Canine Parovirus Immunization: Myths And Realities
Time and again, questions have often been asked related to immunization of dogs. For example: Are vaccines safe? What are the risks where adverse reactions are known to occur, such as with MLV distemper and canine adenovirus vaccines? How soon do vaccines provide protection when a dog is exposed to virulent virus? How long does immunity last? Does a vaccine protect against actual infection and subsequent transmission of a pathogen, or only against disease? Should vaccines be expected to provide protection under all circumstances of breeding and management, or are other methods of disease control of equal or even greater importance?
Distinguishing The Viruses: Canine parvovirus-2 (CPV-2) is a term used to distinguish the highly pathogenic parvovirus from the “minute virus of canines” (CPV-I): CPV-2 is closely related to feline, mink and raccoon parvoviruses, but it is known to infect and cause disease only in members of the dog family. The principal mode of transmission is by fecal-oral spread. Virus is shed in the faeces of infected dogs for about one week, but not longer than two weeks. A carrier state has not been confirmed. CPV-2 is very stable to heat and most common chemical disinfectants, and it can survive in the environment for several weeks or months. The survival time depends on climatic conditions, amount of exposure to sunlight and the degree of hygiene practiced. The prevalent virus now circulating worldwide in the dog population appears to have arisen around 1980, as a mutation of the “original” CPV-2. Evidence suggests that the more recent types are somewhat more virulent for dogs. The period of time between infection and the onset of illness is shorter, and the clinical course is commonly characterized by unusually severe hemorrhagic enteritis and/or collapse of an affected pup in a shock-like state. Most dogs will have fever and reduced numbers of circulating white blood cells (leukopenia), especially those called lymphocytes. Decrease in all types of white cells (panleukopenia) is seen only in severe cases. The decrease in white blood cells is usually transient, and is generally more severe in pups less than four months old. It is important to determine the extent of leukopenia early in the course of the disease, since dogs with markedly low white blood cell counts are more likely to die and rapid treatment is essential. It is widely believed that concurrent infections with other infectious organisms such as coronavirus, certain bacteria or parasites result in particularly severe parvoviral disease. Certain breeds appear to be at greater risk, but increased chances of severe infection have been demonstrated so far only for Dobermans, Rottweilers and English Springer Spaniels.
Immunity: Partial And Complete: Immunization is the adrninistration of viral or bacterial antigens with the expectation of producing immunity. The fact that an animal has been vaccinated with MLV or inactivated parvovirus, however, in no way insures that immunity has been produced. Failure to immunize pups has resulted from faulty vaccine potency or giving inadequate doses (dose-splitting). The principal impediment to control of parvoviral enteritis, however, is that period (the critical period) between weaning and sixteen weeks of life when maternal antibody levels are inadequate to protect the pups, yet are high enough to block the response to vaccination. Immune means increased resistance to infection as a consequence of vaccination or previous exposure to an infectious agent. Immunity to CPV-2 may be complete, where the virus is blocked from gaining entry into an animal and unable to multiply and spread, or partial. In the latter case, viral growth in the body is greatly reduced and it may be restricted to the intestine or respiratory tract. An incompletely immune dog infected with CPV-2 will not have virus circulating in the bloodstream and it will usually not become ill because of the presence of antibodies in the serum. Most MLV and inactivated parvoviral vaccines are available, and can fully protect susceptible dogs from disease for periods that range from few weeks, or months, to at least three years. Complete immunity engendered by MLV vaccines in susceptible pups occurs within two to three days of vaccination and usually persists at least three years. The time between vaccination and onset of protection with inactivated vaccines is not known, but is believed to be longer than that for MLV. On the other hand, inactivated vaccines usually give complete immunity for only a few weeks, even though the vaccinated dogs may be resistant to disease for several months.
