Persistence of ivermectin in plasma and faeces following treatment of cows with ivermectin sustained-release, pour-on or injectable formulations.

Sixteen young dairy cows were randomly allocated to 4 groups of 4 animals each: Group 1 cows were each given a single Ivomec SR Bolus; Group 2 cows were treated with the Ivomec Pour-on formulation; Group 3 cows were injected with the Ivomec Subcutaneous Injection; Group 4 cows were untreated controls. Blood and faecal samples were collected from all cows on the day before treatment and on days 1, 2, 3, 7, 14, 21, 28, 35, 42 and 49 after treatment for HPLC determination of plasma and faecal ivermectin concentrations. Group 1 (SR Bolus) cows had mean (+/- S.D.) plasma ivermectin concentrations ranging from 5.6 +/- 1.8 micrograms l-1 (p.p.b.) at 14 days to 11.0 +/- 4.7 micrograms l-1 at 49 days. Faecal ivermectin concentrations were little changed from 4.0 +/- 2.0 micrograms g-1 (p.p.m.) dry wt (dry weight) [0.5 +/- 0.2 microgram g-1 wet wt (wet weight)] at 14 days to 3.0 +/- 2.0 micrograms g-1 dry wt (0.5 +/- 0.4 microgram g-1 wet wt) at 49 days. Group 2 (Pour-on) cows showed a rapid rise in plasma concentrations to 32.9 +/- 15.7 micrograms l-1 2 days after treatment, followed by a gradual decline to 1.3 +/- 0.07 micrograms l-1 at 28 days. Faecal ivermectin concentrations rose sharply to 18.5 +/- 7.4 micrograms g-1 dry wt (2.8 +/- 1.2 micrograms g-1 wet wt) 2 days after treatment, then fell to 0.04 +/- 0.004 microgram g-1 dry wt (0.006 +/- 0.0004 microgram g-1 wet wt) at 28 days. Group 3 (Injection) cows also showed a rapid rise to an early plasma peak of 46.1 +/- 22.7 micrograms l-1 3 days after treatment, followed by a gradual decline to 1.3 micrograms l-1 at 35 days. Faecal ivermectin concentrations rose to 1.2 +/- 0.34 micrograms g-1 dry wt (0.2 +/- 0.05 microgram g-1 wet wt) at 3 days, declining to 0.08 +/- 0.0001 microgram g-1 dry wt (0.01 +/- 0.0008 microgram g-1 wet wt) at 28 days. No ivermectin was detected in the plasma or faeces of Group 4 (Control) cows. Concentrations of ivermectin potentially toxic to dung-breeding or dung-feeding invertebrates were excreted for the duration of the study in dung of cows treated with the SR Bolus and for 28 days in the dung of cows treated with the Pour-on or injectable formulations.

Control strategies for ostertagiasis.

There are currently four major options for control of ostertagiasis and other ruminant gastrointestinal parasites. These are the traditional, suppressive, integrated and strategic approaches. Strategic dosing is the most realistic and beneficial approach for most regions of the world, including the U.S.A. It is effective, practical, labor-saving and can yield big economic returns. Today this approach is even more appropriate because of important advances in the technology of controlled release and pulse-release devices. Top priority in worm control programs should be given to dairy replacement heifers in their first year at pasture and to beef calves in the immediate post-weaning period. This is preferable to recommendations for blanket treatment of all beef and dairy cattle, regardless of age, immune status or pasture infectivity. An anthelmintic overkill is both wasteful and potentially harmful in that it increases the selection pressure for drug resistance. It is suggested that better dissemination of information is needed to make producers aware of the benefits of strategic dosing programs. This would eliminate much of the confusion about parasite control and focus attention on simple and effective programs that could significantly reduce current losses to the livestock industries.

Control strategies for ruminant and equine parasites to counter resistance, encystment, and ecotoxicity in the USA.

