The cattle tick in relation to animal production in Australia

P.H. Springell

ALL PHOTOS: CSIRO AUSTRALIA

1. Engorged adult female tick laying eggs on the ground.

The cattle tick (Boophilus microplus) is economically the most damaging bovine ectoparasite in Australia (Gee, 1959). Although it is confined to the wetter and warmer regions of northern Australia, about half of the country's 24 million cattle are in the enzootic zone. The parasite is also found in many other parts of the world (Roberts, 1970) and other species, including horses, sheep and deer, are minor natural hosts. Australia is fortunate in having only one tick species of major economic importance. The Australian experience is therefore valuable in that the effects of B. microplus can be accurately assessed in isolation. It must be stressed, however, that the various conclusions that can be made in this situation may not necessarily apply generally, on account of the intricate interplay resulting from the presence, in different combinations, of many other ticks and tick-borne diseases elsewhere.

P.H. Springell is principal research scientist at the Cattle Research Laboratory, CSIRO, Division of Animal Genetics, Rockhampton, Queensland, Australia.

Economic losses to the cattle industry

A survey of economic losses to the cattle industry caused by B. microplus was undertaken by Gee (1959), and it was estimated to be US$8.50 (December 1973 exchange) per beast per year. The estimate was based on questionnaires to producers and on interviews with property owners. The losses within the state of Queensland, in northeastern Australia, were partitioned as shown in Table 1.

The survey may be criticized on several counts, but it is the most comprehensive document available to date. A new inquiry has been held, and the report was to be published in March 1974.

2. A heavily tick-infested beast showing engorging adult females.

3. Adult female ticks attached to animal's skin, in final stages of attachment prior to dropping.

According to a Ministerial statement outlining the major conclusions of the new report, the total loss due to the cattle tick in Queensland alone has not changed greatly since 1959: it increased from US$47 million to $50 million (December 1973 exchange). The present cost to Australia as a whole is estimated to be $62 million; no comparable figures were available in the earlier report.

Table 1. Breakdown of the economic losses caused by the cattle tick

Babesiosis and anaplasmosis

Babesiosis, which is carried and transmitted by the cattle tick, is enzootic in the area covered by the survey. Young calves (one to two months old) are protected by maternal antibodies, but continuous exposure to Babesia-carrying ticks results in the acquisition of long-term immunity. Of the two species of Babesia found in Australia, B. argentina is more prevalent and more pathological than B. bigemina.

The demarcation line between the enzootic and tick-free areas is not rigidly defined, but the tick-infested area is in general north of latitude 30°S, where the annual rainfall exceeds 500 millimetres (Figure 4). Climatic and seasonal fluctuations result in some boundary movements.

In these marginal zones, B. microplus infestations occur only sporadically and, consequently, natural immunity to babesiosis cannot be relied on. Here there are additional economic penalties, either of vaccination against tick fever or of periodic stock losses from the disease. Vaccination is also necessary when stock is moved from a tick-free to a tick-infested area.

Similar considerations apply where tick populations have been reduced artificially. While these considerations have discouraged stock owners from adopting the most effective tick control measures, there is now a greater trend toward reducing and maintaining ticks at minimal levels and protecting cattle by vaccination (Mahoney and Ross, 1972).

Anaplasma marginale, which is also transmitted by B. microplus, is another parasite that causes tick fever. Anaplasmosis has not been studied to the same extent as babesiosis, possibly because of its lower incidence. In a recent evaluation of tick fever outbreaks in northern Queensland from January 1964 to April 1971 (Rogers, 1971), it was found that 140 (77 percent) were caused by Babesia argentina, 37 (20 percent) by A. marginale and 5 (3 percent) by B. bigemina. A vaccine against anaplasmosis is also available. Recent unpublished studies on artificially induced babesiosis without concurrent B. microplus infestation suggest that animal production is affected only if the animal dies.

Animals surviving the disease appear to show no weight loss. Consequently, the production losses described here are most probably attributable to the cattle ticks themselves rather than to the protozoans they sometimes carry.

