Modelling the Effects of Carriers on Transmission Dynamics of Infectious Diseases

CHAPTER ONE

1.0 INTRODUCTION

1.1 BACKGROUND OF STUDY

For certain infectious diseases, there are individuals who are able to transmit their illness but do not exhibit any symptoms. These individuals are called “carriers” and they play an important role in the transmission of the disease. There are two types of carriers. Genetic carriers carry the illness on their recessive genes. They can only pass on their disease to their children and are not contagious. The focus of our study is on infectious disease carriers. These individuals are asymptomatic and are likely unaware of their conditions, and therefore are more likely to infect others. An infectious disease that produces long-term asymptotic carriers is the Typhoid fever caused by the bacteria Salmonella Typhi. Typhoid fever reached public notoriety at the beginning of the 20th century with the cases of “Mr. N the milker” in England and Typhoid Mary in the US. These individuals infected hundreds of people over the decades while they worked  in  the food  production  industry and private homes. Even today,  Typhoid fever infects 21 million people and kills 200,000 worldwide every year. Asymptomatic carriers are believed to play an essential role in the evolution and global transmission of Typhi, and  their  presence greatly hinders the eradication of Typhoid  fever using treatment and vaccination.

Another major infectious disease that causes long-term asymptomatic carriage is hepatitis B, a liver disease caused by the HBV virus of the Hepadnavirus family. Most people infected with HBV recover completely and develop a lifelong immunity to the virus. However, about 5-10% of adults will develop chronic HBV infection, and 15-25% of these will develop liver disease. Hepatitis B’s symptoms include jaundice, abdominal pain, nausea, fatigue and joint pain. About 30% of people with the disease do not show any of these symptoms. A major public-health challenge in the control of hepatitis B infection in many countries is the existence of a large pool of chronic carriers who are responsible for transmitting most of the new infections. Infections of other pathogens are also know to produce asymptomatic carriers. The Epstein-Barr Virus (EBV) of the herpes family is one of the most common viruses in humans.  EBV infection commonly causes infectious mononucleosis, also known as glandular fever. Most people infected with EBV are asymptomatic, as it remains dormant in those who have had it for the rest of their lives in the cells of the throat and the immune system. Clostridium difficile is a bacterium that causes Clostridium difficile-associated diseases (CDAD). CDAD remains the most common cause of acute hospital-acquired diarrhoea, responsible for more than 300,000 cases of diarrhoea annually in acute-care facilities in the United States. Asymptomatic carriage rates of up to 30% have been reported in long-term care facilities. It is believed that carriers are responsible for transmission and large outbreaks of CDAD in Europe and North America.

Despite their public health significance, the effects of carriers on the transmission dynamics of the disease have not received adequate research attention in the mathematical modelling literature. One of the earlier attempts was Kempe, in which a general mathematical model that incorporates disease carriers was developed and analyzed. Medley et al. used a mathematical model for hepatitis B with carriers to discuss the effects of HBV vaccination. Several other studies using large-scale computational models with carriers are specifically aimed at hepatitis B and other diseases

1.2 STATEMENT OF PROBLEM

What really instigated the study was the prevalence of highly infectious diseases in Nigeria. the growth of diseases in Nigeria and most African countries has increased overtime. It is evident that researchers have put in so much effort to come out with model on the effect of carriers on transmission dynamics of infectious diseases but some prove abortive.

1.3 AIMS AND OBJECTIVES OF STUDY

The main aim of the research work is to examine the modelling of the effect of carriers on transmission dynamics of infectious diseases. Other specific aims of the research work are:

  1. To investigate the effects of carriers on the transmission dynamics
  2. To determine the basic reproduction number and through numerical simulations
  3. To investigate on the factors affecting the model
  4. To proffer solution to the above model

1.4 RESEARCH QUESTIONS

The study came up with research questions so as to be able to ascertain the objectives of the study. The research questions are stated below as follows:

  1. What are the effects of carriers on the transmission dynamics?
  2. How to determine the basic reproduction number and through numerical simulations?
  3. What are the factors affecting the model?

1.5 SIGNIFICANCE OF STUDY

The study on the modelling the effects of carriers on transmission dynamics of infectious diseases will be of immense benefit the ministry of health, hospitals, the state government and other researcher that wishes to carryout similar research as the study will be able to discuss the effects of carriers on the transmission dynamics and the factors affecting the model

1.6 SCOPE OF STUDY

The study on modelling the effects of carriers on transmission dynamics of infectious diseases is limited to the modelling of selected infectious diseases like typhoid; how they are transmitted and the model for their transmission.

1.7 DEFINITION OF TERMS

DISEASES:  is a particular abnormal condition, a disorder of a structure or function that affects part or all of an organism. The study of disease is called pathology which includes the causal study of ethology

CARRIERS: individual harbouring specific pathogenic organisms who, though often immune to the agent harboured, may transmit the disease to others

INFECTIOUS: liable to be transmitted to people, organisms, etc. through the environment.