Resistance of St. Croix lambs to Haemonchus contortus in experimentally and naturally acquired infections.

Parasitological and immunological parameters of experimental or naturally acquired infections with Haemonchus contortus were compared in St. Croix and Dorset lambs. In experimental infections, St. Croix lambs developed significantly greater levels of resistance to H. contortus, following primary exposure, as compared with Dorset lambs. This resistance was influenced both by age and by prior exposure to parasites. In grazing experiments on H. contortus-infected pasture, St. Croix lambs shed significantly fewer eggs as early as 5 weeks following initial exposure. Further, St. Croix lambs had more than 99% fewer worms in the abomasum at necropsy compared with age-matched Dorset lambs. Lymphoproliferative assays using peripheral blood mononuclear cells and antigen-specific serological tests demonstrated only minor differences in immune responsiveness between the two breeds despite the dramatic parasitological differences. Similarly, abomasal mucus from both breeds had elevated levels of parasite-specific antibodies and contained substances mediating larval paralysis. In contrast, St. Croix lambs which had become resistant to nematode infection had dramatically higher numbers of globule leukocytes in the abomasal mucosa compared with Dorset lambs.     

The effect of oxfendazole terminated infections with Haemonchus contortus on the development of immunity in sheep

The relative contribution of third (L3), fourth (L4) or adult stages of Haemonchus contortus to the development of immunity was evaluated in three groups of sheep subjected to infections terminated by oxfendazole treatments at the L3, L4 or adult stage. A control group did not receive immunising infections. All the groups were challenged with 5000 L3, to evaluate the protection provided by the different protocols. All sheep were necropsied at the end of the experiment to count the abomasal worm burdens. A marked reduction in egg counts after challenge infection was only observed in sheep in which the infection was terminated in the adult stage (Group 4). A significant reduction in worm burden was also observed in Group 4. The immunising infections and/or the challenge infection resulted in moderately elevated IgG antibody levels against L3, L4 and adult somatic antigens in all the groups. In contrast, a strong IgG response against H. contortus excretory/secretory (ES) antigens was observed in the groups in which the immunising infection was terminated in the L4 and the adult stage. An elevated lymphocyte proliferation response against Haemonchus ES antigens was found only in the group that had their immunising infection terminated at the adult stage. The combined data suggest that exposure to and elicited immunological responses to ES antigens are important for the development of immunity against H. contortus.     

Evaluation of copper supplementation to control Haemonchus contortus infections of sheep in Sweden

A pen study was conducted to assess the effect of providing daily copper mineral supplement, or copper wire particle (COWP) capsules, on established or incoming mixed nematode infections in young sheep. For lambs with established (6 week old) infections, COWP resulted in 97% and 56% reduction of the adult and early L4 stages of H. contortus, respectively, compared with controls (p<0.001). Additionally there was a 74% reduction in Teladorsagia circumcincta infections in the COWP lambs compared with controls (p<0.01). However, no effect was observed when COWP were given at the commencement of a larval dosing period of 6 weeks. There was no significant effect of copper mineral supplement (given at the recommended rate to prevent Cu deficiency) on either established, or developing parasite infections. In addition, a field trial was conducted on a commercial farm to assess the effects of COWP in the management of recurrent H. contortus infections, but lack of parasites during the grazing season prevented an adequate assessment from being made. These results indicate that there is little, if any, benefit from a parasite control standpoint in recommending copper therapy, specifically to control parasites in Swedish sheep flocks.     

Inhibited development of Ostertagia spp. and Cooperia spp. in naturally acquired infections in calves

Abstract

Aujeszky? disease is predominantly a nervous disease of piglets, weaners, and fatteners in which the mortality may vary from 540%. Respiratory disease is also a feature of infection with certain strains of the virus. Infection of pregnant sows may cause abortion, stillbirths, and mummification, and infertility may result in both boars and sows. Traditionally the serum neutralisation test has been used as an indicator of past infection with Aujeszky s's disease virus (ADV). However, cell-mediated immunity has recently been shown to play an important role in recovery and protection from infection with ADV and antibody-dependent cell-mediated cytotoxicity can be used as an alternative diagnostic test for detection of antibody. A number of inactivated, and live, avirulent vaccines are now available for control of the disease but their use is contraindicated in countries such as New Zealand which have a low prevalence of infection. Where possible early eradication is the best policy to follow.     

Serum and abomasal antibody response of sheep to infections with Haemonchus contortus

Serum and abomasal IgA, IgG and IgM antibody response against adult worm, L3 and egg antigens of Haemonchus contortus was monitored by the ELISA technique after one or two infections with this nematode. Following the first infection, antibody levels in serum did not change materially. After administration of a challenge dose of infective larvae, antibodies of the three immunoglobulin classes in infected animals rose slightly, but this rise appeared later than the fall in the faecal egg counts. In contrast, in abomasal mucosa, IgA anti-larval antibody levels, which did not increase materially after the primary infection, rose rapidly after a transient inhibition when sheep were challenged. A close temporal relationship was observed between the rise in local anti-worm IgA antibodies and the self-cure reaction, but antibody levels fell rapidly after worm diminution. The local antibody response was thus considered to be related to immunity of sheep to H. contortus.     

