A Resume Of Investigations On Nematode Parasitism In Sheep In New Zealand
Greatest activity in the field of parasitology, has taken place in those countries possessed of a large and varied vertebrate fauna. Perhaps, therefore, it is not surprising that in New Zealand, with its paucity of Amphibia, Reptilia, and Mammalia, the subject should have attracted little attention from zoologists. It is a matter for regret that, when host material was more readily available, the opportunity was not taken to study its parasitism, for our unique fauna might have provided knowledge, in this connection, of great interest to science.
Although this particular field for parasitological study has been almost lost to science, another field of economic importance has been created in New Zealand through the introduction of domestic animals. So far as sheep are concerned, following their introduction, flocks multiplied rapidly as the forest was converted to grassland and coincidentally the problem of parasitism developed, and for more than half a century has assumed considerable economic proportions. All the species of parasites were brought inadvertently with their hosts, either directly from Britain, or indirectly from there by way of Australia. Probably many of the species now found in this country arrived here with the early importations of sheep.
It appears strange that, though parasitism of sheep has been recognised as a major problem in New Zealand for two or three generations, and though attempts at control have been obviously inadequate, the study of the subject should have been neglected. There seems to have been no appreciation that it is only by understanding the factors which determine the pitch of parasite numbers that known anthelmintics can be applied to the limit of their potentiality, or that other methods of control can be devised.
In taking up the study of parasitism in sheep the writer has adopted a zoological, mainly ecological, approach, because such approach appeared most likely to contribute towards an understanding of the mechanism of the epidemiology of parasitism.
To begin with, a survey(1) was made of the parasites of sheep. Twenty-five species of nematodes were found, several of these being recorded from New Zealand for the first time. The range of species was similar to that found in sheep in other temperate countries. Since nematodes are so intimately associated with their hosts it is to be expected that they had no difficulty in accompanying
their hosts to this country, particularly as no attempts were made to prevent them from gaining a footing.
While conducting the survey of parasites it became apparent that there were rhythms in nematode populations and, from data collected at random in the Manawatu District, some idea of their nature was obtained.(1, 2) In order to study the epidemiology of parasitism more closely the writer established a series of epidemics in small flocks of sheep over which he had a greater degree of control than was practicable under normal farming conditions. From the results obtained it has been possible to piece together a portion of the framework of the underlying causal factors.(2, 4, 9)
The technique of experimentation, in some instances, was to observe the course of parasitism by estimating the numbers of eggs of the various species of parasites found in faeces. This enabled the continued observation of experimental animals, but it involved the problems of the identification of eggs of trichostrongylids as well as the estimation of their numbers. As no method for the identification of trichostrongylid eggs in sheep, which would enable the compilation of a record for comparative purposes, existed, the writer was compelled to devise his own.(2, 5, 7, 9) That no suitable method should have existed previous to this investigation is indicative of the state of knowledge of the epidemiology of nematode parasitism in sheep.
The question arose as to the extent that the populations of parasites contributed to the numbers of eggs found in faeces, at particular points of time. Nematodirus spp. were chosen for experimentation. The method of study was to kill worms freshly removed from sheep with, heat, and to examine the state of segmentation of eggs found in utero. From the results it was concluded that at any time during the period of virile parasitism the whole population of females, with few exceptions, contributed to the egg output, though fluctuations occurred in the tides of egg production. The corollary followed that populations of worms in sheep could not occur without their presence being indicated by eggs appearing in the faeces.
In some experiments post-mortem, examinations of sheep were made. While in these instances accurate data were obtainable on the number of parasites present. the usefulness of the hosts came to a close with killing. A combination, therefore, of the methods of estimating egg numbers in faeces and post-mortem examination was adopted in most experiments.
An initial epidemiological experiment took the form of isolating a small flock of sheep in a paddock, permitting the nematodes to multiply naturally and parasitism to run its course. Regular faecal sampling for egg-counting purposes was carried out on those animals for a number of years and a picture of the parasitism was obtained by analysis of the data. From this experiment(2) and others(1, 9) the rhythmical nature of unimpeded parasitism was clearly indicated. The results for two genera found in these experiments. Nematodirus and Haemonchus, may serve as illustrations of the sort of knowledge that has been gained from this work. Nematodirus was found in sheep mainly under nine months of age; older animals were resistant to parasitism if they had previously passed trough a cycle of infection; free-living stages of this parasite genus were able to survive as long as seven months on pasture that was heavily grazed by resistant sheep, and at the end of this period were able to parasitize lambs in the spring. It was concluded that Iambs are the main factor in contaminating pasture with this genus for lambs of the succeeding year. Parasitism by IIaemonchus differed in a number of respects from that by Nematodirus. Reinfection by the former was possible, but, as shown above, the writhe was unable to demonstrate this for the latter genus; it is not certain that the Haemonchus infection can occur to sheep throughout the year, as with Nematodirus.
