Avian spirochetosis is an acute, febrile, septicemic, bacterial disease that affects a wide variety of birds.
The causal organism of avian spirochetosis, Borrelia anserina, is an actively motile spirochete, ~0.2–0.3 mcm × 8–20 mcm, and consists of 5–8 loosely arranged coils. The genome of B anserina is smaller than the one reported for other spirochetes. Cultivation in vitro is difficult and needs special substrates. Borrelia will grow on Barbour-Stoenner-Kelly medium but loses virulence after 12 passages. It can also be propagated in embryonated duck or chick embryos or in young ducks or chicks.
Spirochetosis is found in temperate or tropical regions, wherever the biologic vectors are found. The most common vector is Argas persicus, the “cosmopolitan” fowl tick, but other Argas spp transmit the bacteria in different geographic areas. In the western USA, a highly efficient vector is A sanchezi.
Diverse immunologic and serologic types of B anserina have been demonstrated in many areas. Recovery from one type confers solid immunity against the homologous types for ≥1 year but not against heterologous strains. Relapses, reported for some human Borrelia infections, are unknown in B anserina infection of birds. Therefore, any reinfection can be attributed to a heterologous type.
Generally, an infected Argas tick can transmit the bacteria at every feeding and maintains the infection throughout larval, nymphal, and adult stages. The ticks also transmit the infection transovarially, ie, the F1 larvae are infective. Ticks remain infected despite feeding on chicks hyperimmune to B anserina or on chicks with high blood levels of chemotherapeutic agents effective against Borrelia. Other vectors (lice, mosquitoes, some species of ticks, inanimate objects) can transmit the spirochete mechanically to a susceptible host whenever the piercing apparatus becomes contaminated with blood that contains Borrelia. Ingestion of bile-stained fecal droppings containing the spirochete, contamination of feed or water, and cannibalism during spirochetemia can result in infection. After the bite of an infected tick, the incubation period is ~3–12 days.
Signs of avian spirochetosis are highly variable, depending on the virulence of the spirochete and the age of the birds, with young birds being more affected. Signs include listlessness, depression, somnolence, moderate to marked shivering, and increased thirst but infection may remain asymptomatic. Ruffled feathers, anemia, and pale combs can be noticed as well, and inappetence can lead to reduced weight. During the initial stages of the disease, there is usually a green or yellow diarrhea with increased urates. The course of the disease is 1–2 weeks. Mild strains are common. However, in many tick-infested geographic areas, morbidity and mortality might approach 100% in exceptional cases. Egg production in layers or breeders may be reduced by 5%–10%, with a higher number of small eggs.
The spleen is enlarged, with petechial or ecchymotic hemorrhages, appearing dark or mottled. However, a contrasting situation may be seen in Mongolian pheasants, in which the spleen is reported to be small and pale. Occasionally, the liver may be swollen and contain focal areas of necrosis. Kidneys may be enlarged and pale. A green, catarrhal enteritis is common.
Specific agglutinins clump the spirochetes in successively larger clumps during the terminal stages of the disease. Agglutination lysis then begins to disintegrate these clumps, and spirochetal degradation products are liberated, which may result in fever. Death occurs most often 1–3 days after Borrelia disappear from the bloodstream.
Diagnosis depends on demonstration of Borrelia in the blood, either as actively motile during darkfield microscopy, as stained spirochetes in Giemsa-stained blood smears, or by PCR. In young birds, the Borrelia may reach vast numbers per oil-immersion field and persist for several days. Older birds usually have low numbers of Borrelia that are detected only with difficulty or not at all, and that persist for only 1–2 days. Increased numbers of immature RBCs are noticed due to the anemia. Silver staining can be used to demonstrate the bacteria in tissues.
Agar-gel diffusion and various serologic tests have been described but are of questionable value because of the diverse serotypes that exist in some localities. Spirochetal antibodies are readily detected in yolks of eggs laid by infected hens.
Several antibacterial agents are effective. The most widely used are penicillin derivatives, but streptomycins, tetracyclines, and tylosin are also effective. The antibiotics can be completely effective if begun when the number of spirochetes per oil-immersion field is low or moderate; however, if large numbers of spirochetes are present in the bloodstream, the sudden liberation of large quantities of spirochetal degradation products can result in higher mortality than with no treatment.
Control must be directed against the biologic vector. Argas ticks are notable for their long lifespan, ability to survive for extended periods without a blood meal, efficiency in transmitting the spirochete, and ability to remain securely hidden in cracks and crevices often beyond the effective reach of pesticides. Accordingly, control is difficult. A combination of tick eradication and immunization is the most effective means of control.
Immunization can be highly successful and, next to eradication of the biologic vector, is the preferred method of control. Bacterins prepared from local strains of Borrelia have been used with success. Vaccines may be prepared from formalin- or phenol-inactivated material from lysates of blood, tissues, embryos, or eggs infected with B anserina, and they may be lyophilized or liquid. Whole-egg propagated bacterins are usually given in one or two IM injections. Little if any cross-protection is afforded to different serotypes. Birds normally have protective immunity after recovering from natural infection.