Prevention and Control Methods and Technical Key Points for Canine Distemper in Foxes, Raccoon Dogs and Mink
In some fox and raccoon dog farms and @#@251@#@ farms, outbreaks of canine distemper have occurred. Regarding the prevention and control of canine distemper, the following views are discussed:
Concerning the prevention and control of canine distemper, after decades of research, a variety of attenuated and inactivated vaccines have been developed in terms of its etiology, pathogenesis, epidemiological characteristics, clinical symptoms, pathological diagnosis and immunoprophylaxis, and many vaccines have already been supplied to the market as official commercial products. However, in recent years, the prevalence of canine distemper and the losses it has caused to the world's dog industry and fur animal breeding industry have far exceeded people's expectations. The disease has been continuously prevalent in recent years, and the age of onset is no longer as previously statistically reported, mainly occurring in puppies and fox cubs within 3 months or 1 year of age. Now even suckling puppies and fox cubs at more than ten days of age, as well as adult dogs and adult foxes, have also experienced outbreaks of canine distemper. The mortality rate was previously considered to be 20%, but is currently considered to reach 40-60%.
Some domestic and foreign vaccines that were originally considered to provide good immunity have now repeatedly shown immune failure after use. Why does this phenomenon occur, and what measures should be taken in the future to control it, has become an issue of concern and attention for the vast number of dog, fox and mink raisers as well as disease prevention and control workers.
(I) Discussion of the Causes of Immune Failure
1. Whether variant strains have emerged: Canine distemper is a member of the genus Morbillivirus in the family Paramyxoviridae and has the basic characteristics of viruses in this genus. The fact that the same vaccine in China has different immune effects in different regions may be related to inconsistencies in canine distemper strains between different regions.
2. The problem of maternal antibody interference: Fox cubs can obtain serum antibodies against canine distemper equivalent to 77% of those of the mother fox through the placenta and colostrum, of which 5% comes from the placenta and 95% from colostrum. The immune level of the mother fox is closely related to the antibody level in the colostrum. The maternal antibodies obtained by fox cubs can protect them from infection by virulent canine distemper virus for a certain period of time, but at the same time also interfere with the immunity produced by attenuated canine distemper vaccines. This interference with attenuated vaccines far exceeds the resistance to virulent virus. Experiments show that only canine distemper neutralizing antibodies of 1:100 or above can resist infection by virulent canine distemper virus, but a neutralizing antibody titer of 1:20 can seriously interfere with immunization by canine distemper vaccine. This brings great difficulty to selecting the most suitable timing for canine distemper immunization. If immunization is given too early, the high level of maternal antibodies interferes with the development of active immunity and cannot resist infection by virulent canine distemper virus. The idea of waiting until maternal antibodies decline below 1:20 before immunizing is good, but because canine distemper infection exists in fox and mink farms in various places, before maternal antibodies decline to 1:20, these foxes and mink have already become ill. For this reason, some people have proposed measuring the maternal antibodies of foxes before parturition and determining the primary immunization time for foxes and mink. Although the practice of measuring maternal antibodies before immunization is scientific, it is actually unworkable, because the vast number of fur animal farms do not have such research conditions.
Some people have also suggested that in canine distemper epidemic areas, an early immunization method can be adopted, that is, before newborn fox cubs have sucked colostrum, inject the fox cubs with vaccine, and after 3-4 hours, when the attenuated vaccine has already infected certain cells, then allow them to suck the mother fox's milk, so as to avoid neutralization of the attenuated vaccine virus by maternal antibodies.
Abroad, it is reported that in canine animals such as dogs and foxes, maternal antibodies have declined by 80% at the eighth week of age and have completely disappeared by the ninth week of age. According to this pattern, the first immunization can be carried out at 60 days of age.
Research by Shandong Agricultural University conducted the first immunization at 45 days of age, followed by second and third immunizations at intervals of 2 weeks. Because after 45 days of age, maternal antibodies decline rapidly. If immunization starts at 60 days of age, then 45-60 days of age is an immune blank zone, and fox cubs at this time are most susceptible to infection. Zhou Xiangyang proposed: increase the maternal antibody level; to increase the maternal antibody level of fox cubs and mink, the method is for mother foxes and mink to be injected with measles vaccine 30 days into pregnancy. Within 5-7 days after injection, protective antibodies against canine distemper can be produced, but they are not neutralized by canine distemper antibodies. One week after injection, canine distemper vaccine is then injected. At this time, the colostrum of the mother fox and mink at parturition can maintain a relatively high level of maternal antibodies against canine distemper, which can protect newborn fox cubs and mink from attack by canine distemper virus for a relatively long time. With the above method, no abnormal conditions such as premature birth, miscarriage, deformed fetuses or stillbirths have been observed.
3. Influence of other exogenous and endogenous factors
(1) Genetic factors: Foxes with incomplete immune function, whether female or male, are susceptible to this disease. A common example is inbred foxes.
(2) Mixed infection: In farms where parvovirus disease, coronavirus disease and other diseases are prevalent, the proportion of canine distemper immune failure is relatively high, because during the epidemic process of these diseases, the immune function of foxes is often suppressed. As for whether parvovirus vaccine and fox encephalitis vaccine interfere with canine distemper vaccine, reports are still inconsistent.
(3) Parasitic infection: In fox farms where toxoplasmosis, eperythrozoonosis, piroplasmosis, ascariasis and demodectic mange occur, although canine distemper immunization is carried out according to routine procedures, canine distemper still occurs. The reason is that these parasites cause immunosuppression.
(4) Nutrition and environmental problems: High temperature and high humidity, malnutrition during long-distance transport, and deficiency of sodium selenite and vitamin E in the daily feed can affect the immune function against canine distemper. In addition to causing foxes to be in a state of stress, the above causes also include that a high-temperature and high-humidity environment can cause the body temperature of foxes to rise, thereby affecting the proliferation of the attenuated vaccine virus.
(5) Severe virulent infection: Although the neutralizing antibodies of some foxes are above 1:100 and can resist infection by a certain amount of virulent canine distemper virus, if such foxes are exposed for a long time to an environment contaminated by virulent canine distemper virus and are attacked by large amounts of virulent virus for a long time, exceeding the immunity of the foxes, immune failure can also result.