Chagas disease, with a high incidence rate, is a resiliant and effective pathogen causing acute and chronic disease. There is no cure, prophylactic treatment or vaccination. Biotechnology research explores several opportunities for vaccination development including genetic vaccines and plant extracts.
Chagas' disease (American trypanosomiasis), is a protozoan pathogen, Trypanosoma cruzi is carried by an blood sucking insect vector, the triatomine bug (Reduviidae)(1) affecting between 11-18 million people each year (2) killing 50,000 (3). In the year 2000 population prevalence was 1 million cases (4), adding hugely to the global burden of disease. There is currently no cure for the disease and available treatments are used selectively due to their cost, efficacy and side effects. the parasite is a zoonoses(5), with a resevoir of infection in most domestic mammals(6), particularly those in close contact with the population of poorer affected South American countries.
The disease is endemic in South American countries, where control methods are bringing down the incidence of the disease to manageble levels(7). However, money spent on vector control and drugs used to allay symptoms could be saved if a cheap, effective and safe vaccine became available;research into genetic vaccines, viable virulence factors of T.cruzi, the T.cruzi genome and plant extracts are opening up new possibilities that may well increase the probability of a vaccine for immunisation against Chagas' disease within the next ten years.
The aetiological agent of Chagas is a protozoan flagellate T.cruzi, a zoonose transmitted to humans via an insect vector, the triatomine bug (Reduviidae). Triatomine bugs bite the human or mammals/birds within the reservoir of infection, leaving behind T.cruzi infected faecal matter collected from a previous bite. The bite causes itching and leads to the invasion of T.cruzi via the broken skin. T.cruzi then circulate in the blood and infect new tissue sites. (See lifecyle diagram)(8)
That T.cruzi is a flagellate may be of great importance when considering the recent testing of Chagas' disease patients testing a vaccine containing purified, extracted T.cruzi paraflagella rod proteins (PFR’s), the results of which have shown that both types of immune response were produced, and the production of interferon-gamma and the required CD lymphocytes increased (9). See treatment and control.
There is a large mammalian reservoir of infection in which the pathogen can reside and later re-infect the local population.(8) Climatic conditions in South American countries are ideal for the triatomine bug as are living conditions; the triatomine bug resides in the thatched roofs and mud walls of poorer domestic dwellings. Elimination of the habitat provided by humans in the form of these 'mud' building by exhanging the roofs and walls for tin may be a useful means of control. It is, however, feared that apart from the domestic species that now inhabit homes and animal shelters such as T.infestans, wild species are moving into domestic dwellings from the surrounding flora and posing further threat to the communities most at risk.(10)
The extreme antigenic variation (11) and the probability of the required activation of both arms of the immune system-{to cope with intra and extracellular pathogens} need to be considered when creating a vaccine for Chagas' disease. Clonal variants within each population of T. cruzi means that not all antigens are shared by allT. cruzi populations.
Trypanosomes have a large genome and can switch between immunological evasion methods to suit the environment of the host, with most mechanisms inhibiting host immune responses (12). Determining the parts of the genome that remain intact and contain specific antigens that can be targeted for use in a vaccine is crucial for effective vaccine production. (13)
Any useful antigen must be trypanosome-specific and since trypanosomes are eukaryotes they have many proteins within their cell structure that are similar to humans, complicating the search.
Immunity to Trypanosoma cruziis likely to require the initiation of both a humoral and cell-mediated immune response to extracellular trypomastigotes and intracellular amastigotes (14). T.cruzi invades host cells some of which are directly responsible for immunity such as macrophages; mobile leukocytes that once infected carry the pathogen to new infection sites e.g. muscle tissues particularly the heart.
Another major problem are 'anti'T. cruzi antibodies, which have been found to react with cardiac muscles in some chronic sufferers producing autoimmune inflammatory cardiopathies (15).