The need for improved parasite control strategies to conserve anthelmintic efficacy and to avoid drug-related problems are addressed. Recent surveys have revealed a trend for sole dependence on ivermectin by livestock owners in the USA, with little regard for epidemiologic-based strategies, or the annual rotation of unrelated anthelmintic groups. Innovative parasite control strategies for cattle, sheep, and horses in northern USA are presented. The importance of closer monitoring and more rational use of anthelmintics is stressed.

Potential problems associated with the controlled release of anthelmintics in grazing animals.

A high, constant and sustained release of drug appears to be a major requirement to avoid a late season rise in pasture infectivity that may result in production losses in calves in autumn or Type II osteragiasis in the following spring. The timing of administration is of crucial importance and will vary in regions of different epidemiology, such as between northern and southern United States. Lack of a standard and reliable technique for pasture larval counts has resulted in some negative or erratic results which are open to question. Although controlled release anthelmintics offer advantages of convenience, a comparison of the economic benefits at present favours prophylactic treatment of dairy heifers with conventional anthelmintics 3 and 6 weeks after spring turnout in northern regions of the Northern Hemisphere. There is a serious risk that boluses based on controlled release by diffusion will behave like slow decaying insecticides and select strongly for drug resistance, especially if farmers administer them in succession throughout the grazing season. There are, however, 2 features in the design of a controlled release device that in theory may minimize the risk of rapid selection for resistance: a high and constant release of anthelmintic followed by a rapid decline to zero as the device becomes exhausted. Under these conditions, the device may even prolong the useful life of an anthelmintic to which resistance has already developed. This paper was presented at Pfizer Symposium on The Application of Sustained Release Anthelmintic Dosage Forms in the Control of Parasites in Grazing Animals at the World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) 10th International Conference, 18-20 August 1983, Perth, W.A., Australia.

Infection pattern of Cryptosporidium and Giardia in calves.

Faecal samples were collected weekly over a 3 month period from 0- to 20-week-old calves on an Ohio dairy farm, and examined for Cryptosporidium oocysts and Giardia cysts by a quantitative direct immunofluorescence assay. Oocysts and cysts were detected as early as 4 days of age. Shedding of Cryptosporidium oocysts peaked at 1 week and was low by 3 weeks. Some calves, however, continued to pass low numbers of oocysts. Shedding of Giardia cysts peaked at 2 weeks of age with high levels maintained until 7 weeks of age. Most calves continued to pass low numbers of Giardia cysts after weaning. Cumulative infection rates for both Cryptosporidium parvum and Giardia sp. were 100%.

Comparison of daily and monthly pyrantel treatment in yearling thoroughbreds and the protective effect of strategic medication of mares on their foals.

Studies on a Thoroughbred breeding farm in Ohio were done to: (1) compare the effects of daily administration of pyrantel tartrate feed pellets with monthly administration of a pyrantel pamoate paste to yearling horses (21 January-3 September); (2) assess the effects of daily pyrantel tartrate given strategically in spring/summer to foaling mares (1 April-16 August) and given for a prolonged period to barren mares (21 January-3 September); (3) determine if strategic medication of foaling mares with daily pyrantel tartrate protected their foals until weaning. There were no differences in cyathostome egg counts, pasture larval counts, body condition scores, or body weights of yearlings treated with daily pyrantel tartrate or monthly pyrantel pamoate. Both treatments failed to maintain fecal egg counts of yearlings below 100 eggs per gram (epg), and mean counts exceeded 400 epg (pyrantel pamoate) and 700 epg (pyrantel tartrate) in August and September, resulting in a sharp, but moderate increase in pasture infectivity in October. By contrast, prolonged or strategic use of daily pyrantel tartrate in mature horses were each highly effective in reducing pasture contamination and infectivity with cyathostome eggs and larvae respectively. Strategic medication of foaling mares provided protection of their foals until weaning and first treatment of foals was delayed until after weaning when mean strongyle counts exceeded 100 epg. Treatment of weanlings with pyrantel pamoate had little effect on egg counts. A comparative anthelmintic study with ivermectin, oxibendazole, and pyrantel pamoate confirmed earlier studies showing reduced efficacy of anthelmintics in young horses.

