Nutritional and immunological interrelationships in response to nematode infections in periparturient ewes

Abstract

The periparturient rise in gastrointestinal (GI) nematode egg output from the ewe is a well-established phenomenon. The immune function of the host is thought to relax during this period. Nutrition may play an important role in this relaxation but the mechanism is still not clear. In this thesis, five experiments were conducted to investigate nutritional and immunological interrelationships in response to nematode infections in periparturient ewes.

The first experiment (Chapter 4) was an indoor trial. Eighty pregnant ewes were divided into four groups and allocated into a 2x2 design with two levels of dietary protein treatment (120% versus 80% of metabolisable protein (MP) requirement) and vaccination treatment (vaccinated with 5,000 T. circumcincta and 3,000 T. colubriformis infective larvae per animal per day during weeks -8 to -3 relative to parturition, versus un-vaccinated). All animals were challenged with 10,000 T. circumcincta and 7,000 T. colubriformis infective larvae per day for 2.5 weeks before parturition. Litter size was recorded at parturition. The worm burden was determined and mucosal tissue samples for local immune cells collected at weeks +1 and +3 relative to parturition, with five animals slaughtered at each time from each group.

The animals in the high protein group had a lower faecal egg count (FEC), lower worm burdens of T. circumcincta and T. colubriformis, more intestinal mucosal mast cells and globule leukocytes than their low protein counterparts. Single bearing ewes lost less bodyweight after lambing, had a lower FEC and more intestinal globule leukocytes than multiple bearing ewes. A significant interaction of protein treatment x litter size on FEC showed that single bearing ewes fed the high protein level markedly suppressed FEC. Vaccination induced a higher parasite specific antibody response (in serum total antibody, IgE and IgA, and saliva total antibody and IgA) in the vaccination period. The vaccinated animals also had a lower number of abomasal eosinophils, and a higher number of abomasal mucosal mast cells, abomasal globule leukocytes and intestinal mucosal mast cells than the un-vaccinated controls. The results of this trial showed that higher dietary protein and smaller litter size significantly increased the resistance of the periparturient ewes against the GI nematodes. Pre-lambing vaccination induced responses in the immune system but did not affect ewes' bodyweight and parasitological parameters under test.

The second experiment was an outdoor trial (Chapter 5). The indistinct effect of pre-lambing vaccination in the previous trial was further investigated. One hundred and twenty twin-bearing ewes were allocated to three groups. A "trickle vaccination" group was vaccinated with a daily dose of 5,000 T. circumcincta and 3,000 T. colubriformis infective larvae during weeks -8 through -3 inclusive before challenge. A "single vaccination" group was infected with 60,000 T. circumcincta and 30,000 T. colubriformis infective larvae in a single dose during week -4. Another group was used as controls. All animals were challenged with 10,000 T. Circumcincta and 7,000 T. colubriformis per day for 2.5 weeks immediately before parturition. Half of the animals from each group were slaughtered for worm burden determination and mucosal sampling.

Vaccination did not affect the ewe bodyweight but significantly affected the FEC, with the back transformed values of FEC in the trickle vaccination group and the single vaccination group reduced, respectively, to 1/3 and 1/14 of that in the control group in week +3. The extent of the periparturient decline of serum and saliva parasite-specific total antibody was reduced by trickle or single vaccination, and the decline of serum T. circumcincta-specific total antibody was completely eliminated by trickle vaccination. Serum specific IgA was elevated in response to trickle vaccination, single vaccination and challenge. There was no response in eosinophils, mucosal mast cells or globule leukocytes, in the abomasal mucosa or intestinal mucosa. The results of this trial revealed that the host immune system could be stimulated by a pre-lambing vaccination and the host resistance against nematodes could be enhanced as shown by the change in FEC, but not in worm burdens or ewe bodyweight.

The third experiment was another outdoor trial (Chapter 6). The effects of dietary protein supplements, litter size, as well as pre-lambing vaccination were studied in this trial. One hundred and ninety-eight pregnant ewes were divided into six groups incorporated into a 3x2 factorial design with three levels of nutritional treatment (supplemented with fish meal-, soya meal-based supplements or no supplementation) and two levels of vaccination treatment (single vaccination with T. circumcincta (60,000) and T. colubriformis (40,000) at week -5, versus non-vaccination). All animals were challenged with infective larvae of T. circumcincta (7,000) and T. Colubriformis (5,000) per animal per day for 3.5 weeks before parturition. Litter size was recorded after parturition. Worm burdens were determined at weeks +3 and +6.

