Overview of HIV/AIDS 

HIV, or the Human Immunodeficiency Virus, is a virus which infects and destroys CD4 cells, also known as T helper cells. Over time, this causes a reduction in the number of CD4 T cells, resulting in the compromise of the immune system, eventually making it harder for the body to fight off infections. Over time, without medical intervention, the viral infection can progress to AIDS (acquired immunodeficiency syndrome), the most severe stage of HIV. The body can no longer defend an individual from infections.

Impact of HIV/AIDS on the immune system

HIV has a major detrimental impact on the immune system through the destruction of CD4 T cells, a type of white blood cell essential for coordinating immune response against infection, working to activate with other cells in the immune system. A depletion in their numbers leads to an ineffective immune system activation when an infection occurs, preventing it from being cleared from the body.

HIV uses CD4 T cells to replicate, leading to their death as well as using the cells to spread across the body whilst escaping other immune system mechanisms, which may pose a threat to the virus. With the decrease in CD4 T cell numbers, the immune system eventually becomes incapable of coordinating an effective response against infections. Since the immune system relies on CD4 T cells to signal to other immune cells to respond to the infection, fewer CD4 T cells leave the immune system weakened, thus allowing infections to take hold, which would otherwise be cleared by the body.

As the immune system becomes less effective, an individual becomes more susceptible to what are known as opportunistic infections. Infections occur more often in individuals with compromised or weakened immune systems compared to those with a healthy immune system. In the absence of CD4 T cells, the body cannot fight off these infections, in extreme cases leading to severe illness or death.

HIV also causes inflammation in the body due to the immune system constantly trying to eliminate the virus; with time, such inflammation can damage tissues and organs. Such ongoing battles against HIV exhaust the immune system, reducing its ability to function.

Overview of parasitic infections 

Common parasitic infections

A parasite refers to an organism that lives on or inside a host, deriving its nutrients from the host at the expense of the host. Parasites can infect any organism, from humans to animals to plants and are classified into three types.

Protozoa or single-cell parasites are microscopic organisms that live inside human cells or tissues where they multiply. Examples are:

• Plasmodium spp
• Toxoplasma gondii
• Giardia lamblia

Helminths are worm-like parasites, larger than protozoa and often visible to the naked eye in their adult form. Helminths live inside the ‘tubes’ of the body, such as the intestines, blood vessels or muscles of the host. Examples are:

• Roundworm
• Blood fluke
• Tapeworm

Ectoparasites, or external parasites, live on the surface of the host and can transmit other diseases through their bites. Examples include

• Ticks
• Fleas
• Lice 

Prevalence of parasitic infections in regions with high HIV/AIDS rates

Prevalence of parasitic infections often coincides with high HIV rates due to overlapping risk factors such as poverty, inadequate healthcare and poor sanitisation. Individuals with HIV are vulnerable to severe complications from infection due to a weakened immune system. Factors contributing to a high prevalence of parasitic infection in regions with high rates of HIV are:

• Geographic overlap - regions across the world with high HIV rates also experience high rates of parasitic disease due to environmental conditions that favour the spread of parasitic diseases.
• Weakened immune systems
• Lack of access to healthcare

Increased susceptibility to opportunistic infection

Parasites are opportunistic pathogens in the case of individuals infected by HIV due to the already weakened immune system. Opportunistic pathogens are organisms not typically cause serious illness in healthy individuals, but in the case of immune-compromised individuals, can lead to severe, life-threatening infections. If left to progress, HIV lowers the body’s ability to fight infection, thus allowing parasites to thrive and cause further health problems.

Since HIV progressively depletes CD4 cells, often essential for mounting an immune response through activation of other immune cells, HIV-infected individuals become more vulnerable to parasitic infections

Common parasitic infections in individuals with HIV/AIDS

Toxoplasmosis

Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii, often contracted by eating undercooked meat or through contact with cat faeces. The infection is often asymptomatic in individuals with a normal immune system.¹ However, the parasite poses a risk to those with a weakened immune system, such as individuals with HIV/AIDS, with the possibility of causing encephalitis, since the primary region it resides in is the brain and retina.¹ Toxoplasmosis can go on to cause cerebral toxoplasmosis, a potentially fatal condition caused by Toxoplasma gondii, leading to seizure, confusion and coma in individuals with HIV who have a low CD4 T cell count.¹ It is commonly detected by the presence of ring-enhancing brain lesions seen on MRI scans

MRI of the brain with a “white ring” due to the presence of Toxoplasma gondii, from wiki images.

Cryptosporidium parasite

Cryptosporidiosis is an infection caused by the Cryptosporidium parasite² and is known to cause diarrhoea. The downstream effect of this is dehydration and malnutrition, leading to further weakening of an already immunocompromised patient. A diagnosis of Cryptosporidiosis can be made by testing stool samples for the parasite, its antigen and its eggs.²

Leishmaniasis

Leishmaniasis is a disease caused by protozoa transmitted by sandflies, caused by the Trypanosomatida parasite. It can cause various diseases, but the one most often seen in those suffering from HIV is visceral leishmaniasis. This condition can lead to damage to the internal organs, severe anaemia and potential death in immunocompromised individuals.³

Malaria

Malaria is caused by the Plasmodium parasite and is transmitted through the bite of a Mosquito. Co-infection with malaria and HIV is common. Symptoms of malaria are fever, chills, headaches, nausea and vomiting, diarrhoea, abdominal pain and muscle or joint pain.⁴ It is often diagnosed through microscopic examination of blood films or Rapid Diagnostic Tests. Malaria is one of the parasitic infections with can accelerate the progression of HIV to AIDS, leading to an increased HIV viral load. Additionally, chronic infection can cause severe immune activation, contributing to the depletion of CD4 T cells and speeding up the deterioration of the immune system.

Summary

The interaction between HIV and parasites is both complex and mutually works to promote the negative effects of each other. Since HIV weakens the immune system by depleting CD4 T cells, the individual becomes more vulnerable to parasitic infection since their immune system cannot effectively mount a response against the parasite. On the other hand, parasitic infections can cause chronic immune activation and inflammation, contributing to HIV replication and CD4 T cell depletion. Parasitic infections compromise the immune system through exhausting immune resources, allowing the entry of opportunistic agents. Overall, co-infection between HIV and the parasite contributed to faster disease progression in both cases, exhausting the individual's immune system and increasing morbidity.