1. EPHEMERAL FEVER
INTRODUCTION
Ephemeral fever is a mild virus disease of cattle. It is transmitted by an insect and is characterised by muscular tremors, limb stiffness, lameness and depression of the milk yield. The disease is present in most of the continent of Africa. It is also known in Australia.
The disease is usually very seasonal, varying with the seasonal prevalence of the insect carrying the disease. The mortality rate is not high, and deaths, when they occur, are usually due to complications from secondary infections.
CAUSE AND TRANSMISSION
The disease is present in the blood and it can be transmitted by inoculation of whole blood. A very minute quantity is sufficient to infect a susceptible animal. Only cattle are affected by this disease.
Transmission occurs only from other infected cattle. Natural transmission is thought to be via midges, although there is no conclusive evidence that midges are the actual genus involved. Transmission does not occur by direct contact between animals or via discharges from infected animals, but it is apparently dependent upon the presence of an insect carrying the disease. The proximity to the habitat of such insects and the direction of the prevailing wind and other ecological factors increase the chance of the virus, the insect and the susceptible host coming into contact with each other.
SYMPTOMS AND DIAGNOSIS
The incubation period is usually between 2 and 10 days and is followed by sudden and high fever. This may be of a very short duration and may easily be overlooked. Loss of appetite and a
marked diminution of milk yield are first noticeable, together with salivation, nasal and ocular discharge. Constipation or diarrhoea may occur. Muscular tremors appear, followed by severe stiffness of the limbs 48 hours thereafter, which then become weak and lame. The animal then goes down, often with the hind legs rigid, sometimes lying on its side.
This may last 2 or 3 days, after which eating and rumination recommence although the leg weakness may last for another 2 or 3 days.
TREATMENT AND CONTROL
Unfortunately, control of the insect carrying the disease has not yet been achieved, but we look forward to it happening in the future. Treatment is therefore generally limited to an injection of salicylate to relieve the muscular stiffness.
2. ‘EPIVAG’
INTRODUCTION:
This is a venereal disease of cattle found in Africa, south of the Sahara which usually shows itself by the hardening and enlargement of the epididymis of bulls and by inflammation of the genital tract in cows.
CAUSE AND TRANSMISSION
The cause is thought to be a virus but there is insufficient evidence to confirm this as yet. Transmission is by coitus and it is thought to be by the direct contact of the tissues. It is possible, however, although not very likely, that semen acts as the medium of transmission.
The disease is restricted to cattle, and mainly to areas where the husbandry is poor, i.e. where the bull runs continuously with the herd and where there is communal grazing. There appears to be a prolonged carrier state and exotic breeds appear to be more susceptible than indigenous breeds.
SYMPTOMS AND DIAGNOSIS
The disease takes 3 – 6 months to develop in bulls. The scrotum in the region of the epididymis becomes asymmetrical in outline with uneven enlargement of the epididymis firstly at the tail and then later at the head. The bull’s fertility and libido is unaffected.
In cows, the appearance of dirty tails and buttocks appears which, upon examination, reveal at first a slight vaginitis which may develop and become severe vaginitis with a thick mucous discharge. The cervix and uterus may also be involved. Recovery and full breeding capacity may be restored if the infection does not involve the ovaries and if the Fallopian tubes do not become occluded by fibrous tissue. Death does not often occur
from this condition. Usually there is a rapid spread when the disease is newly introduced into a herd.
TREATMENT AND CONTROL
There are no means of laboratory diagnosis. However, the commencement of a programme of artificial insemination is usually quite effective in controlling the disease. Clean bulls should be used and the greatest care taken in the sterilisation of artificial insemination equipment between the cows. Affected bulls should be culled. There should also be a total prohibition of the movement of cattle from infected to healthy herds. In many areas, rigid control of breeding stock is not possible and therefore control of the disease may be impossible.
