Anaplasmosis, a rickettsial disease characterized by progressive anemia and caused by Anaplasma marginale, is endemic in many parts of the country, causing significant economic losses to beef and dairy industries.
Practitioners in some areas are reporting that they are diagnosing anaplasmosis more frequently in recent years, possibly
a reflection of increased cattle movement due to drought and other natural disasters.
The causative organism also can infect sheep, goats and some wild ruminants. These animals usually have inapparent infections
and can become a reservoir for the disease.
A. marginale is transmitted from carrier to susceptible cattle via arthropod vectors or iatrogenically (needles, dehorning and castration
equipment, tattoo instruments, etc.).
Ticks serve as biological vectors and are more likely to spread the disease from herd to herd. Horse flies are the other major
arthropod vector. They are mechanical vectors, so transmission requires close contact of animals. Therefore, they are important
in spreading the disease within a herd. Stable flies can transmit the disease, but probably not as effectively as horse flies.
Although it is stated in the literature that mosquitoes are vectors, they probably play little part in spreading this disease
unless very severe infestations occur.
Iatrogenic spread within a herd also can occur. According to one report, a needle used on an infected animal leads to a 60-percent
chance of the next animal getting infected if the same needle is used.
The incubation period can be from one to eight weeks, depending on the infective dose. In natural infections it is usually
three to five weeks. The organism replicates in red blood cells (RBCs), which are then removed from circulation by the reticuloendothelial
(RE) system. This causes an extravascular hemolysis and progressive anemia.
Animals that survive the anemia usually become chronic carriers. It is unknown where in the body the infection persists, but
blood remains infective, even if infected RBCs cannot be detected on a blood smear. Most carrier animals remain immune to
subsequent acute disease.
Transplacental transmission is reported to occur and could play a role in the maintenance of the disease within herds. Calves
that are infected in utero rarely show clinical signs but become carriers.
The severity of disease is age- and possibly breed-related. Calves less than 1 year old show only mild signs or remain asymptomatic.
Cattle up to 2 years of age have acute but rarely fatal disease. Cattle more than 2 years of age are most likely to have severe,
acute and potentially fatal disease.
Bos taurus cattle appear to be more likely to develop severe, acute disease than Bos indicus cattle. The reasons for the age and breed susceptibility differences are not known.
Peracute, acute, chronic and mild forms of anaplasmosis occur. Acute anaplasmosis is most common and usually occurs in summer
and fall during peak vector season. If outbreaks occur at other times, iatrogenic transmission should be suspected.
Fever (in some), anorexia, rapid loss of body condition, severe decrease in milk production, pale and icteric mucous membranes,
increased heart and respiratory rates, muscle weakness and depression are common.
Aggression from cerebral anoxia is common, especially in beef cattle. Abortions can occur in females and temporary infertility
can occur in males. Since the hemolysis is extravascular, hemoglobinuria does not occur.
With peracute anaplasmosis, death occurs within hours of the onset of clinical signs. This is most common in highly susceptible
purebred animals and high-producing dairy cows. Cattle may die before icterus occurs.
Chronic disease occurs in severely affected animals that do not die. It may take weeks to months for animals to recover, during
which time production losses can be significant (decreased calf weaning weights, infertility). Cattle that have hematocrit
levels less than 11 have a poor prognosis. They might live, but severe hypoxia can damage the heart and other organs, leading
to a chronic "poor doer." In mild cases, abortion and decreased milk production can be the only problems, which might go unnoticed.