Changes in gastrointestinal mucosal mass and mucosal and serum gastrin in sheep experimentally infected with Ostertagia circumcincta

Anthelmintic drugs used in equine species

Internal parasites of horses comprise an intractable problem conferring disease, production and performance losses. Parasitism can rarely be controlled in grazing horses by management alone and anthelmintic drugs have formed the basis of therapy and prophylaxis for the last sixty years. The pharmacology of the anthelmintic drugs available dictate their spectrum of activity and degree of efficacy, their optimal routes of administration and characteristics which prevent some routes of administration, their safety tolerance and potential toxicities and as a consequence of their persistence in the body at effective concentrations their use in epidemiological control programmes. Their use has also resulted in the selection of parasites with genetically controlled characteristics which reduce their susceptibility to treatment, characteristics which are often common to whole chemical classes of anthelmintics. Pharmacological properties also confer compatibility in terms of safety and persistence with other anthelmintic drugs and thus the potential of combinations to treat parasites from different phylogenetic groups such as nematodes, cestodes and trematodes and also the potential by agency of their different molecular mechanisms of action to delay the selection of resistant genes. The major groups of anthelmintics now available, the benzimidazoles (BZD), macrocyclic lactones (MLs) and tetrahydropyrimidines are all highly effective against their targeted parasites (primarily nematodes for BZD's and ML's and cestodes for tetrahydropyrimidines) easily administered orally to horses and are well tolerated with wide margins of safety. Nevertheless, some parasitic stages are inherently less susceptible such as hypobiotic stages of the small strongyles (cyathostomins) and for some such as the adult stages of cyathostomins resistance has developed. Furthermore, for some less common parasites such as the liver fluke unlicensed drugs such as the salicylanilide, closantel have been used. A deep understanding of the pharmacology of anthelmintic drugs is essential to their optimal use in equine species.

Pathophysiology of Marshallagia marshalli in experimentally infected lambs

Species of Marshallagia are abomasal parasites in free-ranging and domesticated ungulates in temperate climatic zones throughout the world. Pervasiveness of these nematodes is significant in various parts of the world. There has been limited research in the area of Marshallagi amarshalli pathogenesis. The aim of this study was to investigate the effects of M. marshalli on the acid secretory capacity of the abomasal mucosa and the morphological changes due to parasitic migration to different parts of abomasal tissue in sheep. Ten lambs, approximately around 6 months old, were allotted to two groups of five (A and B). The sheep from group A were infected orally with a dose of 5000 third-stage larvae (L3) of M. marshalli whereas the sheep of group B were not infected. The results indicated that the development of M. marshalli in the abomasal glands of ruminants causes pathophysiological changes, which include a reduced acidity of the abomasal contents, increased abomasal pH and increased serum pepsinogen concentrations. The reduced acid secretion is explained by a replacement of functional parietal cells by undifferentiated cells. Histology changes include mucosal cell hyperplasia, loss of parietal cells and inflammatory cell infiltration, which includes numerous granulocytes and lymphocytes.

Effects of Teladorsagia (Ostertagia) circumcincta infection on lambs selected for high fleece weight