The protein Tc52 has been investigated and identified as one possible specific antigen with effects on T. cruzi virulence. The protein is responsible for reduction of the host immune system with an immunosuppressive effect, blocking the production of interleukin 2 (IL2) an important signalling molecule, and halting production of T-lymphocytes, important cells in the host’s defences against T.cruzi. Both forms of T.cruzi secrete Tc52 and introduction of the antigen via genetic vaccination in Chagas' infected mice have shown promising results (16).Once infected the host remains infected for life leaving asymptomatic patients capable of spreading the disease through blood products via transfusion (17). Infected induviduals do not present aquired immunity after infection and the pathogen remains in the tissues. 20-40% of infected people develop the cardiac form of the disease, Chagasic cardiomyopathy (18), which can cause sudden heart attack or congestive heart failure (19), often arising some years after infection and apparent recovery. Many of the acute cases are children between the ages of 1-5, leaving them asymptomatic carriers. (20) Reduction of child incidence via a vaccination programme would reduce the number of cases in the local populations.
Blood transfusions using unscreened blood are a concern in North America where the high incidence of relocation of South Americans have brought about more cases and a much higher risk of infection via blood transfusion patients, from donated blood previously unchecked. The full force of the North American problem will become more evident in the next few decades when incidences of Chagas related heart disease may increase (21).
There are currently no drugs to cure Chagas'. Benznidazole and nifurtimox are used to remove blood circulating parasites but are only relatively effective in young people; however, they do not rid the patient of the disease. The side effects of these drugs are painful and this and their cost prevents their wide usage. The search for new drug possibilities has been revived due to evidence suggesting benznidazole may prevent heart disease in up to 62% of young children in the early stages of the chronic phase. (22)
At present control of the insect vector has been the most successful method of keeping the incidence of the disease to minimum. Insecticide sprays and constant surveillance of the infection of isolated populations has reduced the incidence of the disease in many South American countries and some have been certified transmission free due to the efforts of control programmes such as the Southern Cone Programme (23).
Traditional vaccines have some disadvantages when dealing with a pathogen such as T. cruzi, though they are particularly useful in preventing many viral and bacterial infections. (table 1)(24)
New technology has provided the opportunity to search more specifically for new vaccine targets and improve the methods of target administration to increase probabilities of immunity against Chagas'. Genetic vaccines and their integration within the vaccinated individual are now more clearly understood bringing new hopes of an effective vaccination. (25)(26)
Several vaccine targets are currently under investigation one of which is the Paraflagella Rod Proteins (PFR's), proteins specific to trypanosoma and necessary for flagella formation allowing the pathogen to travel to the correct tissues. These proteins are highly conserved between species and any vaccine produced using PFR's may be effective against more than one type of trypanosoma. (27)
The T.cruzi trans salidase genes, ASP-1, ASP-2 and TSA-1 are another possibility as target antigens. These proteins are profuse of the surface of the pathogen and appear to be have important enzymatic roles in T.cruzi survival withsimilar specifics to PFR's in relation to antigenicity (28).
Genetic vaccines are made up of DNA fragments, which induce host cells to produce an antigenic protein, alien to the body, which induces an immune response. (29) ‘Memory cells’ within the immune system recognise the protein if it is introduced at a later stage and destroy it, preventing infection in the same way as conventional vaccines.(30)
DNA vaccines are easy to make, and can be produced inexpensively, allowing poorer countries to access the vaccination, they require no special transport conditions and can be stored in various forms which is important in poorer tropical countries. (31)
DNA vaccines have been shown to produce a strong cell-mediated response, which involves cytotoxic CD+ T cells and T Helper cells (Th1) CD4+ cells (32), a response which may prove effective against intracellular pathogens such as the amastigote phase of Chagas disease.
Testing is still in its infancy and the duration of protection of those vaccination so far tried have been variable. The long-term protection of genetic vaccines in general is still under investigation and may have an effect on vaccine production possibilities in the near future. There are still issues that need to be addressed such as, the probability of naked DNA producing an anti-DNA autoimmune disease or, the occurrence of mutations that may lead to cancer. (33)
Technology has also provided the opportunity to investigate vaccines made from plant products, the World Health Organisation currently support a research project investigating plant extracts that may be capable of blocking T.cruzi enzymes and providing another avenue for vaccine research.(34)
New technology has enabled scientists to discover weaknesses within the genes of T.cruzi, which may allow production of an effective vaccine much sooner than anticipated. Sidestepping chemicals and turning to plants has provided further vaccine possibilities. With more opportunities for discovery of an effective typical antigen emerging, the problems involved in creating a vaccine to ease the global burden of the Chagas disease are no longer insurmountable and the probability of effective immunization against Chagas' disease is gradually increasing.