Infectivity of Moniezia benedeni and Moniezia expansa to oribatid mites from Ohio and Georgia.

Six species of oribatid mites belonging to four families and five genera were successfully infected by M. benedeni. They were Scheloribates laevigatus (Scheloribatidae), Exoribatula sp. cf. biundatus (Scheloribatidae), Xylobates capucinus (Haplozetidae) and Zygoribatula undulata (Oribatulidae) from Ohio, and Galumna ithacensis (Galumnidae) and Scheloribates lanceoliger (Scheloribatidae) from Georgia. Cysticercoids were also recovered from S. laevigatus infected with M. expansa. Among the six species of mites, S. laevigatus is the major intermediate host of M. expansa and M. benedeni worldwide, while the other five species have not been previously reported as transmitters of Moniezia spp. This is also the first report of any species from the genera Exoribatula and Xylobates as intermediate hosts of tapeworms. Different species of oribatid mites varied in their efficiency as intermediate hosts of M. benedeni. Moniezia benedeni had higher infection rate and infection intensity in S. laevigatus than in other species. Development in S. laevigatus was also more rapid than in the other species. Mite size was only one of the determinants of intermediate host efficiency. Storage at 4 degrees C for 1 to 5 weeks reduced the viability of M. benedeni eggs. Stored eggs had lower infection rates and infection intensity and slower development in S. laevigatus. They also had lower infection rates in X. capucinus and Exoribatula sp. cf. biundatus.

In vitro assessment of two species of nematophagous fungi (Arthrobotrys oligospora and Arthrobotrys flagrans) to control the development of infective cyathostome larvae from naturally infected horses.

The ability of two species of nematophagous fungi, Arthrobotrys oligospora and Arthrobotrys flagrans (syn. Trichothecium flagrans, Duddingtonia flagrans), to control the development of infective larvae in feces from naturally infected horses was assessed in vitro. The horses were from a farm where it had been previously established that cyathostomes accounted for 100% of the strongyle egg output. The feces from these naturally infected horses were mixed with spores of each fungal species at four concentrations: 0 (control), 1, 10, and 100 spores per egg. Five replicates for each group were incubated for 8 days. Infective larvae were harvested using a Baermann technique and counted. The percentage reduction in infective cyathostome larvae was calculated for each fungal concentration compared to controls. A fungal concentration of 1 spore per egg resulted in 40.5% and 32.1% reduction for A. oligospora and A. flagrans, respectively. A concentration of 10 spores per egg resulted in 87.4% and 90.5% reduction, while 100 spores per egg resulted in 95.8% and 93.9% reduction for A. oligospora and A. flagrans, respectively.

Concurrent infections of Giardia and Cryptosporidium on two Ohio farms with calf diarrhea.

Giardia and Cryptosporidium infections were diagnosed by immunofluorescence assay on two Ohio dairy farms with calf diarrhea problems. On the first farm, all nine diarrheic calves sampled once in June had Giardia cysts in their feces. On the second farm, all five diarrheic calves examined at the beginning of the diarrhea outbreak in March had Giardia infection. When resampled, the overall infection rate of normal and diarrheic calves was 82.4% in April, and 40.0% in August after the diarrhea subsided. Positive calves ranged from 11 to 164 days of age, and 22.2% of them were as young as 1 to 3 weeks of age. Eight of nine diarrheic calves (88.8%) on the first farm had Cryptosporidium infection. Lower infection rates (< 30%) were found on the second farm. Six of 10 positive calves were 11-22 days old, three were 164-177 days old, and one was 71 days old. Five of these 10 positive calves were also positive for Giardia infection. Five diarrheic calves on the northern Ohio farm and one diarrheic calf on the central Ohio farm were treated with metronidazole after failing to respond to antibiotic therapy. Clinical improvement was observed in all calves within 48 h after the start of treatment. The high Giardia infection rates and intensities in calves of a wide age range and the clinical response to metronidazole suggest that Giardia infection contributed to the outbreaks of diarrhea.