The physiological processes leading to the expression of the resilient phenotype, which allow animals to maintain a relatively higher production level during infection, have been investigated in lambs from a closed flock selected for 40 generations for high fleece weight (HFW), but with higher FEC and worm burdens than their unselected control (C) flock run in parallel. After recovery from surgery to implant abomasal cannulae, eight parasite-naive lambs from each flock were infected intraruminally at 4.5 months-of-age with 50,000 Teladorsagia circumcincta L3. Blood, abomasal fluid and faecal samples were collected daily for measurement of serum gastrin and pepsinogen concentrations, blood eosinophils, abomasal pH and FEC. Four lambs from each flock were euthanased on Day 8 post-infection and the other four on Day 28 post-infection. At necropsy, abomasal contents and tissues were collected for worm counts, abomasal lymph nodes and fundic tissue for cytokine gene expression and fundic tissue for histopathology. Expression of resilience appeared to be age-dependent as there were no significant differences in either FEC or worm burden between lambs from the two flocks, unlike older HFW lambs in a previous study. Abomasal secretion did not differ between flocks. Histopathological changes were typical of parasitism: inflammatory cells, mainly eosinophils and lymphocytes, were numerous in nodular areas and there were fewer TGF-alpha positive parietal cells, many of which were vacuolated. By Day 28 p.i., globule leucocytes were present. Mucosal thickness was significantly greater on Day 8 than Day 28 p.i. (p=0.000) and in C than HFW lambs. There were fewer parietal cells on Day 28 than on Day 8 p.i. (p=0.003) for pooled data. Circulating eosinophil counts increased moderately in both groups, significantly less in the HFW lambs. Fewer tissue and blood eosinophils in the HFW than C group on Day 8 p.i. were consistent with cytokine gene expression patterns, particularly lower IL-5 levels. Worm count decreased by 90% by Day 28 p.i., along with declining tissue eosinophil counts and IL-13 gene expression and increasing IL-10 and IL-4 gene expression. Food intake was depressed less in the HFW lambs, suggesting that maintenance of appetite could be an important aspect of the physiological basis for resilience. Although the resilient phenotype was not apparent at the younger age, lesser effects on food intake, differences in ALN cytokine profiles and lower blood and tissue eosinophil numbers in the HFW lambs may lead to the expression of resilience when older.

Effects of Ostertagia ostertagi and omeprazole treatment on feed intake and gastrin-related responses in the calf

Infection with the bovine abomasal nematode, Ostertagia ostertagi, results in a loss of acid-secreting parietal cells and an increase in gastric pH. The effects of an experimental infection with Ostertagia and/or daily treatment with omeprazole (OMP) at 2mgkg(-1) bodyweight for four consecutive days (experiment days 24-27, inclusive) on voluntary feed intake, blood and tissue gastrin concentrations, abomasal G-cell numbers, gastric pH, and blood cholecystokinin (CCK) and pepsinogen concentrations were investigated in the calf. Ostertagia-infected calves demonstrated a significant drop in feed intake between days 24 and 27 post-infection (38%; P<0.001) and in G-cell numbers (42%; P<0.05) and significant increases in abomasal pH (P<0.001), fundic mucosal weight (99%; P<0.01), and blood gastrin (P<0.05) and pepsinogen (P<0.0001). OMP treatment of worm-free animals resulted in a significant drop in intake between days 24 and 27 (30%; P<0.001) and in G-cell numbers (17%; P<0.05) and significant increases in abomasal pH (P<0.01) and blood gastrin (P<0.001). OMP treatment of Ostertagia-infected animals with an existing hypergastrinaemia had no effect on feed intake, abomasal pH, blood gastrin or pepsinogen or abomasal G-cell numbers. Blood CCK concentrations were also unaffected by either Ostertagia infection or OMP treatment. These data suggest that: (a) the depression in feed intake associated with OMP in worm-free calves was not due to a side effect of drug treatment; (b) inappetance in Ostertagia-infected animals is closely associated with the parasite-induced hypergastrinaemia; and (c) the elevation in abomasal pH was a major factor responsible for the elevated blood gastrin concentrations seen in parasitised and OMP-treated animals.

Pathophysiology of abomasal parasitism: is the host or parasite responsible?

Nematode larvae developing within the glands cause local loss of parietal cells and mucous cell hyperplasia whereas reduced acid secretion, increased serum gastrin and pepsinogen concentrations and generalized histological changes are associated with parasites in the abomasal lumen. Parietal cells with dilated canaliculi and/or degenerative changes typical of necrosis are present soon after the transplantation of adult worms, and abomasal secretion is also affected. Anaerobic bacteria survive in greater numbers as the pH rises, with bacterial densities becoming similar to ruminal populations at an abomasal pH of 4 and above. Failure to lyse bacteria may affect adversely the nutrition of the host. The parasites may initiate the pathophysiology through the release of excretory/secretory (ES) products which either act directly on parietal cells or indirectly through enterochromaffin-like (ECL) cells by provoking inflammation or by disrupting the protective mucosal defence system. Parietal cell dysfunction is proposed as a key event which leads to loss of mature chief cells and mucous cell hyperplasia, as well as hypergastrinaemia. Inflammation increases circulating pepsinogen concentrations and may also contribute to increased gastrin secretion. Stimulation of mucosal proliferation and differentiation of parietal cells in the isthmus by the raised serum gastrin levels will be beneficial by generating a new population of active parietal cells and adequate acid secretion.

