Observations on the resumption of development of inhibited Ostertagia, Cooperia and Nematodirus infections in calves stabled overwinter.

Two groups of three month old, parasite-free calves grazed a permanently infected pasture for 14 days, Group A during the first two weeks of September and Group B during early November. Half of each group was killed 14 days after removal from the pasture and the remainder stabled overwinter before slaughter and parasitological examination. Marked inhibition of development occurred for Cooperia oncophora with a variable lower level of inhibition for Ostertagia ostertagi and practically none for Nematodirus helvetianus in those calves grazing late in the fall. Under the conditions of this study, inhibited Cooperia larvae resumed development in several calves soon after they were stabled while small numbers of Ostertagia resumed development regularly during the winter and spring with a considerable number of Ostertagia still present when the calves were slaughtered at the end of the stabling period. On the other hand, Nematodirus and practically all Cooperia worms were lost during the stabling period. In three of seven calves grazing late fall pastures, large Cooperia infections were either not established or failed to become patent.     

Relative importance of winter survival of larval nematodes in pasture and infected carrier calves in a study of parasitic gastroenteritis in calves.

A study was done in Maine to determine the relative importance of winter survival of nematode larvae in pasture and infected carrier animals as sources of infection for susceptible calves. Under the conditions of the experiment, it appeared that winter survivals in pasture of the infective stages of the genera Ostertagia, Cooperia, Nematodirus, and Trichostrongylus were of greater importance than carrier animals as sources of infection for susceptible calves. While animals in plots infected the previous summer and simultaneously allowed to graze alongside infected carrier animals did show more worms than those grazed only in infected plots, these differences were not statistically significant. Both groups had significantly (P greater than 0.01) more worms than did calves grazed only with carrier animals for the period of the experiment (8 weeks). It was also observed that carrier calves with low fecal egg counts (less than 200 eggs/g of feces) introduced in early spring to uncontaminated pasture could produce enough parasitic contamination by early fall to cause fulminating infections in susceptible calves grazing the pasture at the same time. Infected animals that survived clinical disease during their 1st summer developed a strong immunity which limited their acquisition of further infections when they were exposed to severe pasture contamination the following year.     

No foot-and-mouth disease virus transmission between individually housed calves

The foot-and-mouth disease outbreak in The Netherlands in 2001 most likely started on a mixed veal-calf/dairy-goat farm. The outbreak among the 74 calves on this farm appeared to be limited to four animals, and no clinical signs of FMD were reported. Also on a second veal-calf farm minor clinical signs and limited virus transmission were observed. Since FMD is known to be a very contagious disease, and can cause severe lesions, these observations were disputed. Therefore, we carried out two experiments to determine whether the Dutch FMD virus isolate from 2001 does spread among individually housed calves with limited contacts, either indirect (experiment 1) or direct (experiment 2). In experiment 1, four pairs of calves were housed in an individual box at 1m distance from each other. In experiment 2, two groups of three calves were housed in individual boxes, directly bordering each other. We infected one animal per pair in experiment 1, and the calf in the middle in experiment 2. We recorded clinical signs, virus shedding in saliva and the development of antibodies. In addition, we determined whether the virus was transmitted from the inoculated calves to the neighbour(s). All inoculated calves showed mild signs of FMD--fever, and some vesicles on hooves and/or in the mouth--but only one calf showed signs that were visible without physical examination. All inoculated calves shed virus in the saliva and developed neutralising antibodies. None of the contact animals seroconverted, indicating that virus transmission did not occur. These experiments showed that no virus transmission among individual housed calves can occur. This finding supports the hypothesis of the route of virus introduction to The Netherlands in 2001 and show that the observations on the two veal-calf farms were not impossible.     