Hypergastrinaemia of sheep infected with Haemonchus contortus

Radioimmunoassay of plasma gastrin has shown that a hypergastrinaemia occurs in yearling sheep, after infection with 10(6) infective third stage larvae of Haemonchus contortus. Plasma gastrin concentrations began to rise two to four days after infection, that is, at about the same time as, or slightly before, the abomasal pH was observed to increase.     

The seasonal prevalence of inhibited development of Haemonchus contortus in sheep in Senegal

Throughout a period of 13 months, the abomasa of 1024 sheep, living in a sahelian region, were examined for adult Haemonchus contortus and their inhibited L4 larvae. Of these 796 (78%) were found harbouring H. contortus, but with low worm burdens (geometric mean 37). Significant numbers of inhibited larvae appeared early in the dry season (November) and they represented 49% of the total in December. A similar proportion of inhibited larvae were present until April and after that their number declined abruptly. It was concluded that Haemonchus sp. survives the dry season as hypobiotic larvae as well as adults.     

Haemonchus contortus with low inhibited development in sheep from the highveld of Zimbabwe

The abomasa of sheep grazing on natural pastures in the highveld of Zimbabwe were examined for Haemonchus contortus. Of 304 abomasa, 213 (70%) harboured H. contortus. The worm burden increased during the rains to reach the peak in February-March. This was followed by a decline with low worm loads throughout the dry season. The fourth stage larvae (L4) accounted for 1-7% of the total H. contortus population, except during July and August when they comprised 22 and 54% of the worm burden, respectively. It appears that the inhibition of H. contortus is not common in the commercial farming sector where sheep are treated very frequently.     

The abomasal histology of worm-free sheep given primary and challenge infections of Haemonchus contortus

The histological changes in the abomasal mucosa of pairs of worm-free ewes and lambs were compared at various stages after infection and reinfection with 350 Haemonchus contortus L3 per kg bodyweight. In terms of worm burdens the ewes were relatively resistant to reinfection compared with the lambs and the histological changes in the abomasum at most stages after infection were more marked in the adult animals. These included increases in the numbers of mast cells, globule leucocytes, eosinophils and IgA plasma cells in the mucosa.     

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.     

Effectiveness of strategic anthelmintic dosing in controlling Haemonchus contortus infections in sheep in the United Kingdom

The strategic use of anthelmintics in the control of infections of Haemonchus contortus in ewes and lambs, was investigated in a series of paddock trials. The levels of infection and clinical signs associated with the presence of either benzimidazole-resistant or benzimidazole-susceptible strains of H contortus in lambs were controlled by regular drenching with levamisole or mebendazole, respectively, or by the strategic use of closantel in combination with mebendazole. In the latter case, control was achieved by dosing either the ewes in the early part of the grazing season, or the lambs from June onwards. It was concluded that worm control strategies based on closantel could provide effective control of both benzimidazole-susceptible and benzimidazole-resistant strains of H contortus on farms in the United Kingdom.     

Effectiveness of clean grazing strategies in controlling Haemonchus contortus infections in sheep in the United Kingdom

The effectiveness of clean grazing systems for controlling the sheep abomasal parasite Haemonchus contortus under experimental conditions was monitored for a grazing season in ewes and lambs carrying either benzimidazole-susceptible or resistant populations of the parasite. Both preventive and evasive strategies failed to give effective control of H contortus. Lambs grazing with ewes that had received no anthelmintic showed clinical signs of haemonchosis by early July. Lambs from ewes dosed with a benzimidazole (mebendazole) before turn out, developed clinical signs of haemonchosis later in the season. The presence of anthelmintic resistant genotypes had a significant effect on the appearance of clinical signs, and on the epidemiology and control of disease. It was concluded that alternative worm control strategies will be required for the effective control of H contortus in the United Kingdom.     

Investigation of Haemonchus contortus infections in sheep. Comparison of irradiated larvae and transfer factor treatment

A comparison is made between the resistance conferred to Haemonchus contortus challenge by irradiated larval treatment and transfer factor (TF) treatment in four-month-old lambs and seven-month-old lambs. As in previous investigations the irradiated larval treatment failed to confer resistance to the four-month-old lambs challenged with 10,000 third stage larvae while similar irradiated larval treatment in seven-month-old lambs reduced the worm burdens by 40 per cent compared to controls. The TF treatment produced a 34 per cent reduction in the challenge infection in the four-month-old lambs and a 45 per cent reduction in the seven-month-old lambs compared to the control lambs. It is concluded that TF activity operates independently of immune competence.     

Serial passage of Haemonchus contortus in resistant and susceptible sheep

A recently isolated field strain of Haemonchus contortus was passaged through resistant (repeatedly reinfected) and susceptible (immunosuppressed) sheep for six and nine generations, respectively. Infectivity of the resulting serially passaged strains was not significantly different when tested in groups of 1-year-old susceptible sheep.