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PROSPECTS FOR THE UTILIZATION OF VARIATION IN PARASITE RESISTANCE

AMONG INDIVIDUAL SHEEP WITHIN A FLOCK  

  

M.A. Snyman 


Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900

Email: Gretha Snyman

 


INTRODUCTION

Internal parasites in livestock are becoming a serious problem, especially under conditions of intensive production and in higher rainfall regions. Apart from the fact that severe infections of these parasites can cause actual stock losses, it often leads to less visual losses such as a decrease in milk production of more than 22% (Muller, 1966), decreased wool production in sheep of between 8% and 40% (Muller, 1966; Horak et al., 1976) and decreased weight gain of 20% and 40% at weaning and nine months of age respectively (Snijders et al., 1971). According to Gray & Raadsma (1986), up to one in seven bales of wool could be lost because of internal parasites. This infection also increases the animal's susceptibility to other diseases (Muller, 1966). Hence, parasites should be controlled effectively to prevent economic losses in livestock production.

 

Over the last couple of decades new and better drugs have been developed and these have often been used as the only means of control against gastro-intestinal worms in sheep.  There is, however, growing evidence that worm strains have gradually developed a natural resistance to existing groups of chemicals. This is caused by the prolonged use of one chemical group at frequent intervals, or by continuous under-dosing (Van Wyk et al., 1989; Van Wyk et al., 1991). Surveys of anthelmintic resistance suggest that South Africa may be one of the world’s worst affected countries. 

 

The threat and reality of anthelmintic resistance have focused attention on sustainable parasite control and the development and implementation of systems that do not rely on drenching alone to achieve control. While some animals may die during an outbreak of worm infection, others are unaffected.  At this stage all animals are drenched, although many of them do not need treatment.  Worm eggs excreted thereafter by animals are of worm strains resistant to anthelmintics and selection for resistance begins. Only 20% of the animals in a flock carry 80% of the worms. If these 20% susceptible animals can be culled, a considerable advantage can be gained with regard to worm control. 

 

Parasite Resistance in Host Animals

The breeding of sheep that require minimal anthelmintic treatment is one of several options to manage anthelmintic resistance. During the past decades the possibility of selection for animals resistant to internal parasites has been investigated and selection programmes based on faecal egg counts have been implemented in Australia and New Zealand (McEwan et al., 1995).

 

A proportion of the individual animals in a flock are less affected by internal parasites, as they are either resistant or resilient to the effects of the parasites. Resilience is the ability of the animal to maintain production levels during infection. Resistance refers to the ability of the animal to reduce the number of worms in the gut, by reducing the establishment of incoming larvae, arresting or delaying larval development, accelerate expulsion of adult worms and reduce fecundity of female worms.

 

Incidence of Host Resistance

Genetic variability of host resistance to internal parasites has been recorded among different sheep breeds. Mugambi et al. (1997) found that following infection with Haemonchus contortus, sheep of the Red Maasai produce fewer eggs and have lower mortality than sheep of the Dorper and Blackheaded Somali breeds.  The Red Maasai was also substantially more resistant than the Romney Marsh. Consistent with these findings, Baker et al. (1999) indicated the Red Maasai to be more resistant to endoparasites than Dorper sheep. Wanyangu et al. (1997) found faecal egg counts in Red Masaai sheep to be significantly lower than in the Dorper during both artificial and natural challenge.

 

Within breed differences in parasite resistance also exist and this enabled the selection of lines with improved resistance (Hooda et al., 1999). Eady et al. (1996) indicated that there is little genetic variation in nematode resistance between Merino strains and bloodlines.  The major source of genetic variation for faecal egg count is found within bloodlines, with individual sires showing a wide range in resistance in their progeny.

 

Resistance to one species of internal parasites appears to confer a degree of cross-protection to other species of parasites, despite evidence of differences in the immune mechanisms contributing to resistance to individual species of internal parasites in sheep (Colditz et al., 1996).

 

Measurement of Resistance

For practical reasons, apart from being strongly correlated with parasite resistance, the parameter/s used as indicator of parasite resistance should be easy to measure or sample and its assay should be inexpensive (Douch et al., 1996). There are various ways to determine internal parasite burdens in living animals. Of these, faecal egg counts and degree of anemia are the most commonly employed.