A model for study of lungworm (Dictyocaulus sp.) and gastrointestinal nematode infection in young red deer (Cervus elaphus)

A model of sub-clinical parasitism in young red deer, using concurrent trickle infections of lungworm (Dictyocaulus sp.) and mixed gastro-intestinal (GI) nematodes of deer-origin was evaluated. 20 parasite-free deer calves were artificially reared indoors from 4 days of age. A further five calves were naturally reared on pasture with their dams, treated with anthelmintic and brought indoors at 3-4 months. At 4-4.5 months of age they were individually housed and allocated to five groups (n=5). Groups were dosed 3 x per week, for 9 weeks with 0, 100 and 500, 200 and 1000 (2 groups), 400 and 2000 infective larvae of lungworm and mixed GI nematodes, respectively, cultured from deer faeces. Liveweight and voluntary feed intake measurements and faecal and blood samples were taken weekly. In the fourth week following cessation of trickle infection, deer were euthanased and lung and GI nematodes recovered. Both lungworm and GI nematode infections became patent at Week 4 of infection. Maximum group arithmetic mean faecal egg counts were 100-190 epg. Maximum group arithmetic mean faecal lungworm larval counts were 58-123 lpg. Group arithmetic mean nematode counts at slaughter ranged from 439-806 for GI nematodes and 31-73 for lungworm, respectively. Despite low nematode counts, reduced liveweight gain, voluntary feed intake and serum albumin concentration, elevated serum pepsinogen, gastrin and globulin concentrations and elevated peripheral eosinophil counts and slight haemoconcentration, but no clinical signs, were observed. The reduction in liveweight gain was related to the reduction in voluntary feed intake (r2=0.83; p<0.088). Naturally-reared deer had similar liveweight gains, voluntary feed intake and nematode counts to artificially-reared deer. Thus, methods of infection to produce concurrent sub-clinical lungworm and GI nematode burdens for study of sub-clinical parasitism in young deer have been defined.

Infection of sheep with adult and larval Ostertagia circumcincta: gastrin

Gastric endocrine cell populations and serum and tissue gastrin have been examined in sheep which were infected either intraruminally by tube with 150,000 Ostertagia circumcincta larvae followed by a trickle infection of 10,000 larvae thrice weekly for 8 weeks or by the transfer of 15,000 adult worms directly into the abomasum and killed 8 days later. Depletion of both antral gastrin and somatostatin was evident in both groups: tissue gastrin concentrations were reduced by 85% in the trickle infection and both G cells (gastrin-containing) and D cells (somatostatin-containing) were pale and fewer after adult worm transfer. The concurrent depletion of antral gastrin and somatostatin supports the contention that the hypergastrinaemia in parasitised sheep is largely secondary to the increase in abomasal pH. Although there was no change in the proportions of G34 and G17 in the tissues, there was an increase in the longer form of gastrin in the circulation of the larval-infected sheep, suggesting that there may be differential secretion of G17 and G34 which may be exaggerated as the rate of secretion increases. Although the fundic mucosa was thicker following trickle infection, there was no evidence of enterochromaffin-like cell hyperplasia in either infected group. It is suggested that hyper-gastrinaemia may be beneficial to the host, as it may allow the abomasum to regain the ability to acidify its contents during continued exposure to the parasites.