Use of the sustained-release morantel bolus in stocker calves in southern United States

Two groups of 21 mixed-breed heifers were wintered on separate permanent pastures. Each heifer from one group was administered a sustained-release morantel bolus on October 7 (day 0), and the other group remained as untreated controls. Body weights were determined and fecal samples were taken at 28-day intervals. At the onset of the trail and at every 56 days, 6 heifers were removed from each group for slaughter to determine the developmental stages and the number of gastrointestinal nematodes. In addition, 3 tracer calves that were free of gastrointestinal nematodes were released on each pasture for 28 days at the beginning of the trail and after the last experimental-group calves had been removed. The 6 calves slaughtered on day 0 of the trail had a mean of 5,544 gastrointestinal nematodes. Tracer calves released on day 0 and removed on day 28 of the trial acquired 31,143 and 30,530 gastrointestinal nematodes from the pastures containing the treated and control heifers, respectively. Throughout the trial, the number of nematodes in the control calves increased at each sampling date (mean, 126,168 worms), whereas the mean number of worms in the treated heifers was 45,458. Tracer calves placed in the pastures after the 168-day trail acquired significantly more worms (9,632 vs 2,899; P less than 0.05) from grazing the pastures with control heifers than from grazing the pastures with treated heifers. Counts of eggs per gram of feces were significantly different (P less than 0.01) between the 2 groups from day 28 through day 112.(ABSTRACT TRUNCATED AT 250 WORDS)     

Efficacy of levamisole against Ostertagia ostertagi in Louisiana cattle during maturation of inhibited larvae (September) and during minimal inhibition (December/January).

Levamisole (LEV) was tested in four experiments to compare efficacy values against Ostertagia ostertagi when larval maturation was occurring (September), following inhibition and also when populations were expected to be largely adult (winter). A primary objective was to determine the importance of developing fourth-stage larvae (DL4) and inhibited, early fourth-stage larvae (EL4) in replacing adult worms lost through treatment and the effect of this on reduced efficacy against adult worms. Young crossbred beef calves ranging in weight from 150 to 230 kg were used in the first (September 1981), second (September 1983) and third experiments (January 1987). Jersey calves of 110 kg average weight were used in the fourth experiment (December 1988). Calves were randomized to groups according to weight and group sizes ranged from three to five calves. All parasite infections were naturally acquired, but a mixture of nematode third-stage larvae (L3) (22,500 per calf), including 20% Ostertagia ostertagi, was inoculated into Jersey calves of Experiment 4 following a 2 week exposure to natural infection. All LEV treatments were by subcutaneous injection at dosages of 6 and 8 mg kg-1. Treatment with ivermectin was used only in Experiment 3 as an efficacy reference. All calves were killed at 8-10 days after treatment. The efficacy of LEV against all developmental stages of Ostertagia ostertagi was consistently low in all experiments and a dose-dependent response was not evident. Large numbers of all Ostertagia ostertagi developmental stages were present in non-treated calves in both September experiments. Percent reduction of adults, DL4 and EL4 at the 6 mg kg-1 and 8 mg kg-1 dosages, respectively, were adults, 51.7 and 23.6 (1981), 8.7 and 51.3 (1983); DL4 40.3 and 13.2 (1981), 37.9 and 33.1 (1983); EL4, 19.6 and 0 (1981), 59.6 and 42.9 (1983). Smaller numbers of Ostertagia ostertagi were present in winter experiments and adult worms greatly outnumbered larval stages. Percent reductions of adults, DL4 and EL4, respectively, were (1987) LEV 6 mg kg-1, 40.2, 0 and 0; ivermectin 200 micrograms kg-1, 98.7, 97.7 and 100.0; (1988) LEV 6 mg kg-1, 62.4, 100.0 and 100.0; LEV 8 mg kg-1, 49.1 65.0 and 74.1. Too few larval stages were present in the latter experiment for valid efficacy values.(ABSTRACT TRUNCATED AT 400 WORDS)     

Development of immunity to Ostertagia ostertagi (Trichostrongylidae: Nematoda) in pastured young cattle.

