Viral Zoonoses: Diagnosis, Treatment & Control Measures

Viral zoonoses are infectious diseases caused by viruses that are transmitted from animals to humans. These diseases can involve multiple-host infections and are typically referred to as zoonotic because they originate in non-human vertebrate animals such as mammals, birds, reptiles, and potentially amphibians. These animals can serve as reservoirs or amplifying hosts, often carrying the viruses without exhibiting significant symptoms.

Zoonotic viruses may spill over into human populations through direct contact with animals, environmental exposure, or intermediary hosts. Some well-known viral zoonoses include rabies, Ebola, hantavirus, and avian influenza. Mitigating the risk of viral zoonoses involves understanding the transmission pathways, promoting good hygiene, controlling animal populations, and developing vaccines where applicable.

This thread shares information about viral zoonoses of significance to the food system.

 

[Kev]

Rabies​

Viral zoonoses are acute and affect all warm-blooded animals and humans. The major characteristics of these diseases include hyper-excitability, paralysis, and death.

Aetiology of rabies​

They are caused by an RNA virus that is very sensitive and will be inactivated by:

• Getting it out of the host
• Ordinary environmental conditions like heat and light
• Disinfectants

It is however resistant to freezing. Freezing it actually preserves it.

Epidemiology of rabies​

• When a viral zoonosis occurs in humans or animals, it will cause acute symptoms.
• Carnivores tend to be the natural hosts of viral diseases. Infected animals have very high concentration of the virus in their saliva before any clinical sign of infection occurs.

Viral zoonoses have a global presence.

Transmission of rabies​

• Viral zoonoses spread through bites from the natural host or contact with body fluids from an infected warm-blooded host.
• The GIT may also be a route though there aren’t any documented cases.

Pathogenesis of rabies​

• The virus is introduced through a bite. It into the peripheral nerve and moves to the central nervous system through motor action.

Clinical signs of rabies​

• The signs are typically manifested in dogs.
• Onset of the disease varies with the site of the bite. Incubation period takes long period (3 – 8 weeks on average though it can be as short as 10 days or as long as 6 months).
• The symptoms of the disease takes two forms in dogs:
  • Furious form
  • Dumb form

Furious form​

• This is the most common form of rabies in carnivores. It appears in three stages:

Prodromal phase​

• Takes 2 – 3 days
• A slight increase in temperature
• Dilation of the pupils

Excitable phase​

• Lasts 3 – 7 days
• The animal is very alert and excited. It can move over very long distances while barking and biting until its voice becomes coarse.

Paralysis phase​

• Starts from the head moving down to the limbs.
• Paralysis of the mouth leads to excessive salivation, dropped jaw, and the animal appear to choke whenever it tries to swallow anything.
• When paralysis reaches the limbs, the animal will exhibit uncoordinated movement and it flops. Death will follow in 3 – 4 days.

Dumb form of rabies​

• The dog starts with the prodromal phase but skips the excitable phase. It goes straight to paralysis phase.

Rabies in cattle​

• Rabid cattle are furious. They bellow a lot and are aggressive but they do not bite. Paralysis and death follows eventually.
• Cattle are dead-end hosts of rabies because they do not transmit it to any other host.

Rabies in humans​

• People get the disease from a carnivore’s bite. Incubation period is between 30 – 60 days, which also depends on the site of the bite.
• Initial symptoms are usually general, e.g. headache, general malaise, slight fever, nervousness, anxiety, and a tingling sensation at the bite site.
• After these signs, the characteristic symptoms for humans will set in. They include:
  • Hydrophobia (the victim fears water) due to the painful spasms in the pharyngeal muscles on attempt to swallow anything, including saliva.
  • Excitability
  • Convulsions will set in followed by paralysis and death.

Diagnosis of rabies​

• Confine animals suspected to have rabies. After their death, use their brain for diagnosis (check the negri bodies).
• You can also do serological tests.
• In humans, the patient’s history is very important in diagnosis.

Treatment of rabies​

• Once the disease has started in animals, it is untreatable. It will progress until the host dies.
• In humans, give repeated vaccinations as soon as you realize the patient is suffering from rabies.

Control measures for rabies​

1. Prevent occurrence
2. Vaccinate all carnivores
3. Destroy all non-vaccinated wild and domestic carnivores when an outbreak notification is issued
4. People at risk should receive regular vaccinations
5. In case of an outbreak, issue a notification and pronounce quarantine then begin vaccination of all animals.