Types of Vaccines: Killed viral vaccine consists of virus that has been chemically inactivated so as to prevent its growth, but not altered to the extent that it fails to engender an immune response. Such vaccines contain, in addition to virus, plentiful amounts of foreign protein in the form of cell debris and animal serum. Some contain adjuvants that may cause a brief sting. Two doses given at least three weeks apart are required. Protection is usually more variable with inactivated vaccines since it depends upon the amount of virus – the actual mass of viral antigen. Inactivated vaccines are very safe, but they usually do not result in enduring immunity. The actual duration of immunity to inactivated CPV-2 vaccines has not been reported but it may last several months. If a dog encounters virulent virus during that period when it is resistant to disease, but not completely immune, it will develop a post-infection type immunity which may last several years. The only way to determine the immune status of a dog is to determine the level (titer) of serum antibody. Dogs given inactivated vaccines should be revaccinated at least annually, even better at six-month intervals during the first year of life, unless it is known that a vaccinated dog has been naturally exposed to parvovirus. Modified live virus vaccines consist of mutant viruses which are weakened in their ability to cause disease as a result of prolonged laboratory propagation and selection for reduced virulence. CPV-2 is grown in canine or feline cell cultures. Even when diluted, such vaccines often contain several thousand times the minimal immunizing dose. Live vaccine viruses replicate in the inoculated dog and produce immunity, but they must not grow to the extent that they cause unacceptable illness. Acceptable reactions are those such as slight temperature elevation, brief muscle soreness or decrease in appetite. Unacceptable reactions or adverse reactions may still occur on rare occasions with some vaccines such as canine distemper (CD). Combined or muitivalent vaccines contain more than one virus, or several viruses plus one or more bacterial antigens. Animals given multivalent vaccines that contained as many as six different viruses or bacterial organisms respond to individual components in the same manner as if each one were administered separately. The notion that a dose of combined vaccine could overload a dog’s immune system is unfounded. Also vaccines are manufactured as a standard dose, and a dose should never be split. The dose for a Chihuahua or Yorkie should be the same as that for a Newfoundland or Great Dane. Vaccines are not formulated on a body weight basis. One practical way that has been used to deliver a full dose of vaccine to smaller pups or toy breeds is to deliver a smaller volume by reducing the amount of dilution (usually distilled water) when reconstituting freeze-dried vaccines. In that way, an entire dose can be given, but in a smaller volume. However, this is not possible with inactivated vaccines where the entire dose must be given.
Myths And Realities:
Myth: All living strains of CPV-2 are the same with respect to the degree of weakened virulence and efficacy.
Reality: Living parvovirus vaccinal strains do differ somewhat to the extent they have been cultivated in the laboratory. For example, certain MLV vaccine strains that had undergone prolonged culture in the laboratory were found to grow well in cell cultures, but not in the dog. Such vaccines were inconsistent in their ability to immunize dogs.
Myth: Some modified live vaccines “break through” maternal antibody and immunize pups at a time when they are still protected against disease.
Reality: Of the five different commercial vaccines studied including the ones where maternal antibody breakthrough has been claimed – none immunized pups until maternal antibody titers had declined to levels that indicate susceptibility to infection. The biological role of maternal antibodies is to protect pups against infection during gestation and early life. Maternal antibodies are, however, the principal mechanism for immunization failures. Antibody levels sufficient to block successful immunization may fail to suppress infection by virulent CPV-2.
Myth: Some living CPV-2 vaccine viruses are not shed in the faeces.
Reality: All vaccines tested spread to contact animals within one week of vaccination, presumably by the fecal-oral route. Spread to susceptible littermates is believed to be the reason for increased immunication rates of live virus vaccines in breeding kennels.
Myth: Attenuated CPV-2 vaccines suppress the immune system so as to render dogs more susceptible to other pathogenic agents, provoke postvaccinal reactions and even precipitate immune disorders.
Reality: Experimental and field evidence has not provided any indication that CPV-2 vaccines cause immuno-suppression or immune disorders. There is no scientific basic for such speculative concerns.
Myth: Certain MLV vaccines may boost pre-existing antibody titers and reinforce immunity.
Reality: Do not expect dogs to have booster responses, except those animals with very low titers.
Myth: Live parvo vaccines may precipitate bloat and seizures. They are suspected as a cause of autoimmune hemolytic anemia, uterine inertia, ulcers, skin disorders…. Using killed parvo decreases chances of postvaccinal illness.
Reality: Postvaccinal reactions have indeed occurred following vaccination with several viral vaccines.
Myth: More frequent vaccination will control or even eliminate parvovirus from contaminated breeding kennels with sustained episodes of illness.
Reality: Frequent vaccination alone will not reduce parvo-virus in kennels with a high incidence of disease. Meticulous attention must also be paid to management and hygiene. Common sense cleanliness is vital to any success in disease control. Pups should be isolated as much as possible; use rubber boots when entering a whelping/rearing area; wash hands going in and out and use disposable outer clothing.
Focus of Immunization Programme: The principal viral diseases that threaten a pup’s health during the first four months of life are parvovirus infection and distemper. By comparision, disease caused by hepatitis (canine adenovirus-1) or corona-virus is rarely seen. Therefore, the focus of any immunization programme during the first three months of a pup’s life should be directed toward prevention of distemper and parvovirus infection. Studies have indicated that a large majority of vaccinated dogs (more than ninety-five percent) had high levels of antibody to distemper, hepatitis and parvovirus for at least three years. Annual vaccination should, therefore be viewed as a form of term insurance, rather than as an established requirement. Nevertheless, the placement of physical barriers against disease transmission – together with use of vaccines that interrupt viral transmission are essential to reduce parvovirus disease in breeding kennels.