This experiment comprised 3 groups of calves, (+P2), (-P2) and (-P1), which all started their first grazing season as parasite-free calves. The (+P2)- and (-P2)-group grazed 2 seasons. In the first season the (-P2)-group of calves was grazing a pasture with no detectable trichostrongyles and treated with anthelmintics every second week. The untreated (+P2)-group grazed an Ostertagia ostertagi contaminated pasture. During the second grazing season these 2 original groups grazed together with a new group of first-year grazing calves (-P1) on paddocks infected with O. ostertagi. Parasitological analyses showed that (+P2)-group had negligible egg excretions in the second year in comparison with (-P2) and (-P1). This indicated, that the egg output may be regulated through acquired immunity. The difference in egg excretions was not reflected in the serum pepsinogen levels, which were only slightly elevated for all groups in the second year. Post mortem examination at the end of the experiment showed that only the (-P1)-group harboured relatively high numbers of worms in the abomasa at that time. Antibodies of 3 immunoglobulin classes were investigated: IgA, IgG1 and IgG2. The IgA and IgG1 responses correlated with the presence of developing and adult worms in the abomasa and they remained elevated in the (+P2)-group throughout the experiment, perhaps indicating an involvement of these antibodies in a protective immune response. In the (-P2)-group the IgA and IgG1 showed fast and sharp rises during the second season that most likely were age-related and as such a result of maturation of the immune system. The role of IgG2 is unclear as the IgG2 response was weak in all groups of calves and difficult to relate to the parasitological data.     

Evaluation of the morantel sustained release bolus in cows and calves

The efficacy of the morantel sustained release bolus (MSRB) in controlling gastrointestinal parasitism in beef cattle was assessed during the 1982 spring-autumn grazing season. Forty-eight cows and their calves were allotted to three equal groups. One group (T-1) served as a nonmedicated control group. One MSRB was administered to each calf of the T-2 group, and to each cow and calf of the T-3 group at the beginning of the study. The efficacy of the bolus was assessed by comparison of weight gain performance and parasitological data (fecal worm egg counts, herbage larval counts, worm counts from tracer and principal trial calves, and plasma pepsinogen level determinations). Though not statistically significant, treated calves from Group T-2 had a numerical mean weight gain advantage of 2.6 kg, and those from Group T-3 of 4.7 kg, over control calves. Average daily gains (ADG) for the three groups of calves were 0.69, 0.72, and 0.73 kg, respectively. Untreated cows from Group T-2 and treated cows from Group T-3 outperformed the control cows by 12.3 and 7.5 kg, respectively. Fecal worm egg counts from both groups of treated calves were significantly (P less than 0.01) lower than counts from control calves during the entire 169-day trial; notably, egg counts were reduced by 99% 28 days after MSRB administration to both groups of calves. There were no significant differences in the number of eggs counted from the three groups of cows, probably because of the very low numbers of eggs encountered. Mean total worm burdens of principal calves (six per group) necropsied at trial termination indicated a 91% (P less than 0.01) reduction in Group T-2 and an 87% reduction (P less than 0.01) in Group T-3. Worm-free tracer calves were introduced onto pastures every 28 days to monitor availability of infective larvae. The mean number of worms recovered at necropsy from tracer calves that grazed with control cattle increased as the season progressed. However, the numbers of parasites recovered each month from mid-August through mid-October from tracers that grazed pastures with treated cattle were lower (P less than 0.05) than those levels displayed at trial initiation. In addition, the mean numbers of worms from treated group tracers were lower than from the controls for each necropsy period.(ABSTRACT TRUNCATED AT 400 WORDS)     

Attempts to produce protection against Ostertagia ostertagi in cattle.

Various single or multiple doses of Ostertagia ostertagi were administered to young calves, and the production of protection phenomena elicited by single challenge inoculations ranging from 50,000 to 300,000 larvae or multiple challenge inoculations totaling 98,000 and 300,000 larvae was investigated. With some regimens, the vaccinations apparently resulted in protection against challenge exposure, as reflected by 36 to 56% fewer worms becoming established in challenge-exposed vaccinated calves than in challenge-exposed nonvaccinated, control calves. Other protection phenomena were elicited by some vaccinated calves of significantly more female worms lacking the distinctive vulval flap of O ostertagi and harboring significantly fewer eggs per female. Challenge exposure with a pathogenetic dose of 300,000 larvae produced the same degree of retarded weight gain in vaccinated as in nonvaccinated calves, and at necropsy, visceral lesions and pathologic alterations were equally severe in both groups of calves.     

