Irradiated vaccines for helminth control in livestock.

For nearly 40 years, irradiated larval vaccines have been available for the control of parasitic bronchitis in cattle and sheep caused by Dictyocaulus spp. Despite research on a number of other host/parasite systems, no other vaccines have been commercially successful. Vaccination could provide a useful addition to other control methods in an integrated parasite management system where the criteria for vaccine success may not be complete control and sterile immunity, but a sufficient reduction in worm burden to decrease overall reinfection levels at the flock/herd level and, hence, prevent clinical disease and subclinical effects including production loss. Indeed, vaccination against Dictyocaulus spp. relies on continued natural infection to maintain levels of immunity. However, the difficulties of producing live larval vaccines are often cited as a reason why this line of research should not be pursued. This paper discusses some of the difficulties in vaccine production and offers some solutions and recommendations for those wishing to develop and register irradiated larval vaccines for other helminth diseases.

Availability of Haemonchus contortus L3 larvae on pasture at Kiboko: a semi-arid warm agro-climatic zone in Kenya.

A study was carried out to determine the availability of Haemonchus contortus L3 larvae on pasture in a semi-arid warm agro-climatic zone of Kenya. By means of tracer sheep, it was shown that no H. contortus L3 larvae were available on pasture during the dry periods of the year (July-October and February). They were only available on pasture during the rainy season (November-January and March-June). Sheep permanently grazed on the same pasture however, harboured adult H. contortus in their abomasa throughout the year indicating that the perpetuation of haemonchosis in livestock in this agro-climatic zone was greatly dependent on the ability of the parasite to survive in the host throughout all seasons.

Resistance of four sheep breeds to natural and subsequent artificial Haemonchus contortus infection.

The response of Red Maasai sheep to natural and artificial Haemonchus contortus infections was compared with sheep of Blackheaded Somali, Dorper and Romney Marsh breeds. Significant breed differences in egg count, packed cell volume (PCV), and mortality rates showed that the Red Maasai sheep were more resistant to natural H. contortus infection than sheep from the other three breeds. Of the initial groups of 15 wethers of each breed, two animals from each of the Dorper and Blackheaded Somali groups and nine from the Romney Marsh group died with haemonchosis during a 12 month field study. Following artificial infection of the Red Maasai, Dorpers and Blackheaded Somalis, with 10000 H. contortus L3, the Red Maasai sheep maintained a lower egg output and a higher PCV than animals of the other two breeds. The results clearly showed that breed substitution with the Red Maasai is a control option in areas where sheep are kept for meat and H. contortus is endemic.

Response to artificial and subsequent natural infection with Haemonchus contortus in red Maasai and Dorper ewes.

Maiden Red Maasai and Dorper ewes were kept indoors and artificially infected with a single oral dose of 5000 infective larvae of Haemonchus contortus. Their faecal egg counts (FEC) and packed red cell volumes (PCV) were monitored for 9 weeks. They were then treated with an anthelmintic and turned out to graze together on a pasture contaminated with H. contortus. They grazed this pasture for 14 months and were allowed to mate and lamb. While at pasture the ewes were monitored for FEC, PCV and peripheral eosinophilia. Red Maasai ewes had significantly lower FEC, and for certain periods, significantly higher PCV and peripheral eosinophilia. During the periparturient period, FEC were about twice as high in the Dorper breed as the Red Maasai. These results confirm and extend previous reports on the superiority of the Red Maasai breed in East Africa.

Parenteral vaccination of calves against the cattle lungworm Dictyocaulus viviparus.

Protection against challenge with Dictyocaulus viviparus larvae was studied in two groups of calves. The first group was vaccinated orally with an irradiation-attenuated larval vaccine on two occasions, 28 days apart, as recommended by the manufacturer. Each dose contained 1000 larvae. The second group was vaccinated by two subcutaneous injections of vaccine, also 28 days apart. Compared with unvaccinated calves the protection in both groups of vaccinated calves was similar, ie, over 95 per cent reduction in adult worm burdens after an oral challenge of 3000 to 4000 larvae. These results indicate that the passage of irradiated larvae through the intestine and mesenteric lymph nodes is not necessary for the stimulation of a high degree of immunity and opens up the possibility of parenteral vaccination against this and related diseases.

Acute phase response in calves following infection with Pasteurella haemolytica, Ostertagia ostertagi and endotoxin administration.

The concentrations of bovine acute phase proteins were monitored in plasma following experimental infection with Pasteurella haemolytica and Ostertagia ostertagi and after endotoxin administration. Raised levels of haptoglobin, alpha 1 proteinase inhibitor and seromucoid were detected after pasteurella infection and endotoxin administration. Ceruloplasmin levels increased after endotoxin administration but not during pasteurella infection. Raised levels of the four acute phase proteins were found in eight of 19 calves infected with ostertagia but showed a variable pattern and did not correlate with plasma pepsinogen increases. Bovine alpha 1 antichymotrypsin and alpha 2 macroglobulin were identified as acute phase reactants.

Epidemiology of parasitic gastrointestinal nematode infections of ruminants on smallholder farms in central Kenya.

In order to establish the infection pattern with gastrointestinal nematodes in ruminants in the central Kenya highlands, a study was carried out in 58 smallholder farms. The study involved monthly faecal examinations from sheep, goats and cattle and pasture sampling from eight communal grazing areas. Each month, six Dorper worm-free tracer lambs were introduced and four locally grazed cross-bred sheep were purchased for parasite recovery. The mean faecal egg counts (FEC) for cattle were low throughout the study period, whereas those for sheep and goats showed a seasonal pattern with high levels of infection occurring during the two main rainy seasons, especially in March, April and October. There were significant differences in egg counts over time and among farms. Haemonchus contortus was the most prevalent nematode in the tracer lambs whereas the previously exposed locally grazed sheep had significantly lower numbers of H contortus but significantly higher numbers of Trichostrongylus species The highest levels of infection in the tracer lambs occurred in November 1995 and January, May and June 1996. Based on this study, it is now possible to explore the possibility of using strategic treatments for the control of parasitic gastroenteritis in this area of Kenya.

Response of Dorper and red Maasai lambs to trickle Haemonchus contortus infections.

Six-month-old red Maasai lambs were more resistant than Dorper lambs to repeated infections at one to two week intervals with 1000 Haemonchus contortus infective larvae. Resistance after infection was assessed by means of faecal egg counts, packed cell volumes, eosinophil counts, total serum protein concentrations and mortality rates. The weight gains of the infected animals were only marginally lower than those of their uninfected controls, most probably because of their significantly higher feed consumption, and evidently the infected lambs were not utilising all of the extra feed for growth. This absence of anorexia in spite of the infection was probably due to the palatability of the high protein diet fed to the lambs.