Prevalence of Cryptosporidium and Giardia infections on two Ohio pig farms with different management systems.

The prevalence of Cryptosporidium and Giardia infections in pigs was investigated by the use of a direct immunofluorescence assay on two Ohio farms with different management systems. Cryptosporidium and Giardia infections were detected only in weanlings on the farm with slotted and wire floors, but in both weanlings and nursing piglets on the farm with porous concrete floors. Giardia infection was also detected in sows on the latter farm. The farm with porous concrete floors had a significantly higher Cryptosporidium infection rate in nursing piglets and Giardia infection rates in weanlings than the farm with slotted and wire floors. Sows were implicated as the source of both Cryptosporidium and Giardia infections for nursing piglets.

Comparative efficacy of moxidectin and ivermectin against hypobiotic and encysted cyathostomes and other equine parasites.

Efficacies of moxidectin and ivermectin were compared in four groups of eight ponies with natural parasite infections: placebo (Control), oral moxidectin gel at 0.3 mg kg-1 of body weight (Mox 0.3), oral moxidectin gel at 0.4 mg kg-1 of body weight (Mox 0.4), and oral ivermectin paste at 0.2 mg kg-1 of body weight (Ivermectin). Fecal samples were taken 0 and 2 weeks after treatment. Animals were necropsied and worms were collected 2 weeks after treatment. Moxidectin and ivermectin showed similar efficacy (99%) against adult cyathostomes, Strongylus spp., Triodontophorus spp. and Habronema muscae. Both drugs were also more than 98% effective against luminal cyathostome and Oxyuris equi fourth stage larvae (L4). Neither drug was effective (0-10.1%) against hypobiotic early third stage cyathostome larvae (EL3). Moxidectin was moderately effective (62.6-79.1%) in removing encysted cyathostome late third stage larvae (LL3) and L4, whereas ivermectin was ineffective (0%) against these stages. By contrast, ivermectin was 95.4% effective against Gasterophilus spp. third instar stage, whereas moxidectin was only 0-20.4% effective.

Efficacy of albendazole and fenbendazole against Giardia infection in cattle.

Efficacies of albendazole and fenbendazole in suppressing Giardia cyst output of infected calves were evaluated in two clinical trials. In the first trial, 18 naturally infected calves were allocated to an untreated control group (n = 9) and an albendazole-treated group (n = 9). Calves in the treated group were given 20 mg/kg-1 oral albendazole once daily for 3 days. Compared to controls, treated calves showed 98.5%, 97.6% and 90.8% reductions in cysts per gram of feces (cpg) 1, 2 and 6 weeks respectively after the start of treatment. In a second trial, 13 infected calves were allocated to an untreated control group (n = 6) and a fenbendazole-treated group (n = 7). Calves in the treated group were given 10 mg kg-1 fenbendazole orally twice daily for 3 days. Compared to the control group, treatments reduced cpg counts by 100%, 98.5% and 59.5% 1, 2, and 3 weeks respectively after the start of treatment. Both albendazole and fenbendazole appeared to be effective in suppressing cyst excretion by Giardia-infected calves.

Diagnosis of Cryptosporidium on a sheep farm with neonatal diarrhea by immunofluorescence assays.

An outbreak of diarrhea in neonatal lambs occurred on a sheep farm in northern Ohio. Diarrhea commenced as early as 1 week of age and lasted for about 3-4 days. Although 100% of the newborn lambs were affected, most had recovered by 3 weeks of age. Cryptosporidium infection appeared to be the cause of diarrhea. Fecal examination of nine diarrheic newborn lambs (5-10 days old), 23 older lambs (2-3 weeks old, six with diarrhea) and 23 clinically normal ewes by immunofluorescence assays revealed infection rates of 100%, 78.3% and 17.4%, respectively. Most newborn lambs had high oocyst counts. Ewes were considered to be an important source of infection for lambs.

