Emerging Zoonotic Diseases - An Insight

Introduction:

The majority of the infectious diseases reported worldwide every year are of zoonotic origin. Nearly one billion zoonotic cases and millions of deaths because of the same have been reported globally. Statistics have documented 30 new human pathogens in the last three decades, of which 75 percent of them are zoonotic. The recent emergence of the Middle East respiratory syndrome coronavirus (MERS-CoV) has shown that these infections are unpredictable as they are of zoonotic origin and, unfortunately, are detected only during outbreaks. The eastern Mediterranean region is vulnerable to zoonotic infections because of the close proximity of the human population to animals and the increased volume of international trade involving livestock movement within neighboring countries.

What Is a Zoonotic Disease?

A zoonotic disease is defined as an infectious disease that spreads naturally through interaction between the human population and vertebrate animals.

What Are the Types of Zoonotic Diseases?

1. Endemic zoonoses which are already present in geographical areas and affect people and animals.

2. Epidemic zoonoses are sporadic in distribution.

3. Emerging and re-emerging zoonoses are new with respect to the population or might have existed previously, showing a sudden increase in cases in a geographic range.

How Serious Are the Zoonotic Diseases?

• The new emerging pathogens, especially viruses of animal origin, remain unpredictable and have the potential to spread across the countries.

• Because of the high epidemic potentiality, high case fatality, and the absence of specific treatment and vaccines to control the spread of these zoonotic diseases pose a major threat to global health in the interconnected world.

• Unfortunately, these zoonotic diseases lead to high morbidity and mortality rates and cause a burden on healthcare delivery systems. Also, the affected country’s economy is hit severely as the animal trade and other trading opportunities through the livestock are affected badly.

What Are the Emerging Zoonotic Diseases Encountered in Recent Times?

Influenza H1N1:

• The influenza A (H1N1) virus was never associated with human infections until the pandemic H1N1 occurred in 2009.

• WHO declared influenza a (H1N1) virus a pandemic in 2009 as it had spread to 74 countries and territories, which was confirmed by laboratory investigations.

• Etiology:

  • H1N1 influenza is also called swine flu. It is basically a highly contagious respiratory disease seen in pigs caused by swine influenza A virus.

  • The swine influenza virus is regarded as a zoonotic disease as it causes infections in humans through infected pigs or through areas contaminated with H1N1 viruses.

• Pathophysiology:

  • H1N1 swine flu is an acute infectious disease affecting the upper respiratory tract and trachea and sometimes involves the lower respiratory tract.

  • The virus causes inflammation of the upper respiratory passages.

  • The incubation period varies from one to four days, with an average of two days among most of the patients.

• Clinical Features:

  • Fever.

  • Myalgia.

  • Cough and chills.

  • Sore throat.

  • Runny nose.

  • Watery eyes.

  • Eye pain.

  • Body ache.

  • Headache.

  • Tiredness and weakness.

  • Diarrhea.

  • Vomiting.

• Diagnosis:

  • A nose or throat swab is taken to get the respiratory sample.

  • The sample is subjected to virus isolation tests such as RT-PCR (reverse transcriptase- polymerase chain reaction), and assays detecting a four-fold increase in influenza antigens in the sample are regarded as confirmatory tests in diagnosing H1N1.

• Treatment:

  • The mainstay of the treatment lies in providing symptomatic measures such as adequate bed rest, rehydrating with enough fluid intake, eating a light diet, and staying at home to avoid frequent exposure to the environment.

  • Acetaminophen is given to relieve fever and reduce body aches.

  • Antivirals like Oseltamivir or Zanamivir are prescribed to destroy the virus in the body.

Avian Influenza (H5N1):

• Etiology:

  • Human avian influenza is widely known as bird flu.

  • Humans are infected with avian viruses, such as avian influenza virus subtypes such as A(H5N1), A(H7N9), and A(H9N2).

  • The majority of human cases of influenza A (H5N1) and A(H7N9) virus infection are caused by transmission of the infection by infected live or dead poultry foods.

• Pathophysiology:

  • The infection is mostly commonly acquired by ingestion of poultry food contaminated with bird flu virus.

  • Once the virus gains entry into the respiratory system, it infects pulmonary epithelial cells, leading to diffuse alveolar damage and hemorrhage in the lungs.