 

[Kev]

Rift Valley Fever​

• This disease was first diagnosed in Naivasha in 1931. It caused abortions in sheep and had high mortality rates of up to 95 percent.
• In 1951, a similar syndrome occurred in South Africa in sheep. People who came into contact with the sick animals also got sick. One hundred people died and about 100 000 sheep and cattle were infected.
• The disease then spread to other parts of Africa. No incident of this disease has ever been reported outside Africa.

Aetiology of rift valley fever​

This disease is caused by the insect-borne arbovirus. This virus has high affinity for the liver hence it majorly affects the liver.

Transmission of rift valley fever​

• In animals, mosquitoes are majorly responsible for the transmission, though other insects are suspects as well.
• People can be infected by handling aborted material from sick animals. Sick animals also shed the virus in milk, which becomes a route of transmission in humans. Human to human transmission is not documented.

How to recognize rift valley fever in animals​

• Incubation period lasts 1 – 2 days
• High rates of abortion in sheep
• High mortality rates (90 – 100%) in lambs. Most of the deaths occur within 36 hours of the onset of the disease.
• Disease may also occur in mild form (no abortions, the animal just gets some diarrhea).
• Discharge from the nostrils.

Manifestation of rift valley fever in humans​

1. People are susceptible to this disease once exposed to the infected material / affected animals. The most common transmission routes include handling of aborted material, the sick animals themselves, and the lab samples.
2. Incubation period is 4 – 6 days
3. Influenza (flu) like symptoms
4. Flushed face for the light-skinned people
5. Epigastric pain occurs in the advanced stages of the disease
6. The eyes will be affected, and the victims may experience blurred vision, photophobia, and blindness if death does not occur.
7. Death is a very common occurrence in humans

Diagnosis of the rift valley fever​

• Clinical signs are very clear and provide a strong tentative diagnosis.
• Take post-mortem lesions in dead cases.
• Use necrotic hepatitis cells (dead cells) of the liver for diagnosis
• Confirm the virus through viral isolation
• You can also perform serological tests.

Control measures for the rift valley fever​

1. Prevent occurrence in animals by vaccination.
2. Control insects, especially in breeding zones
3. Carefully handle animals and animal products and byproducts
4. Ensure minimal contact between animals and humans.

 

[Kev]

Haemorrhagic fevers (a group of diseases)​

Majorly associated with monkeys. Cause haemorrhagic syndrome in humans. They include:

1. Yellow fever​

Common in Africa, South America, and several other Asian countries

Aetiology of yellow fever​

• Caused by yellow fever virus. Natural hosts are primates. The virus is transmitted by insects from the primates to humans.
• Forest workers and hunters are more susceptible.
• The disease is apparent in animals but humans get severe symptoms such as:
  • Generalized haemorrhage affecting internal organs
  • High fever
  • Liver and kidney failure leading to severe jaundice and haemoglobinuria (haemoglobin in urine)
  • Death is common

Control measures for yellow fever​

1. Control mosquitoes
2. Take precaution when dealing with primates
3. Vaccinate in face of exposure

2. Burbury fever​

It is caused by a primate-associated virus. Lab workers are mostly affected, and they contact the virus through aerial contamination.

Recognition of burbury fever​

The disease is inapparent in animals, but people experience acute fever, vomiting, and diarrhoea. Lymphadenopathy (swollen lymph nodes), haemorrhage, and death is quite common.

Control measures for burbury fever​

Take a lot of care when handling patients

3. Lassa fever​

Lassa virus is responsible for causing Lassa fever. The virus is common in rodents and people contact it from them via aerial contamination.

Symptoms of lassa fever in humans​

• High fever
• Vomiting and diarrhoea
• Serious pneumonia, which starts as an inflammation of the upper respiratory tract
• Haemorrhage
• Death due to circulatory failure

Control measures for lassa fever​

• Control rodents
• Vaccination
• Ensure safety while handling patients

4. Ebola fever​

Ebola is a very severe and fatal disease in humans. It is associated with primates and is caused by the ebola virus, which has many strains including:

1. Zaire sub-type (extremely severe)
2. Reston
3. Sudan
4. Cote de Ivoire

It was first confirmed in the Democratic Republic of Congo (DRC) but has since spread to other parts. In DRC, it occurs mainly as the Zaire sub-type.

Transmission of ebola fever​

It is transmitted through contact with the body fluid of an infected patient.

Symptoms of ebola fever​

1. Very high fever
2. Severe weakness
3. Back pain
4. Headaches
5. Diarrhoea
6. Blood may ooze from body orifices
7. People get rash
8. Liver and kidney failure followed by death

Control measures for ebola fever​

1. Take a lot of care while handling patients
2. Vaccination
3. Reduce risk of sexual transmission
4. Minimize contact with primates
5. Practice outbreak containment e.g. proper burial (or cremation) of the dead