Interactions between lungworms and gastrointestinal worms in calves

Interactions between gastrointestinal worms (Ostertagia ostertagi, Cooperia oncophora) and lungworms (Dictyocaulus viviparus) in calves were studied by assessing the effect of primary infections with either group of worms on the development of homologous or heterologous challenge infections. Primary infections with lungworms resulted in some degree of resistance to challenge with gastrointestinal worms, but this resistance was lower than that found after homologous infection. Primary infections with gastrointestinal worms did not confer any resistance to challenge with lungworms. On the contrary, an indication was found of some enhancing effect of previous gastrointestinal worm infection on the establishment of lungworms. The highest degree of resistance against lungworm challenge was found where calves have been primarily infected with lungworms. Lungworm infections produced some elevation of serum pepsinogen levels. Gastrointestinal worms evoked a rise in circulating eosinophils, although this rise was smaller and occurred later than in lungworm-infected calves. Under the conditions of the experiment, the effect of 6000 infective lungworm larvae on weight gain was larger than the effect of 100,000 L3 of Ostertagia ostertagi and 100,000 L3 of Cooperia oncophora.     

Infection and immune kinetics of Trichostrongylus axei in calves.

In one experiment, 21 calves were given daily oral immunizing inoculations of 1,000 infective larvae (L3) of Trichostrongylus axei, an abomasal nematode parasites, for 35 weeks. Calves were euthanatized in groups of three every 5 weeks to determine infection kinetics. Worm populations steadily increased through week 30, but the percentage of total inoculum that became established was about the same through week 30. At week 35, the number of worms dropped markedly. In a second experiment, 27 calves given daily oral inoculations of infective larvae were allotted to three groups comprised of three subgroups each: (A), challenge-exposed vaccinated; (B), nonchallenge-exposed vaccinated; and (C), challenge-exposed nonvaccinated. Calves of subgroups A and C were given single challenge inoculums of 200,000 L3 after the 10th, 20th, and 30th weeks. All calves were necropsied 35 days after challenge exposure. When immunity was determined from the equation: [(No. of worms in [(B + C)-A])/No. of worms in [B + C])] X 100, immunity was 35% at 10 weeks, 52% at 20 weeks, and 100% at 30 weeks.     

Levels of serum IgE in response to gastrointestinal nematodes in cattle

Serum IgE concentration has been considered a valuable measurement in parasitic infections, yet little has been reported regarding cattle. This study examines the association of IgE levels of nematode-na?ve Holstein steer calves and the level of gastrointestinal parasitism acquired by grazing irrigated pasture for 30 days. Total IgE levels were determined by enzyme-linked immunosorbent assay (ELISA) specific for bovine IgE on serum collected both before and after exposure to parasite-infected pastures. Following necropsy, parasite loads were determined by direct count from the contents of the abomasum, small intestine, cecum and large intestine; species of Ostertagia and Cooperia being the most common helminths found. Significant increases of IgE in the serum of calves with light infestations were seen, whereas calves with moderate to heavy infestations showed only mild IgE increases. With increased parasite burden, the frequency of increased IgE levels was reduced. Additionally, there appeared to be a seasonal correlation relating the level of serum IgE detected to the number of worms counted and to the course of parasite development.     