  • The virus can also infect the trachea, intestine, and the brain. In pregnant women, the virus can pass through the placental barrier and infect the fetus.

  • The impairment of cytokine and chemokine activities is regarded as the main cause behind the pathogenesis of the H5N1 influenza virus.

• Clinical Features:

  • Fever.

  • Cough.

  • Myalgia.

  • Sore throat.

  • Nausea and vomiting.

  • Diarrhea.

  • Stuffy nose.

  • Conjunctivitis.

  • Dyspnea (difficulty in breathing)

  • Pneumonia.

  • Acute respiratory distress.

  • Respiratory failure.

• Diagnosis:

  • A rapid antigen detection test is advised to detect the viral antigen, and the results are available within 15 to 20 minutes.

  • A virus culture is done to detect clinically important respiratory viruses, including the avian influenza virus, and the results can be obtained in two to ten days. Both shell-vial and standard cell-culture methods can be used.

  • Polymerase chain reaction and Real-time PCR assays using primer sets that are specific for the hemagglutination (HA) gene of circulating influenza A/H1, A/H3, and B viruses are employed to confirm the diagnosis.

  • Results can be obtained within hours from either clinical swabs or infected cell cultures.

• Management:

  • Adequate rest is advised.

  • Painkillers like Acetaminophen or Ibuprofen are prescribed for relieving fever and body aches.

  • Antivirals like Oseltamivir, Peramivir, and Zanamivir are given to arrest the viral activity.

  • Fluid replenishment to combat dehydration and weakness.

  • Maintaining good hygiene helps in fighting the disease and aids in speedy recovery.

Middle East Respiratory Syndrome Coronavirus (MERS-CoV):

• Etiology:

  • Middle East respiratory syndrome (MERS) is a respiratory disease of viral etiology attributed to being caused by a novel coronavirus (MERS‐ CoV), and the first case came into light in 2012 in Saudi Arabia.

  • Coronaviruses are a large family of viruses that can cause diseases with varied symptoms, from the common cold to severe acute respiratory syndrome (SARS).

  • Recent outbreak investigations have revealed that the majority of human cases of MERS have been caused by human-to-human transmissions. However, camels serve to be a major reservoir host for MERS-CoV and a cause of zoonotic infection in humans.

  • The virus cannot be transmissible easily from person to person unless there is close contact by providing unprotected care to a patient.

• Pathophysiology:

  • MERS-CoV enters the human body, replicates in the airway epithelial cells, and suppresses the production of interferons.

  • However, unlike SARS-CoV, the MERS virus shows wider tissue tropism.

  • MERS-CoV also stimulates pro-inflammatory cytokines.

  • MERS-CoV virus is more pathogenic than SARS-CoV as it causes delayed pro-inflammatory response, which leads to diminished innate immunity.

• Clinical Features:

  • Fever.

  • Cough.

  • Shortness of breath.

  • Pneumonia.

  • Diarrhea.

  • Respiratory failure.

  • Death.

• Diagnosis:

  • Molecular tests like real-time reverse-transcription polymerase chain reaction (rRT-PCR) assays help detect viral RNA in clinical samples.

  • ELISA, or enzyme-linked immunosorbent assay, is a screening test used to detect specific antibodies that bind to the viral protein.

  • If a sample is tested antibody-positive by ELISA, then a microneutralization test is employed for a confirmatory diagnosis.

  • The microneutralization assay is a highly specific confirmatory test that measures neutralizing antibodies. The microneutralization assay is the gold standard test used for confirmatory diagnosis of MERS-CoV virus by detecting specific antibodies in serum samples.

• Treatment:

  • Painkillers like Acetaminophen and Ibuprofen are given for treating fever and joint pains.

  • Adequate bed rest is advised to fight against the illness.

  • IV fluids are administered to keep the body hydrated.

  • Supplemental oxygen or assisted ventilation is given to keep the body oxygenated and to avoid complications like respiratory distress.

Conclusion:

Emerging zoonotic infections are unexpected and unpredictable. Any disease outbreak anywhere today can be a problem for the world tomorrow. These novel diseases will continue to encounter and challenge the healthcare delivery systems’ resilience and responsiveness to respond on time. Global efforts should be made to bridge the current gaps in knowledge associated with the origin and transmission of many zoonotic infections, of which the majority of them are novel in origin. Hence, greater regional cooperation is required to protect the human population from all types of zoonotic infections.