4. Distribution of Boophilus microplus in Australia

Variations in tick burden

While the tick problem varies considerably in different climatic areas, a number of other factors influence the load of ticks carried by an animal (Table 2). Nutritional stress results in greater tick burdens in European (Bos taurus) cattle (O'Kelly and Seifert, 1969). Male cattle have higher tick counts than females; lactating European cows are also more heavily infested than dry ones, but pregnancy has no effect (Seifert, 1971). Tick numbers are subject to seasonal differences (Turner and Short, 1972), while cattle breeds also differ in their resistance to ticks (Seifert, 1971). Zebu (Bos indicus) cattle and their crosses not only carry fewer ticks, but their resistance is also more highly heritable than in European breeds. The attainment of tick resistance by selection in European cattle is therefore more difficult than in the zebu. In the years since the 1959 survey there has been an increase in cattle numbers, as well as an increase in the proportion of zebu cattle, in northern Australia. Despite increases in the costs of labour and of acaricides, the total economic loss caused by ticks to the industry has not changed greatly, while the importance of this loss in the economy of the industry as a whole has diminished.

Table 2. Some factors influencing the tick burden carried by cattle

5. Larval ticks poised on a blade of grass awaiting passing cattle for attachment.

Production losses

Early studies on production losses caused by the cattle tick seemed to indicate that an average of one mature tick per day caused a growth rate depression equivalent to at least 450 grams a year. However, such average figures are of little practical use in view of the various factors influencing tick burdens. The effects of ticks on beef production have recently been documented more fully (Turner and Short, 1972).

Weight gains following dipping were greatest in European breeds (46 percent) and not significant (< 10 percent) in zebu crosses. There is a lack of comparable data for milk production. This is unfortunate, as they would be of particular relevance to the dairy industry in affected regions. Presumably there must also be some detrimental effects on young beef calves because of the likely depression of appetite following tick infestation (Seebeck, Springell and O'Kelly, 1971) which would affect maternal milk output.

Loss of appetite in heavily tickinfested European cattle was found to be responsible for 65 percent of the body weight reduction (Seebeck et al., 1971). The remaining 35 percent was attributable to interference with the growth process, possibly through the mediation of a tick toxin which could come from the saliva known to be injected into the host.

6. Harness used for artificial tick infestation. Arrow indicates vital containing tick larvae.

Not only is the weight gain retarded as a result of infestation, but there appear to be some after-effects also. Following an infestation period, cattle were freed of ticks by dipping and their subsequent growth examined (Seebeck et al., 1971). Animals which had not been infested and which were restricted in feed intake to the level of the infested animals showed typical compensatory growth (0.78 kg per day), whereas previously infested animals did not grow significantly faster (0.56 kg/day) than a control group (0.45 kg/day) which had been kept tick-free on an ad libitum diet throughout.

This suggests that there may be a “memory” effect or some permanent impairment of the growth process following earlier infestation. The higher respiratory quotients of cattle that had previously been tick-infested (0.732 as against 0.711) suggest that an effect on protein metabolism persists for at least four weeks after dipping (Vercoe and O'Kelly, 1972). Indeed, Turner and Short (1972) showed that some of the differences in growth rate observed between temperate and tropical breeds of cattle in the tropics are due to an exposure of susceptible temperate cattle to ticks. It is tempting to speculate that future growth patterns are set following exposure to parasites early in life.

Blood and composition changes

It has been known for a long time that B. microplus induces anaemia. Seifert et al. (1968) were able to show by radioactive means that the average blood uptake (0.3 ml) of an engorged adult female amounted to about twice its own weight. Subsequently it was found that infested animals had apparently lost their ability to adequately replace haemoglobin and plasma albumin, while globulin synthesis was increased (Table 3) presumably because of an immunological response (Springell et al., 1971). Tick infestation also resulted in a change in body composition. Affected animals had relatively more fat and less muscle than their parasite free controls, as reflected by the muscle/fat ratios of 3.23 and 4.52 respectively. This again points to a possible interference by the ticks with protein synthesis.

Table 3. Loss and replacement of blood proteins by tick-infested cattle

¹ (+) = gain, (-) = loss.
² (+) = production above that withdrawn by ticks, (-) = produc-tion below that withdrawn by ticks.
³ Not significantly different from zero.
⁴ P < 0.001.

Tick control

Much research effort has gone into cattle tick control. Eradication in Australia has never been considered a practical proposition, except in a relatively small area at the southern extremity of the tick-infested sector of northern New South Wales. Efforts were made at about the turn of the century to prevent the southward spread of the tick along the eastern coast, and the ultimate objective in New South Wales has always been eradication. Success was achieved in small areas, but a serious setback occurred during 1956–57, when the largest and most ambitious programme failed. However, control is now maintained at such a high level in northern New South Wales that production losses are negligible.