Infection of sheep with adult and larval Ostertagia circumcincta: abomasal morphology

The infection of parasite-naive sheep with approximately 15,000 adult Ostertagia circumcincta via abomasal cannulae resulted in marked changes in the structure and function of the abomasum. The functional changes, which have been characterised previously, included elevated abomasal pH and increased serum concentrations of pepsinogen and gastrin. Eight days after the transplant of adult worms, the abomasa of recipient animals were significantly heavier than those of controls (P < 0.001), the thickness of the fundic mucosa was greater (P < 0.01), there were fewer parietal cells (P < 0.01) and increases in the numbers of mitotic figures and mucus-producing cells. Mucous cell hyperplasia was also evident in the fundic mucosae of sheep receiving a trickle infection of infective, third-stage O. circumcincta larvae and was prominent within nodules associated with larval development. In non-nodular mucosa, there was hyperplasia of mucous cells and changes in the distribution of parietal cells. Decreases in the number of parietal cells at the gland base were offset by increases at a mid-gland level, probably due to chronic hypergastrinaemia, so that, overall, total parietal cell number was unaffected. Mucous cell hyperplasia and the diminution of parietal cell number are seen in a diverse range of disease states and may be mediated by host growth factors such as Transforming growth factor-alpha. Alternatively, the cellular and/or the secretory changes in response to the presence of adult worms are mediated by chemicals that are cytotoxic/inhibitory for parietal cells, and released by the parasites themselves.

Pathophysiology of infection with gastrointestinal nematodes in domestic ruminants: recent developments

Infection with gastrointestinal nematodes, particularly Ostertagia species in domestic ruminants, continues to represent an important cause of impaired productivity in temperate parts of the world. The mechanisms responsible for such losses include changes in feed intake, gastrointestinal function, protein, energy and mineral metabolism, and body composition, and were described in detail at the last Ostertagia Workshop (Fox, M.T. 1993. Pathophysiology of infection with Ostertagia ostertagi in cattle. Vet. Parasitol. 46, 143-158). Since then, research into the pathophysiology of infection has focused on three main areas: mechanisms of appetite depression; changes in gastrointestinal function; and alterations in protein metabolism. Studies on the mechanisms responsible for appetite depression in Ostertagia-infected cattle have continued to support a close association between impaired feed intake and elevated blood gastrin concentrations. Alternative explanations will have to be sought, however, to account for the drop in feed intake associated with intestinal parasitism in which blood gastrin levels normally remain unaltered. Such work in sheep, and more recently in laboratory animals, has shown that central satiety signals are associated with inappetance accompanying intestinal infections, rather than changes in peripheral peptide levels. Changes in gastrointestinal function have also attracted attention, particularly the mechanisms responsible for increases in certain gut secretions, notably pepsinogen and gastrin. Elegant experimental studies have established that the gradient in pepsinogen concentration between abomasal mucosa and local capillaries could alone account for the increase in blood concentrations seen in Type 1 ostertagiosis. Additional factors, such as increases in capillary permeability and in surface area, probably contribute to such responses in cases of Type 2 disease. The increase in blood gastrin concentrations that accompanies Ostertagia infections in cattle is associated with the concurrent rise in abomasal pH. However, in sheep, additional factors appear to contribute to the hypergastrinaemia which may occur independent of parasite-induced changes in gastric pH. Alterations in protein metabolism have been well documented in ruminants harbouring monospecific infections with either abomasal or intestinal nematodes. More recently, however, the effects of dual abomasal and intestinal infections have been investigated and demonstrated that the host is able to compensate for impaired abomasal digestion provided that the intestinal parasite burden does not occupy the main site of digestion and absorption in the latter organ. An alternative method of improving the host's protein balance, dietary supplementation, has been shown not only to improve productivity, but also to enhance the innate resistance of susceptible breeds of sheep to Haemonchus and to accelerate the development of immunity to Ostertagia in lambs.

