Modes of Transmission of Diseases

Introduction

Any deviation from the normal functioning of the body is known as a disease. Based upon the transmission, there are two types of diseases; viz., Communicable diseases and Non- communicable diseases. The transfer of any communicable disease from a diseased person to a normal person is known as disease transmission. Diseases are transmitted by two modes −

• Direct mode of transmission

• Indirect mode of transmission

In direct transmission, there is direct physical contact between the infected person and the normal person whereas in indirect transmission lack of physical contact between the two is observed.

Direct Mode of Transmission

A disease can be transmitted by direct means through air droplets, direct physical contact (includes sex), contact with the soil, skin or mucosal inoculation, and through the placenta. Diseases transmitted by direct contact include HIV, genital herpes, syphilis, gonorrhea, chlamydia, etc.

HIV (Human Immunodeficiency Virus)

Genome

It contains 2 copies of ss RNA and belongs to the Retrovirus family.

Transmission

It is transmitted by contaminated fluid which includes having unprotected sex, infected blood transfusion, and from mother’s milk. Also transmitted through needles and from mother to fetus. It cannot be transmitted via tears or saliva.

Symptoms

It shows a considerable incubation period ranging from weeks to months. Symptoms are more like influenza. In case of severe infection, there occurs secondary immunodeficiency disorder which is known as AIDS. In AIDS, the immune system becomes weak, and cancer can develop. HIV-infected individuals become infected with Mycobacterium, Plasmodium, etc.

Structure

Important proteins

RTase

It is RNA-dependent DNA polymerase that converts viral RNA into cDNA.

Integrase

It integrates cDNA into host nuclear DNA (provirus).

TAT

It binds to the Transcription activity region (TAR) of viral RNA to promote transcription.

Protease

It is required for viral packaging.

Target Cell and Process

Target cells are CD4 T- helper cells, monocytes, macrophages, and dendritic cells. GP-120 recognizes CD4 on the surface of Thelper cells and the viral genome enters by endocytosis. HIV also involves coreceptors on immune cells- CXCR4 and CCR5. Viral RNA is converted into cDNA by RTase. Proofreading is absent in the RTase enzyme which increases the rate of mutation in the genome. Viral components are produced and assembled with the help of protease. Viral cDNA is integrated into the host genome and viral particles infect each other. CD8 Cytotoxic- T cells are activated and start killing T- helper cells and other HIV-infected cells.

Diagnosis

ELISA of p24 capsid protein detection, ELISA of anti-gp-120 Ab detection, qRTC - PCR for viral RNA detection. The ELISA is based on antigenantibody interaction. In qRTC-PCR cDNA is formed from RNA by the action of reverse transcriptase and this cDNA is used as a template in this PCR.

Treatment

• Highly active antiretroviral therapy (HAART).

• RTase inhibitor: E.g.: Zidovudine/Azidothymidine, Tenofovir.

• Protease inhibitors: Virocept, Crixivan, etc.

Immunization − Recombinant vector vaccine phase 4 trial.

Indirect Mode of Transmission

Indirect modes of transmission can be Vehicle-borne, Airborne, Vector mediated (Biological and Mechanical vectors), Fomite bearing, etc. Examples of diseases that are transmitted by indirect mode include Acute diarrhea, Typhoid, Cholera, Polio, Hepatitis A, B, Malaria, etc.

Tuberculosis (TB)

It is caused by Mycobacterium tuberculosis and is an airborne disease. It imposes a high economic burden on Asian countries.

• Symptoms − Chronic sneezing, blood in the mucus, cavities in lungs, fever, light chills, weight loss.

• Problems Associated with TB − Development of Multidrug resistant (MDRTB) strain. BCG vaccine does not work.

• Feature and Process of Infection− Division time is 20h and contains high lipid content in the cell wall. It primarily infects alveolar macrophages (lungs). Extrapulmonary TB can occur in the intestine, brain, liver, etc., (in mostly immunocompromised patients).

• Detection − X-ray examination, ELISA, Mantoux tuberculin skin test, Acid-fast staining of a lung sputum sample, qRT-PCR.

• Treatment − Two important drugs are used:

• Isoniazid: It inhibits the cell wall synthesis of bacteria.

• Rifampicin: It blocks bacterial RNA synthesis.

• Immunization − BCG is used worldwide for immunization.

Influenza

• Genome − Segmented copy of ss RNA.

• Infection and Symptoms − The virus is airborne.

• Common Symptoms − High fever, running nose, headache, muscle pain, etc.

• Extreme Symptoms − Diarrhea, heart attack, brain infection.

• Family − Type A, B, and C cause infection in humans and other animals like pigs, horses, and birds.

• Common Virus in Humans − H1N1 type A virus. It is weak but spreads faster and H5N1 is strong but spreads slowly.

• Target Cells and Process −Primary target of the virus is lung epithelial cells but later it can infect other organs. Two important viral glycoproteins are Hemagglutin (H) and Neuraminidase(N).

• Process − Hemagglutin helps in the entry of viruses into the host cells. It recognizes surface sugars like sialic acid/NANA on the plasma membrane. Neuraminidase helps in the release of viral particles from host cells after removing Neuraminic acid/NANA/Sialic acid from the viral surface. During successful infection, the virus exploits the host M2 protein ion channel for creating an acidic environment required for viral fusion.

• Diversity in Influenza Virus − Two processes are involved

• Antigenic drift − It is due to mutation in H and N coding genes.

• Antigenic shift − Due to the re-assortment of H and N.

• Diagnosis − X-ray of lungs, qRT-PCR for detecting viral RNA, Ab testing.

• Treatment − Drugs like Neuraminidase inhibitor Oseltamivir, and M2 protein inhibitors like Adamantane.

• Immunization − Heat-killed virus is given.

Conclusion

A disease is any harmful deviation from normal functionality of the body and the spread of disease from an infected person to a healthy person is called transmission. A disease can be communicable and non-communicable. A disease can be transmitted directly or indirectly. Direct transmission occurs through direct contact with air droplets, through the skin, body fluids, and blood. Examples include AIDS etc. Indirect transmission occurs through contaminated substances, vectors, animals, and environmental factors. Examples include malaria, typhoid, influenza, T.B, and Chickenpox.

FAQs

Q1. Which disease is caused due to contact with soil?

Ans. Tetanus and mycosis. In tetanus, there is an acute and painful contraction of muscles. Contractions especially involve the jaw and neck whereas mycosis is a fungal infection that affects organs like skin and lungs.

Q2. Which disease can be transmitted from mother to fetus through the placenta?

Ans. Syphilis and AIDS are the diseases that can be transmitted through the placenta and can infect the fetus.

Q3. What are fomites and name the diseases associated with them?

Ans. Fomites are substances that are infected by some infectious secretions from an infectious patient and have the full capacity to harbor and transmit that infectious agent to a normal healthy person. Diseases associated with fomites include diphtheria, typhoid fever, bacillary dysentery, and hepatitis A.

Q4. What is DOTS?

Ans. It is a program against TB in India for creating awareness about the treatment and spread of TB.

Q5. What does, BCG stands for?

Ans. BCG stands for Bacillus Calmette-Guerin vaccine, given for TB. BCG contains an attenuated strain of Mycobacterium bovis.