A recent study by Wharton et al. (1969) compared dipping, rotational grazing and the use of tick-resistant cattle. The last of these is perhaps the most promising solution at the present time. Another form of biological control, the use of natural predators, has so far been disappointing. While such predators may be expected to occur in areas where B. microplus originated, there is little evidence to date to suggest that the parasite is any more under natural control in these areas than elsewhere.

Although the industry has in the past depended on control by acaricides, this inevitably involved high labour costs (Gee, 1959). In any case, the development of acaricide-resistant tick strains (Wharton and Roulston, 1970) has, with time, rendered one chemical agent after another ineffective.

Much of the work on resistant tick strains has nevertheless provided information on the genetics of resistance and the possible mechanism involved, all of which may in the future provide a sound basis for effective biological control. Parallel work on the inheritance of resistance to ticks in cattle may also contribute toward eventual biological control.

Conclusions

The cattle tick (Boophilus microplus) has an adverse effect on beef production and almost certainly also on milk production. Among the effects observed in cattle are loss of weight and anaemia. These are partly explained by loss of appetite and failure to replace lost proteins. Infested animals may also suffer a change in body composition. Effects of an earlier infestation may persist after removal of the parasites.

Babesiosis and anaplasmosis, which are transmitted by the cattle tick, pose additional problems in regions of sporadic infestation. Outbreaks of these diseases in such areas can cause stock losses unless vaccination is resorted to.

The tick is found in humid tropical and subtropical areas, but temperate breeds of cattle are the most susceptible. Various costly control measures are available, but the use of resistant cattle will probably provide the most effective long-term solution in countries like Australia, where there is only one tick species of major economic importance. However, in many tropical countries where genetic improvement is being sought through the introduction of European stock, there is the danger that replacement of Bos indicus by Bos taurus genes could result in increased susceptibility to the cattle tick. The presence of ticks other than B. microplus and of several associated tick-borne diseases is an added complication in tropical areas outside Australia.

References

Gee, G.F. 1959. The economic importance of the cattle tick in Australia. Canberra, Bureau of Agricultural Economics.

Mahoney, D.F. & Ross, D.R. 1972. Epizootiological factors in the control of bovine babesiosis. Aust. vet. J., 48: 292–298.

O'Kelly, J.C. & Seifert, G.W. 1969. Relationships between resistance to Boophilus microplus, nutritional status and blood composition in Shorthorn X Hereford cattle. Aust. J. biol. Sci., 22: 1497–1506.

Roberts, F.H.S. 1970. Australian ticks. Melbourne, Commonwealth Scientific and Industrial Research Organization.

Rogers, R.J. 1971. An evaluation of tick fever outbreaks in Northern Queensland in recent years. Aust. vet. J., 47: 415– 417.

Seebeck, R.M., Springell, P.H. & O'Kelly, J.C. 1971. Alterations in the host metabolism by the specific and anorectic effects of the cattle tick (Boophilus microplus). I. Food intake and body weight growth. Aust. J. biol. Sci., 24: 373–380.

Seifert, G.W. 1971. Variations between and within breeds of cattle in resistance to field infestations of the cattle tick (Boophilus microplus). Aust. J. agric. Res., 22: 159–168.

Seifert, G.W., Springell, P.H. & Tatchell, R.J. 1968. Radioactive studies on the feeding of larvae, nymphs and adults of the cattle tick Boophilus microplus (Canestrini), Parasitol, 58: 415–430.

Springell, P.H., O'Kelly, J.C. & Seebeck, R.M. 1971. Alternations in the host metabolism by the specific and anorectic effects of the cattle tick (Boophilus microplus). III. Metabolic implications of blood volume, body water and carcass composition changes. Aust. J. biol. Sci., 24: 1033–1045.

Turner, H.G. & Short, A.J. 1972. Effects of field infestations of gastrointestinal helminths and of the cattle tick (Boophilus microplus) on growth of three breeds of cattle. Aust. J. agric. Res., 23: 177–193.

Vercoe, J.E. & O'Kelly, J.C. 1972. The fasting metabolism of Africander and British crossbred cattle and the effect of previous infestation with cattle tick (Boophilus microplus Canestrini). Proc. Aust. Soc. anim. Prod., 9: 356–359.

Wharton, R.H., Harley, K.L.S., Wilkinson, P.R., Utech, K.B. & Kelley, B.M. 1969. A comparison of cattle tick control by pasture spelling, planned dioping and tick-resistant cattle. Aust. J. agric. Res., 20: 783–797.

Wharton, R.H. & Roulston, W.J. 1970. Resistance of ticks to chemicals. A. Rev. Ent., 15: 381–404.