How is hepatitis A transmitted?

Source of Hepatitis A Infection

Hepatitis A is a typical anthroponotic infection, i.e. the source of infection is a person with an obvious or latent form of the disease. The indications of other authors of the discovery of the hepatitis A virus in some species of monkeys cannot shake the confidence in the anthroponotic nature of the infection, since, on the one hand, these findings are quite rare, and on the other hand, in these cases the possibility of infection of monkeys from humans is not excluded.

The main role in the active maintenance of the epidemic process is played by patients with hepatitis A, especially atypical forms - latent, anicteric and subclinical. Remaining unrecognized in most cases, patients with these forms continue an active lifestyle, attend organized children's groups and thus become a hidden - and therefore often powerful - source of infection. It should also be taken into account that even with incomplete accounting, the number of patients with latent, anicteric and subclinical forms significantly exceeds the number of patients with typical icteric forms of the disease. This is especially characteristic of organized children's groups.

The greatest epidemiological danger is posed by patients starting from the end of the incubation period and throughout the pre-icteric period. The duration of the active replication and virus secretion phase varies from 15 to 45-50 days, most often does not exceed 2-3 weeks. With the onset of jaundice, the concentration of the virus in the blood quickly decreases, and it usually cannot be detected by conventional research methods. Using highly sensitive methods for detecting the hepatitis A virus antigen, it was found that at the end of incubation and prodrome it is determined in almost all patients, in the first week of the icteric period - only in half, in the second week - in 20-21%, in the third - in 5% of patients. Similar data were obtained by numerous researchers on a large clinical material and in experimental hepatitis in animals (marmosets). It is also important to take into account the fact that there is no significant difference in the frequency of detection of hepatitis A virus antigen in the feces of patients with various forms of hepatitis A (icteric, anicteric, erased, inapparent). This once again confirms the enormous importance of atypical forms of the disease in maintaining a continuous epidemic process in hepatitis A.

Mechanisms of transmission of hepatitis A

The causative agent of hepatitis A is transmitted exclusively by the feco-oral route, through contaminated food, water and through household contact. The virus is excreted only with feces, and it usually gets into food when basic hygiene rules are violated during processing and preparation; water is infected by contamination of water sources with household discharges. The literature has widely described food, water, and contact-household outbreaks of hepatitis A among both adults and children. In this case, various food products (oysters, orange juice, dried melon, milk, ice cream, cottage cheese, etc.), water from open reservoirs, wells, and ditches acted as transmission factors for the infection. Water outbreaks are usually characterized by an explosive nature, when mass diseases occur simultaneously within a short period of time, united by the use of water from one source.

The role of flies as a factor in the transmission of infection is probably exaggerated, but under conditions of low sanitary culture and overcrowding, flies can carry the infectious agent to food products or drinking water. Large epidemics allegedly caused by the fly factor were mainly described before the use of highly sensitive specific methods for verifying hepatitis A and therefore require critical re-evaluation. The same can be said about the airborne transmission of the hepatitis A virus. Studies in recent years have not confirmed the possibility of finding the hepatitis A virus on the mucous membranes of the respiratory tract, which excludes the airborne transmission of infection.

Parenteral transmission of infection is allowed, which, naturally, can only occur if the patient's blood containing the virus enters the recipient's bloodstream. Theoretically, this is, of course, possible, but it is realized, apparently, extremely rarely due to the instability of the virus in the blood.

Transplacental transmission of hepatitis A virus from mother to fetus is excluded by almost all researchers due to the inability of the hepatitis A virus to overcome the transplacental barrier.

Hepatitis A can rightfully be called the "disease of dirty hands". This is how epidemic foci of hepatitis A arise in kindergartens, schools and other organized groups, as well as on playgrounds and in isolated families. Diseases occur simultaneously in small groups with the consistent involvement of new contingents of susceptible children in the epidemic process. In schools, pioneer camps, mobile kindergartens, hand infection most often occurs in public places (public toilets, playgrounds, canteens), as well as through contact with door handles, railings, shared toys, etc. With this route of infection, it can be difficult to establish the source of infection, which is why hepatitis A has a fairly high percentage of sporadic morbidity. This is also facilitated by the low infectious dose of the pathogen.

Hepatitis A is characterized by seasonal increases and periodicity of incidence. Seasonal increases are more clearly observed among children. The increase in incidence occurs in the autumn-winter period with a predominant maximum in October-December, which is typical of the feco-oral mechanism of infection transmission. The relatively later onset of the peak incidence, compared to classic gastrointestinal infections (shigellosis, salmonellosis, etc.), can be explained by a longer incubation period for hepatitis A. The increase in hepatitis A incidence in the autumn-winter period can also be explained by a sharp increase in mass contacts due to the start of school, an increase in the occupancy of preschool institutions at this time of year, and the accumulation of children in closed spaces, which, as is known, complicate the implementation of generally accepted sanitary and hygienic measures.

Periodic increases in morbidity are characterized by strict cyclicity, they are repeated at intervals of 10-12 years. In the territory of the former USSR, an increase in morbidity was noted in 1960-1962, then until 1970 a gradual decrease was observed, but already starting in 1971, the incidence began to gradually increase and reached a maximum in 1983. The reason for such periodicity is the fluctuation of collective immunity, which is confirmed by the results of a study of the dynamics of accumulation of specific antibodies both in children in the age aspect and in adults. At present, the incidence of hepatitis A in our country has a pronounced downward trend.

Hepatitis A virus susceptibility

Experiments on volunteers have shown that susceptibility to the hepatitis A virus is determined solely by the presence or absence of specific antibodies. The dose and virulence of the pathogen are important, as well as the genotypic and phenotypic heterogeneity of some people with respect to hepatitis A infection. The contagiousness index for hepatitis A approximately fluctuates between 0.2 and 0.8 (0.4 on average), i.e. out of 100 contact children who have not had hepatitis A, about 40 become ill. In children in their first year of life, the contagiousness index approaches 0 due to the presence of transplacental immunity. At the age of 3 to 7-9 years, the contagiousness index increases to 0.6-0.8, in adults it decreases to 0.2 or more.