Experimental Haemonchus contortus infection in three breeds of sheep.

Nine- to 10-month-old Florida Native, St. Croix and Dorset/Rambouillet sheep were infected with Haemonchus contortus. The primary infection was terminated by anthelmintic treatment 9 weeks later and animals were then reinfected with H. contortus. Dorset/Rambouillet sheep showed higher fecal egg counts and decreased packed cell volumes and plasma protein levels compared with the other two breeds during the primary infection. However, no breed differences were found in total worm burdens in either primary or secondary infection. Differences between breeds were also not detected in lymphocyte responsiveness to parasite antigen, H. contortus specific mucosal antibody levels, numbers of abomasal mast cells and globule leukocytes or abomasal histamine levels.

Dung dispersal and grazing area following treatment of horses with a single dose of ivermectin.

Environmental consequences of treating horses with recommended dosages of ivermectin paste were studied in two controlled experiments with 29 horses in Ohio. In 1988, dung dispersal rates were measured by changes in dry weight over time of 48 copromes (300 g) formed from feces taken from four treatment and four control horses 3 days post ivermectin treatment. There was delayed dispersal of copromes from horses treated with ivermectin in June, resulting in significantly heavier ivermectin copromes compared with those of control horses by September. There was no difference in ivermectin or control copromes after treatment in August. In 1989, the effects of treating horses with oral ivermectin or oxibendazole compared with untreated controls were quantified in a study with 21 horses under natural grazing conditions after treatment on 7 June. Sequential measurements of dung pat circumferences from 20 June 1989 to 7 March 1990 showed highly significant differences between ivermectin and control pats and between ivermectin and oxibendazole pats. Reductions in mean dung pat diameters did not occur in ivermectin pats until 11 October, when there was a 4.1% reduction compared with 35.1% for control pats and 37.2% for oxibendazole pats. By the end of the study, there was a 24.7% reduction in ivermectin pats compared with 59.1% and 59.9% for control and oxibendazole pats respectively. In addition, there were significantly more pats showing complete dispersal in oxibendazole or control plots than in ivermectin plots. The calculated grazing area lost to feces was three times greater for ivermectin plots than for oxibendazole or control plots. It is suggested that environmental effects of the avermectins can be reduced by more rational use of anthelmintics.

Effect of environmental stimuli on pre-infective and infective stages of Haemonchus contortus in the northern United States for the induction of hypobiosis.

The effects of both natural and artificial conditioning of pre-infective and infective stages of Haemonchus contortus were examined in two studies with 108 crossbred lambs, using an Ohio isolate of H. contortus known to exhibit up to 100% winter hypobiosis. The intensity of hypobiosis varied from 0 to 36% in 17 lamb groups given larvae subjected to various temperature and photoperiod conditions. The results in lambs given larvae conditioned for 4-8 weeks were not significantly different from those in lambs given freshly cultured larvae. It is suggested that winter hypobiosis of H. contortus in the northern U.S.A. is an obligatory survival mechanism that occurs without the need for external stimuli to trigger the onset of hypobiosis.

Sex-related susceptibility of bulls to gastrointestinal parasites.

The effects of sex and anabolic implants on fecal egg counts and pasture contamination were examined in 77 naturally infected yearling bulls, steers, and heifers of mixed beef breeds grazing the same contaminated pasture in southern Ohio in the spring of 1990. Fecal egg counts were compared in seven groups of 10-14 animals each, twice before anabolic implants and twice after implants. Comparisons were made between untreated bulls, steers, and heifers, between steers with and without implants, and between heifers with and without implants. Bulls had significantly (P < 0.01) higher fecal egg counts than steers, and counts from steers were not significantly different to those from heifers. There were no significant differences between counts from implanted and control steers or heifers. The results have important practical implications both for parasite control and for bovine parasite research. There is a need to pay special attention to young bulls when designing parasite control programs, and to distribute the sexes equally between groups when doing research trials with cattle of mixed sexes. Some previous studies on worm population dynamics and anthelmintic evaluation may need to be re-examined in the light of sex differences.

