Why is it difficult to control vector-borne diseases?

Controlling the diseases produced by arbovirus requires to act on the vectors that transmit the infection. However, this can be very complex due to the characteristics of the biological cycles of the vectors. In addition, it is not possible to totally eliminate the vectors on farms or in the wild, and many of these vectors are spread throughout the world, as is the case with the Culex mosquitoes or the culicoides midges. Therefore, the control and prevention of these diseases must include,

in addition to the control of vectors, other aspects such as: reducing the risk of animals becoming infected, increasing their resistance by vaccination, and controlling the natural reservoirs that perpetuate the virus in nature. In the event that they are zoonoses, it will also be necessary to prevent contagion to people. Vector control relies on reducing the population of vectors or the exposure of susceptible animals.

Some measures are: Reduce the places or containers that accumulate stagnant water, which are where the females lay their eggs, for example, in puddles, old tyres, buckets, etc. Insecticides and insect repellents on the animals, stables and transport vehicles. Mosquito screens in windows of farms Mosquito repellent lamps with ultraviolet light in stables Housing animals in the hours of the day with increased vector activity, especially at sunrise and sunset. And restrict the movement of animals in the periods of increased activity of vectors. The vectors present in an area can be monitored by placing traps in farms, analysing the species of mosquito or of culicoide midges trapped and the presence of the virus.

Thus, it is possible to detect quickly if the population of a given vector is increasing in an area and whether a virus is circulating. This will suggest increasing control measures on farms and reducing the risk of infection of animals. This measure has proved very effective in the fight against bluetongue and West Nile fever. If the arboviruses have reservoirs in wildlife, it is still more difficult to eliminate the disease once it appears in a region. For this reason, it is necessary to monitor wild animal groups. In West Nile fever, which has many species of wild birds as reservoirs, periodic analyses of these birds are carried out to detect the presence of antibodies or viruses in blood.

This surveillance in wild birds is very effective to detect the circulation of virus in an area quickly, and to take action before cases of the disease appear in horses and people. It is also important that people report if they find dead birds or those with neurological signs. When the incidence of infected animals or the disease itself increases alarmingly, it is very difficult to control its spread. For that reason, susceptible animals need to be vaccinated. This is usually the most effective measure in these cases, but always accompanied by vector control. Usually vaccines are inactivated due to biosecurity, since attenuated live vaccines can cause vaccine-associated viremia in animals, with there being the risk of transmission to new carriers.

The design of vaccines and the protection that they confer depends greatly on the characteristics of the virus. Let’s take a look at the examples of bluetongue and West Nile fever. Bluetongue virus has up to 26 different serotypes that do not induce cross-immunity. This implies that vaccines must include the serotypes that have been isolated in the area, which greatly complicates their development. Currently, the vaccines available are used in sheep and cows in areas with high incidence of the disease. The case of West Nile fever virus is easier, since this virus does not have different serotypes, and vaccines induce a strong immunity against all viral strains. Nowadays, there are inactivated vaccines for horses that are recommended in endemic areas.

At the moment, there are no vaccines marketed for people or birds. In summary, although it is impossible to fully eliminate the risk of diseases by arboviruses, in animals we can apply measures to have the infections under control. It must be borne in mind that these measures must be adapted to each disease and vector in particular to be effective.