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6. Investigating a possible gastro intestinal parasite problem


6.1 Introduction
6.2 Diagnosing a herd/flock problem
6.3 Long-term monitoring of a herd/flock problem or of a control programme
6.4 Plot experiments


6.1 Introduction

This chapter considers strategies for investigating a possible gastro-intestinal parasite problem in a herd or flock, and methods for applying the techniques described in previous chapters. This will be considered in three stages:

(a) Diagnosing a herd/flock problem.
(b) Long-term monitoring of a herd/flock problem or of a control programme.
(c) Plot experiments.

The units of investigation in the first two cases should be the herd or flock. Where groups of herds are communally grazed, the whole group should ideally be the unit of investigation. Gastro-intestinal parasite problems generally involve an entire herd or flock, and to be effective, diagnosis, treatment and control measures should be directed at the entire herd or flock.

The unit of investigation for plot experiments is a representative and convenient area of pasture in a similar environment to that in which animals are grazed.

6.2 Diagnosing a herd/flock problem


6.2.1 Sampling of live animals
6.2.2 Sampling of dead (moribund or sacrificed) animals


As many animals as possible, including those showing clinical signs, should be examined, and any abnormalities, such as diarrhea and anaemia, recorded.

To understand the significance of the gastro-intestinal parasite problem in a particular group of animals, samples should be taken from live animals and dead (moribund or sacrificed) animals.

6.2.1 Sampling of live animals

(a) Which animals should be sampled?

If there are animals with obvious clinical disease suggestive of gastro-intestinal parasitism, these should be sampled. However, it is also important to sample animals suspected of subclinical disease and those that appear healthy in order to fully understand what is occurring in a herd/flock.

(b) How many animals should be sampled?

There is no magic sampling number, but in general, the more animals sampled, the better the understanding of the problem and the greater the validity of the results. Table 6.1 should serve as a guide. It is based on both general principles and practical/logistical constraints.

Table 6.1 SUGGESTED SAMPLE SIZE FOR GIVEN TOTAL HERD/FLOCK NUMBERS

Number in herd/flock

Number to sample

1-10

All

11-25

At least 10 animals

26-100

At least 20 animals

101-200

At least 30 animals

Over 200

At least 15 per cent

Over 500

At least 10 per cent

(c) What samples should be taken?

A faecal sample for a faecal egg count and/or a flotation procedure (see sections 3.3 and 3.4).

A blood sample in anticoagulant for a haematocrit (packed cell volume, PCV; see section 5.3).

6.2.2 Sampling of dead (moribund or sacrificed) animals

(a) Which animals should be sampled?

Any animal which dies of suspected gastro-intestinal parasitism. If a considerable number of the herd or flock are affected, one or two very sick or moribund animals should be sacrificed for examination.

(b) How many animals should be sampled?

As many as possible of those that die. However, economics and other factors will not normally allow more than one or two very sick or moribund animals to be sacrificed for examination.

(c) What samples should be taken?

A total worm count on the abomasum, small intestine and large intestine should be carried out (see section 4.3).

6.3 Long-term monitoring of a herd/flock problem or of a control programme


6.3.1 Sampling of live animals
6.3.2 Sampling of dead (moribund or sacrificed) animals
6.3.3 Sampling of pasture
6.3.4 Sampling of tracer (sentinel) animals


By sampling only the diseased animals as described for the diagnosis of a herd/flock problem, only a limited understanding of the epidemiology will be gained. If circumstances allow, a longer-term monitoring of herds or flocks should be carried out.

Samples should be taken from live animals, dead (moribund or sacrificed) animals, pasture and tracer (sentinel) animals.

Climatic data should also be recorded.

6.3.1 Sampling of live animals

(a) Which animals should be sampled?

The animals to be sampled will normally be healthy animals identified at the beginning of the monitoring period. The same identified animals must be sampled at specified intervals (see below) for the period of monitoring.

(b) How many animals should be sampled?

Again, there is no magic sampling number. However, with the repeated sampling of the same animals, fewer numbers may be required than in the diagnosis of a herd/flock problem. The guidelines in Table 6.1 are recommended.

(c) What samples should be taken?

A faecal sample for a faecal egg count and a faecal culture. A blood sample in anticoagulant for a haematocrit (packed cell volume, PCV: see section 5.3).

(d) How often should samples be taken?

(i) Ideal

Under ideal circumstances, these procedures should be carried out every two weeks during the rainy season and every 4-6 weeks after one month into the dry season. Data should be collected over three calendar years.

(ii) Acceptable

If circumstances do not allow, it is acceptable to sample once a month during the rainy season, and once every 6-8 weeks after one month into the dry season. Data should be collected for a MINIMUM of one calendar year.

6.3.2 Sampling of dead (moribund or sacrificed) animals

(a) Which animals should be sampled?

