J. Armour

The epidemiology of helminth disease in farm animals.

Abstract In this paper the factors which precipitate production loss due to helminth disease in farm animals are classified within accepted epidemiological principles. In endemic areas, outbreaks of helminth disease occur for three basic reasons. The first and most frequent is where there is an increase in the infecting mass which usually occurs seasonally and after at least one parasitic generation. The factors which influence such an increase are those affecting contamination of the environment with eggs or larvae and those controlling the development, dissemination, survival and availability of the free-living larval stages, i.e. translation. Important contamination aspects include biotic potential of the helminth, management practices such as dates of turning out to grazing of housed animals and density of stocking, immune status of the host and hypobiosis. Translation depends primarily on suitable temperature and humidity levels but is also influenced by general factors such as soil structure, vegetation type and drainage. Certain rotational grazing practices can also favour translation. The second is where the susceptibility of existing stock is altered either to the effects of an existing infection or to the acquisition of a new infection. The former may occur following a change in diet, particularly if the nutritional value is reduced or where deficiency of certain minerals or trace elements occur, e.g. phosphorus or cobalt, while the stress of pregnancy and lactation can exacerbate production loss without further infection occurring. The acquisition of new infections is also enhanced during late pregnancy and lactation due to the peri-parturient relaxation in immunity, and by intercurrent infection with other agents, drug therapy with steroids and occasionally anthelmintics. Sometimes hypersenstive reactions to reinfection of immune animals also result in impaired productivity. The third is where susceptible stock are introduced into an infected area. Helminthnaive juveniles are particularly susceptible although the absence of a significant age immunity to many helminths ensures the continuing susceptibility of helminth-naive adults. Genetic factors such as breed, sex, haemoglobin type and hereditary susceptibility of certain breeding lines may exacerbate this category. The longevity of helminth infective stages also contributes by maintaining infection between successive batches of livestock. Finally, helminthiases may occur in a non-endemic area due to the introduction of infected stock. This situation is less common and usually occurs where the recipient environment is suitable for the free-living development of the helminth introduced. Infection may also be introduced by the application of manure products such as slurry and by the introduction or the movement of vectors.

The influence of host immunity on the epidemiology of trichostrongyle infections in cattle

The acquisition of immunity to the important trichostrongyles of cattle is reviewed and the subsequent effect on the epidemiology of trichostrongylosis is discussed. Examples of factors which can delay or impair the development of immunity to these nematode infections are presented. These include the interactions with anthelmintic prophylactic programmes, nutrition, intercurrent infections, pregnancy and lactation.

A study of some factors influencing the immunization of sheep against Haemonchus contortus using attenuated larvae

Abstract The respective roles of age of lamb, size of immunizing dose, previous exposure to infection and anthelmintic therapy on the immune response of Scottish Blackface sheep to gamma-irradiated H. contortus larvae were studied. As previously shown, young lambs immunised at 10 and 14 weeks of age failed to develop a significant resistance to subsequent challenge. In contrast, parasite-naive lambs immunised at 9 and 10 months of age with 10 000 H. contortus larvae irradiated at 60 kilorontgens developed a highly significant immunity to subsequent challenge. This degree of immunity was not significantly altered by the administration of spaced doses of normal larvae from 10 weeks of age until the point of vaccination. Worm-free lambs vaccinated at 9 and 10 months of age failed to develop any immunity to subsequent challenge when the anthelmintic thiabendazole was administered 3 weeks after each immunizing infection.

The sequential development of type I and type II ostertagiasis in young cattle with special reference to biochemical and serological changes.

The sequential development of Type I and Type II ostertagiasis over a 2-year period in the same naturally infected cattle is described for the first time. Particular reference is made to biochemical and serological changes. Positive relationships were demonstrated between the clinical signs of both Type I and Type II disease, and marked increases in the levels of plasma pepsinogen, plasma gastrin and antibody titres to adult Ostertagia antigen. At necropsy, there were significant relationships between the combined total of adult and developing 5th stage larvae of Ostertagia spp. and the levels of both plasma pepsinogen and gastrin. By the end of the second grazing season the cattle had acquired an immunity to infection with Ostertagia spp. and had very low burdens of this parasite at necropsy. However some of these cattle maintained elevated plasma pepsinogen levels when under natural challenge by Ostertagia spp. larvae and the aetiology of these changes and the problems of diagnosis using this parameter are discussed. Similar trends of infection were observed for Cooperia oncophora, although resistance to the parasite developed more rapidly.

Anthelmintics for ruminants

Abstract There is now a wide range of potent broad-spectrum anthelmintics for cattle and small ruminants, including the benzimidazoles, the avermectins, tetrahydropyrimidines and imidothiazoles which are highly efficacious against parasite gastroenteritis and bronchitis. Fascioliasis can be controlled with the salicy-lanilide and related phenolic compounds, the benzimidazoles, albendazole and triclabendazole the latter being the first flukicide with excellent activity against all stages of liver fluke infestation. The major developments for anthelmintic therapy have been and will be in the use of intraruminal boluses with either sustained release or pulse release of anthelmintic and other methods for group medication. The implications of such strategies for helminth resistance and individual immunity require further evaluation.