Effects of Ostertagia ostertagi on gastrin gene expression and gastrin-related responses in the calf

1. Infection with the bovine abomasal nematode Ostertagia ostertagi results in a loss of acid-secreting parietal cells and an increase in gastric pH. The effects of an experimental infection on gastrin mRNA expression, blood and tissue gastrin concentrations, the different molecular forms of gastrin in each, and pyloric mucosal chromogranin A-derived peptides were investigated in the calf. 2. An increase in blood gastrin concentrations in the infected group reached a peak by day 28 postinfection (635 pg ml-1; P < 0.01). Gel chromatography analysis of blood samples revealed that the hypergastrinaemia comprised largely gastrin-34 (G-34) in parasitized calves while gastrin-17 (G-17) predominated in control animals. 3. An 11-fold increase in gastrin mRNA expression was recorded in the parasitized animals which was accompanied by a 23.8% reduction in pyloric mucosal gastrin content and an apparent drop of 24.7% in the number of gastrin-producing G cells detected. There was no major change in the relative abundance of G-17 and G-34 in the pyloric mucosa of infected calves. No significant differences in the concentration of pyloric mucosal chromogranin A-derived peptides were recorded between infected and control groups. 4. These data suggest that the hypergastrinaemia seen in parasitized calves results largely from an increase in gastrin synthesis and that depletion of previously stored peptide makes virtually no contribution to elevated blood gastrin concentrations.

Control of gastrointestinal parasitism in calves with albendazole delivered via an intraruminal controlled-release device

The efficacy of albendazole in an intraruminal controlled-release device against gastrointestinal nematodes in calves was evaluated under field conditions. Calf productivity was monitored during the first and second grazing seasons. Two groups of parasite-naive Holstein Friesian heifer calves were grazed from May to October on adjacent, similarly contaminated paddocks. One group was given a Captec bolus at turnout, the group was left untreated. Pasture larval counts peaked on the paddock grazed by the untreated calves at 23,000 1 kg-1 dry matter in October. This precipitated clinical parasitic gastro-enteritis in the untreated calves with a mean peak faecal egg count of 335 epg, high blood pepsinogen and gastrin concentrations and a mean worm burden of 79,614 at 100 days post-turnout. Pasture larval counts on the paddock grazed by the treated calves remained less than 3000 1 kg-1 DM and the mean faecal egg count was zero until mid-August, peaking at 146 eggs per gram of faeces (epg) in October. The treated calves had body liveweight advantage of 19 kg at the end of September. During the second grazing season the first season untreated calves were refractory to infection. The first season treated calves showed signs of parasitic gastro-enteritis, although not to the extent exhibited by a group of first season parasite-naive tracer calves which were grazed on the same paddock.

Relationship between pharmacological properties and clinical efficacy of ruminant anthelmintics

The purpose of this review article is to establish a relationship between pharmacological properties and clinical efficacy of the most widely used broad-spectrum veterinary anthelmintics. The impact of drug pharmacokinetics/pharmacodynamics on anthelmintic efficacy and resistance is discussed. We review the clinical pharmacokinetics of the currently available anthelmintics used in livestock-imidazothiazoles, tetrahydropyrimidines, benzimidazoles/pro-benzimidazoles, salicylanilides (closantel) and avermectin-type compounds. Understanding the pharmacokinetic and metabolic behaviour of broad-spectrum anthelmintics in the host, and factors modulating that behaviour, is highly important for maximizing anthelmintic utility and efficacy.

Gastrin and gastrin-related responses to infection with Ostertagia ostertagi in the calf

The effects of a single challenge with 60,000 infective Ostertagia ostertagi larvae on blood and gastrointestinal mucosal gastrin concentrations, gastrin-producing G-cell numbers in the pyloric mucosa and growth of different parts of the gut were investigated in 16, two-and-a-half-month-old calves. Infected calves exhibited a rise in abomasal pH which was accompanied by a 145 per cent increase in wet weight of the fundic mucosa (P < 0.05) and a significant rise in blood total gastrin concentrations (P < 0.01). Circulating little gastrin (G-17) was unaffected. Pyloric mucosal total gastrin concentrations remained unaltered in the infected calves until day 28 when levels fell to 36.9 per cent of control group values (P < 0.01). Pyloric mucosal G-cell numbers declined during the experiment in the infected group. It is suggested that release of previously stored tissue gastrin and not a change in G-cell numbers contributes to the hypergastrinaemia associated with ostertagia infection in the calf.