A comparison of the efficacy of four different long-acting boluses to prevent infections with Dictyocaulus viviparus in calves

A field study of calves in their first grazing season tested the efficacy of four long-acting devices--a morantel sustained-release bolus, a levamisole sustained-release bolus, an oxfendazole interval bolus, and an albendazole interval bolus--against Dictyocaulus viviparus. The pasture had been previously contaminated by four calves orally inoculated with infective lungworm larvae. The calves were grazed together with four bolus-treated groups, each comprising four calves. Lungworm infection became patent in the experimentally inoculated calves between 22 and 26 days. Infection in the bolus-treated groups became patent after 54 days. The morantel bolus group excreted the most larvae, followed by the albendazole bolus group, and the levamisole bolus group. The oxfendazole bolus group excreted by far the least larvae. Eosinophil curves and ELISA titres showed that treated groups had essentially the same course of infection. The heavy infection to which the treated calves were exposed produced complete immunity in all groups. Challenge infection of 10,000 larvae at housing did not change any of the test parameters. Post-mortem examination showed only one positive calf with few worms. We concluded that when pastures are heavily infested with lungworm larvae, all boluses prevent severe clinical signs and allow build up of solid immunity, although none completely prevent excretion of larvae.     

The effect of treatment with eprinomectin on lungworms at early patency on the development of immunity in young cattle

An experiment was carried out to study the effect of topical application of eprinomectin at early patency on the build up of infection and development of protection against Dictyocaulus viviparus in young cattle. Three groups of six calves were used and parasitological and blood variables were monitored at weekly intervals throughout the trial. At the start of the experiment calves in groups A and B were experimentally inoculated with 100 D. viviparus infective third-stage larvae (L3) for five consecutive days, whereas calves in group C served as uninfected controls. The calves in group A were each treated with eprinomectin (0.5mg/kg bodyweight) in a pour-on formulation at early patency at day 24 post the first inoculation, whereas the calves in groups B and C were left untreated. Seven weeks following anthelmintic treatment all groups were challenged with 1500 L3. Another 4 weeks later the animals were sacrificed and established worms in the lungs were counted. Moderate transient signs of lungworm disease occurred both in groups A and B. However, group B calves were found to be about 8 times more resistant than those in group A, whereas the naive infection controls in Group C was found to be about 35 times more susceptible to infection. Also the ELISA values showed that the course of infection was different between experimental groups. The eosinophil counts prior to and at the time of slaughter indicate that immunity was involved in the protection and the response was correlated with previous exposure and worm load. Weight gains differed significantly, but only between groups A and C and between groups B and C that on an average were approximately 13kg heavier at the termination of the experiment. It was concluded that eprinomectin was effective against established adult lungworms. However, the untreated calves (group B) developed a more marked resistance to lungworms compared to those that were subjected to anthelmintic treatment at early patency (group A). On the other hand, the cumulative number of excreted larvae was on an average 43 times higher in group B as compared to group A. Consequently, infected calves that remain out on pasture should be treated. This will restrain transmission of the parasite despite the fact that immunity is deteriorated.     

Mast cell and eosinophils in the wall of the gut and eosinophils in the blood stream during Toxocara vitulorum infection of the water buffalo calves (Bubalus bubalis)

Toxocara vitulorum is a pathogenic nematode from the small intestine of very young buffalo calves. To understand the development of the inflammatory responses in the wall of the gut, samples of tissues were removed from the duodenum, jejunum and ileum of buffalo calves naturally infected with T. vitulorum during the beginning of the infection, at the peak of egg output, as well as during the periods of rejection of the worms and post-rejection. Two additional control groups of uninfected calves (by anti-helminthic therapy of their mothers and after the birth) were also necropsied on days 30 and 50 after birth. Blood samples were fortnightly collected from birth to 174 days post-birth. Blood smears were prepared and stained with Giemsa for eosinophils. The parasitological status of buffalo calves was evaluated through weekly fecal egg counts (EPG) from 1 to 106 days after birth, which revealed that T. vitulorum egg shedding started on day 11, reached the peak of the infection on day 49 and finally expelled the parasites between days 50 and 85 after birth. In the infected buffalo calves, the mast cell population increased significantly, by two-fold in the mucosa (villus-crypt unit (VCU)) of the duodenum and four-fold in the proximal jejunum; but these increases were statistically significant only at the peak of the infection. Although mast cell numbers increased in the mucosa of the ileum as well as in both the submucosal and muscle tissues of the duodenum, proximal jejunum and ileum, the data was not significantly different from the controls. Eosinophil numbers increased in the mucosa of the duodenum (two-five times higher than the control) and proximal jejunum (three-five-fold) during the period of the infection (beginning, peak and rejection). The relative numbers of eosinophils increased in the blood stream from the second to the seventh week. In conclusion, T. vitulorum infection elicited mastocytosis and tissue eosinophilia in the duodenum and proximal jejunum, as well as eosinophilia in the blood stream, during the beginning, at the peak and during the rejection of the worm. After the rejection of the worms, the numbers of these cells returned to normal levels suggesting that these cells may have a role in the process of rejection of T. vitulorum by the host.     