The prevalence and intensity of internal parasites of horses in the U.S.A.

Fifty-five adult horses were necropsied over a 15-month period, and their worm burdens counted and speciated. Twenty-one species of Cyathostominae were recovered. Ten species: Cyathostomum catinatum, Cylicocyclus nassatus, Cylicostephanus longibursatus, Cyathostomum coronatum, Cylicostephanus goldi, C. calicatus, C. minutus, Cylicocyclus leptostomus, C. insigne and Cyathostomum pateratum, comprised 98.9% of the total cyathostome burdens. These same 10 species also demonstrated high prevalences in 4 previous surveys. Eight of these 10 species have been shown to be resistant to benzimidazole anthelmintics. Anthelmintic resistance apparently had little effect on the prevalence of a species, but appeared to increase its relative abundance within a mixed population. The prevalence of other internal parasites was similar to previous reports, but Strongylus vulgaris adults and arterial lesions were less common.

Periparturient rise in the excretion of Giardia sp. cysts and Cryptosporidium parvum oocysts as a source of infection for lambs.

The possibility of a periparturient rise (PPR) in the excretion of Cryptosporidium parvum oocysts and Giardia duodenalis-type cysts and its implications in the epidemiology of these 2 pathogens were investigated. Fecal samples were taken from 32 lambing ewes every 2 wk from 4 wk before lambing until 8 wk after the start of lambing. Samples from both ewes and lambs were examined with a quantitative immunofluorescence assay. Excretion of Giardia sp. cysts by ewes increased at 2 wk prepartum, peaked at zero and 4 wk postpartum, and returned to low levels at 6 and 8 wk postpartum. Some ewes also excreted C. parvum oocysts at the start of lambing. Excretion rates and intensities of Giardia sp. cysts by lambs were high, whereas those of C. parvum oocyst excretion were light. The PPR in excretion of Giardia sp. cysts by ewes was considered to be the major source of giardiasis for lambs. Excretion of C. parvum oocysts by ewes at parturition also played a role in initiating cryptosporidiosis in lambs.

Epidemiology and control of equine strongylosis at Newmarket.

Seasonal rises in mean faecal egg output were observed in grazing ponies in spring (578 eggs per gram) and in summer (930 epg) on 30 April and 2 September, respectively, in untreated ponies. Pasture infectivity reached a peak of 18,486 third stage larvae (L3)/kg on 17 September, two weeks after peak egg counts, coincidental with abundant September rainfall (103.0 mm). Differentiation of infective larvae from pasture showed the cyathostomes (small strongyles) to be predominant, but Trichostrongylus axei assumed major importance from late August to October. The large strongyles were rarely detected: Strongylus vulgaris was found only once and S edentatus only twice. The most effective parasite prophylaxis was achieved by twice weekly removal of faeces. In this group, concentrations of infective L3 on pasture reached a maximum of 1000 L3/kg, compared to 18,486 L3/kg for a control group and 4850 to 10,210 L3/kg for anthelmintic treatment groups. The removal of faeces increased the grazing area by about 50 per cent, by eliminating the characteristic separation of horse pasture into roughs and lawns. Spring and summer anthelmintic treatments of mature ponies with oxibendazole were effective in reducing the late season rise in pasture infectivity to 4850 L3/kg, but treatment of young ponies (mainly yearlings) with ivermectin every eight weeks or oxibendazole every four weeks resulted in pasture infectivity as high as 10,000 L3/kg. There was evidence of cyathostome resistance to benzimidazole drugs.