Canine distemper: diagnostic use of antibody titres

It is often difficult to make a clinical diagnosis of distemper in the dog, particularly when the presentation is atypical; laboratory investigations are, therefore, indispensible. Infection by canine distemper virus (CDV) induces a specific antibody response although it is often not possible to use this information to make a diagnosis because the demonstration of antibodies by tests such as seroneutralisation is not able to distinguish between antibodies raised to an infection from those produced in response to vaccination. It is, therefore, necessary to use specific investigations such as polymerase chain reaction (PCR)_¹ or examination of cerebrospinal fluid_², which enable a definite diagnosis to be made, but which can only be carried out in specialised laboratories.

The search for IgM antibodies is widely used in human medicine to diagnose infections caused by measles virus, a virus closely related to CDV. Likewise, tests based on indirect immunofluorescence can be used to search for anti-CDV IgM in the serum of infected dogs^_4-5.

IgM are present in the serum of both infected dogs and vaccinated dogs, but in these latter, they appear only after the first vaccination and remain for about 2 months. According to some reports, indirect immunofluorescent detection of IgM in a single sample of blood can enable a diagnosis of distemper to be made, thus becoming an alternative diagnostic method for CDV infections, particularly among populations of dogs in which vaccination is usually performed.

The evaluation of serum IgM antibodies is based on the observation of the kinetics of IgM after vaccination. IgM antibodies are produced only after the first dose of an attenuated CDV vaccine and they persist for about 2 months before disappearing definitively, not reappearing even after booster vaccinations. This situation is the same as that generally seen with measles virus in humans. In fact, in a study of the antibody response in children vaccinated against measles, it was found that subjects receiving two consecutive vaccinations produced IgM only after the first vaccination_⁶.

In vaccinated dogs, IgM antibodies appear in the serum on the fifth day after vaccination and their titre increases until the 14_^th day and then remains stable for 1 week before decreasing gradually until disappearing definitively around the second month after vaccination. Re-vaccination carried out 1 year after the first dose does not lead to the reappearance of IgM antibodies.

Most dogs with clinically suspected infection have titres between 1:40 and 1:320, clearly higher than those in asymptomatic animals which have not been in contact with sources of infection. One possible limitation to the evaluation of IgM antibodies is that similar antibody titres have been found in both symptomatic and asymptomatic animals which have a history of contact with a potential source of infection.

Studies on the use of different techniques for the detection of antibodies have shown that there is a strong correlation between the results of seroneutralisation, indirect immunofluorescence for IgG and an enzyme-linked immunosorbent assay (ELISA), which suggests that they could be used equivalently to demonstrate antibody reactivity to CDV.

An interesting finding concerning antibody titres in vaccinated or infected animals was that 56% of correctly vaccinated animals showed typical symptoms of distemper. Furthermore, there are reports of symptomatic subjects with antibody titres below the threshold of positivity for seroneutralisation, indirect immunofluorescence for IgG and ELISA, even though correctly vaccinated, which raises the problem of lack of immunisation after vaccination^7-11. Various hypotheses have been made to explain this lack of induction of immunity by vaccines: (i) persistence of maternal immunity, which can interfere with the immune response raised to vaccination given too early; (ii) poor storage of the vaccine; (iii) immunodepression of subjects because of stress, malnutrition, parasitic infections, or use of cortisones; (iv) presence of an immunodepressant antigen, such as canine parvovirus, in the vaccine. 

References

1. Shin Y.-S., Mori T., Okita M., Gemma T., Kai C., Mikami T. Detection of canine distemper virus nucleocapsid protein gene in canine peripheral blood monuclear cells by RT-PCR. J. Vet. Med. Sci., 57: 439, 1995.
2. Appel M. Canine distemper virus. In: Appel M.J. (Ed.), Virus infections of carnivores, Elsevier Science Publisher B.V., Amsterdam, 133, 1987.
3. Oshitani H., Suzuki H., Mpabalwani M., Mizuta K., Kasolo F.C., Luo N.P., Numazaki Y. Laboratory diagnosis of acute measles infections in hospitalized children in Zambia. Trop. Med. Int. Hlt., 2: 612, 1997.
4. Blixenkrone-Moller M., Pedersen I.R., Appel M.J., Griot C. Detection of IgM antibodies against canine distemper virus in dog and mink sera emplying enzyme-linked immunosorbent assay  (ELISA). J. Ve t . Diagn. Invest., 3: 39, 1991.
5. Marsilio F., Tiscar P.G., Palucci O., Gatti A. Diagnosi di laboratorio di cimurro del cane mediante la ricerca delle IgM specifiche;  Veterinaria, Anno 13, n. 1, Febbraio 1999
6. Erdman D.D., Health J.L., Watson J.C., Markowitz L.E., Bellini W. J . Immunoglobulin M antibody response to measles virus following primary and secondary vaccination and natural virus infection. J. Med. Virol., 41: 44, 1993.
7. Blinxenkrone-Moller M., Svansson V., Have P., Orvell C., Appel M.J.G., Pedersen I.R., Dietz H.H., Henriksen P. (1993) - Studies on manifestations of canine distemper virus infection in a urban dog population. Veterinary Microbiology, 37: 163-173.
8. Ek-Kommonen C., Sihoven L., Pekkanen K., Ricula U., Nutio L. (1997) - Outbreak of canine distemper in vaccinated dogs in Finland. Veterinary Record, 141: 380-383
9. Marsilio F., Tiscar P.G., Palucci O., Gatti A. (1996) - Valutazione dell’immunità umorale di cani vaccinati per canine distemper virus (CDV). Atti SISVet: 303-304.
10. Povey R.C. (1986) - Distemper vaccination of dogs: factors wich could cause vaccine failure. Canadian Ve t e r i n a ry Journal, 27: 321-323.
11. Sagazio P., Cavalli A., Buonavoglia D., Cirone F., Pratelli A., Buonavoglia C. (1998) - Correlazione tra vaccinazioni e titoli anticorpali nei confronti del virus del cimurro e dell’adenovirus del cane tipo 2 in cani adulti e in cuccioli. Veterinaria, 12: 23-25.