 

Faecal Egg Counts

Comparing the faecal egg count of individual sheep with that of their contemporaries, gives an indication of resistance. An egg count of 500 eggs per gram (epg) means nothing in itself in terms of resistance. An animal with a faecal egg count of 500 epg could be resistant if the mean for the group is 1500 epg or susceptible if the mean is 250 epg.  The relative faecal egg count of an animal is very important in worm resistance because of the massive variation in faecal egg count from area to area or year to year. Faecal egg counts is extremely variable, with coefficients of variation often in excess of 100% and has a moderate heritability of 0.2-0.3 (Gray et al., 1995).

 

Degree of Anemia

An important clinical sign of parasitic infection for some gastro-intestinal nematodes is the degree of anemia that can be measured by estimating haematocrit. Haematocrit is a measure of the volume occupied by the red blood cells in relation to the total volume of circulating blood. With H. contortus infection, haematocrit is a trait equally heritable as FEC and strongly correlated with resistance (Woolaston & Piper, 1996).

 

Anemia has also been suggested as an alternative for supporting selection criteria along with FEC. The most practical method for a quick evaluation of the anemic status of a flock is by inspecting their eye mucous membranes.  Unless animals suffer from eye infection, the colour of their membranes roughly corresponds with the respective haematocrits. Correlations between the clinical classification of the state of anemia in sheep and haematocrit are highly significant.

 

The FAMACHAã technique enables quick detection of anemic individuals that may require drenching. Results with the evaluation of the FAMACHAã technique on commercial farms indicated that the efficacy of clinical anemia assessment was adequate to be implemented as guide to identify animals that require anthelmintic treatment (Malan et al., 2000).

 

Data recorded in a project that involves the development of a protocol for the evaluation of internal parasite resistance of breeding sires, were analysed to investigate the relationship among faecal egg counts, haematocrit and FAMACHAã-score.

 

MATERIAL AND METHODS

Experimental animals

The project started in March 1998. Since then, lambs in the Afrino flock at the Carnarvon Experimental Station were infected annually with third stage H. contortus larvae at 6-8 months of age for data collection under artificial challenge procedure.

 

Artificial Challenge Procedure

Culturing of larvae

For the practical implementation of the artificial challenge procedure, it is essential to use a dependable population of anthelmintic susceptible H. contortus larvae, in order to avoid infecting susceptible farms with resistant populations. Third stage larvae of a susceptible population of H. contortus were used. Five donor animals, kept under conditions that preclude unintentional infection with worms, were treated with anthelmintics and faecal egg counts done to ensure that the animals were as far as possible free of internal parasites. These five animals were infected with the larvae obtained from Onderstepoort Veterinary Institute, and faeces were collected for culturing of larvae to be used for the artificial infection of experimental animals.

 

Artificial infection

Experimental animals for the artificial challenge were drenched daily with Levamisole at 15 mg/kg live weight and Trichlorfon at 62.5 mg/kg for two consecutive days before artificial infection to ensure that they were free from natural H. contortus infections. The animals were then infected with 4000 to 6000 third stage H. contortus larvae, administered in three equal doses over three days. Twenty-eight days after artificial infection, faecal egg counts (FEC), haematocrit (HEMA) and FAMACHAã-score (FAM) were done for each animal, according to the procedures described below. After faecal sampling, the animals were treated with an effective anthelmintic.

 

Data collection

Haematocrit

One blood sample per animal was collected by venipuncture into heparinized vacuum tubes. Haematocrit was measured by means of micro-centrifugation on an aliquot of this blood sample.

 

Faecal sampling

At least 5 g of faeces was collected directly from the rectum of each lamb. Faecal samples were placed in individual plastic bags and immediately placed on ice for transportation to the Provincial Veterinary Laboratory at Grootfontein for faecal egg counts.

 

FAMACHAã-score

FAMACHAã-score was done according to the method described by Van Wyk et al. (1997) and Malan et al. (2000).

 

Analysis of data

Data collected from 1998 to 2006 on the Afrino flock at the Carnarvon Experimental Station under artificial challenge procedure were analysed. Proc GLM-procedures of SAS (SAS, 1996) were used to obtain least-squares means for FEC. The frequency distribution of animals and their average HEMA and FEC over the five FAMACHAã-categories were calculated for the overall data set.