The fish meal supplemented group had the heaviest post lambing bodyweight and the lowest FEC. The soya meal supplemented animals had an intermediate bodyweight and FEC between the fish meal supplemented and non-supplemented groups. The soya meal supplemented group had the highest total worm burden and tended to have a lower serum parasite-specific total antibody response than the other groups. The litter size was negatively associated with ewes' bodyweight, and negatively associated with FEC and worm burden. The ewes that lost their lambs had a rapid post-lambing bodyweight recovery while those rearing two lambs did not change in 14 weeks post lambing. The FEC for ewes not rearing a lamb did not rise in contrast to those rearing one or two lambs. Vaccination did not have any significant effect on bodyweight, parasitological parameters or antibody responses. The results of this trial confirmed the results of Chapter 4 that showed that host resistance was affected by nutrition supply and demand and extended it to outdoor conditions. However, these conditions did not give consistent results in the antibody response, except for a decline of serum parasite-specific total antibody coinciding with parturition. It was also shown that supplementation with animal protein (fish meal) was more efficient than plant protein (soya meal). Soya meal might have a negative effect on the host immunity as measured by worm burden and serum total specific antibody.

The effect of cold stress on the immunity of the periparturient ewes was explored in another outdoor trial (Chapter 7). One hundred and twenty ewes were used in a 2x2x2 factorial design incorporating pregnancy status (twin or single bearing), nutrition treatment (high or low pasture allowance) and stress treatment (shearing followed by cold exposure or unshorn and non-exposed). Differential nutritional allowances were imposed during weeks -8 and -5 relative to parturition. High and low (120 and 80% of metabolisable energy (ME) requirement) allowances were calculated for a ewe carrying or suckling 1.5 lambs. The cold-exposed ewes were shorn in week -5 and exposed to artificial wind and rain for 4 h on each of four consecutive days during week -4. All animals were challenged twice with 150,00 T. circumcincta and 15,000 T. colubriformis infective larvae during week-4.

The bodyweight of the cold exposed ewes was lower than the non-exposed ones, starting after cold exposure (3.47 kg) and was maintained lower until week +5 (4.66 kg). The cold exposed group had higher FEC, lasting for four weeks (1.6-4.4 fold), and lower levels of serum IgA against T. circumcincta and T. colubriformis, lasting for two weeks after cold exposure. The low allowance group had lower bodyweight, with the difference between the nutrition treatment groups being greatest at the end of the nutrition treatment (6.09 kg), gradually converging thereafter and remaining at 2.36 kg at week +5. The low allowance group had higher FEC (by 1.5-10.6 fold) and a lower JgA response to both T. circumcincta and T. colubriformis only at the end of nutrition treatment. The twin bearing ewes lost more bodyweight (about 4.5 kg after parturition), had a long-lasting higher FEC (up to 8.8 fold), and had lower total antibody against both T. circumcincta and T. colubriformis in the later stage of the trial, compared to the single bearing ewes. The results suggested that cold stress impaired the host immunity against GI nematodes resulting in a marked loss in bodyweight. The host immunity could be manipulated by general nutrition through pasture allowance. The effect of nutritional stress was further confirmed.

The fifth experiment was an indoor trial designed to focus on the local immunity against nematodes using abomasal cannulation. Ten pregnant ewes were fitted with an abomasal cannula at week -10 relative to parturition. They were divided into two groups, cold exposed or non-exposed. All animals were fed the 80% allowance during weeks -8 to -6, 100% during weeks -5 and -4 and then gradually increased up to over 150% in lactation. The cold exposed group was shorn and exposed to cold as in the previous trial, i.e. during week -4 relative to parturition. Blood, saliva, faecal samples and abomasal biopsy samples were collected weekly from week -8 to week -5 and twice a week from week -4 to week +4 relative to parturition. All the animals were challenged with 17,000 infective larvae of T. circumcincta daily from week -7 to the time of parturition.

Time was the dominant factor affecting almost all parameters tested. Abomasal globule leukocytes and mucosal mast cells significantly decreased in the periparturient period but eosinophils did not. The cold exposed group tended to have a lower FEC and lower serum total parasite-specific antibody. Cold exposure did not appear to affect the parasite-specific antibody in saliva or eosinophils, mucosal mast cells and globule leukocytes in abomasal mucosa as shown in this small-sample trial.

Serum total antibody and interleukin-5 declined concurrently with parturition. The former was consistently observed in all trials. The latter was only tested in the first experiment (Chapter 4).

It is concluded from this project that an increase in nutritional supply and a decrease in nutritional demand significantly enhanced the immunity of a periparturient ewe against GI nematode infections. The interrelationship between nutrition and immunity appears to be multi-layered with the local immunity responding to a greater degree than the systemic immunity.