3. FOOT AND MOUTH DISEASE
INTRODUCTION
Foot and Mouth disease is a highly contagious viral disease, predominantly affecting cattle, but also all even-hoofed animals, including pigs. Initially, there is a fever and lesions on the mouth and feet. It is not usually fatal but the prohibition of the free movement of infected livestock and livestock products means that it is one of the most serious animal diseases in the world. Foot and Mouth disease occurs in all African countries to a greater or lesser extent. In Asia the disease is widespread. Sporadic outbreaks also occur in the U.S.A., the United Kingdom and over the rest of Europe. It is not present in Australia or New Zealand.
CAUSE AND TRANSMISSION
The virus which causes the disease appears in 7 serotypes and many sub-types. There is no cross- immunity among types. The virus is highly resistant to freezing and even to some very strong disinfectants and it can persist for over a year where infected animals have been kept. It can survive for months on clothing, hair and personal equipment, on straw and hay and on vegetables. Transmission of the virus over quite long distances by motor vehicles is known. It is destroyed by sunlight but can persist in bright sunshine for some time at low temperatures. It can be killed by boiling, provided it is not in animal tissue. Wild animals, rodents and hedgehogs have been found to carry the virus. Recovered animals have a natural immunity which can last from 11 months to 4 ½
years. The virus is present in milk and in saliva, before clinical signs appear. It can also be found in the faeces and urine of infected animals.
The main method of transmission is by ingestion via feeds. Wind-borne transmission can occur. It can be carried by birds and by meat products.
SYMPTOMS AND DIAGNOSIS
Clinically this disease runs a standard course regardless of the animal affected, or the virus type. The incubation period is usually between 3 and 8 days. The first sign is a high fever, less pronounced in older animals, followed by loss of appetite and indications of stomatitis which shows in the slow, careful chewing and swallowing, that indicates a painful state of the mucous membrane. Rumination is protracted and this is when the animal stands motionless with glazed eyes and with long, viscid masses of saliva hanging from the mouth. At this stage, the inner surface of the lips is hot, dry and reddened.
By the third day the characteristic vesicles appear; they are 1 to 2cm in diameter and appear on the inner lips, the gums, the dental pad, and the back and sides of the tongue and inner cheeks. At the same time, one or more of the feet become affected. The animal becomes lame with pain in the foot. When more than one foot is affected the animal goes down and has difficulty in rising. Vesicles can be seen in the cleft of the foot and sometimes at the base of the foot. The rapid loss of milk yield in milking cows and the condition in all animals cannot be accounted for by the visible lesions alone.
Variations in the virulence of the infection account for this, and there is a malignant form of the disease which causes myocarditis and consequent heart failure.
At times, there is profuse diarrhoea and even convulsions. In sheep and goats, the course of the disease is usually fairly mild with no salivation and it is of shorter duration. In pigs, the feet are mainly affected.
TREATMENT AND CONTROL
Usually slaughter of the infected animals is compulsory depending on the regulations governing the control of the disease. It is essential with any vaccination programme that the virus-type is first identified and that the vaccine contains specific antibodies. When the incidence of the disease has been reduced by vaccination to the stage where the slaughter policy becomes exclusive to that, it is essential to ensure that re-introduction of the disease from outside is prevented. This means that there must be a complete prohibition of the movement of animals and of animal products. The latter include hay, straw and similar material which may carry the infection. Vehicles travelling from infected areas are a prime source of danger. The prohibition of such movement of vehicles, together with disinfection of tyres, footwear, etc., should be enforced.
4. LUMPY SKIN DISEASE (LSD)
INTRODUCTION
This disease is mainly confined to East, Central and South Africa. It is an acute infectious disease of cattle and is characterised mainly by the eruption of large numbers of cutaneous nodules and by lymphadenitis. Its shortened name is LSD.
