Experiment 3: Natural infection study Fig. 5. illustrates the average number of A. galli (adults, larvae and total number) found at the time of slaughter in the group of chickens from the Stalosan F - treated pen compared to the group from the non-treated pen (control). On average, statistically significant (P < 0.05) more adult worms were found in the control group. In contrast however, the average number of larvae were found to be significantly higher (P < 0.05) in the Stalosan F - treated group. The number of all recovered worms (adults + larvae) were almost the same in both groups (the two groups to the right), with an average of 8.6 ± 1.0 in the control group and an average of 8.3 ± 1.3 in the chickens from the Stalosan F - treated eggs. No difference in weight gain was found between the two groups of chickens (P > 0.05). Discussion The observed loss of between 58 % and 83 % of the incubated parasite eggs during the in vitro experiments probably occurred during the procedure of washing the eggs free of the disinfectant or the sand, rather than during the incubation time as the washing procedure involved several steps where eggs could be lost. Some might have been impossible to wash out of the agar in the Petri dish and others might have been lost during centrifugation. Although between 50 % and 75 % of the total number of recovered eggs were dead or appeared abnormal after exposure to Stalosan F a similar proportion of the eggs in the control groups also died or became abnormal during exposure to commercial bird sand. Furthermore, the only time significant difference was found, the proportion of normal appearing eggs was higher in the Stalosan F -treated group. Additionally, time of exposure did not have an effect as the proportion of normal appearing eggs did not change during the 3 weeks of incubation. Thus, in vitro application of Stalosan F apparently did not have an effect on parasite eggs either originating from the uteri of female A. galli worms or from faeces with a mixture of A. galli , H. gallinarum and Capillaria spp. However, when the infectivity of the batch of Stalosan F - treated mixed parasite eggs was evaluated by inoculating chickens, we found that infection was only established in two chickens out of twenty- five, each harbouring only one A. galli . In contrast, significant more worms established in the control group, which received non-treated eggs. This suggests that, although the in vitro examination could not detect any difference between treated and non-treated eggs, the exposure to Stalosan F may have had a sub-lethal effect on the parasite eggs which was reflected in a lower infectivity. However, the average worm burden was also lower at slaughter in the control group than normally seen with similar infection doses (Permin et al. , 1997b). In the natural infection study we introduced tracer animals into two pens contaminated with A. galli eggs. The chickens from the Stalosan F - treated pen were found to harbour significantly fewer adult worms than the chickens from the untreated pen. However, we also found that the Stalosan F - group harboured significantly more larvae and when comparing the total number of worms (larvae and adults) recovered, no difference between the two groups was found. It therefore seems that the development from larvae to adult worm was arrested in the chickens from the Stalosan F - treated pen. Whether or not these larvae would develop into adults at a later stage remains uncertain, but even a short period of arrested development could be of importance in the epidemiology of the worms, since this would decrease the reproduction rate of the worms hereby reducing the number of parasite eggs in the pen. The average number of A. galli recovered at the time of slaughter in the natural infection study was relatively low in both groups, indicating that the pens might have been less contaminated than expected. It is therefore not surprising that no differences in weight gains were found between the two groups of chickens. However, it is generally agreed that severe infections with parasites such as A. galli causes impaired feed conversion resulting in reduced growth and egg production. It is thus possible that a stronger contamination of the pens with A. galli eggs would have resulted in a difference in weight gain between the two groups of chickens. Management practices largely determine the extent of helminthosis in chickens (Morgenstern and Lobsiger, 1993; te Winkel, 1997). Total enclosure, improvement of cleaning, disinfection procedures and production according to the “all in - all out” principle have decreased the significance of helminth infections in the modern industrialised poultry production (te Winkel, 1997). With the ban on battery cages, new free-range systems have developed in which the prevention of helminth infections has proved to be difficult (Permin et al., 1999). The use of outdoor areas, where parasite eggs may persist in the environment for years have increased the risk of helminth infections (Ackert, 1931). Management practices including alternate use of the pen and strict disinfection of the house might subsequently reduce problems with helminth infections (Permin and Han-sen, 1998).
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