Last update: April 3, 2012 11:56:35 AM E-mail Print

 

Rift Valley fever and Wesselsbron disease in Angora goats

EM van Tonder

 

 

 1. Introduction

As a result of the extremely favourable climatic conditions encountered during the past two years, a variety of disease problems has again occurred or showed an increased incidence amongst Angora goats. Two of these diseases which deserve the urgent attention of all goat farmers and against which timeous preventative measures must be taken are undoubtedly Rift Valley Fever and Wesselsbron disease. Both these diseases have occurred in Angora goats during the past two seasons and it can be stated without doubt that deaths caused by them were often mistakenly attributed to other causes.

 

2. Causes

Both diseases are caused by viruses which are transmitted by mosquitoes.

As they mainly breed in stagnant waters, the incidence of mosquitoes is in turn dependent upon favourable climatic conditions. A good rainfall with the formation of open pools and dams affords additional breeding sites which together with prevailing hot weather is responsible for the rapid multiplication of mosquitoes. The incidence of mosquitoes as well as the above-mentioned diseases is therefore dependent upon the normal rain season and would in these parts only make their appearance during late summer and autumn. On account of the changed rainfall pattern and the presence of stagnant waters and vleis, outbreaks were experienced even in early spring of the previous year. As result of a suppression of the adult mosquito population, both diseases usually disappear rapidly after the first frosts. It should however be pointed out that a significant decrease in incidence or even a complete disappearance of these diseases, only takes place under extremely severe winter conditions. During the previous year when mild conditions were experienced, both diseases occurred almost uninterruptedly throughout the winter.

Although not sufficiently known it is nevertheless suspected that certain wild animals act as reservoirs or latent carriers of these viruses, from where mosquitoes acquire the infection and transmit it to susceptible animals.

 

3. Symptoms and Post Mortem Lesions

Rift Valley Fever and Wesselsbron disease cannot easily be differentiated on symptoms and post mortem lesions and can only be accurately diagnosed on laboratory examination.

In general however, Rift Valley Fever can be regarded as a disease affecting all age groups of goats and sheep, while Wesselsbron disease is a typical abortion disease which also affects the newly born as well as the young kid within the first week of its life. A large percentage of pregnant ewes usually abort without showing symptoms. The only exception was an outbreak of Wesselsbron disease in the Karoo during 1956-1957 where adult sheep also showed symptoms and died.

In the case of Rift Valley Fever young kids are most susceptible and mortalities of up to 95 per cent occur within a short period. The first signs of the disease are usually the sudden death of young kids without perceptible symptoms beforehand and a daily increase in the number of deaths. Symptoms that can be observed include high fever, shivering and general weakness with death following within a day or two. In older goats the mortality varies round about 30 per cent and usually occurs within 2 to 4 days. The symptoms here include a dry red and scaly muzzle which peels within a few days, nasal discharge, and diarrhoea which could be bloody, pale and even yellow mucus membranes, shivering, a staggering or even stiff gait and sudden lagging behind and death when animals are driven. Pregnant ewes that become affected nearly always abort while weak kids die within a day or two after birth.

In case of Wesselsbron disease, the adult non-pregnant ewes do not usually show symptoms. The most important feature however, is that a large percentage of pregnant ewes abort within a short period without showing visible symptoms themselves. An occasional ewe might succumb due to complications such as retained afterbirth, uterine infection etc. Some kids are also born alive but are weak and usually die within a day or two after birth.

Both diseases, but more specifically Wesselsbron disease, can therefore easily be confused with other abortion diseases such as enzootic abortion and habitual abortions which are generally encountered in Angora goats.

In both diseases the liver is mainly affected with the result that on post mortal examination this organ is found to be enlarged and brittle with a dark reddish-brown, yellowish-brown and even golden yellow colour. Small, dull, greyish-white spots and haemorrhages appear on the surface as well as throughout the liver tissues. Other lesions include a swollen spleen, haemorrhages on the mucus membranes, heart and other organs, accumulation of a reddish-brown fluid in the abdominal and chest cavities and heart sac and a red discolouration of the lining of the intestinal canal, the contents of which often consist of pure blood. A mild to a more pronounced jaundice is also often present.

As these diseases can often not be distinguished without laboratory examination, it is recommended that the nearest veterinarian should be consulted in order to obtain an accurate diagnosis.

 

4. Prevention

As treatment of affected animals is not worth attempting, it is essential that meticulous preventive vaccination should be carried out timeously.

There are excellent vaccines available against both diseases and only one inoculation is sufficient to protect an animal for the rest of its life. Once all goats have been inoculated, it will only be necessary to do the offspring annually. Kids born from ewes that have not been vaccinated previously, can be vaccinated at an early age, usually from 14 days and older, while those born from ewes that have already been vaccinated, should not be vaccinated before the age of 5 to 6 months. Kids born from vaccinated ewes acquire a passive immunity from their mothers via the colostrum milk within the first day of their lives. This immunity gradually disappears up to the age of 5 to 6 months. If these kids are vaccinated within this period, they will become fully susceptible as result of a neutralisation effect of the vaccine. Pregnant ewes should not be vaccinated especially within the first three months of pregnancy.

On account of the fact that Angoras are mated during autumn i.e. from March to May, and that these diseases, depending on climatic conditions, could already occur in early spring or summer as was also experienced the previous year, vaccination presents some difficulties.

The best time for vaccination is still in late winter or early spring before these diseases have made their appearance, especially under the present prevailing climatic conditions. The problem, however, is the fact that Angora ewes are then either pregnant or in the process of kidding.

When animals are vaccinated for the first time it would be best to inoculate all non-pregnant animals during the above-mentioned. period and to do the pregnant ewes after they have kidded and can be handled without the danger of losing kids. The kids can then also be done when they are approximately 14 days to one month old. If these diseases make an earlier appearance i.e. September - October however, it will probably be warranted to risk vaccinating the remainder of pregnant ewes during late pregnancy as well as the kids at an earlier age.

The annual vaccination i.e. the vaccination of kids from ewes that have been vaccinated previously, also presents some problems in Angora goats. Such kids will only reach the age of 5 to 6 months during January to April the next year, i.e. within the period when these diseases are usually prevalent. Should these diseases be prevalent at that stage losses would be experienced in these kids before they are actively immunised. It should be borne in mind that the passive immunity acquired from their mothers, does not suddenly disappear at the age of 5 to 6 months but that this is a gradual process subject to extreme variation with the same group of kids. Should this be the case all kids must still be vaccinated while other precautionary methods must also be employed.

These methods are not very effective and should therefore not be regarded as the alternative for preventative vaccination. They can at most only assist in reducing losses to a certain extent and should be applied until vaccination is completed and for at least 14 days thereafter. These measures include the transfer of susceptible goats from low-lying to higher grazing areas and light spraying with insect repellents, once to three times a week.

Both vaccines can be applied simultaneously and should preferably not be given within 14 days to 3 weeks of any other vaccinations.

The aforegoing only represents a guideline for the prevention of these diseases and since circumstances and management vary from farm to farm, individual farmers are recommended to contact the nearest veterinarian in order to obtain the best advice in their specific cases.

 

Published

Angora goat and mohair journal 18 (1)