 

RESULTS AND DISCUSSION

Means for artificial challenge FEC and FEC range for the different tests carried out over the experimental period in the Carnarvon Afrino flock are presented in Table 1. It is obvious that there were large differences in FEC between years/tests. For example, there were a lot of animals that had a zero FEC in 1998, compared to the high FEC’s in 1999 and 2001 (Test 1). It is obvious that there were large differences in actual FEC between animals within each test. In a test with a low average FEC, there usually are also a lot of animals with zero FEC. This makes it difficult to distinguish between animals with high or low resistance. It is important that the level of infection is high enough for differences among animals to be expressed.

Table 1. Means (±s.e.) for artificial challenge FEC and FEC range for the different tests carried out over the experimental period in the Carnarvon Afrino flock

Year of birth

FEC±SE

FEC range*

n**

1997

1598±490

0 (2) – 14800

216

1998

 329±500

0 (41) – 2400

236

1999

15665±520

0 (5) – 87800

201

2000

2944±498

0 (8) – 12100

225

2001 (Test 1)

10597±496

0 (22) – 76000

264

2001 (Test 2)

3475±497

0 (10) – 20200

264

2001 (Test 3)

3149±500

0 (11) – 16400

252

2002

2263±500

0 (4) – 12400

251

2003 (Test 1)

 2059±471

0 (18) – 19200

303

2003 (Test 2)

1357±473

0 (40) – 13000

294

2004

1351±489

0 (3) – 6700

291

2005

3240±511

0 (5) – 7900

218

* Range of FEC within each test (value in brackets is number of animals with 0 epg)

** n = number of animals in group

 

Relationship among FEC, FAM and HEMA

The distribution of animals and their average HEMA and FEC over the five FAM-categories are illustrated in Figure 1 for all the artificial challenge tests done over the experimental period on the Afrino lambs at Carnarvon. It is obvious that there is a definite relationship between FEC, HEMA and FAM, where animals with higher FEC had higher FAM-scores and lower haematocrit values. These relationships are also evident from the estimated correlations among these traits (Table 2).

 

Table 2. Heritabilities of, and genetic and phenotypic correlations among FEC, FAM and HEMA

 

FEC

FAM

HEMA

FEC

0.19±0.03

0.29±0.13

-0.67±0.14

FAM

0.29

0.17±0.03

-0.50±0.15

HEMA

-0.41

-0.49

0.19±0.03

 


Figure 1. Distribution of Carnarvon Afrino lambs over FAM categories

(Average of all artificial challenge tests over the experimental period)

 

When data of all the different tests carried out over the experimental period were considered, the following general trends could be observed:

 

CONCLUSIONS

Apart from the results presented above, the following conclusions with regard to the proposed protocol for the evaluation of sires on breeding values for parasite resistance, could be made at this stage of the project:

  1. For the purpose of developing a protocol to evaluate breeding values for resistance of sires born in the extensive sheep grazing areas, breeding values for parasite resistance based on the Famacha©-system, will not be feasible. The reason for this is that the recording period is too long before differences between animals are expressed.
  2. The data collected during this study, however, confirm that the Famacha©-system is an excellent management tool to identify animals that need anthelmintic treatment.
  3. Although HEMA has a high genetic correlation with FEC, it is more difficult and expensive to measure, while other factors could also influence the anemic status of an animal. Therefore, HEMA as such would not be a feasible selection criterion as indicator of resistance.
  4. For the purpose of developing a protocol to evaluate sires born in the extensive sheep grazing areas, FEC after artificial challenge procedure seems the best option as selection criteria for resistance against internal parasites. The following should be taken into account:

 

ACKNOWLEDGEMENTS

The author wishes to convey her sincere appreciation to the following people for their valuable contribution in the execution of the project: M van Heerden and farm aids at Grootfontein, A Karstens from the PVL- Middelburg and T. Buys and Farm Aids at the Carnarvon Experimental Station

 

REFERENCES

Baker, R.L., Mwamachi, D.M., Audho, J.O., Aduda, E.O. & Thorpe, W., 1999. Genetic resistance to gastro-intestinal nematode parasites in Red Maasai, Dorper and Red Masaai X Dorper ewes in the sub-humid tropics. Anim. Sci. 69, 335-344.

Colditz, I.G., Watson, D.L., Gray, G.D. & Eady, S.J., 1996. Some Relationships between Age, Immune Responsiveness and Resistance to Parasites in Ruminants. Int. J. Paras. 26(8/9), 869-877.

