Viral diseases of animals


VIRAL DISEASES OF DOMESTIC ANIMALS

SWAMP FEVER

Equine Infectious Anaemia (EIA), also known as swamp fever is a horse disease caused by a retrovirus and transmitted by bloodsucking insects. The EIA virus is mechanically transmitted from one horse to another by the bloodsucking horse flies, deer flies (Tabanus), stable flies (Stomoxys spp.), mosquitoes and possibly midges. Symptoms include recurrent fever, weight loss, an enlarged spleen, anemia, and swelling of the lower chest, abdominal wall, penile sheath, scrotum, and legs. Horse tires easily due to a recurrent fever and anemia, may relapse to acute form even several years after the original attack.

The EIA virus is a slow acting virus of the lenti-retrovirus group. Retroviruses cause leukemia in cats, mice and cattle, arthritis, pneumonia and neurological diseases in small ruminants and acquired immune deficiency syndrome (AIDS) in humans. These viruses localize and multiply in macrophages of many organs, especially in the spleen, liver, kidney, and lymph nodes, where they take over the cell and sit and wait to become activated. Upon activation, the cell reproduces more viruses, which bursts free from the cell to infect other cells. This causes recurring cycles, in which the horse seems normal and then ill.

There is no known treatment that can eliminate the virus from the body. To date there are no satisfactory vaccines for EIA. The Coggins’ test is an agar gel diffusion (AGID) test, which is a practical diagnostic test for identifying horses infected with EIA. This test is used to detect the EIA antibody.

EQUINE ENCEPHALOMYELITIS

Equine encephalomyelitis is an inflammation of the brain and spinal cord that affects horses but is also deadly for humans. The virus was isolated, characterized, and vaccines were produced in the 1930s. The viruses responsible for causing these diseases are members of a family of viruses called the alphavirus. The mosquito transmits the virus from small infected animals such as birds and rodents to horses.

The warm, humid weather of the summer is good for mosquito breeding and this is when outbreaks are more common. Transmission of EEE is not horse to human, but bird or rodent to human via the mosquito.

Approximately two days after equine infection with encephalomyelitis, there is an infection and low-grade fever. The first apparent signs are at four to five days. At that time, the animal generally has a fever and rapid heart rate, is showing signs of anorexia, depression, and variable other neurological signs. As the illness progresses the brain stem and spinal cord are affected. Muscle weakness becomes apparent and there are behavioural changes and dementia. Notable symptoms include aggression, head pressing, wall leaning, compulsive circling, and blindness. Other signs might include uncontrolled twitching of the eyeball, and facial muscle paralysis. As the disease progresses, a semi-comatose and convulsive state occurs. Death usually follows two or three days later. If the animal survives, residual nervous system problems result.

Encephalomyelitis vaccines are available for horses from several different companies. They are packaged as single or combination vaccines.

AFRICAN HORSE SICKNESS

African horse sickness (AHS) is a highly infectious non-contagious, vector born viral disease affecting all species of Equidae. It is classified as an Orbivirus of family Reoviridae, of which there are 9 serotypes. All serotypes are distributed throughout Africa, although there is a variation in their temporal distribution. It is endemic to the African continent, and is characterised by respiratory and circulatory damage, accompanied by fever and loss of appetite. The disease manifests in three ways, namely the lung form, the heart form and the mixed form.

The lung (dunkop) form is characterised in the following manner:

  • Very high fever (up to 41 degrees).
  • Difficulty in breathing, with mouth open and head hanging down.
  • Frothy discharge may pour from the nose.
  • Sudden onset of death.
  • Very high death rate (90%).

The heart (dikkop) form is characterised in the following manner:

  • Fever, followed by swelling of the head and eyes.
  • In severe cases, the entire head swells.
  • Loss of ability to swallow and possible colic symptoms may occur.
  • Terminal signs include bleeding in the membranes of the mouth and eyes.
  • Slower onset of death, occurring 4 to 8 days after the fever has started.
  • Lower death rate (50%).

 This disease is spread by insect vectors such as midges but can also be transmitted by species of mosquitoes including Culex, Anopheles and Aedes, and species of ticks such as Hyalomma and Rhipicephalus.

There is currently no treatment for AHS. Control of outbreak in an endemic region involves quarantine, vector control and vaccination.

JAPANESE B-ENCEPHALITIS

 This is a fatal disease of pigs, horses, sheep, birds and man. The infection is caused by a flavivirus, a single stranded RNA virus. It is transmitted by the bite of the Culex tritaeniorhynchus mosquito. The virus multiplies at the site of the bite and in regional lymph nodes before viraemia develops that can lead to inflammatory changes in the heart, lungs, liver, and reticulo-endothelial system. The endemic area for Japanese encephalitis spreads across Asia from Pakistan to the coast of Siberia and includes Japan.

