An anthelmintic is a drug or combination of drugs which destroys or removes parasites in or from the host animal.
An ideal anthelmintic for domestic animals should have the following properties:—
(1) Safety for the animal to which it is administered.
(2) Efficiency in expelling or destroying the particular species of helminth against which it is used.
(3) Simplicity of administration on a large scale.
Determination of the Efficiency of Anthelmintics.
There are two chief kinds of tests: in vitro tests where the action of a drug is observed on parasites which have been removed from the host animal and kept alive in a dilute saline solution, and in vivo tests where the drug is administered to an infested animal.
A great many tests of drugs have been carried out by in vitro methods but owing to difficulties in deciding whether a worm is dead or not the method is in general not very satisfactory and results obtained by this method cannot be directly applied to tests with infested animals.
Whatever the results of the in vitro tests it is essential to carry out tests in vivo before accepting or rejecting any particular drug. The question of the toxicity of the drug for the host animal is a very important one.
There are three chief kinds of in vivo tests. Two or more of these may be combined.
(a) Egg Counts on Faeces. This can be carried out only where the parasite concerned produces eggs or larvae and passes them in the faeces at a fairly regular rate. Variations occur in faecal egg counts from a variety of causes including the nature of faeces, food intake. etc. These variations must be allowed for in assessing the significance of egg counts. In order to overcome some of the variations related to technique it is essential to carry out a series of faecal egg counts before and after administration of the drug. A few counts can give very misleading information. The usual practice is to carry out ten counts, on consecutive days, before treatment, then allow four days to elapse and carry out n more counts. If at the end of 14 days after treatment there is suggestion that egg counts have been temporarily depressed by the drug and are beginning to increase, daily counts should be continued until they reach a steady level.
When faecal egg counts are being used, the host animals should be infested with a single species or genus of worm in order to avoid complications arising from variations in egg production by different kinds of worms. The worms against which treatment is directed should be of uniform age, for it has been found that young worms, less than 15 days old, tend to be less readily killed than adult worms.
The faecal egg count method has a number of advantages over the other methods of testing anthelmintics:—
(1) It avoids the necessity for killing the experimental animals. These can be used again for further tests.
(2) It is essential when dealing with the smaller worm parasites, particularly those living in the upper parts of the bowel. These worms. e.g., Trichostrongylus spp., when killed by a drug are likely to be digested or may disintegrate before they are passed in the faeces. They cannot, therefore, be collected from faeces, counted and identified.
In sheep this method is used for tests of drugs against Haemonchus, Trichostrongylus, Ostertagia, Nematodirus and Dictyocaulus.
(b) Critical Test. This is the ideal test and gives clear cut results. One can draw up a balance sheet showing the number of worms removed and the number found at post-mortem examination. The result can then be expressed as percentage of worms removed. The method can be used only for large worms, or for worms living in the large bowel. Small worms and those living higher up in the bowel may be digested or disintegrate before they are passed in the faeces.
The method consists of collecting the worms passed during a week or more after treatment and identifying and counting them. The animal is then killed and the worms remaining are collected, identified and counted.
In sheep this method is used for tests of drugs against the large bowel parasites, Trichuris, Oesophagostomum and Chabertia.
(c) Controlled Test: For this type of test two groups of infected animals are selected, the animals in one group are treated with the test drug, and after a variable period all the animals are killed and worms present in each animals are collected, identified and counted. The worm burdens of the treated and untreated animals are then compared.
The great variations in worm burdens, even between animals of the same age run under identical conditions, make the interpretation of results or controlled tests difficult. Even when apparently identical animals are artificially infested under controlled conditions, wide variations occur in the proportions of worms which develop from a given number of larvae. It is therefore essential to have fairly large numbers of animals in test groups and, where possible, test groups should be selected on a basis of faecal egg counts so that variations between groups may be minimised.
In the controlled test many anomalies may occur. For example, suppose that the average number of worms in the control (untreated) group was 2,000, and that at post-mortem examination one of the treated animals harboured 2,000 worms. It appears that in this animal no anthelmintic efficiency was shown. However, this animal may have harboured 10,000 worms, of which the drug had actually removed 80 per cent.
Criteria of Efficiency.
A standard of effeciency must take into consideration the habits and location of the parasite in the host. Some parasites are readily destroyed, others are resistant or, more correctly, live in situations where adequate contact with the anthelmintic is only occasionally achieved.
In sheep Haemanchus contortus is killed by a wide variety of drugs provided they are swallowed into the abomasum, and thus for any drug possessing any anthelmintic properties a reasonable "kill" of this parasite is expected. On the other hand Trichostrongylus spp. in the small intestine is much less readily killed and therefore does does not expect as great a degree of efficiency. For example, if a drug destroys 60 per cent of Trichostrongylus spp. it is considered to show promise; but if it removes only 60 per cent. of H. contortus it scarcely warrants further attention.
For the expression of anthelmintic efficiency, the "percentage of worms removed" can be used when the "critical test" has been carried out. When faecal egg counts are used. "percentage reduction in egg count" is used and it is assumed that a given percentage reduction indicates a similar reduction in the number of female worms. There is experimental evidence that this is a valid assumption.