CAUSE AND TRANSMISSION
LSD is caused by a pox virus. Sheep and cattle can be protected against true LSD by prior inoculation with virulent sheep pox virus. There are at least three agents associated with LSD, but true LSD is caused by the virus known as ‘Neethling’. The virus is present in both the blood and saliva. It can be
transmitted from skin lesions and can survive for more than a month inMhaidigensawnti:thv skyinv luelseiontnos which have been salted. However, it can produce immunity in recovered annfeimctiaolus, although the actual
duration of this immunity is still in doubt. It is probable that mosquMitoyoecs radriet sr:eisnpfloamnsmibaltei fnor the transmission of the disease. It is also possible that infected feeding andodf rihnekhinegarttromuugshclse.are involved in the transmission. However, the disease has been found in areas where mosquitoes could not be involved in the transmission of the disease.
SYMPTOMS AND DIAGNOSIS
Pure-bred exotic cattle under high production conditions are more susceptible than indigenous cattle. The incubation period is 4 – 14 days, followed by the rise of body temperature and by loss of appetite, increased salivation and a nasal discharge. These are followed by the appearance of nodules or lymphs in the skin. These are usually of uniform size on the animal but vary in number from a few to many hundreds. These cover the whole body but are predominantly found on the neck, brisket, back, thighs, legs, udder or scrotum and around the muzzle and eyes. Hairs over the lumps stand erect thereby clearly defining the affected area. The lumps are firm, round, raised areas in the skin usually measuring 4 – 5cm across.
The skin nodules may persist for many years, or they may develop into what are termed sitfasts which is a dry section which separates from the original nodule, leaving a granular crater of varying depths and sizes. These heal with scar tissue. This may take 1 – 2 months to occur. Animals lose condition and the milk yield drops. Secondary infection of the udder is likely to occur and pregnant cows abort and bulls become sterile. With the Allerton virus, the infection is benign. There is no sitfast- type lesion, and with the virus only the superficial epidermis is affected. After 7 to 9 days, the swelling subsides, leaving a hard, dry epidermis. The dried epidermis peels off in 10 – 14 days, leaving a hairless area with no scar. The hair then grows again. The diagnosis is made on the appearance of the skin lesions. The virus does not respond to treatment, but the secondary infections should be treated with antibiotics and sulphonamides.
TREATMENT AND CONTROL
Quarantine has not been effective in East and South Africa. The use of a sheep pox virus tissue vaccine can be effective in producing immunity and restricting the spread of the disease, but this vaccine is obviously limited to countries where sheep pox occurs. A vaccine from an attenuated virus on lamb kidney tissue-culture has been effective and is now in general use.
5. RABIES
INTRODUCTION
Rabies is a dangerous and often fatal disease. All mammals are susceptible. It mainly affects carnivores. Its danger to man is through the bite from a rabid dog. It affects the nervous system with clinical signs of mania and paralysis. Domestic cats, wild and feral carnivores including jackals, foxes and wolves and bats can all be affected and be of direct danger to man. The disease is a very ancient one and is known over most of the tropical world except
on some islands protected by the barrier of the sea. Australia and New Zealand are free from rabies.
CAUSE AND TRANSMISSION
The rabies virus is large. It is susceptible to destruction by standard disinfectants. There are various strains, the two most common being street virus and fixed virus. The usual transmission is through bites, although it is possible for the virus to be transmitted by inhalation and very possibly ingestion.
The virus can be found in the salivary glands for 5 days before clinical signs appear in the infected animal. This is a factor which is of vital importance in the control of the disease.
SYMPTOMS AND DIAGNOSIS
After the bite of the rabid animal, the incubation period varies. The length of time is roughly related to the distance from the central nervous system to the place and the severity of the bite.
There are two recognised syndromes in dogs i.e. the ‘dumb’ and the ‘furious’. In ‘dumb’’ rabies the symptoms are paralysis of the peripheral nerves. In ‘furious’ rabies the chief manifestation is mania due to irritation of the central nervous system. In ‘dumb’ rabies, dogs show a characteristic staring expression caused by the paralysis of the facial muscles.