Exposure of calves to aerosols of parainfluenza-3 virus and Pasteurella haemolytica.

The present study was undertaken to investigate whether sequential exposure to aerosols of parainfluenza-3 virus followed by Pasteurella haemolytica, or P. haemolytica followed by parainfluenza-3 virus, could lead to the production of pulmonary lesions in conventionally-raised calves. Twenty male calves with low serum antibody titres to both organisms were placed in five equal groups. Synergism of parainfluenza-3 virus and P. haemolytica was not demonstrated in any of the sequentially infected groups and pulmonary lesions were mild in all challenged calves. Clinical signs of disease were not present after exposure to parainfluenza-3 virus although the virus was repeatedly isolated from nasal secretions of all inoculated calves. Exposure to P. haemolytica produced a transient response which consisted of increased rectal temperatures and respiratory rates, with a mild neutrophilic leukocytosis and a mild left shift present six hours postinoculation and returning to normal within 24 hours. Results from this study suggest, although do not confirm, that reduced pulmonary clearance of inhaled P. haemolytica in parainfluenza-3 virus infected calves does not necessarily lead to production of severe pulmonary lesions and that previous exposure to aerosols of P. haemolytica may not enhance secondary parainfluenza-3 virus infection.     

Comparison of a levamisole sustained-release bolus and ivermectin treatment to prevent bovine lungworm infection

The efficacy of a levamisole sustained-release bolus to prevent parasitic bronchitis in calves in their first grazing season was compared to ivermectin treatment at three, eight and thirteen weeks after turn out. Contamination of the pasture was established by experimentally infected seeder calves. Twenty calves were split into two groups. Ten calves of one group received a bolus at the start of the experiment. In the other group the calves were treated with ivermectin at 21, 56 and 91 days. Two principal calves from each group were killed during the experiment to study histopathological changes. Pairs of tracer calves were introduced on both pastures at intervals of four weeks throughout the grazing period. The permanent calves were challenged with lungworm larvae at housing and slaughtered four weeks later. Both systems prevented parasitic bronchitis. Larval output was completely reduced in the ivermectin-treated calves while all bolus-treated calves excreted larvae at certain times. The highest group average was 4 larvae per gram faeces. Eosinophilia, ELISA-titres and histopathological changes confirmed the differences in larval uptake. Challenge infection was not successful in either group and no worms were found at slaughter. Weight gain was significantly different at housing in favour of the ivermectin-treated calves, but after challenge this was reduced due to a higher weight gain in the bolus-treated calves. The practical consequences of the results have been discussed.     

Dictyocaulus viviparus in calves: effect of rotational grazing on the development of infections.

A study was made of the possibility of reducing lungworm infections in young grazing calves by rotational grazing for weekly periods on six paddocks. For this purpose three groups of four calves each were grazed on separate pastures in 1989, whereas a fourth group served as a permanently housed control group. Two groups of calves were infected experimentally with six doses of 10 larvae of Dictyocaulus viviparus during the first 3 weeks on pasture. In the third group, low natural infections with overwintered larvae occurred. One of the experimentally infected groups was rotationally grazed for weekly periods on six small plots while both other groups were set-stocked. Faecal larval counts and worm counts in tracer calves demonstrated lower lungworm infections in the rotationally grazed group than in both set-stocked groups. However, the numbers of worms found after challenge infection and subsequent necropsy were relatively high in the rotationally grazed group, indicating that development of immunity was less than in both other groups. Owing to the dry weather conditions in the summer of 1989, no serious clinical signs of husk developed in any of the three groups. These dry conditions, however, did not prevent the build-up of heavy pasture infectivity with gastrointestinal nematodes resulting in heavy worm burdens and serious clinical signs in tracer calves grazing for 4 days in August and September-October, respectively. This implies that rotational grazing did not have a clear effect on build-up of gastrointestinal nematode infections.     

