has been shown to regulate parasite numbers. Ingestion of host antibodies has been reported to have effects on arthropod parasites including reduction in the size of the blood meal, reduced fecundity and hatchability of eggs, increased duration of the first instar, gut rupture and death (Ratzlaff and Wikel, 1990; Tellam et al., 1992; Ben Yakir et al., 1994; Heath et al., 1994). Sheep lice are obligate parasites, spend their whole life on the host and appear to be relatively sensitive to regulating influences (Murray and Gordon 1965). As such they would seem to be attractive targets for an immune based approach to control. The recent development of a commercial vaccine for cattle tick demonstrates the concept of vaccinating to protect against ectoparasites (Willadsen et al., 1995). In addition the use of nutritional supplements or prebiotics to enhance immune response is an area of growing interest in other animal industries (Blecha, 1988). Further studies are needed to investigate these approaches for the control of sheep lice. In addition, it is clear that there is significant variability in susceptibility to lice in sheep populations, and furthermore that this resistance may be related to resistance to other sheep parasites. Improved resistance to lice could play an important part in an integrated approach to lice control. Studies are needed to determine the heritability of susceptibility to lice and its genetic association with production parameters and other disease resistance traits. References Balic, A., Bowles, V. M., and Meeusen, E. N. (2000). The immunobiology of gastrointestinal nematode infection in ruminants. Advances in Parasitology 45,

11 Bany, J., Pfeffer, A. and Phegan, M. D. (1995). Comparison of local and systemic responsiveness of lymphocytes in vitro to Bovicola ovis antigen and concanavalin A in B. ovis-infested and naive lambs. International Journal for Parasitology 25, Ben-Yakir, D., Mumcuoglu, K. Y., Manor, O., Ochanda, J. and Galun, R. (1994). Immunisation of rabbits with a midgut extract of the human body louse Pediculus humanus humanus: the effect of induced resistance on the louse population. Medical and Veterinary Entomology 8, Blecha, F. (1988). Immunomodulation: a means of disease prevention in stressed livestock. Journal of Animal Science 66, Dahl, M. V. (1996). Clinical Immunodermatology (Mosby: St Louis). Dantzer, R. and Kelley, K. W. (1989). Stress and immunity: an integrated view of relationships between the brain and the immune system. Life Sciences 44, Eady, S. J., Woolaston, R., Ward, L. J., Gray, D. G., Karlsson, J. and Greef, J. (1997). Nemesis systems incorporating resistance to worms in Merino Breeding Programs. Proceedings of the Australian Society of Animal Breeding and Genetics 12, Eisemann, C. H., Pearson, R. D., Donaldson, R. A., Cadogan, L. C. (1994). Ingestion of host antibodies by Bovicola ovis on sheep. International Journal for Parasitology 24, Heath, A. W., Arfsten, A., Yamanaka, M., Dryden, M. W. and Dale, B. (1994). Vaccination against the cat flea Ctenocephalides felis felis. Parasite Immunology 16, Heath, A. C. G., Cole, D. J. W., Bishop, D. M., Pfeffer, A., Cooper, S. M., Risdon, P. (1995). Preliminary investigations into the aetiology and treatment of cockle, a sheep pelt defect. Veterinary Parasitology 56, James, P. J. and Moon, R. D. (1998). Pruritis and dermal response to insect antigens in sheep infested with Bovicola ovis. International Journal for Parasitology 28, James, P. J., Moon, R. D., Brown, D. R. (1998a). Seasonal dynamics and variation among sheep in densities of the sheep biting louse, Bovicola ovis. International Journal for Parasitology 28, James, P. J., Moon, R. D. and Ragsdale, D. W. (1998b). Skin surface antibodies and their associations with sheep biting lice, Bovicola ovis, on experimentally infested sheep. Medical and Veterinary Entomology 