The epidemiology and control of gastrointestinal nematode infestation of sheep and cattle in South Africa. I. The historic role of Onderstepoort and a short discussion of present research priorities

The research activities of the Veterinary Research Institute, Onderstepoort, from its inception in 1908 until recent times are reviewed in this first article of a series on nematode epidemiology of sheep and cattle. While the taxonomic and certain biological aspects of the subject and the testing of anthelmintic compounds have been well covered on the whole in various parts of the country, the testing of control methods under field conditions has largely been neglected. It is suggested that all known methods of integrated worm control should be scrutinized and the most promising methods be tested under practical farming conditions in the country to reduce our dependence on anthelmintic compounds for worm control. Furthermore, unless we are able to employ the available anthelmintics to better advantage and thus reduce the tempo of selection for resistance, the situation may well develop where highly effective remedies are no longer available for worm control.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi. Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format

An apparatus for the restraint of sheep during infestation with schistosomes

With this adjustable apparatus sheep aged from five months to five years and varying in mass from 16-95 kg could be restrained effectively. Cercarial suspension was spilt by only 2/44 sheep during infestation with Schistosoma mattheei. Using a battery of seven cages, it was possible to infest 23 sheep, each for 30 minutes, in one day. No cercariae were more than 4 and a half hours old at the end of exposure.The articles have been scanned in colour with a HP Scanjet 5590; 300dpi. Adobe Acroabt XI was used to OCR the text and also for the merging and conversion to the final presentation PDF-format

Studies on schistosomiasis. 4. Differential staining of live and dead cercariae after immobilization with physostigmin

A modified eosin technique similar to that used for differential staining of live and dead spermatozoa stained dead cercariae of Schistosoma mattheei red while those that were alive remained colourless and transparent. Prior to exposure to the dye live cercariae were immobilized with physostigmin, which was better for that purpose than five other substances tested. The lowest concentration of physostigmin to immobilize cercariae quickly and effectively was found to be 3,3 x 10-⁶ M.The articles have been scanned in colour with a HP Scanjet 5590; 300dpi. Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format

A rapid method for differentiating between the infective larvae of Oesophagostomum columbianum and Chabertia ovina

When freshly-collected, exsheathed infective larvae of Oesophagostomum columbianum and Chabertia ovina were frozen in liquid nitrogen and subsequently thawed, conspicuous vesicles appeared in the intestinal cells of the larvae. The mean number of vesicles, which differed in size according to the species was 20 for O. columbianum and 32 for C. ovina.The articles have been scanned in colour with a HP Scanjet 5590; 300dpi. Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format

A comparison of the infectivity of cryopreserved versus unfrozen infective larvae of Haemonchus contortus, Trichostrongylus colubriformis and Trichostrongylus axei. Results of the Onderstepoort Veterinary Institute and collaborators from 1977 to the present

The infectivity for sheep of cryopreserved infective larvae (L3) of various strains of Haemonchus contortus, Trichostrongylus colubriformis and Trichostrongylus axei is compared using previously published results of trials conducted at the Onderstepoort Veterinary Institute laboratories, and of collaborators. The means and ranges of development were similar for both frozen and unfrozen larvae of two of the three worm species reviewed. A mean of 33,4% (range, 12,7-63,0 %) of cryopreserved H. contortus L3 developed, compared to a mean of 43,7% (range 2,4-78,7 %) of unfrozen L3 of this worm species. The corresponding values for T. colubriformis were 33,0% (range 10,3-62,7%), and 33,5% (range 8,3-52,2 %), respectively. In the case of T. axei, the development of the cryopreserved L3 (tested in only three trials) was markedly lower than that of unfrozen L3 in the single trial in which the latter was evaluated. It is concluded that development of cryopreserved L3 is probably similar to that of unfrozen L3 and that, for several reasons, maintaining nematode larvae in the frozen state in liquid nitrogen is a much superior method to that of one which entails cycling worm strains continually in their final hosts.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi. Adobe Acrobat v.9 was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.mn201

Refugia - overlooked as perhaps the most potent factor concerning the development of anthelmintic resistance

Anthelmintic resistance involving particularly the gastrointestinal nematodes of small ruminants is escalating globally, to the extent that in certain countries, such as South Africa, it has already reached alarming proportions, and is affecting practically all the anthelmintics. In this paper it is argued that the high levels of resistance in nematodes of veterinary importance indicate that the drugs have been used incorrectly. It is suggested that the phenomenon of refugia plays a much more important role in the selection of anthelmintic resistance than other phenomena that are more frequently investigated and recommended for counteracting it, such as reduced drenching frequency and avoiding under-dosing. While refugia is commonly mentioned in passing in most papers on anthelmintic resistance, it is, almost without exception, not incorporated in the final control/management proposals. On the strength of the conclusions arrived at in the present paper strategies such as the drench-and-move system in which all the animals in a flock are drenched before they are moved to pastures containing few or no worms in refugia, and the system of strategic drenching on safe pastures should be condemned and never recommended. If such strategies are indeed unavoidable, the farmer should be warned that the farming system would probably not be sustainable even in the short term, in view of the generally high levels of resistance already present in most of the important sheep-producing regions. Farmers should be educated to consider refugia above all else when designing worm management programmes. Finally there seems to be too much complacency concerning the possibility that anthelmintic resistance may also escalate in cattle, eventually to reach the proportions that it has in sheep.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi. Adobe Acrobat v.9 was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.mn201