Anthelmintic resistance and the control of ovine ostertagiasis - a drug-action model for genetic selection

A site-specific genetic prediction model is presented, which examines the influences of different anthelmintic treatment regimens on selecting for drug resistance within a sheep management system. The model exploits the power of modern microcomputers to integrate factors such as parasite strain, geographic location, management practice and genetic fitness to identify effective control regimens which do not lead to resistant Ostertagia circumcincta strains over a 30-year horizon. The potential use of the model as a farm level management-support tool is discussed.

A 4-year study of the effectiveness of alternate grazing of cattle and sheep in the control of bovine parasitic gastro-enteritis

In many farming enterprises, animal management systems which could provide a practical and effective alternative to chemotherapy for the control of bovine helminthosis would be readily accepted. One system which has been proposed and shown to be effective in the short or medium term involves grazing different host species on a rotational basis. The study described here examined the effect of alternating cattle and sheep annually over an extended period of 4 years. Up to the second grazing season the system appeared to be successful, with a marked reduction in the cattle worm burdens. However, by the end of the study period the parasite burdens in calves grazed on the alternated pasture were equal to, or greater than, those of set-stocked control animals. It was thus clear that the alternate grazing strategy had failed. Data obtained from other parameters measured, i.e. faecal egg counts, pasture larval numbers and plasma pepsinogen levels, confirmed this observation.

Efficacy of some anthelmintics on an ivermectin-resistant strain of Haemonchus contortus in sheep

Following evidence of reduced efficacy of ivermectin in a field population of Haemonchus contortus in Brazil, this strain of the parasite was submitted to a controlled anthelmintic test. Eighty worm-free lambs were randomly split into two groups of 40. Each lamb in the first group was infected with 5000 third stage larvae (L3) of the ivermectin-resistant strain; the remaining 40 lambs were each infected with 5000 L3 of a H. contortus strain of known susceptibility to the major groups of anthelmintic compounds used in sheep. On Day 28 post-infection, each group was subdivided according to egg counts and at random into four sub-groups of ten lambs, each of which was treated with albendazole (ABZ) at 3.8 mg kg-1, levamisole (LEV) at 7.5 mg kg-1 or ivermectin (IVM) at 0.2 mg kg-1, or was left as untreated control. At slaughter, 7 days later, all the anthelmintics reduced worm burdens in animals infected with the susceptible strain (ABZ 98.9%, LEV and IVM 100%). By contrast, in the lambs infected with the ivermectin-resistant strain, LEV was 99.8% effective, but ABZ reduced worm counts by only 14.7% and IVM by only 10.4%. Interestingly at necropsy on Day 7 post-treatment, the egg counts in the resistant strain treated with ABZ had been reduced by 92.5%, although worm counts were reduced by only 14.7%.(ABSTRACT TRUNCATED AT 250 WORDS)

Level of Benzimidazole resistance in a strain of Ostertagia circumcincta studied over several infections in lambs

A benzimidazole-resistant strain of Ostertagia circumcincta (HFRO) was used experimentally to infect lambs. The level of resistance, measured by an egg hatch assay, was studied throughout each infection and also after treatment of the lambs with fenbendazole. The HFRO strain was highly resistant to benzimidazoles. There was day-to-day variation in the level of resistance throughout a single infection with a high level of resistance in the early part of the infection, around Day 27 post-inoculation of infective larvae, falling to a lower level later in the infection. Egg hatch assays on the 3 days immediately post-treatment with fenbendazole showed the resistance level was high then resistance fell to the pre-treatment level after 7 days. Selection for benzimidazole resistance using fenbendazole treatment at the normal dose rate of 5 mg kg-1 over five passages of the HFRO strain in lambs failed to increase the resistance level. Storage of larvae over a 5-month period at 4 degrees C, prior to infection of lambs, did not produce any alteration in the resistance level. The possible reasons for the variations in resistance found with the HFRO strain are discussed along with the implications for sheep parasite control and further development of benzimidazole resistance.

Ostertagia ostertagi in calves: A growth, nitrogen balance and digestibility study conducted during winter feeding following thiabendazole anthelmintic therapy

Abstract Calves naturally infected with O. ostertagi and treated at housing with thiabendazole (Tbz) were compared with similar calves reared under parasite-free conditions during the first winter housing period. The diet of both groups contained an excess (125 per cent) of protein, but not of metabolizable energy, over that recommended for an average growth of 0·6 kg per day. There was a marked lag in growth for the first 6 weeks of housing for the Tbz dosed calves; thereafter the growth rates were similar. Nitrogen (N) balance and digestibility studies conducted at intervals over a 17-week period following treatment showed little differences in digestibility of the diet but an overall significantly greater N retention in the parasite-free calves. Daily water output in both faeces and urine was greater in the Tbz treated calves. Total protein was lower but plasma urea concentration was higher in Tbz treated calves. It is speculated that only a short-term impairment of post-absorptive metabolism may have occurred in the Tbz calves and that the high protein intake may have reduced the severity of the lowered N balance observed.