The measure of efficiency of an anthelmintic usually adopted by the stockowmer is recovery of the infected animals or improvement in their condition. This is a very unreliable measure because it cannot make allowances for the damage produced in the host animal and it is liable to be complicated by changes in the plane of nutrition.
It is important to consider the criterion of efficiency in relationship to the epidemiology of outbreaks and the degree of infestation. For example, consider a sheep harbouring 2,000 female H. contortus and suppose that an antheimintic removes 80 per cent. of them. This reduction will cure the disease, but it leaves 400 females, each of which continues to lay 5,000 eggs per day. Such a sheep will contaminate the pasture with 2,000,000 eggs every day. The disease has been cured, but conditions for its recurrence persist.
Consider now a sheep infested with 30,000 Trichostrongylus spp. and suppose that an anthelmintic removes 70 per cent of them, leaving 9,000. Mortality may be prevented but sufficient worms remain to prevent rapid and complete recovery.
It is clear, therefore, that under outbreak conditions and where severe infestations are present, anthelmintics must be highly efficient in order to achieve complete control.
Special Problems of Ruminants.
The complex ruminant stomach system provides means for the dilution, alteration or absorption of drugs before their anthelmintic effects can be exerted. In general, most of the recognised anthelmintics exert their effects on helminth parasites of sheep only when the forestomachs (rumen and reticulum) have been by-passed and the drug swallowed into the abomasum.
In early tests of drugs for anthelmintic properties in sheep this was apparently overlooked with the result that observations were often contradictory, and some drugs may have been rejected simply because they may have been swallowed into the rumen in most of the animals. This is very likely to have occurred with tests on small groups of sheep.
With the demonstration of the effects of copper sulphate on the oesophageal groove reflex it followed that a retesting of many anthelmintics for sheep was necessary. Before any anthelmintic can be rejected it must be given the opportunity of acting from the abomasum.
(a) Oesophageal groove reflex and copper salts. The only method by which the oesophageal groove reflex can be influenced is by stimulation with ionized copper salts. According to Watson (in the press) the site of stimulation is the posterior region of the mouth cavity, the pharynx and first few centimeters of the oesophagus. Stimulation of the reflex results in closure of the oesophageal groove and passage of swallowed fluids direct to the abomasum. Fluids given with or within 15 seconds of copper salts are swallowed into the abomasum in about 90 per cent of sheep. On an average 10 per cent. fail to respond to copper salts, and some of the individuals in this group may fail repeatedly.
Thus, when copper salts are used there is still considerable hazard in the destination of drugs being tested for anthelmintic properties, and still difficulties in interpreting results.
(b) Dilution of drugs in the rumen; absorption from the rumen. Drugs swallowed into the rumen may be intimately mixed with the contents and greatly diluted before passing on through the abomasum. Dilution may destroy or reduce anthelmintic effects. Sometimes, if the drug is not immediately incorporated with the ruminal contents, and if the rumen empties fairly rapidly, the drug may reach the parasites in sufficient concentration to exert anthelmintic effects. Some drugs may be absorbed from the rumen, or may be modified to a non- anthelmintic form. All of these things lead to variations in anthelmintic efficiency.
(c) Drugs which are effective when swallowed into the rumen. In the usually prescribed doses there are only two anthelmintics which are highly effective when swallowed into the rumen. These are phenothiazine and carbon tetrachloride. The reasons offered for these exceptions to the general rule are for carbon tetrachloride—that it is absorbed, circulates in the blood stream and destroys blood sucking parasites (liver fluke, Haemonchus); and for phenothiazine—that owing to its low solubility the concentration, when it does pass on into regions of the alimentary tract inhabited by the worms, is, in the region of each particle of the drug, still relatively high, and as a bulky dose is used there are ample particles to favour the chance contacts between the drug and the parasites.
Some other anthelmintic drugs, notably arsenical compounds, exert marked anthelmintic effects, although swallowed into the rumen, provided the usual dose rate is increased. For example, double doses of sodium arsenite or arsenic pentoxide, injected into the rumen, are effective in killing Haemonchus, and are non-toxic for the host. Similar doses injected into the abomasum are usually toxic for the host. The ordinary anthelmintic dose, injected into the rumen, is only very occasionally effective in destroying Haemonchus.
(d) Use of abomasal injections to overcome the uncertainty of the oesophageal groove reflex. In the past two years, in testing drugs for anthelmintic properties in sheep, injections were made directly into the abomasum through the abdominal wall. The abomasum can be located in a high proportion of sheep, and by examination of some of the contents withdrawn through the hypodermic needle used, it can be determined with a high degree of precision whether the injection was in fact made into the abomasum. In this way several of the variables which arise if the drug is adminstered by mouth are overcome.
(e) Testing of drugs first by injection into the abomasum, and then, if efficient, into the rumen. If a drug shows promising efficiency when tested by abomasal injection, it has then to be compared with carbon tetrachloride and phenothiazine in their respective spheres, and to equal or better these drugs it must exert its efficiency when injected into the rumen. Therefore, new drugs are tested first by abomasal injection in order to determine whether anthelmintic properties are present, and then, if of promise, they are compared by rumenal injection with the two outstanding anthelmintics which are effective when so administered.