Douch, P.G.C., Green, R.S., Morris, C.A., Mcewan, J.C. & Windon, R.G., 1996. Phenotypic Markers for Selection of Nematode resistant Sheep. Int. J. Paras. 26(8/9), 899-911.

Eady, S.J., Woolaston, R.R., Mortimer S.I., Lewer, R.P., Raadsma, H.W., Swan, A.A. & Ponzoni, R.W., 1996.  Resistance to nematode parasites in Merino sheep: Sources of genetic variation. Aust. J. Agric. Res. 47, 895-915.

Gray, G.D. & Raadsma, H.W., 1986. Resilience to Haemonchus contortus infection in crossbred lambs between Merino strains and bloodlines. Proc. 3rd World Congr. Appl. Livest. Prod. Vol. XI.

Gray, G.D., Woolaston, R.R. & Eaton, B.T., 1995.  Breeding for Resistance to Infectious Disease in Small Ruminants. Australian Center for International Agricultural Research Monograph Series No. 34.

Hooda, V., Yadav, C.L., Chaudhri, S.S. & Rajpurohit, B.S., 1999.  Variation in resistance to haemonchosis: selection of female sheep resistant to Haemonchus contortus.  J. Helminth. 73, 137-142.

Horak, I.G., Honer, M.R. & Schröder, J., 1976. Live mass gains and wool production of Merino sheep: Three treatment programmes for parasite control. J. S. Afr. Vet. Ass. 47, 247-251.

Malan, F.S., Van Wyk, J.A. & Wessels, C.D., 2000.  Clinical Evaluation of Anaemia in Sheep; Early Trials.  FAO TCP Workshop : Sustainable Worm Control Programmes for Sheep and Goats,  Onderstepoort Veterinary Institute, 12-14 June 2000, 34-39.

Mcewan, J., DODDS, K.G., WATSON, T.G., GREER, G.J., HOSKING, B.C. & DOUCH, P.G.C., 1995. Selection for host resistance to roundworms in the New Zealand sheep breeding industry : the WormFEC service. Proc. Austr. Ass. Anim. Breed. Gen. 11, 70-73.

Mugambi, J.M., Bain, R.K., Wanyangu, S.W., Ihiga, M.A., Duncan, J.L., Murray, M. & Stear, M.J., 1997. Resistance of four sheep breeds to natural and subsequent artificial Haemonchus contortus infection. Vet. Parasitol. 69, 265-273.

Muller, G.L., 1966. Die gesondheidsprobleme van diere met betrekking tot verhoogde skaap- en wolproduksie in Wes-en Suid-Kaapland. In: Die Skaap en sy Vag..

SAS INSTITUTE. INC. 1996. SAS Procedures Guide, Version 6, 3rd Ed., Cary, NC : SAS Institute. Inc.

Snijders, A.J., Stapelberg, J.H. & Muller, G.L., 1971. Low level Thiabendazole administration to sheep. I. Susceptibility of medicated sheep to natural infestations to Outeniqua. J. S. Afr. Vet. Ass. 42, 155-167.

Van Wyk, J.A., Malan, F.S. & Bath, G.F., 1997. Rampant anthelmintic resistance in sheep in South Africa – what are the options? In: managing anthelmintic resistance in Endoparasites. Workshop held at the 16th International Conference of the World Association for the Advancement of Veterinary Parasitology, Sun City, 10-15 August 1997, 51-63.

Van Wyk, J.A., Malan, F.S., Gerber, H.M. & Alves, R.M.R., 1989. The problem of escalating resistance of Haemonchus contortus to the modern anthelmintics in South Africa. Onderstepoort J. Vet. Res. 56, 41-49.

Van Wyk, J.A., Van Schalkwyk, P.C., Bath, G.F., Gerber, H.M. & Alves, R.M.R., 1991. The threat of wide dissemination of anthelmintic resistance by veld ram performance testing units. J. S. Afr. Vet. Ass. 62, 171-175.

Wanyangu, S.W., Mugambi, J.M., Bain, R.K., Duncan, J.L., Murray, M. & Stear, M.J., 1997. Response to artificial and subsequent natural infection with Haemonchus contortus in Red Maasai and Dorper ewes.  Vet. Parasitol. 69, 275-282.

Woolaston, R.R. & Piper, L.R., 1996.  Selection of Merino sheep for resistance to Haemonchus contortus: genetic variation.  Anim. Sci. 62,  451-460.

 

Published

Grootfontein Agric 7 (1), 29-34