The presence of an early L4 larvae population in relation to the immune response of calves against Ostertagia ostertagi

The influence of different levels of infection with Ostertagia ostertagi on the development of a protective immune response in calves was investigated. Four groups of calves were infected with either 5000 (Group A), 10,000 (Group B), 20,000 (Group C) or 40,000 (Group D) infective larvae (O. ostertagi L3) weekly until treatment began. Group E functioned as controls. All animals were treated with oxfendazole (9 mg ml-1) at Week 17 (Groups A, B and E) or Week 18 (Groups C and D). Sixteen days post-treatment all calves received a challenge infection of 150,000 O. ostertagi L3 spread over 10 consecutive days. Faeces and blood were collected weekly for egg counts and to assess levels of pepsinogen, gastrin and IgG1 and IgG2 Ostertagia antibodies. All calves were necropsied 31 days post-challenge for worm counts. Egg counts and pepsinogen levels were proportional to the infection level during the first few weeks of the experiment. Only in the high-dosed Group D was a gastrin response evoked. Ostertagia IgG1 antibodies increased between Day 25 and Day 95, and in the non-infected control group an antibody rise was observed from Day 67 onwards. All measured parameters except Ostertagia antibodies showed a gradual decrease from Day 70 until the day of treatment. At necropsy there was no significant difference between the groups in the total worm populations. Only the composition of the worm populations differed, with 35% early L4 (EL4) larvae in the previously infected Groups A, B, C and D and only 5% in the control Group E. The results indicate a slow immune response against O. ostertagi in cattle and question the possible role of the EL4 stage in developing immunity.

Pathophysiology of infection with Ostertagia ostertagi in cattle

Infection with the abomasal nematode, Ostertagia ostertagi, is an important cause of impaired productivity in young cattle in temperate parts of the world. Such losses have been associated with marked changes in feed intake, gastrointestinal function, protein, energy and mineral metabolism, and in body composition. The reduction in feed intake is an important factor in the pathogenesis of infection and may account for a large part of the difference in weight gain between ad libitum fed control and infected calves. Despite the obvious importance of inappetance, only recently has an association been made between reduced intake, altered gut motility and elevated levels of certain gastrointestinal hormones, such as gastrin. It has been suggested that the elevated gastrin levels accompanying abomasal parasitism may impair reticulo-ruminal motility and slow down abomasal emptying, leading to a stasis of ingesta and a reduction in feed intake. The rise in blood gastrin levels may also be partly responsible for the marked hyperplasia of the fundic mucosa seen in abomasal infections. Pronounced changes in protein metabolism have also been associated with Ostertagia infection. Radioisotopic studies have demonstrated increased losses of albumin into the gastrointestinal tract which are accompanied by an increase in the rate of synthesis in the liver. Dietary protein breakdown in the abomasum is also likely to be impaired, although there is evidence of a compensatory increase in protein digestion in the lower gut of parasitised calves. Increased losses of albumin are not always accompanied by increases in faecal nitrogen, suggesting that albumin is broken down and recycled as ammonia. Radioisotopic studies in animals with intestinal nematode infections have demonstrated a marked reduction in muscle protein synthesis and an increase in protein synthesis in gastrointestinal tissue. Such changes in the balance of protein synthesis are likely to be brought about by alterations in the balance of certain metabolic hormones. Marked changes in energy metabolism also accompany Ostertagia infection. Parasitised calves exhibit a marked increase in non-esterified fatty acid levels, resulting from the mobilisation of adipose tissue, and a reduction in digestive efficiency of energy, probably associated with the increase in cycling of protein through the gastrointestinal tract and the compensatory increases in protein synthesis. Mineral metabolism may also be affected although relatively little work has been conducted in cattle. Changes in body composition reflect a reduction in deposition of muscle protein and fat, and an increase in bone content and water retention.