Chapter 8 Epidemic and Prevention of Parasitic Diseases

For parasitic diseases to be prevalent in an area, the area must have various conditions required to complete the development of parasites, that is, There are three basic links of parasitic diseases: source of infection, route of transmission and susceptible groups. In addition, it is also affected by biological factors, natural factors and social factors. When these three factors are conducive to the spread of parasitic diseases, a considerable number of people in this area can become infected, causing the prevalence of parasitic diseases.

1. Basic links in the epidemic of parasitic diseases

(1) Source of infection

The source of infection of human parasitic diseases refers to those infected by human parasites Humans and animals, including patients, carriers and reservoirs (domestic animals, domestic animals and wild animals). As a source of infection, it exists in the body and can be excreted at a certain developmental stage in the parasite's life cycle, and can continue to develop in the outside world or in another host. For example, carriers or patients infected with multiple helminths can excrete worm eggs in their feces; carriers of Entamoeba histolytica can excrete cysts; eggs or cysts are infectious when they are excreted, or in a suitable external environment. develop to the infective stage (infectious phase). The infection stage refers to the developmental stage when the parasite invades the host and can continue to develop or reproduce.

(2) Transmission route

Refers to the process by which parasites are transmitted from the source of infection to the susceptible host. Common transmission routes for human parasites include the following:

Soil Intestinal parasites survive in the soil on the ground during the infection stage. For example, roundworm eggs and whipworm eggs develop into infective eggs in fecal-contaminated soil; hookworm and Strongyloides stercoralis eggs develop into infective stage larvae in soil. Human infections are associated with soil exposure.

Water Various parasites can reach the human body through fresh water. For example, water may contain amoeba and Giardia cysts in the infective stage, Taenia solium eggs, certain infectious nematode eggs, schistosomiasis cercariae and Fasciola brucei metacercariae, etc.

Food is mainly vegetables, fish and other foods. Since most rural areas use fresh manure for fertilization, vegetables often become the main route for parasite transmission. For example, infective roundworm eggs, whipworm eggs, Taenia solium eggs and infective stage larvae of hookworms, as well as original cysts, etc., can all be transmitted by eating unwashed or undercooked vegetables. Trichinella spiralis and Taenia solium tapeworm It can be spread by eating raw or undercooked pork. Some freshwater fish can transmit Clonorchis sinensis, etc.

Arthropod vectors, many medical arthropods can serve as vectors for a variety of parasites. For example, mosquitoes are Plasmodium and filarial worms, sandflies are Leishmania, and fleas are the vectors of hymenometa tapeworms.

Direct human-to-human transmission Direct human-to-human contact can directly transmit certain parasites. For example, Trichomonas vaginalis can be transmitted through sexual intercourse, and scabies mites can be transmitted through direct contact with the patient's skin.

The main routes and methods of human parasite infection are as follows:

Oral infection of various parasites during the infection stage can be through food, drinking water, contaminated fingers, toys or Other agents enter the body through the mouth, which is the most common way of infection. Such as roundworms, whipworms, pinworms, Clonorchis sinensis, cysticerci, etc.

Transdermal infection Some parasites actively invade the human body through the skin during the infection stage, such as hookworm filamentous larvae in the soil, schistosomiasis cercariae in the water, scabies mites, demodex, etc. directly invade the skin. Some parasites enter the human body through the skin through the stings of blood-sucking arthropod vectors. For example, mosquitoes transmit Plasmodium, filarial worms, and sandflies transmit Leishmania.

Self-infection Some parasites can cause self-reinfection in the host. For example, the eggs of Hymenometa brevis can hatch into six hookworms in the small intestine, and the larvae can develop into adult worms in the small intestine; The Taenia solium tapeworm parasitizes in the small intestine. The shed gestational segments are refluxed into the stomach due to vomiting and are digested. After the eggs reach the small intestine from the stomach, six hookworms hatch out, burrow into the intestinal wall and travel to various parts of the body with the blood circulation, causing Autoinfection with cysticerci.

Retrogradely infected pinworms lay eggs around the anus of the human body. The eggs can hatch near the anus, and the larvae enter the parasitic site in the intestine through the anus and develop into adult worms.

Transplacental infection Some parasites can pass through the placenta with the maternal blood and infect the fetus, such as Toxoplasma gondii, Plasmodium, hookworm larvae, etc.

In addition, some parasites can enter the human body through the respiratory tract, such as Pneumocystis carinii; such as Trichomonas vaginalis through the vagina; such as Plasmodium through blood transfusion and other ways to enter the human body.

(3) Susceptible groups

Susceptible people refer to people who lack immunity to parasites. After the human body is infected with parasites, it can usually develop acquired immunity, but it is mostly vector immunity. When the parasite disappears from the human body, the immunity gradually decreases and subsides. Therefore, it is easy to become infected with this parasite when there is an opportunity for infection. When people from non-endemic areas or areas where malaria has been eradicated enter malaria areas, they become susceptible due to lack of specific immunity. Susceptibility is also related to age, and children generally have lower immunity than adults.

2. Factors affecting the prevalence of parasitic diseases

(1) Natural factors

Including climatic factors such as temperature, humidity, rainfall, light, as well as geographical environment and Biological populations, etc. Climatic factors affect the growth and development of parasites in the outside world. For example, a warm and humid environment is conducive to the development of helminth eggs and larvae in the soil; climate affects the breeding activities and reproduction of intermediate hosts or vector arthropods, and at the same time, it also affects the growth and development of parasites in their bodies. The development and growth of parasites are restricted. If the temperature is lower than 15~16 degrees Celsius or higher than 37.5 degrees Celsius, malaria parasites cannot develop in mosquitoes. Warm and humid climate is not only conducive to the growth and reproduction of mosquitoes, but also suitable for mosquitoes’ blood-sucking activities, increasing the chances of spreading malaria and filariasis. Temperature affects the invasiveness of parasites. For example, the infectivity of Schistosoma cercariae to the human body is related to temperature. The geographical environment is closely related to the growth and development of intermediate hosts and the breeding and habitat of vector arthropods, which can indirectly affect the prevalence of parasitic diseases. Soil properties directly affect the development of eggs or larvae of soil-transmitted worms.

(2) Biological factors

The life history of parasites is indirect, and the existence of intermediate hosts or arthropods is a necessary condition for the prevalence of these parasitic diseases. For example, in my country The prevalence of schistosomiasis in the area south of the Yangtze River is consistent with the geographical distribution of oncomelania; the prevalence of filariasis and malaria is consistent with the geographical distribution and activity seasons of their mosquito hosts or mosquito vectors.

(3) Social factors

Including social system, economic status, scientific level, culture and education, medical and health care, epidemic prevention and health care, and people’s production methods and living habits, etc. The impact of these factors on the prevalence of parasitic diseases has received increasing attention. The natural factors and biological factors in a region are relatively stable within a certain period, while social factors are often variable, especially with changes in political and economic conditions, and can affect natural and biological factors to a certain extent. Economic and cultural backwardness is inevitably accompanied by backward production methods and lifestyles, as well as poor hygiene habits and sanitary environment. Therefore, it is inevitable that many parasitic diseases will become widespread and seriously endanger human health. Therefore, social factors are crucial in influencing the prevalence of parasitic diseases.