The control and prevention of disease is a much better policy than trying to cure a disease after it has appeared on a farm or a district. There are a number of steps a farmer can take to control and prevent disease and these steps are covered in this lecture.
One of the questions that a farmer or Veterinary surgeon must always consider is whether or not, it is more economical to slaughter, or treat a sick animal which will bring the animal back into good production, and will the production justify the cost of the treatment? Furthermore, if the animal is suffering from an infectious disease, the risk of the disease spreading might far outweigh the value of the animal itself.
In Southern Africa, where farms can be situated long distances from Veterinary services, slaughtering is sometimes the best method of dealing with a sick animal.
In some countries an official policy of slaughter has been adopted to control certain diseases. An example of this is the control of Foot and Mouth Disease in the U.K. As soon as an outbreak of this disease is discovered, all infected stock and all stock that has been in contact with infected animals is slaughtered, the carcasses buried in pits and covered with lime. Compensation is paid to the farmer for the loss of his stock.
This is the method of keeping an infected animal away from its fellows until the infection has been treated and cured. It is a good practice to carry out with new animals when they come onto a farm so that any diseases they may be carrying will not spread to the rest of the stock. Quarantine can be carried out by placing the animals in a paddock or pen away from other stock. With small stock, pigs or poultry, animals in quarantine should always be fed after the healthy stock has been fed so that germs are not carried to them via the stockman’s clothes and boots.
In the case of outbreaks of Foot and Mouth Disease or Anthrax in this country, the area containing the infected animals is designated to a quarantine area, and no animals can be moved away. The reason for this is to keep the infection inside the area to prevent it from spreading.
In the case of all infectious diseases, early and accurate diagnosis is most important. The early stages of disease are more easily treated as the germ causing the disease has not gained a strong foothold in the body of the animal, and it is less likely to have produced tissue damage which may be difficult to repair. Moreover, as a disease develops in an infected animal, the longer it is left untreated the greater the risk of the disease spreading to other animals on the farm.
Diagnosis of disease depends to a large extent on the skill and experience of the farmer or the Veterinary surgeon, but it can be helped by taking samples of blood, mucus, sputum, milk or dung from the animal and sending these to a Veterinary laboratory to be examined. Dung samples will often show the presence of worms and other internal parasites.
Blood samples will confirm many of the tick-borne diseases, and mucus from an aborted calf will show the presence of the bacteria causing Contagious Abortion.
HOW TO TAKE A BLOOD SMEAR
Micro-organisms, on an average, measure only from 1/500 to 1/1000th part of a mm. Therefore it can be appreciated that light dust or other fine dirt which may be mixed with them could effectively hide them from view, for both dust and germs are equally magnified by the microscope. Moreover, fine dust actually resembles the germ of gall sickness, and sometimes, only its position in relation to the blood cells enables a diagnosis to be made. Fine dust falling indiscriminately on the smear makes it doubtful whether one is spotting the gall sickness germ or another dust particle.
In order for such small organisms to be seen, and the minute differences between them identified, it is essential that the smear should be spread out very thinly on the glass of the microscope slide.
Therefore, in making a blood smear, first thoroughly clean two or three microscope slides (smear glasses), if necessary with spirits to make sure that no dust or grease or the slightest suspicion of “film” is present. Clean the ear of the animal with a damp cloth and nick its edge with a pair of scissors or a sharp knife, sufficiently deep to cause the blood flow rather than merely to produce a half-clotted bead of blood. It is impossible to make good smears with blood which has started to clot. A drop of blood about the size of a millet seed (no larger) is picked up on the second slide near one end. Holding the first glass at an angle to the second, push it lightly along its length to the other end, drawing the blood behind in a thin film.
Figure 1: How to prepare a blood smear.
Do not allow the smear to dry in the sun. Keep it in the shade. When it is thoroughly dry, wrap it in clean paper, upon which particulars of the smear are recorded.
AVOID THE FOLLOWING MISTAKES: DO NOT:
- Use dirty or greasy glass;
- Allow dirt or dust to settle on glass;
- Use too big a drop of blood;
- Wrap until dry;
- Wrap in dirty paper;
- Press two wet slides together and;
- Omit to write full particulars.
SMEARS OF PUS AND DISCHARGE
The same procedure is carried out as for blood smears.
Material for smears from a suspected quarter-evil case is obtained by cutting into the swelling and discoloured muscle of the affected quarter of the carcass and picking up a small drop of fluid from the surface. Spleen smears should also be taken.
Smears are made by puncturing one of the large glands (in flank or front of the shoulder) with a clean, large, hypodermic needle and spreading the exuding fluid on a clean slide.
Smears are obtained by cutting into the spleen of a carcass and picking up a small piece from the cut surface with one corner of the slide. Rub this into a pulp on the second slide near one end, and then draw (not push) the second slide over the first. To push (as with a blood smear) would render the smear made with spleen substance too thick.
