Viral Causes

Viral Causes: BVD, Border disease, Infectious Bovine Rhinotracheitis, Rift Valley Fever

Bovine Viral Diarrhoea (BVD)

This virus is also an important cause of diarrhea. For more information Link to Diarrhea of adults

This infection is a problem in cattle. BVD is the most commonly diagnosed virus in bovine abortion cases in Europe and North America.

The virus can cause:

Early embryonic death
Mummification of the foetus
Abortion
Stillbirth
Birth of calves with congenital defects of the eye/brain
Birth of weak calves that do not thrive, even if they reach adulthood

Persistently infected animal are the source of infection for susceptible animals. Some persistently infected animals die early, others live long enough to join the adult herd and produce persistently infected calves.

Diagnosis is established through blood tests.

Prevention focuses on the removal of infected animals and herd vaccination. There are live virus vaccines and inactivated viral vaccines. The latter should be used in pregnant cattle.

Border Disease

This disease only affects sheep. It is caused by a virus related to the one that causes Bovine Viral Diarrhea disease in cattle. (For more information please Link to Diarrhea of adults)

The virus usually enters a flock through the purchase of a persistently infected sheep. Problems are seen at the next lambing, i.e. barren ewes, stillbirths, abortions and persistently infected lambs which will continue to act as a source of infection for the rest of the flock. Permanently infected sheep that reach breeding age may have reduced fertility and can either abort or produce live permanently infected lambs.

Signs of border disease

In non-pregnant sheep the infection is inapparent, leading to long-lived immunity.

Early foetal death with subsequent resorption (only sign: lot of barren ewes)
Mummification, abortion or stillbirth.
Birth of the so-called 'hairy shaker' lambs, with coarse wool over the head and neck, permanent trembling, and an abnormal skeleton. They show very poor growth rate.

Diagnosis

This is possible on samples of blood and brain tissue.

Prevention and Control

Prevention is achieved by operating a closed flock system and testing all replacements. As there is no local laboratory capable of testing for presence border disease, any system here must depend on close observation. If ?hairy shaker? lambs are born in large numbers, then segregate in-lamb ewes from those with affected lambs and, as infection during pregnancy results in an immunity to re-infection, mix susceptible animals retained for breeding with surviving hairy shakers through the grazing period to maximize their chances of becoming immune.

There ie is no vaccine.

Infectious Bovine Rhinotracheitis

(For more information please see also under Respiratory diseases)

This infection is a problem in cattle. The disease is caused by a herpes virus and usually results in respiratory disease, but may also cause sporadic abortions in cows. Abortions generally occur in late pregnancy and aborting cows may or may not have shown respiratory signs previously. When there is a high fever resulting from systemic or respiratory disease this can also cause abortion.

Cows may be carriers of the virus.

Control is by herd vaccination and intranasal, modified live virus and killed vaccines are available.

Rift Valley Fever

Introduction

Aborted foetus after Rift Valley Fever

Rift Valley Fever (RVF) is an acute mosquito-borne viral disease affecting domestic ruminants (cattle, sheep, goats, camels, domestic buffaloes) and man. It occurs mainly in Eastern and Southern Africa. Outbreaks have also occurred in Egypt, West-Africa (Mauretania), Saudi Arabia and Yemen. During epidemics the sudden occurrence of mass abortions at all stages of pregnancy especially in sheep and camels, deaths in new-born animals and severe influenza like occasionally also fatal haemorrhagic disease in humans are characteristic. Apart from Rabies, RVF is probably the most deadly zoonotic disease transmitted from animal to man. RVF outbreaks in Eastern Africa, including Somalia, have caused hundreds of deaths in humans.

Formerly, apart from mass abortions, most cases in adult animals were mild, but in recent times a haemorrhagic form of the disease has emerged with rapid death in mature animals, including even cattle.

There is a remarkable age-related innate resistance to RVF virus; mortality rate in lambs less than 1 week old exceeds 90% whereas the rate in lambs over 1 week drops to 20%.

Widespread flooding causing mosquito eggs to hatch

Major RVF outbreaks in animals and humans occur in 5 to 20 year cycles. Between epidemics, the virus survives in mosquito eggs laid on vegetation at the edge of flood zones When these are flooded the eggs hatch. This only occurs when the water table rises following prolonged and very heavy rain, for example during El-Nino. The mosquitoes hatch in massive numbers and start infecting livestock.

Mode of spread

Epidemics in domesticated animals are started by the bites of infected mosquitoes. After this, infective droplets generated by aborted foetuses and abortion fluids spread the disease rapidly through the flock or herd.

Humans are usually infected by inhaling infectious droplets when handling infected animals or tissues (meat!) or abortion materials. This occurs when slaughtering animals during a RVF outbreak and when assisting infected animals during abortion. Until recently the disease in man was considered to be a non-serious influenza type illness, but a fatal haemorrhagic form has emerged and now RVF in man is considered to be one of the most dangerous diseases known.

The virus may spread over large areas by windborne mosquitoes and also through movement of infected animals.

Signs of Rift Valley Fever

The incubation period in lambs is 12 to 36 hours. A fever of up to 41°C (106°F) may develop.
Peracute infections occur in new-born lambs which die within hours. Acute reactions occur in older lambs and calves and occasionally in adult sheep.
In very severe infection in calves, death may occur in 2 days after infection without their showing any clinical signs.
A haemorrhagic syndrome was observed during the last outbreak in Kenya, affecting adult cattle.
In its severe form, calves will develop high fever, and may vomit. Some nasal discharge may also be seen followed by prostration and mortality may reach up to 70%.
Almost all infected pregnant sheep and camels abort within a short period and at very different stages of pregnancy. Cattle are more resistant but may also abort.
Subacute reactions occur in adult sheep, cattle and camels. There is a low-grade fever, partial lack of appetite and general weakness. Jaundice is prominent and foul smelling diarrhoea can occur.
In pregnant ewes the mortality and abortion rates varies from 5 - 100%. Abortion rates in pregnant camels are between 80% and 100%

Diagnosis

When suspecting RVF do not touch abortion materials, keep a distance from aborting animals, do not attempt any post-mortems and stop all slaughter of animals for meat. Blood tests are used to confirm a RVF diagnosis. The DVO or vet department should immediately be alerted when unusually high numbers of abortions occur soon after strong rainfall and flooding.

In sick humans, there is a lack of appetite, nausea, severe headache, joint pains, dizziness and nose bleeding. Encephalitis, retinitis, photophobia, loss of central vision, irritation, stupor and coma can occur. Haemorrhagic RVF is characterised by a very acute febrile illness, accompanied by jaundice. Widespread haemorrhages develop within 2-4 days and death usually occurs within another 3-6 days. There is no treatment for RVF in livestock and humans. Patients who are immune-suppressed or malnourished are at particular risk of death.

Recovery from RVF is followed by lifelong immunity.

Prevention and Control

Outbreaks of Rift Valley Fever generally occur after periods of prolonged and very heavy rain. Such being the case livestock owners should be aware that if long rains are excessively heavy and cause widespread flooding, an outbreak of Rift Valley Fever is likely to occur.
Control of mosquitoes through use of insecticides, acaricides and pour-ons, movement of stock to dry areas or to higher cooler altitudes can all help in preventing the RVF outbreak from reaching your herd.
Immunisation remains the only effective way to protect livestock but vaccination of animals with suitable RVF vaccines can only be carried out under authority of the DVS by government veterinary vaccination teams. Some live RVF vaccines can also cause abortions and should NOT be used in pregnant animals.

Treatment

There is no known medical treatment for Rift Valley Fever.