The role of temperature and light on inhibition of development of Cooperia oncophora.

Infective Cooperia oncophora larvae were temperature and light conditioned by holding them at various temperatures and daily light intensities for up to 56 days prior to administration to parasite-free calves via stomach tube. The calves were killed either on day 20 or day 21 postinfection. A marked (up to 80.5%) inhibition of development occurred in larvae held at 4 degrees C while little or no inhibition occurred in larvae held at room temperature or in fresh larvae. Marked inhibition also occurred in larvae held at 15 degrees C for 56 days while a low rate of inhibition occurred in larvae held at 17 degrees C for 42 days. Low incidence of inhibition was recorded in two of four calves given larvae held at ambient temperatures of mid-summer while appreciable inhibition of development of larvae occurred in two calves permitted to graze during the second and third week of September in 1975. Temperature conditioning of C. oncophora eggs for 27 days did not result in inhibition of development of infective larvae subsequently cultured from the eggs. Photoperiod or the presence of light did not have any appreciable effect on the development of inhibition of C. oncophora under the conditions of these investigations.     

Sustainable use of anthelmintics in grazing animals

It is suggested that the major factor in avoiding the development of anthelmintic resistance is the percentage of worms that do not encounter the anthelmintics (worms in refugia). This in turn is determined by the numbers of larvae on pasture, the percentage of animals treated and whether any stages in the host can avoid the action of anthelmintic. To maintain anthelmintic efficacy the percentage of worms in refugia must be sufficiently large. In cattle, this should involve treating only first-year animals and using a different pasture each year for calves. For sheep, only animals that have to be treated should be dosed with anthelmintic and clean grazing strategies that involve the use of anthelmintics should be avoided. For horses, reliance should be placed on the removal of faeces from pasture and only treating when the animals' condition requires it. Without a change in anthelmintic use there is the likelihood of increasing numbers of cases for which no anthelmintic is effective and animal welfare may be compromised.     

Genetic control of immunity to gastrointestinal nematodes of cattle

Previous studies have indicated that host genetics significantly affects the number of gastrointestinal nematode eggs per gram (epg) in the feces of calves during their first grazing season. An entire calf crop of approximately 190 animals was monitored monthly until weaning to verify these earlier results, and to begin to discern the basis for this phenomenon. A significant genetic effect on fecal epg values was not observed until calves had been on pasture for 2-3 months, and was demonstrable until late in the grazing season when the effect was lost. The loss of a genetic effect coincided with the appearance of significant numbers of the more highly fecund nematode species Haemonchus placei and Oesophagostomum radiatum, and with an apparent increase in Ostertagia ostertagi transmission, indicating that the observed genetic control of epg values may be species specific, dose dependent or both. Calves were selected from the population, and grouped according to their epg phenotype over the grazing season as either high or low epg calves. Postmortem examination of some of these calves indicated that worm burdens in the low epg calves were 60% of those of the high epg calves. Experimental challenge inoculation of the remaining calves indicated that: (1) challenge with Cooperia oncophora resulted in low epg calves harboring worm numbers that were 65% of those of high epg calves; (2) challenge with O. ostertagi resulted in similar numbers of worms in both groups, but the fecundity of worms in the low epg groups was significantly lower (P less than 0.05) than in the high epg group. Analysis of serum anti-Ostertagia antibody levels in the grazing calf population showed rises in serum IgG1, IgG2, IgM and IgA antibody levels during the grazing season. Peak serum IgG2 and IgG1 anti-Ostertagia antibody levels were found to be significantly affected by host genetic factors while IgA and IgM levels were not under such control.