The incubation period is 6 to 16 days. There is fever, headache, nausea, diarrhea, vomiting, and myalgia, which may last for several days. This may be followed by a spectrum of neurological disease ranging from mild confusion, to agitation, to overt coma. It is more common in children, while headache and meningism are more common in adults. Tremor or other involuntary movements are common.

Japanese Encephalitis-VAX was a formalin-inactivated vaccine derived from mouse brain against Japanese B Encephalitis, produced since 1992 by BIKEN (Japan). The new vaccine available in the UK is the Japanese Encephalitis Green Cross vaccine (GC vaccine).

SWINE POX

Swinepox is a worldwide disease of pigs caused by a virus of the family Poxviridae and the genus Suipoxvirus, which can survive outside the pig for long periods of time and is resistant to environmental changes. Symptoms include small circular red areas 10-20mm in diameter that commence with a vesicle containing straw-coloured fluid in the centre. After two to three days the vesicle ruptures and a scab is formed which gradually turns black.

The disease is most frequently seen in young pigs but all ages may be affected. After an incubation period of 1 week, small red areas may be seen most frequently on the face, ears, inside the legs, and abdomen. These develop into papules and, within a few days, pustules develop that change into small vesicles. The centres of the pustules become dry and scabbed and are surrounded by a raised, inflamed zone. Later, dark scabs form, giving affected piglets a spotted appearance. These eventually drop or are rubbed off without leaving a scar. The early stage of the disease may be accompanied by mild fever and dullness. Virus is abundant in the lesions and can be transferred from pig to pig by the biting louse (Haematopinus suis). The disease also may be transmitted, possibly between farms, by other insects acting as mechanical carriers. Recovered pigs become immune. There is no specific treatment. Eradication of lice is important.

FOWL POX

Fowl pox is a worldwide disease of poultry caused by viruses of the family Poxviridae and the genus Avipoxvirus. There are two forms of the disease. The first is cutaneous form(dry pox) that is spread by biting insects, especially mosquitoes that causes lesions on the comb, wattles, and beak. The second form is diphtheritic form(wet pox), which is spread by inhalation of the virus and causes a diphtheritic membrane to form in the mouth, pharynx, larynx, and sometimes the trachea. Symptoms include weight loss, reduced egg production, lesions, small whitish or yellowish areas, nodules or scabs, raised white or opaque nodules which may join to form yellow, cheesy, necrotic lesion. The virus, abundantly present in the lesions is transmitted by contact to pen mates through abrasions of the skins. Mosquitoes and other biting insects can have a mechanical role in the transmissions. Modified live fowl pox virus vaccines are available commercially.

COW POX

Cow pox is a disease of the skin that is caused by a virus known as the Cow pox virus. The pox is related to the vaccinia virus, and got its name from dairy maids touching the udders of infected cows. The cow pox virus is within the family Poxviridae and the genus Orthopoxvirus. The ailment manifests itself in the form of red blisters and is transmitted by touch from infected animals to humans. When it is gone, the person is immune to small pox. Cow pox virus has been found only in Europe and in adjacent parts of the former Soviet Union. Despite its name, the reservoir hosts of cow pox virus are rodents, from which it can occasionally spread to cats, cows, humans, and zoo animals, including large cats and elephants. Transmission to humans has traditionally occurred via contact with the infected teats of milking cows. However, currently, infection is seen more commonly among domestic cats, from which it can be transmitted to humans.

The pathology of the skin lesions caused by cow pox virus is similar to that of small pox. However, there is greater epithelial thickening and less rapid cell necrosis. There is also more involvement of the mesodermal tissues. The most significant pathological feature of cow pox is the presence of two types of cytoplasmic inclusion bodies: irregular B-type inclusion bodies, and numerous large, homogenous, acidophilic, A-type inclusion bodies.

Human cow pox usually responds to treatment with antivaccina immunoglobulin. However, this should be restricted to the most severe cases. Usually, the lesions regress spontaneously. Identification and isolation of animals infected with cow pox can help decrease the incidence of human infections.

FOOT & MOUTH DISEASE

Foot-and-mouth disease (FMD) or hoof-and-mouth disease is a highly contagious and sometimes fatal viral disease of domestic animals such as cattle, water buffalo, sheep, goats and pigs, as well as antelope, bison and deer. It is caused by foot-and-mouth disease virus. Seven main types of Foot and Mouth Virus are believed to exist that belong to the genus Aphthovirus of the family Picornaviridae. Picornaviruses are tiny viruses (27-30 nm across) that are not enveloped with an icosahedral capsid and contain a single strand of positive sense RNA.

The disease is characterised by high fever that declines rapidly after two or three days; blisters inside the mouth that lead to excessive secretion of stringy or foamy saliva and to drooling; and blisters on the feet that may rupture and cause lameness. Adult animals may suffer weight loss from which they do not recover for several months as well as swelling in the testicles of mature males, and in cows, milk production can decline significantly. Symptoms of the Foot and Mouth in Cattle include, Slobbering and smacking lips, Shivering, Tender and sore feet, Reduced milk yield, Sores and blisters on feet and Raised temperature.




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