Every opportunity should be taken of sampling animals that die for whatever cause. Sacrificing animals is not advocated for long-term monitoring programmes if tracer (sentinel) animals are used (see below).

(b) What samples should be taken?

Those animals that die should be subjected to a total parasite count on the abomasum, small intestine and large intestines (see section 4.3). Data on the quantitative seasonal availability of infective larvae on pasture can be gathered by pasture larval counts and/or the use of tracer animals.

6.3.3 Sampling of pasture

(a) From where should pasture samples be taken?

During the dry season, pasture samples for larval counts should be taken regularly from the same grazing areas within "high risk" sites where larval survival is likely. These may include areas of impeded drainage and river beds. Additional data may be gathered from the following sites:

(i) Areas regularly grazed by the herd or flock.

(ii) Areas frequently or periodically grazed by the herd or flock at high stocking density.

During the rainy season, a representative sample from the largest possible area of grazing activity should be taken.

(b) How often should samples be taken?

Pasture sites should be sampled at the same intervals as animal sampling. Samples should be taken at the same time of day on each occasion.

(i) Ideal

Under ideal circumstances, this should be every two weeks during the rainy season, and every 4-6 weeks after one month into the dry season. Data should be collected over three calendar years.

(ii) Acceptable

If circumstances do not allow, it is acceptable to sample once a month during the rainy season, and once every 6-8 weeks after one month into the dry season. Data should be collected for a MINIMUM of one calendar year. During the dry season and in arid regions with limited grass cover, herbage larval counts may be of limited value. Under these circumstances, the use of tracer animals (see below) is recommended.

6.3.4 Sampling of tracer (sentinel) animals

Tracer animals are intended to provide an indicator of the availability of infective larvae on the pasture. Tracers should be parasite-free and non-immune animals. These can be produced by:

(a) Rearing parasite-free animals from birth in animal houses. This is the best method, but it requires good animal housing facilities.

(b) Using young (6-9-month-old calves; 3-4-month-old lambs) animals which have been treated twice at 14-day intervals with a larvicidal anthelmintic before starting as tracers. This is the most satisfactory method of producing tracer animals where animal housing facilities are not available.

Tracer animals should be introduced to a herd or flock at intervals of 4 weeks over a minimum period of one year. The group of tracers introduced each month to join the herd or flock should not be treated with an anthelmintic and should be removed after one month. The tracer animals should then be held in a house or pen with no access to pasture, fresh or cut, for a period of at least 21 days, following which they should be slaughtered. (The larvae remaining in the abomasal and intestinal mucous membrane after this period will be considered inhibited larvae.) Slaughtered tracer animals should then be subjected to a total parasite count for the abomasum (including larval stages) and small and large intestines (see section 4.3). Normally it is sufficient to introduce two tracer animals to a herd or flock each month, but the accuracy of the data will be improved by introducing more than this.

6.4 Plot experiments


6.4.1 Procedure
6.4.2 Monitoring the climate in plot experiments


Useful epidemiological information can be obtained from studying the seasonal dissemination and survival of L3 on herbage surrounding faecal pats containing a known number of nematode eggs per gram of faeces which are deposited on pasture at regular intervals throughout a minimum period of one year.

6.4.1 Procedure

(a) Identify a parasite-free grass covered area (600-900 m²). Construct fencing to prevent grazing if necessary. If such an area is not available, consider preparing one by planting/sowing similar grass species as are found in grazing areas. If neither alternative is possible, monitor herbage larval counts in the study plot prior to and during the study to establish the background contamination of (L3).

(b) Identify a source of infected animals for continuous supply of faecal material containing nematode eggs. If not available, calves can be infected for the supply of faeces

(c) Collect as much faeces as possible (10 kg) from the source animals. Mix the faecal material well. If it is too dry, add water.

(d) Determine the approximate parasite egg count by running five McMaster tests. Calculate the mean count in e.p.g.

(e) Prepare 4-6 plots of 9 m² each. Ensure that no traffic of personnel occurs directly from one plot to another.

(f) Deposit a faecal pat of 1 kg in the centre of each plot.

(g) Repeat procedures (c)-(f) every 4-6 weeks for a minimum of one year.

(h) From each plot with faecal pats of the same age, collect a small amount of herbage by hand from the grass growing near the faecal pat. Combine these samples and place in a plastic bag/container. Repeat this procedure, collecting grass 40-50 cm from the faecal pats. Grass sampling should be carried out at two-weekly intervals for a period of one year.

(i) Samples from plots containing faecal pats of different age must be kept separate.

(j) Process the herbage samples for isolation of L3 (see section 5.2).

6.4.2 Monitoring the climate in plot experiments

The two parameters important for monitoring the climate are:

· ambient temperature
· rainfall

These should be monitored daily, if possible, for the entire period of the sampling/study.


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