Freezing of sheep faeces invalidates Haemonchus contortus faecal egg counts by the McMaster technique

Faecal pellets from a sheep that was artificially infected with a monoculture of Haemonchus contortus were collected over a 2-h period in the morning. In the laboratory the faeces were thoroughly mixed by hand and 48 by 1 g aliquots of the pellets were sealed in plastic bags, from which the air had gently been expressed. The faecal worm egg count of the sheep was about 14 000/g. Varying numbers of the bags were either processed for faecal worm egg counting (FEC) by the McMaster technique on day 0, or were stored at one of the following temperatures: about 4 degrees C, -10 degrees C or -170 degrees C before processing. The faecal aliquots that were frozen were thawed at room temperature after having been frozen for either 2h or 7 days, and processing of aliquots maintained at 4 degrees C proceeded shortly after the samples had been removed from the refrigerator. A dramatic reduction in egg numbers was found in all the aliquots that were frozen at -170 degrees C before faecal worm egg counts were done, as well as in those frozen for 7 days at about -10 degrees C. Numerous empty, or partially empty, egg shells were observed when performing the counts in faeces that had been frozen. In contrast, there was no significant reduction in the numbers of eggs in aliquots maintained for 7 days in a refrigerator at about 4 degrees C before examination, when compared with others examined shortly after collection of the faeces. Since H. contortus eggs in faeces are damaged by freezing, some methods that can be used for short term preservation are outlined. It is concluded that all nematode egg counts from cryopreserved faeces (whether in a freezer at -10 degrees C or in liquid nitrogen) should possibly be regarded as being inaccurate, unless the contrary can be demonstrated for different worm genera. However, exceptions are expected for the more rugged ova, such as those of the ascarids and Trichuris spp.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi. Adobe Acrobat v.9 was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.mn201

A field strain of Haemonchus contortus showing slight resistance to rafoxanide

A field strain of H. contortus, already resistant to benzimidazole anthelmintics, was also found to be slightly resistant to rafoxanide. This is apparently the first report of resistance to rafoxanide in a field strain of H. contortus.The articles have been scanned in colour with a HP Scanjet 5590; 300dpi. Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format

Benzimidazole-resistant Haemonchus contortus-the effect of cryopreservation on the resistance of Successive generations

Infective larvae (L3) of a strain of benzimidazole-resistant Haemonchus contortus were stored in the gas phase of liquid nitrogen, thawed and used to infest worm-free sheep from which, in turn, larvae were cultured, frozen and thawed. This cycle was repeated 5 times. Thereafter, the progeny of the cryopreserved larvae were compared with the progeny of the original untreated larvae for susceptibility to benzimidazole anthelmintics. Repeated freezing of consecutive generations of L3 in liquid nitrogen did not appear to affect their relative resistance to benzimidazoles and, although other strains must also be tested, it would seem that cryopreservation may be used for storing resistant strains.The articles have been scanned in colour with a HP Scanjet 5590; 300dpi. Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format

Survival and development of larvae of the common nematodes of ruminants after long-term cryopreservation and investigation of different routes of infestation

Exsheathed infective larvae (L3) of 16 species of nematodes were tested for infectivity in either sheep or cattle after they had been frozen and stored in 0,09% NaCl solution in the gas phase of liquid nitrogen for periods of up to 59 months. A mean of >90% of the L3 of Haemonchus contortus, Ostertagia circumcincta, Trichostrongylus axei, Trichostrongylus colubriformis, Nematodirus spathiger, Oesophagostomum columbianum and Chabertia ovina of sheep and Haemonchus placei, Ostertagia ostertagi, Cooperia spp. (C. pectinata and C. punctata), Nematodirus helvetianus and Oesophagostomum radiatum of cattle was alive when they were thawed, after having been frozen for 52-59 months. These L3 as well as those of Marshallagia marshalli and Trichostrongylus falculatus, which had been frozen for 30-33 months, were infective to sheep or cattle when dosed per os or inoculated into the abomasum or the duodenum. Thawed Dictyocaulus filaria L3, frozen for 31 months, developed poorly when injected intravenously into sheep. This appears to be the first report showing infectivity of L3 of O. circumcincta, T. colubriformis, N. spathiger and, possibly, of O. radiatum by the oral route after cryopreservation in liquid nitrogen.The articles have been scanned in colour with a HP Scanjet 5590; 300dpi. Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format