In order to demonstrate the role of sheep of different age-groups in the epidemiological cycle, an experiment was carried out in which sheep of three age-groups were used. Ewes, together with their newly-born lambs and their lambs of the previous spring, were isolated in a paddock and by periodical slaughterings the entire flock was examined post mortem for their populations of parasites. A succession of parasites was found in which each age-group of sheep displayed a characteristic cycle of parasitism.
The foregoing experiments gave some indication that breeding ewes played a part in the epidemiological cycle in lambs. To test this an experiment was designed in which the extent that ewes contaminated pasture with eggs was determined. From the beginning of pregnancy until after weaning of lambs
the faeces of the ewes were periodically sampled for egg-counting purposes. Among other results(6) it was found that the ewes conveyed increased numbers of eggs of Ostertagia and Haemonchus to the ground during lactation. Since Ostertagia, is prone to infect lambs in large numbers from soon after birth, it would appear that the lactating ewes of this experiment were significant vectors of this species. In regard to Haemonchus it is probable that the ewes were the initiating cause of parasitism in lambs, though it would seem that the large autumn populations of this species that may be found in lambs must be the result of a self-augmentation process of parasitism, arising mainly from lamb-conveyed contamination of pasture.
The question arose as to the mode of accumulation of populations of parasites in their hosts, and experiments planned to show this were carried out. So far as Nematodirus, Haemonchus and Ostertagia, were concerned it was found that the parasites increased rapidly in numbers, but that development of resistance to super-infection intervened after a comparatively short period of exposure to risk of parasitism. The writer could not secure evidence in support of a popular belief that parasites are continually being acquired and lost, or that in the aggregate many more come to maturity in sheep than ever are found at postmortem, examination.
In order to obtain information upon the epidemiology of parasitism in larger flocks and at the same time to obtain a guide as to the comparative value of epidemiological facts, the course of parasitism in two neighbouring flocks of lambs was studied.(9) Essentially the two epidemics resembled one another and gave support to the view that there was a reasonable chance that results demonstrated in one locality may be of comparative value over an extended area of similar ecological character.
The elimination of parasites from a single sheep, demonstrated by examination of faeces for worms, was studied in the belief that some understanding might be obtained of the manner in which a cycle of parasitism came to a close and indirectly of the way in which the population of parasites was built up. A noteworthy fact from this work was that the elimination of a number of species was able to take place coincidently, thus indicating that some non-specific factor operated. A further fact, possibly bearing on the mode of accumulation of parasites, was that the total numbers of the various species found in this animal were comparable to those found in individuals at autopsy; if this sheep be accepted as a valid index, it would seem that there could not have been a continuous process of turnover of mature parasites, otherwise much larger numbers would have been found.
Aspects of the epidemiological studies have from time to time required testing under specially controlled experimental conditions. Thus the nature of resistance to Haemonchus has been the subject of experimentation in which sheep that have been raised indoors free from previous exposure to parasitism have been used. Such anomalous results have been obtained that it is evident all aspects of this problem are complex and are a fertile field for investigation.
Incidental investigations have been made in conjunction with the foregoing experiments. In two of these(3, 6) the distribution of nematodes in the small intestine of sheep was studied. It was found that a number of species of nematodes of the small intestine of the sheep have a common area of parasitism but that each species has a normal or skew frequency, of which the peaks maintain a particular sequence in reference to each other. The conclusion was drawn that the station of each species was determined by the rate of response of its invading larvae to stimuli contained within the alimentary tract; specific-differences existed in this respect.
1. Tetley, J. H., 1934. J. Helminth, vol. 12. no. 4, pp. 183–196.
2. —— 1935. Ibid., vol. 13, no 1, pp. 41–58.
3. —— 1937. N.Z. J. Sc. and Tech., vol. 18, no. 11, pp. 805–817.
4. —— 1941. Cornell Vet., vol. 31, no. 3, pp. 243–265.
5. —— 1941. J. Parasit., vol. 27, no. 5, pp. 453–463.
6. —— 1941. Ibid., vol. 27, no. 5, pp. 449–452.
7. —— 1941. Ibid., vol. 27, no. 6, pp. 473–480.
8. —— 1941. Ibid., vol. 27, no. 6, pp. 481–491.
9. —— N.Z. Dept. Sc. and Indust. Res. Bulletin (in preparation).