In cattle, the incubation period is usually 3 weeks, but can be from 2 weeks to 6 weeks or extend into months. In the paralytic form the animal has no co-ordination of gait, with periodic collapse of the hindquarters. The animal in both the ‘dumb’ and the ‘furious’ form of the disease is unable to drink because of the paralysis and may die within 48 hours, or a few days later.
In sheep, the picture is very similar, and a number of sheep may be infected at one time because of the ease with which they can be attacked by a rabid animal. They will then attack each other, man or any animate object. They attempt to bleat but no sound is produced.
Negri-bodies can be found in various parts of the brain of a rabid animal, and their presence confirms the diagnosis of rabies. These negri-bodies are spherical objects in the cytoplasm of certain brain cells, particularly those of the cerebellum. Although their relationship to the virus in unknown they are accepted as certain evidence of its presence. The head of an animal which has died from suspected rabies should be severed under strict aseptic conditions and sent to a laboratory for further inspection. Accurate diagnosis of rabies is one of the
most difficult tasks because it is not usually possible to follow the course of the disease from beginning to end. Failure to diagnose rabies may result in human deaths which could be prevented.
TREATMENT AND CONTROL
No treatment is yet available which has any effect on the course of rabies. Unqualified people should not attempt to
cure rabies. Control measures are usually the subject of strict government legislation. Such regulations vary from country to country. When a country is free of the disease, for example, the United Kingdom, the imposition of a strict quarantine period for imported carnivores appeared to be an effective measure in preventing the entry of the disease. However, there is obviously difficulty in ensuring these measures in countries which have land borders. There are two basic principles involved in the control of rabies. One is the prevention of exposure to infection and the other is vaccination. In general, this consists of strict control of dogs, wild feral carnivores and bats. The most frequent danger is from infected domestic dogs, although these in turn, draw their infection from wild and feral carnivores. There are numerous vaccines but they are essentially of four basic types. The well-tried Pasteur type of vaccine, which is an inactivated product from sheep or goat nerve tissue, is still very effective and available in most countries.
6. RIFT VALLEY FEVER
INTRODUCTION
Rift Valley Fever (RVF) is an acute febrile viral infection mainly found in sheep and cattle and may
occur in man. In sheep and cattle it is characterised by abortion, in lambs and calves by hepatitis and in man by an influenza-like disease. It is a disease only found in Africa. Its name arises from the fact that it was first identified in the Rift Valley in East Africa. Generally, it is of seasonal occurrence.
CAUSE AND TRANSMISSION
The RVF virus is small with a specific affinity to the liver, and because it is a fixed virus it is useful for producing vaccine. Not only cattle, but man, sheep, rats and mice are highly susceptible to infection from the virus while other large animals are free from it. Goats are only moderately susceptible and transmission from animal to animal is usually by mosquitoes. Transmission from animal to man is by direct contact. Transmission is more likely in areas where warm, moist conditions are ideal for a high incidence of the disease-carrying insect.
SYMPTOMS AND DIAGNOSIS
In lambs and calves, the incubation period is from 12 hours to 3 days. The characteristic clinical signs are a high fever and vomiting, a muco-purulent nasal discharge and prostration. In lambs the death rate from RVF can be as high as 100%. In calves, the symptoms are similar, but less obvious and the mortality rate is about 70%. In both adult sheep and cattle, abortion is the prominent symptom and a high fever followed by death. The infection in cattle and sheep, together with a high mortality rate in lambs and calves and a characteristic liver lesion will appear and thereby permit a diagnosis. A diagnosis of this disease can be made when the characteristic liver lesion appears in the animal. Laboratory tests are needed for a definite diagnosis.
TREATMENT AND CONTROL
No treatment for animals has yet been discovered. However, the prohibition of the movement of susceptible species across national borders in Africa, and from Africa to other countries would be needed to prevent new introductions. Such prohibitions would be impractical while the control of the insects carrying the disease is at present not possible. In man, a formalin-killed-tissue-culture vaccine has been successful, and in the treatment of sheep, an attenuated vaccine has proved successful.