12, James, P. J., Carmichael, I. H. C., Pfeffer, A., Martin, R. R. and O Callaghan, M. G. Variation among merino sheep in susceptibility to lice (Bovicola ovis) and association with susceptibility to trichostrongylid gastrointestinal parasites (submitted for publication). Jenkinson, D., Lloyd, D. H. and Mabon, R. M. (1979). The antigenic composition and source of soluble proteins on the surface of the skin of sheep. Journal of Comparative Pathology 89, Kettle, P. R. and Lukies, J. M. (1982). Long-term effects of sheep body lice (Damalinia ovis) on body weight and wool production. New Zealand Journal of Agricultural Research 25, Kemp, D. H. and Bourne, A. (1982). Boophilus microplus: the effect of histamine on the attachment of the cattle tick larvae, studies in vivo and in vitro. Parasitology 80, McGuirk, B. J., Atkins, K. D., Kowal, E. and Thornberry, K. (1978).Breeding for resistance to fleece rot and bodystrike the Trangie programme. Wool Technology and Sheep Breeding 26(4):

12 Murray, M. D. (1960). The ecology of lice on sheep II The influence of temperature and humidity on the development and hatching of the eggs of Damalinia ovis (L) Australian Journal of Zoology 8, Murray, M. D. (1968). Ecology of lice on sheep VI The influence of shearing and solar radiation on populations and transmission of Damalinia ovis Australian Journal of Zoology 16, Murray, M. D. and Gordon G. (1969). Ecology of lice on sheep VII. Population dynamics of Damalinia ovis (Shrank). Australian Journal of Zoology 17, Nelson, W. A. (1962). Development in sheep of resistance to the ked Melophagus ovinus (L.) I. Effects of seasonal manipulation of infestations. Experimental Parasitology 12, Niven, D. R. and Pritchard, D. A. (1985). Effects of control of the sheep body louse (Damalinia ovis) on wool production and quality. Australian Journal of Experimental Agriculture 25, Pfeffer, A., Phegan, M. D., and Bany, J. (1997). Detection of homocytotropic antibody in lambs infested with the louse Bovicola ovis using a basophil histamine release assay. Veterinary Immunology and Immunopathology 57, Raadsma, H. W. (1999). Genetic aspects of resistance to ovine cutaneous myiasis. In Breeding for Disease Resistance in Animals, 2 nd edition (Ed. R.F.E. Axford, S.C. Bishop, F.W. Nicholas and J.B. Owen) pp (CABI Publishing, New York). Ratzlaff, R. E.and Wikel, S. K. (1990). Murine responses and immunisation against Polypax serrata (Anoplura:Polyplacidae). Journal of Medical Entomology 27, Roberts, E. M., Jackson, N. and Phillips, J. M. (1975). A revised list of family groups of Australian Merino stud flocks. Wool Technology and Sheep Breeding 22, 6-9. Scott, M. T. (1952). Observations on the bionomics of the sheep body louse (Damalinia ovis). Australian Journal of Agricultural Research 3, Sinclair, A. N., Butler R. W. and Picton J. (1989). Feeding of the chewing louse Damalinia ovis (Shrank) (Phthiraptera:Trichodectidae) on sheep. Veterinary Parasitology 30, Stromberg, P. C. and Fisher, W. F. (1986). Dermatopathology and immunity in experimental Psoroptes ovis (Acari:Psoroptidae) infestation of naïve and previously exposed Hereford cattle. American Journal of Veterinary Research 47, Tellam, R. L., Amith, D., Kemp, D. H. and Willadsen, P. (1992). Vaccination against ticks. In Animal Parasite Control Utilising Biotechnology (Ed. W.K. Yong) pp (CRC Press, Inc.: Boca Raton, Fla.). Utech, K. B. W., Tierarzt, R. H., Wharton, R. H. and Wooderson, L. A. (1969). Biting cattle louse infestations related to cattle nutrition. Australian Veterinary Journal 45, Willadsen, P., Bird, P., Cobon, G. and Hungerford G. (1995). Commercialisation of a recombinant vaccine against Boophilus microplus. Parasitology 110, Wilkinson, F. C., de Chaneet, G. C., Beetson, B. R. (1982). Growth of populations of lice, Damalinia ovis, on sheep and their effects on production and processing performance of wool. Veterinary Parasitology 9,