Particulars required with smears:
- Owner’s name;
- Date taken;
- By whom forwarded;
- Nature of preparation;
- Breed, age and sex of animal and;
- Disease suspected.
5. POST MORTEM
If an animal dies on the farm and there is no obvious cause of its death the farmer can carry out a post mortem on the carcass. This means cutting it open and examining the internal organs for signs of abnormality caused by disease.
POST MORTEM EXAMINATION OF A SHEEP
The steps to follow when carrying out a post mortem examination of the carcass of a dead sheep are as follows:
- A wheelbarrow covered with a piece of steel mesh makes a convenient examination table and the instruments required are: a hacksaw, a pair of scissors; a sharp knife; a few microscope slides and several plastic bags for specimens.
- Before any incision is made, a careful examination of the outside of the body should be carried out, looking for wounds or the presence of external parasites such as ticks, scab mite or lice.
- The nostrils should be carefully examined for any evidence of a mucus discharge which may indicate a nostril-fly infestation or pneumonia. If the former is suspected, then the head should be cut through from front to back down the centre, using the hacksaw. If nostril flies are present the larvae will show up as white or black maggots in the cavities of the nose.
- The lips and mouth are examined to check that there are no abnormalities.
- The throat is examined for enlargement of the thyroid gland, commonly called goitre. This shows as a large swelling just below the lower jaw and is more common in new-born lambs. The two swollen lobes can be felt easily.
- The ears should be examined for the presence of ticks.
- In young lambs, special attention should be given to the scrotum and tail to check for the presence of an elastrator ring or subsequent infection. This could be the cause of tetanus.
- The lower portions of the legs, the armpits and the groin should be checked for the presence of red-legged ticks which could cause paralysis and death in younger sheep.
- Scorch marks on the hair or wool, extending down the legs as fairly thin lines, would indicate that the animal has been struck by lightning.
- Now the skin must be removed from the whole carcass, taking care to keep the initial cuts shallow and through the skin only, to not go too deep and so cut through the body wall as well.
- The inner surface of the skin and the carcase itself should now be examined for damage by bruising or by grass-seeds.
- A swelling in front of the shoulder or hind leg may be due to an infection in the gland, commonly called “cheesy gland”.
- Lay the carcase on its right side.
- Cut through the muscles which hold the fore-limb in place and lay the front leg back.
- Cut into the groin, sever the ligaments of the hip joint and lay the hind leg back.
- Cut away the side of the abdominal wall and examine the content of the abdominal cavity (normally a small amount of clear, serous fluid). If blood-tinged, it may indicate peritonitis or inflammation of the intestines. Abdominal fluid is also excessive when anaemia is present.
- The abdominal organs are held in place mainly by thin membranes which tear easily. Use the knife carefully in removing them so as to avoid cutting the stomach and large veins.
- Remove the organs in the following order: spleen (attached to the stomach); left kidney (attached to the backbone); stomach and intestines; liver and right kidney.
- Next remove the left chest wall by cutting the ribs along the breastbone and along the back with a hacksaw.
- The heart is then opened, exposing the two upper and two lower chambers. The valves and the interior of the heart can now be examined together with the big blood vessels. Where death has been due to Blue Tongue, a dark blood-stained area can always be seen in the wall of the vessel carrying blood to the lungs.
- The windpipe is now opened using a pair of scissors, and the cut should be continued down into the smallest air passages entering the lung.
- Now the liver is examined. If the surface is uneven, this may be the result of Bilharzia or Liver Fluke infestation.
- The liver is examined by making a cut through the middle of the organ. The consistency should be friable, almost crumbly, breaking up easily under light pressure. Liver tapeworms are found in nearly all sheep livers.
- The kidneys are the next examined by making a cut right through both of these organs. The most common change in kidneys is due to Pulpy Kidney, when the kidney becomes soft and pulpy.
- The stomach and bowels are now examined. The paunch or rumen is cut from end to end starting at the gullet, and the contents and wall are examined. Adult conical fluke may be seen attached to the wall of the rumen where the gullet enters this organ. The honeycomb or reticulum is now opened; the contents which are always dry and abnormalities rarely found.
- The abomasum or true stomach is now opened which is where wireworms may be found.
- The bowel is released from the fine supporting tissue, the mesentery, and is now examined by splitting it down with scissors. In the first part of the bowel, the small bankrupt worm may occur. These are too small to detect easily as they are hidden in the intestine. They are more easily seen if the inner surface is scraped with one glass slide and the mucus then spread and trapped with a second slide which are held up to the light. Tapeworms also occur in the small intestine and are quite obvious. The nodular worm is another common parasite. The immature stages of this worm cause nodules on the outer surface of the bowel wall. The blind gut itself is now opened where the adult nodular worms can be seen if they are present.
Samples of any diseased organs found during the examination should be placed, fresh, in separate plastic bags and taken to the nearest Veterinary Officer as soon as possible for advice.