African swine fever (ASF) is an infectious disease caused by African swine fever virus (ASFV) that is clinically characterized by high fever, skin congestion, and bleeding from internal organs such as lymph nodes, spleen, and kidneys. ASFV can infect domestic pigs, wild boars and soft ticks and is the only DNA virus that uses arthropods as a biological vector. Because the virus is very harmful to the pig industry, the World Organisation for Animal Health (OIE) lists it as an animal disease that must be reported, and China lists it as a class of animal diseases. In the face of the menacing ASF epidemic, it is urgent to do a good job in biosecurity prevention and control, and study the transmission mechanism of its only insect vector, blunt-edged tick in China.
Ticks are commonly known as grass crawlers, ticks, and flat lice. There are 117 known species of ticks in China, including 13 species of soft ticks and 104 species of hard ticks, accounting for about 13% of the total number of ticks in the world. The biological vector of African swine fever is a species of tick, belonging to the arachnid order, tick tick order, tick family, soft tick family, blunt tick tick genus.
Soft ticks are large nest-dwelling blood-sucking animals that usually occur in burrows or protected host habitats for a short period of time, lasting up to 10 minutes. The blunt-edge tick can shed blood from the host after it is saturated with blood, and its blood-sucking behavior occurs only at night when the animal is resting. The blunt-edge tick is a multi-host parasitism, pigs can be their host but do not appear often, often in the nest or body of some rodents such as wild rats, hares, etc., with the activity of animals to move.
The blunt-edged tick is highly adaptable to dry and hot environments, which is why they can be widely distributed on the African continent south of the Sahara Desert. Temperature and humidity in different environments have a great impact on the development of tick bodies, low temperature or starvation environment, larval development slows down, nymph molting time is prolonged, female worms delay spawning. The characteristics of the blunt-edge tick in the host such as multiple blood sucking, multiple spawning and super hunger tolerance make it generally survive for up to 5 to 10 years, and its survival time can be as long as 15 years under suitable conditions.
There are three main ways of asfFV transmission: 1) direct contact transmission. Transmission through direct contact between healthy pigs and sick or recessively poisoned pigs. 2) Biomedia transmission. The blunt-edge tick is an extremely important natural and storage host for ASFV, which can not only be preserved in the blunt-edge tick, but also replicated in its body. The blunt-edged tick is a multi-host parasite that sustains infection in nature by sucking animal blood. 3) Other transmission routes. Other small rodents, mosquitoes, stable flies, etc. can also transmit ASFV through contact with infected pigs; contaminated water, pork and pork products, blood products, contaminated feed, swill, bedding, utensils, short-distance aerosol transmission, etc.
There are two main ways in which ticks transmit ASFV: biological and mechanical. Among them, biological transmission, ASFV can be replicated in the blunt-edge tick and can exist for several months to several years, infected with ticks and then bite pigs will spread the virus, once the blunt-edge tick infected with ASFV, it is easy to form a natural source of epidemic in the region; but over time, the virus titer and transmission capacity of ticks will decline.
According to the results of epidemiological investigation and laboratory infection studies at home and abroad in recent years, ticks that can be used as ASFV vectors are limited to the genus Obtuse tick in the family Soft tick family. The distribution of soft ticks as the biological vector has been found in most of the areas where African swine fever occurs. At least 9 species of blunt-edge ticks have been internationally confirmed to infect and transmit ASFV.
Before the occurrence of African swine fever, There was less attention paid to its vector, the blunt-edge tick, and almost no survey of the distribution of soft ticks in China was conducted. The records of the blunt-edged tick in China are limited to the 1970s, and the distribution survey area is also limited to the desert and semi-desert areas of Xinjiang, Shanxi, Shaanxi and other places in China, and there are no reports in the humid and rainy central China, south China and north China. At present, there are two kinds of blunt-edge ticks known to be distributed in China: the special blunt-edge tick and the left-sided tick, of which the left-sided tick may be a potential medium for ASFV.
At present, the distribution and species of soft ticks in China are missing, and the potential threat of soft ticks, especially blunt-edge ticks, to ASF outbreaks can be underestimated but not yet discovered, once there is a blunt-edge tick in some areas and infected with ASFV, as the natural host and storage host of ASFV in the epidemic area for a long time, the area is easy to form the natural source of ASFV. At present, China should immediately start to investigate and monitor the species and distribution of ticks in various regions, especially the blunt-edge tick, and establish big data of ticks to facilitate a more accurate judgment on the development of the epidemic situation of ASFV in China, provide bio-vector data support for the prevention and control of ASF epidemic, and timely and effectively prevent and control the epidemic.
There is no vaccine against ASF at home and abroad. China is a big pig country, and the number of pigs has grown rapidly in these decades. However, the development of the industry is differentiated, on the one hand, the traditional, small-scale, self-sufficient breeding methods are still scattered; on the other hand, the intensive, large-scale, industrialized breeding methods are increasing. These two completely different breeding methods attach completely different importance to biosecurity prevention and control, the former due to the low degree of biosecurity attention, backward technology, insufficient understanding of animal health regulations, the invasion and spread of African swine fever and other diseases play an important role. ASFV is transmitted by ticks, which is also one of the biggest threats to the prevention and control of the ASF epidemic in China, the lack of targeted testing measures in China, the killing of the African swine fever vector - ticks is not in place, and the risk of ASFV infection cannot be ignored. Doing a good job of disinfection and tick extermination is an important measure to cut off the transmission route of ASFV.
For the effective tick-killing drugs used in pig farms, pig wholesale markets and slaughterhouses, the state has not yet issued corresponding guidelines or standards, for which the Guangdong Provincial Bureau of Animal Husbandry and Veterinary Medicine organized relevant experts to formulate and issued the "Guidelines for the Prevention and Control of African Swine Fever Disinfection and Tick Killing Process and Medication Guidelines" on September 11, 2018, pointing out that the drugs that can be used for effective ticks such as pig farms, pig wholesale markets and slaughterhouses are 40% octylthion pouring solution, cypermethrin solution, etc.
Under the current situation, the first is to do a good job in eliminating ticks in the area where ASF outbreaks have occurred and in the surrounding adjacent areas, and eliminate the natural and storage hosts of ASFV from the source of the epidemic. Second, it is necessary to study, investigate and monitor the species, distribution and biological characteristics of ticks, especially blunt-edge ticks in China. The third is conditional experiments to carry out in-depth research on the infection mechanism of ASFV on blunt-edge ticks, proliferation in vivo, and transmission mechanism between populations. The fourth is to conduct drug screening for blunt-edge ticks, find out more safe and efficient insecticidal drugs, and establish a drug prevention and control mechanism. The fifth is to carry out the development of vaccines for blunt-edge ticks and avoid the excessive use of drugs to make them resistant. In addition to the prevention and control of ticks, the role of stable flies, mosquitoes and other blood-sucking animals in the environment on the transmission of ASFV can not be ignored, and measures to eliminate mosquitoes and flies should be done well, and the management of tick-bearing wild animals (wild mice, hares, foxes, birds) in the environment should be strengthened, and the migration activities of the above animals should be avoided as much as possible to spread ASFV.
Source: Veterinary Guide, No. 5, 2019. Author: Weng Yabiao, work unit: South China Agricultural University
